Development of Ethyl Formate Delivery System for Active ...

Development of Ethyl Formate Delivery System for Active Packaging and

Insect Fumigation Applications through Precursor and Electrospinning

Technologies

by

Amr Zaitoon

A Thesis

presented to

The University of Guelph

In partial fulfillment of requirements

for the degree of

Doctor of Philosophy

in

Food Science

Guelph, Ontario, Canada

© Amr Zaitoon, April 2021

ABSTRACT

DEVELOPMENT OF ETHYL FORMATE DELIVERY SYSTEM FOR ACTIVE

PACKAGING AND INSECT FUMIGATION APPLICATIONS THROUGH

PRECURSOR AND ELECTROSPINNING TECHNOLOGIES

Amr Zaitoon Advisors:

University of Guelph, 2021 Prof. Loong-Tak Lim

Prof. Cynthia Scott-Dupree

Ethyl formate (EF) is a generally regarded as safe (GRAS) volatile compound which is

naturally occurring in many food products (e.g., rice, barley, grapes, beer, cheese). It is a potent

insecticide and antimicrobial promising as an alternative to the fumigant methyl bromide, which

is being phased out due to its ozone depletion in the stratosphere and toxic properties. However,

EF is highly volatile, flammable, and susceptible to hydrolytic degradation. These properties pose

substantial end-use delivery challenges. In this research, a novel solid-state EF precursor (EFP)

compound was synthesized via the condensation reaction of adipic acid dihydrazide and triethyl

orthoformate to form diethyl N,N'-adipoyldiformohydrazonate, as confirmed by Fourier

transformed infrared and solid-state nuclear magnetic resonance spectroscopies. EFP was non-

volatile and remained stable under dry conditions but could be hydrolyzed readily in the presence

of moisture and acid to trigger the release of EF vapor. To facilitate the end-use applications, EFP

was encapsulated in free-surface electrospun ethylcellulose/poly(ethylene oxide) (EC-PEO)

nonwovens after optimizing the polymer concentrations to obtain bead-free electrospun fibers. The

release kinetics of EF vapor from the neat EFP powder and EFP-loaded EC-PEO nonwovens were

evaluated at different conditions using gas chromatography. Scanning electron microscopy

revealed that EFP powder was physically entrapped within the electrospun EC-PEO fibers. Fourier

transformed infrared spectroscopy detected no specific interactions between EFP and the polymers

in the nonwovens. Preliminary studies on strawberries showed that the release of EF vapor from

the EFP-loaded nonwovens delayed the growth of spoilage microorganisms on the fruits.

Furthermore, EFP powder was used as an EF activated release system for in-packaging fumigation

of blueberries to control an invasive insect pest in fruits – spotted-wing drosophila (SWD).

Complete control of SWD eggs, larvae, pupae, adults was achieved after EF exposure. There were

no significant (p > 0.05) differences in blueberries quality parameters between EF treated and

untreated berries. This research shows that the conversion of the highly volatile EF into a solid-

state precursor, in conjunction with the activated release strategy, can be useful for active

packaging applications of fresh produce to mitigate insect pest risks and prevent microbial growth

during distribution.

iv

ACKNOWLEDGEMENTS

First and foremost, all praises and thanks be to God, for without his showers of blessings,

this study would not have been possible. My deepest gratitude goes to Professor Loong-Tak Lim,

who gave me an amazing opportunity to join his research group and expertly guided me through

my graduate education. His persistent enthusiasm and keen interest in science and technology kept

me engaged in my research work with a constant passion to learn more and helped me to a very

extent to accomplish this work. It is also my genuine pleasure to thank Professor Cynthia Scott-

Dupree for generously supporting and guiding me with her immense knowledge and expertise. Her

guidance helped in all the time of my research to achieve the objectives of my studies. Besides my

advisors, I would like to thank Professor Gopinadhan Paliyath from my thesis committee for his

insightful comments and encouragement. My gratitude also goes to Dr. Mohamed Abdel-Wahab,

Dr. Sameer Al-Abdul-Wahid, and Mr. Robert Harris for their assistance in my research studies. I

am grateful to all staff in the Department of Food Science for their kind help. I would like also to

acknowledge all of my colleagues in the Packaging and Biomaterial Group: Ayesha, Charles,

Caihua, Dongning, David, Emeli, Fabiana, Liguo, Matthew, Raymond, Ramazan, Singam, Tabrez,

Xiuju, and Yibo for giving me a pleasant work experience, leading discussions, and brainstorming

ideas. I would like to thank the financial support from Ontario Agri-Food Innovation Alliance and

iFood Packaging Systems Corp. It is my privilege to thank my beautiful wife, Reem, for her love,

caring, and encouragement throughout my research studies. I also would like to extend my thanks

to my siblings, Hossam and Hatem, and brother-in-law, Ahmed, and my dear friends, Ali and

Mohamed for their valuable support. Last but not least, I am extremely grateful to my parents for

their love, prayers, and sacrifices for educating me for my future. I feel very blessed and lucky

every day that I have wonderful parents like you.

v

Publications included in this thesis:

a) Peer-reviewed papers:

1) Zaitoon, A., Lim, L. T., & Scott-Dupree, C. (2019). Synthesis and Characterization of

Ethyl Formate Precursor for Activated Release Application. Journal of agricultural and

food chemistry, 67(50), 13914-13921. https://doi.org/10.1021/acs.jafc.9b06335.

2) Zaitoon, A., & Lim, L. T. (2020). Effect of Poly (ethylene oxide) on the Electrospinning

Behavior and Characteristics of Ethyl Cellulose Composite Fibers. Materialia, 100649.

https://doi.org/10.1016/j.mtla.2020.100649.

3) Zaitoon, A., Lim, L. T., & Scott-Dupree, C. (2020). Activated release of ethyl formate

vapor from its precursor encapsulated in ethyl Cellulose/Poly (Ethylene oxide)

electrospun nonwovens intended for active packaging of fresh produce. Food

Hydrocolloids, 106313. https://doi.org/10.1016/j.foodhyd.2020.106313.

4) Zaitoon, A., Jabeen, A., Ahenkoraha, C., Scott-Dupree, C., & Lim, L. T. In-Package

Fumigation of Blueberries Using Ethyl Formate Vapor Released from a Solid-State

Precursor: Effects on Spotted-Wing Drosophila (Drosophila suzukii Matsumura)

Mortality and Fruit Quality. Food Packaging and Shelf Life (Under revision).

5) Zaitoon, A., & Lim, L. T. Controlled and Triggered Release of Active Gaseous/Volatile

Compounds for Active Packaging Applications of Agri-Food Products: A Review

(Manuscript in preparation)

b) Patent:

1) Zaitoon, A., Lim, L. T., and Scott-Dupree, C. 2020. Precursor compounds of ester

compounds. International Patent application PCT/CA2020/050725.

vi

TABLE OF CONTENTS

Abstract ........................................................................................................................................... ii

Acknowledgements ........................................................................................................................ iv

Publications included in this thesis ................................................................................................. v

Table of Contents ........................................................................................................................... vi

List of Tables ................................................................................................................................. xi

List of Figures .............................................................................................................................. xiii

List of Abbreviations .................................................................................................................. xvii

Chapter 1: Introduction ................................................................................................................... 1

Chapter 2: Controlled and Triggered Release of Active Gaseous/Volatile Compounds for Active

Packaging Applications of Agri-Food Products: A Review# .......................................................... 5

2.1 Abstract ................................................................................................................................. 6

2.2 Introduction ........................................................................................................................... 7

2.3 Active gaseous/volatile compounds ...................................................................................... 9

2.4 Controlled and triggered release of active gases/volatiles through encapsulation ............. 14

2.4.1 Cyclodextrins ............................................................................................................... 16

2.4.1.1 Molecular structure and properties of cyclodextrins ............................................. 16

2.4.1.2 Encapsulation of gases/volatiles into cyclodextrins .............................................. 18

2.4.1.3 Release properties of gases/volatiles from cyclodextrin complexes ...................... 23

2.4.1.4 Modified cyclodextrins .......................................................................................... 25

2.4.2 Metal-organic frameworks ........................................................................................... 31

2.4.2.1 Molecular structure and properties of metal-organic frameworks ......................... 31

2.4.2.2 Encapsulation of gases/volatiles into metal-organic frameworks .......................... 33

2.4.2.3 Release properties of gases/volatiles from metal-organic framework complexes . 35

2.4.3 Other supramolecular assemblies ................................................................................ 41

vii

2.4.4 Encapsulation in polymer films ................................................................................... 44

2.4.5 Encapsulation in electrospun nonwovens .................................................................... 49

2.5 Controlled and triggered release of active gases/volatiles through Precursor compounds . 55

2.6 Applications of gases/volatiles controlled release systems in active food packaging ........ 62

2.7 Conclusion .......................................................................................................................... 66

Chapter 3: Justification, hypothesis, and objectives ..................................................................... 69

3.1 Ethyl formate as a fumigant ................................................................................................ 69

3.2 Hypothesis........................................................................................................................... 70

3.3 Objectives ........................................................................................................................... 71

Chapter 4: Synthesis and Characterization of Ethyl Formate Precursor for Activated Release

Application# .................................................................................................................................. 72

4.1 Abstract ............................................................................................................................... 73

4.2 Introduction ......................................................................................................................... 74

4.3 Materials and Methods ........................................................................................................ 77

4.3.1 Materials ...................................................................................................................... 77

4.3.2 Synthesizing of ethyl formate precursor ...................................................................... 78

4.3.3 Fourier transformed infrared analysis .......................................................................... 78

4.3.4 Nuclear magnetic resonance spectroscopy .................................................................. 78

4.3.5 Differential scanning calorimetry ................................................................................ 79

4.3.6 Particle size analyzer .................................................................................................... 79

4.3.7 Scanning electron microscopy ..................................................................................... 80

4.3.8 Headspace analysis ...................................................................................................... 80

4.3.9 Ethyl formate release studies ....................................................................................... 82

4.3.10 Stability study ............................................................................................................ 83

4.3.11 Data analysis .............................................................................................................. 83

viii

4.4 Results and discussion ........................................................................................................ 84

4.4.1 Ethyl formate precursor formation ............................................................................... 84

4.4.2 Thermal analysis .......................................................................................................... 88

4.4.3 Particle size distribution and morphology ................................................................... 90

4.4.4 Activated release of ethyl formate ............................................................................... 91

4.4.5 Stability of the precursor .............................................................................................. 95

4.5 Conclusion .......................................................................................................................... 96

Chapter 5: Encapsulation of the ethyl formate precursor into electrospun nonwovens—Part 1:

Effect of Poly(ethylene oxide) on the Electrospinning Behavior and Characteristics of Ethyl

Cellulose Composite Fibers# ......................................................................................................... 97

5.1 Abstract ............................................................................................................................... 98

5.2 Introduction ......................................................................................................................... 99

5.3 Materials and Methods ...................................................................................................... 102

5.3.1 Materials .................................................................................................................... 102

5.3.2 Spin-dope solutions and rheological measurements .................................................. 102

5.3.3 Electrical conductivity and surface tension measurements ....................................... 103

5.3.4 Electrospinning process ............................................................................................. 103

5.3.5 Microstructural and infrared analyses ........................................................................ 105

5.3.6 Data analysis .............................................................................................................. 106

5.4 Results and discussion ...................................................................................................... 106

5.4.1 Polymer solution parameters ...................................................................................... 106

5.4.1.1 Apparent viscosity ............................................................................................... 106

5.4.1.2 Electrical conductivity ......................................................................................... 108

5.4.1.3 Surface tension ..................................................................................................... 110

5.4.2 Infrared vibrational analysis of electrospun nonwovens ........................................... 111

5.4.3 Spinnability and morphology of the electrospun fiber .............................................. 114

ix

5.5 Conclusion ........................................................................................................................ 122

Chapter 6: Encapsulation of the ethyl formate precursor into electrospun nonwovens—Part 2:

Activated release of ethyl formate vapor from its precursor encapsulated in ethyl

Cellulose/Poly(Ethylene oxide) electrospun nonwovens intended for active packaging of fresh

produce# ...................................................................................................................................... 124

6.1 Abstract ............................................................................................................................. 125

6.2 Introduction ....................................................................................................................... 126


6.3.1 Materials .................................................................................................................... 128

6.3.2 Ethyl formate precursor formation ............................................................................ 129

6.3.3 Spin-dope solution preparation .................................................................................. 129

6.3.4 Spin-dope solution properties .................................................................................... 130

6.3.5 Electrospinning process ............................................................................................. 130

6.3.6 Scanning electron microscopy analysis ..................................................................... 131

6.3.7 Differential scanning calorimetry and Fourier transformed infrared spectrometry ... 132

6.3.8 Ethyl formate release from encapsulated precursor ................................................... 132

6.3.9 Stability study ............................................................................................................ 134

6.3.10 Preliminary studies on preservation of strawberries ................................................ 134

6.3.11 Data analysis ............................................................................................................ 135


6.4.1 Effect of ethyl formate precursor loading on polymer solution properties ................ 136

6.4.2 Characterization of the electrospun nonwovens ........................................................ 138

6.4.3 Activated release of ethyl formate from its precursor-loaded nonwovens ................ 145

6.4.4 The efficacy of ethyl formate precursor for delaying spoilage in strawberries ......... 151

6.5 Conclusion ........................................................................................................................ 154

x

Chapter 7: In-Package Fumigation of Blueberries Using Ethyl Formate Vapor Released from a

Solid-State Precursor: Effects on Spotted-Wing Drosophila (Drosophila suzukii Matsumura)

Mortality and Fruit Quality #....................................................................................................... 156

7.1 Abstract ............................................................................................................................. 157

7.2 Introduction ....................................................................................................................... 158


7.3.1 Materials .................................................................................................................... 161

7.3.2 Ethyl formate precursor formation and capsules preparation .................................... 162

7.3.3 Headspace concentration of ethyl formate ................................................................. 164

7.3.4 Spotted-wing drosophila rearing conditions .............................................................. 165

7.3.5 Spotted-wing drosophila eggs, larvae, and pupae ...................................................... 165

7.3.6 Fumigant toxicity bioassays ....................................................................................... 166

7.3.7 Fruit quality evaluation .............................................................................................. 167

7.3.8 Data analysis .............................................................................................................. 169


7.4.1 Headspace concentration and cumulative EF exposure ............................................. 169

7.4.2 Mortality of spotted-wing drosophila adults exposed to ethyl formate vapor ........... 172

7.4.3 Mortality of spotted-wing drosophila eggs, larvae, and pupae exposed to ethyl

formate vapor ...................................................................................................................... 176

7.4.4 Effect of ethyl formate fumigation on fruit quality ................................................... 177

7.5 Conclusion ........................................................................................................................ 181

Chapter 8: Conclusion and future work ...................................................................................... 182

8.1 Overall conclusion ............................................................................................................ 182

8.2 Future works ..................................................................................................................... 184

Chapter 9: References ................................................................................................................. 187

xi

LIST OF TABLES

Table 2.1: Active gaseous/volatile compounds for active food packaging applications. ............. 12

Table 2.2: Some important properties of cyclodextrins (Ho et al., 2014; Marques, 2010). ......... 18

Table 2.3: Inclusion complexes of various active gaseous/volatile compounds into cyclodextrins.

....................................................................................................................................................... 27

Table 2.4: Adsorption of various active gaseous/volatile compounds by metal-organic

framework. .................................................................................................................................... 38

Table 2.5: Various solid matrices used to encapsulate active gaseous/volatile compounds. ....... 43

Table 2.6: Polymer-based films containing active gaseous/volatile compounds with/without prior

encapsulation................................................................................................................................. 47

Table 2.7: Recent studies on the encapsulation of volatile active compounds into electrospun

nonwovens. ................................................................................................................................... 53

Table 2.8: Precursor compounds used to stabilize and control the release of gaseous/volatile

active compounds.......................................................................................................................... 61

Table 2.9: Some commercially available controlled release systems for active gaseous/volatile

compounds. ................................................................................................................................... 65

Table 4.1: Ethyl formate (EF) released from the precursor (A and B) at different temperatures

using 0.1N citric acid, and the fitted model parameters. .............................................................. 94

Table 4.2: Ethyl formate (EF) released from the precursor (B) at 25°C using different RH with

acidified substrate, and the fitted model parameters. .................................................................... 94

Table 5.1: Polymer solutions with different EC-PEO blend ratios in different aqueous ethanol

prepared for electrospinning. Electrical conductivity, surface tension, consistency coefficient (k),

flow behavior index (n), and apparent viscosity (µ) at 1000 s-1 were indicated. ........................ 104

Table 5.2: Spinnability of the polymer solutions and the average fiber diameter. Sample codes

(F1−F9) are described in Table 5.1. ............................................................................................ 115

Table 6.1: Electrical conductivity, surface tension, flow behavior index (n), consistency

coefficient (k), coefficient of determination (R2) for fitting shear stress versus shear rate data

using Eq. 6.1, and apparent viscosity (µ) at a shear rate of 1000 s-1 of EC-PEO solutions loaded

with different ethyl formate precursor (EFP) contents. The average fiber diameters of resulting

electrospun fibers were also indicated. ....................................................................................... 139

xii

Table 6.2: Onset of melting (To) and melting peak (Tm) temperatures, and enthalpies of melting

(Hm) of pristine ethyl formate precursor (EFP) particles and that encapsulated in EC-PEO

electrospun nonwovens at 10, 30, 50, 70, and 100 % (w/w, polymer content basis). ................ 145

Table 6.3: The EF released from its precursor embedded in EC-PEO electrospun fibers at 120

min and the fitted model parameters as affected by CA concentration, temperature, and EFP

loading capacity. ......................................................................................................................... 149

Table 7.1: Quantities of ethyl formate precursor (EFP, mg) incorporated inside the 0.5 L

thermoformed PET container and EF vapor (mg) released for fumigation and fruit quality

experiments. Corresponding EF dosages (mg/L) were calculated based on the empty volume of

the container. ............................................................................................................................... 163

Table 7.2: Effect of exposure to ethyl formate (EF) vapor at different doses at 22 ± 1 °C on fruit

quality parameters. Firmness (N), total soluble solids (TSS, %), pH, and titratable acidity (TA,

%) were measured after 24 h of exposure to EF vapor, and after another 24 h of aeration (48 h

post-treatment). ........................................................................................................................... 180

Table 7.3: Effect of exposure to ethyl formate (EF) vapor at different doses at 22 ± 1°C on fruit

volatiles content (i.e., ethanol and EF, mg/kg). Volatiles were measured after 24 h of exposure to

EF vapor, and after another 24 h of aeration (48 h post-treatment). ........................................... 180

xiii

LIST OF FIGURES

Fig. 2.1: The chemical structure of cyclodextrins (CDs) and their geometrical dimensions. ....... 17

Fig. 2.2: A schematic illustration of 3-D metal-organic framework (MOF). ............................... 32

Fig. 2.3: Schematic representation of conventional single needle electrospinner (A) and free

surface wire electrospinner (B). .................................................................................................... 51

Fig. 2.4: Concept of the formation of precursors for volatile compounds and their subsequent

release through selective bond cleavage by a triggering agent. .................................................... 57

Fig. 4.1: Schematic representation of the automatic headspace analysis system for studying EF

release kinetics. GSV; gas sampling valve. SV0 and SV1; stream selection valves. ................... 81

Fig. 4.2: Formation of ethyl formate precursor, diethyl N,N'-adipoyldiformohydrazonate, via

condensation of adipic acid dihydrazide and triethyl orthoformate. The hydrolysis of the

precursor, catalyzed by an acid, releases EF. ................................................................................ 86

Fig. 4.3: Fourier transformed infrared (FTIR) spectra of triethyl orthoformate, adipic acid

dihydrazide, and precursors derived from method (Ӏ) and method (П) (see Section 4.3.2) as a

function of reaction time. .............................................................................................................. 87

Fig. 4.4: Solid-state 13C nuclear magnetic resonance (NMR) spectra for precursors derived from

methods (A) and (B) (see Section 4.3.2)....................................................................................... 88

Fig. 4.5: Representative differential scanning calorimetry (DSC) thermograms of adipic acid

dihydrazide and precursors from Methods (A) and (B) (see Section 4.3.2). ................................ 89

Fig. 4.6: Scanning electron microscope (SEM) micrographs of precursor (B). (І); for particles,

and (П); for dried dilution. ............................................................................................................ 90

Fig. 4.7: Particle size distribution of precursor (B). ..................................................................... 91

Fig. 4.8: Ethyl formate (EF) release of precursors from methods (A) and (B) at different

temperatures, activated using 0.1 N citric acid solution. .............................................................. 93

Fig. 4.9: Ethyl formate (EF) released from the precursor (B) at 25°C using different RH with

acidified substrate; or using 0.1N citric acid solution. .................................................................. 93

Fig. 4.10: Ethyl formate (EF) released from the precursor stored at 25°C under different RH.

Different alphabets (a–e) indicate statistical significant difference (p < 0.05). ............................ 95

Fig. 5.1: Schematic diagram of the free surface electrospinning for forming EC-PEO nonwovens.

The polymer solution was loaded into the carriage which glided back-and-forth along a

positively charged wire electrode to evenly coat it with the spin dope solution. When the critical

xiv

voltage was supplied to the wire electrode, the solution ejected as multiple jets toward the

collector substrate positioned in between the spinning and grounded electrodes. ..................... 105

Fig. 5.2: Effects of solvent (A), PEO content (B), and PEO molecular weight (C) on the apparent

viscosity values of EC solutions at different shear rates. Sample codes (F1−F9) are described in

Table 5.1. .................................................................................................................................... 108

Fig. 5.3: Fourier transformed infrared (FTIR) spectra for PEO powder, neat EC fiber (F1), and

EC-PEO fibers, with 1 % PEO (F4), 2 % PEO (F6), and 3 % PEO (F7). A: In the region from

600 to 4000 cm-1. B: In the region from 700 to 1300 cm-1, indicating the shift in PEO

characteristic bands of EC-PEO fiber. ........................................................................................ 113

Fig. 5.4: Scanning electron microscope (SEM) micrographs and histograms of fibers diameter

distribution of neat EC nonwoven (F1) and EC-PEO nonwovens generated using 1 % PEO100 in

70 % (F2), 80 % (F3), 90 % (F4),and 95 % (F5) aqueous ethanol as a solvent. Sample codes

(F1−F5) are described in Table 5.1. Electrospinning process conducted at a carriage speed of 100

mm/s and voltage of 45 kV. ........................................................................................................ 119

Fig. 5.5: Scanning electron microscope (SEM) micrographs for EC-PEO nonwovens, generated

using 70, 80, 90, and 95 % aqueous ethanol (F2, F3, F4, and F5) at 30000x magnification,

showed the skin wrinkles. The underlying fibers were also wrinkled, but they are out of focus in

this image .................................................................................................................................... 120


distribution of EC-PEO nonwovens with 2 and 3 % PEO 100 kDa (F6, and F7), 1 % PEO 300

kDa (F8), and 1 % PEO 900 kDa (F9) in 90 % aqueous ethanol. Sample codes (F6−F9) are

described in Table 5.1. Electrospinning process conducted at a carriage speed of 100 mm/s and

voltage of 45 kV. ......................................................................................................................... 121


distribution of EC-PEO nonwovens with 1 % (w/w) PEO 300 kDa (A) and 1 % (w/w) PEO 900

kDa (B) in 80 % aqueous ethanol. Electrospinning process conducted at a carriage speed of 100

mm/s and voltage of 45 kV. ........................................................................................................ 122

Fig. 6.1: Schematic diagram of the free surface electrospinning setup based on a stretched wire

as a spinning electrode ................................................................................................................ 131

Fig. 6.2: Schematic diagram of the setup used for studying the efficacy of ethyl formate (EF)

vapor, released from its precursor-loaded nonwovens, on extending the shelf-life of strawberries.

..................................................................................................................................................... 135


distribution of EC-PEO nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w,

polymer content basis). ............................................................................................................... 143

xv

Fig. 6.4: Fourier transformed infrared (FTIR) spectra of pristine EFP particles and EC-PEO

electrospun nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w, polymer content

basis). .......................................................................................................................................... 144

Fig. 6.5: Differential scanning calorimeter (DSC) thermographs of pristine EFP particles and EC-

PEO electrospun nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w). ............. 144

Fig. 6.6: Ethyl formate precursor, diethyl N,N'-adipoyldiformohydrazonate, and its acid

hydrolysis, releasing ethyl formate vapor. .................................................................................. 150

Fig. 6.7: The release of ethyl formate from its precursor (EFP) embedded in EC-PEO electrospun

fibers as affected by the CA concentration (A), temperature (B), and EFP loading capacity (C).

..................................................................................................................................................... 150

Fig. 6.8: Ethyl formate (EF) released from the 10% EFP nonwovens stored for 30 days at 25°C

under 0, 60, and 100 % RH. Different alphabets (a–d) indicate statistical significant difference

(p < 0.05). .................................................................................................................................... 151

Fig. 6.9: Photographs of strawberries fumigated with 1 and 2 mg ethyl formate (EF) released

from EC-PEO nonwoven loaded with 70 % (w/w) EF precursor. Samples were stored in PET

packages for 10 days at 5°C, showing delayed mold growth for treated samples compared to the

control. ........................................................................................................................................ 153

Fig. 6.10: Headspace concentrations of O2 (A) and CO2 (B), fruit weight loss percentages (C),

and ethyl formate (EF) release/depletion profiles inside PET packages for 10 d. ...................... 154

Fig. 7.1: Schematic representation of: [A] the ethyl formate (EF) capsule and its activation by

adding citric acid solution to release EF vapor; and [B] the experimental setup for fumigation of

blueberries in thermoformed PET container. .............................................................................. 163

Fig. 7.2: Headspace concentration of ethyl formate (EF) (mg/mg.L; milligram of EF vapor per

milligram of EFP per liter of headspace air) and cumulative EF exposure (concentration

(mg/mg.L) × time (h) product) in PET containers with blueberries at 0 [A], 3 [B], 6 [C], 20 [D],

and 30 % (w/v) [E] loading factor. EF capsules containing 1.32 mg of EFP were used (Section

7.3.3). .......................................................................................................................................... 171

Fig. 7.3: Effect of fruit loading factor (%, w/v) on the cumulative ethyl formate (EF) exposure

(mg.h/mg.L) at 2 and 4h. ............................................................................................................ 172

Fig. 7.4: Mortality of spotted-wing drosophila (SWD) adults at 22 ± 1 °C as affected by EF

dosage and fruit loading factor: At 2 h [A] and 4 h [B] for 0 – 6 % (w/v) load factor, and at 2 h

[C] and 4 h [D] for 20 – 30 % (w/v) load factor. Different alphabets (a–k) indicate statistical

significant difference (p < 0.05).................................................................................................. 175

Fig. 7.5: Mortality of spotted-wing drosophila (SWD) adults after exposure to similar cumulative

ethyl formate (EF) exposures (mg.h/L) for load factors 0 – 30 % (w/v). Cumulative EF exposure

xvi

values (concentration “C” × time “T” products) were generated from Figs. 7.2 and 7.3 for the

different dosages (Table 7.1). ..................................................................................................... 176

Fig. 7.6: Spotted-wing drosophila larvae, pupae, and adults emerged from eggs, larvae, and

pupae, respectively, in blueberries exposed to ethyl formate (EF) vapor at 22 ± 1°C for a 24 h

period. Different alphabets (a–e) indicate statistical significant difference (p < 0.05). .............. 177

xvii

LIST OF ABBREVIATIONS

1-MCP 1-Methylcyclopropene

AITC Allyl isothiocyanate

ANOVA Analysis of variance

AP Active packaging

ATR Attenuated total reflection

CA Citric acid

CAC Cellulose acetate

CD Cyclodextrin

CMC Carboxymethyl cellulose

DLS Dynamic light scattering

DS Degree of substitution

DSC Differential scanning calorimeter

EC Ethyl cellulose

EF Ethyl formate

EFP Ethyl formate precursor

xviii

EOs Essential oils

EVOH Ethylene vinyl alcohol

FDA Food and Drug Administration

FTIR Fourier transformed infrared

GC Gas chromatograph

GRAS Generally recognized as safe

LDPE Low-density polyethylene

NMR Nuclear magnetic resonance

PCL Poly(ε-caprolactone)

PEG Poly(ethylene glycol)

PEGDA Poly(ethylene glycol diacrylate)

PEO Poly(ethylene oxide)

PET Poly(ethylene terephthalate)

PHBV Poly(hydroxybutyrate-co-hydroxyvalerate)

PLA Poly(lactic acid)

PLG Poly(lactide-co-glycolide)

xix

PP Polypropylene

PS Polystyrene

PVA Poly(vinyl alcohol)

PVC Poly(vinyl chloride)

PVP Poly(vinyl pyrrolidone)

SAS Statistical software suite

SEM Scanning electron microscope

SWD Spotted-wing drosophila

TA Titratable acidity

TSS Total soluble solids

1

Chapter 1: Introduction

Ethyl formate (EF) is an FDA-approved food flavoring agent naturally occurring in soil,

water, vegetation, and many food products (e.g., beer, wine, grapes, wheat, barley, raisin, rice,

cheese). EF is a potent insecticidal and antimicrobial agent promising as an alternative to toxic

fumigants (e.g., methyl bromide, phosphine) that are presently being used in quarantine treatments

of fruits/vegetables. Since quarantine and pre-shipment treatments of fruits/vegetables are essential

to prevent microbial growth, quality losses, outbreaks of foodborne pathogens, and

infestation/invasion by insect pests that can be detrimental to both products and environment,

rigorous regulations have been established to ensure that fruits/vegetables are free of insect pests

and pathogenic microorganisms. Typical batch treatments involve the use of fumigation equipment

and compressed gas cylinders that are inconvenient to transport and storage. Fumigation

quarantine of fruits/vegetables is typically carried out in enclosed chambers filled with gaseous

fumigant and can take from several hours up to a day. This extra handling step not only can

potentially inflict product injuries but also delay product shipment, thereby shortening the

available product shelf-life at the retail level. By contrast, in-packaging fumigation can be

beneficial to eliminate the holding of products in the fumigation area, thereby streamlining the

supply chain and reducing the cost. To this end, EF may be incorporated within the individual

packaging unit to fumigate the product to overcome the limitations of the conventional batch

fumigation. However, EF is highly volatile, flammable, and susceptible to hydrolytic degradation,

making its direct end-use application very challenging. This research focused on developing an EF

activated release system intended for in-packaging fumigation of fresh produce and other active

packaging (AP) applications.

2

In Chapter 2, various approaches for stabilizing and controlling the release of

gaseous/volatile active compounds were reviewed and discussed, including encapsulation (e.g.,

into solid matrices, polymer-based films, electrospun nonwovens) and triggered release systems

involving precursor technology. Although precursor technologies have been developed and

applied in cosmetic and perfume industries for the controlled release of volatile compounds, their

uses in AP applications are relatively limited. To the best of our knowledge, controlled release

systems for EF, based on encapsulation or precursor technologies, have not been reported before

in the literature. Therefore, in Chapter 3, a literature review was conducted on EF fumigant for

fresh produce and grain commodities. The challenges associated with its use are also discussed.

The hypothesis and the objectives of this research were presented.

To enhance the storage stability and control the release of EF, a method to synthetize the

EF precursor (EFP) was presented in Chapter 4. The EFP was synthesized via reacting adipic acid

dihydrazide with triethyl orthoformate to produce diethyl N,N'-adipoyldiformohydrazonate, the

molecular structure of which was confirmed by Fourier transformed infrared and nuclear magnetic

resonance spectroscopies. The EFP, appeared as white powder, had a melting point of 174°C and

particle size distribution ranging from 0.15 to 4.62 µm. EFP was non-volatile and remained stable

under dry conditions but could be hydrolyzed readily in the presence of moisture and acid to trigger

the release of EF vapor. The release kinetic of EF vapor from its precursor was studied using

headspace gas chromatography. Upon the exposure to 0.1 N citric acid (CA) solution,

approximately 0.38 mg EF vapor was released from 1 mg of EFP (98 % of the theoretical release)

in 2 h at 25°C. The trigger release behaviour of EF from EFP is promising in applications where

the fumigant release rate must be controlled to achieve optimal efficacy and prevent phytotoxicity.

3

To facilitate the end-use application and prevent premature hydrolysis of EFP when

exposed to ambient moisture, the EFP must be encapsulated within a delivery carrier. In Chapter

5, a method to develop bead-free electrospun ethylcellulose/poly(ethylene oxide) (EC-PEO)

nonwovens for the encapsulation of EFP was presented. Fibers were electrospun using a free

surface electrospinning technique. The EC-PEO spin-dope solutions were prepared by dissolving

EC and PEO in ethanol-water binary solvents. The effects of solvent ratio, PEO molecular weight

and content on the properties (viscosity, electrical conductivity, and surface tension) of the spin-

dope solutions, electrospinning behaviors, and fibers morphology were investigated. The

electrospinning of 10 % (w/w) EC, when dissolved in 90 % aqueous ethanol, resulted in irregular

particulates interweaved with ultrafine fibers, instead of forming continuous fibers. The addition

of 1 % PEO (100 kDa) in EC solution greatly enhanced the electrospinning process, resulting in

bead-free EC-PEO fibers. These polymer concentrations were chosen for the encapsulation of EFP

at elevated concentrations in a study as reported in Chapter 6. Here, the release kinetics of EF

vapor from the precursor-loaded EC-PEO nonwovens were investigated using gas

chromatography. Scanning electron microscopy revealed that EFP particles were entrapped within

the electrospun EC-PEO fibers. Fourier transform infrared spectroscopy did not show any specific

interaction between EFP and the polymers in the nonwovens. When exposed to 0.1 N CA solution

for 2 h at 25°C, the nonwoven loaded with 10 % EFP (w/w) released 0.037 mg EF/mg nonwoven

(96 % of the theoretical release). The EF release rate/amount decreased significantly (p < 0.05)

with decreasing either the CA concentration or the temperature. A preliminary study was

conducted to evaluate the antimicrobial properties of the EFP-loaded nonwovens on strawberries

packaged in thermoformed poly(ethylene terephthalate) containers. The study showed that the EF

4

released from the nonwovens delayed the growth of spoilage microorganisms on strawberries for

up to 10 d at 5°C, suggesting the potential of deploying the EFP-loaded nonwoven for AP

applications.

Additionally, in Chapter 7, EFP was evaluated for in-package EF fumigation of blueberries

infested with various life stages of an insect pest – spotted-wing drosophila (SWD). Here, a capsule

delivery prototype was developed to facilitate the activation EFP. To activate the release of EF

vapor, 0.1 N citric acid solution was added to the capsules. The effects of EF exposure on mortality

of SWD eggs, larvae, pupae, adults in blueberries were studied. Complete control of all life stages

of SWD was achieved after EF exposure (6 mg/L for 4 h for adults and 16 mg/L for 24 h for eggs,

larvae, pupae). No significant (p > 0.05) differences in blueberries quality parameters (firmness,

total soluble solids, pH, and titratable acidity) were observed between EF treated and untreated

berries.

Finally, in Chapter 8, an overall conclusion of this research was presented, along with a

number of proposed topics for future studies to address the knowledge gaps related to this research.

5

Chapter 2: Controlled and Triggered Release of Active Gaseous/Volatile

Compounds for Active Packaging Applications of Agri-Food Products: A

Review#

This chapter is prepared in a manuscript format, which is to be submitted for publication.

6

2.1 Abstract

In active packaging (AP) applications, gaseous and volatile active compounds are being

deployed to enhance safety and preserve quality of many agri-food products during storage and

distribution. However, the use of these compounds is limited by their high vapor pressure and/or

chemical instability. Various approaches for stabilizing and controlling the release of

gaseous/volatile active compounds have been developed, including encapsulation (e.g., into

supramolecular matrices, polymer-based films, electrospun nonwovens) and triggered release

systems involving precursor technology, thereby allowing their safe and effective use in AP

applications. In this review, encapsulation of gases (e.g., CO2, ClO2, SO2, ethylene, 1-

methylcyclopropene) and volatiles (e.g., ethanol, essential oils and their constituents) into different

solid matrices was reviewed in regard to encapsulation mechanisms and release properties. The

incorporation of these compounds into polymer-based films and electrospun nonwovens was

presented as well. The recent development in utilizing precursor compounds of gases/volatiles was

also discussed as a valuable strategy to stabilize and control their release. The potential

applications of these controlled release systems in AP of agri-food products were presented. This

review will assist in exploring innovative controlled release systems for a wide variety of

gaseous/volatile active compounds.

7

2.2 Introduction

Packaging plays a significant role in the food supply chain to protect and ensure the safe

distribution of food products. Traditional packaging systems act as a physical barrier between the

food products and the external environmental conditions. Moreover, they provide ease during

storage and handling. However, changes in consumer preference for fresh, minimally processed,

and additive-free products have led to considerable challenges in using traditional packaging

systems due to reduced product stability. Various AP technologies have been developed, which

are capable of interacting with the product and/or the headspace to enhance the shelf life and

preserve the quality of the food products through the activation of certain mechanisms (Biji et al.,

2015).

One variant of AP is capable of actively releasing active compounds to the food/package

headspace. Common active compounds utilized in AP primarily include antimicrobials to ensure

food safety, antioxidants to increase food stability, and plant growth regulators to control fresh

produce's ripening. The molecular weight, shape, size, and polarity of these compounds are

essential properties that define their stability/release behaviors (Nerin et al., 2016). Non-volatile

active compounds are often stable, thereby can be applied in AP applications through their direct

addition to the packaging materials (e.g., polymer films). In comparison, active gases/volatiles are

highly mobile compounds, i.e., susceptible to undesirable losses by evaporation and/or degradation

during end-uses. Besides the undesirable losses, limitations regarding safety, convenience,

handling, and storage are also problematic. For example, 1-methylcyclopropene (1-MCP) is a

hazardous gas under normal environmental conditions and poses an explosive risk if pressurized

8

in compressed gas cylinders (Neoh et al., 2007). ClO2 gas is normally generated on-site from its

stabilized form due to its high explosiveness, reactivity, and corrosiveness (Chen et al., 2020).

Therefore, stabilization and controlled release of gaseous/volatile active compounds is needed to

allow for their safe and efficient end-use deployment for food preservation and AP applications.

There has been an increasing interest in developing controlled release technologies of

gaseous/volatile active compounds, as demonstrated by the numerous research papers published

in recent years. Encapsulation into solid matrices, polymer-based films, and electrospun

nonwovens have been extensively used/studied to stabilize these compounds. The resulting

materials can be activated by water/moisture or other mechanisms to release the active

gases/volatiles in a controlled manner. Another novel approach is via substituting the

gaseous/volatile active compounds with their chemical precursors. These precursors should be

non-volatile and stable during storage, while sufficiently labile for triggered release of the

gas/volatile through selective cleavage of covalent bonds using triggering reactions (e.g.,

hydrolysis, enzymatic, oxidative) (Levrand et al., 2007). Although precursors have been used

extensively in cosmetic and perfume industries to stabilize volatile compounds (Herrmann, 2007,

2012), their uses in AP are still in the early stage of the investigation.

The objectives of this review are to: (1) present the current status of encapsulation of

gases/volatiles into different encapsulants; (2) discuss the mechanism of encapsulation and release

properties; (3) review the use of chemical precursors of gases/volatiles and their synthetic/release

techniques; and (4) present the potential applications of gases/volatiles controlled release systems

and commercially available systems used in AP of agri-food products.

9

2.3 Active gaseous/volatile compounds

Many active compounds, including antimicrobials, antioxidants, and plant growth

regulators, have been investigated for AP applications to maintain food products safety and quality.

The molecular weight/structure and polarity of these compounds are important determinants that

define their chemical stability and evaporation/diffusion properties (Nerin et al., 2016). Based on

the nature of active compounds, they can exist in the form of gaseous, volatile, or non-volatile

compounds. The choice between these three forms depends mostly on the type of contact between

the food and packaging materials. Where there is a direct contact (e.g., vacuum packaging of solid

food and liquid packaging), volatile and non-volatile compounds can be used, where they can

migrate/diffuse from the packaging material to the food. For indirect contact applications wherein

a substantial package headspace is present (e.g., fresh produce packaging), only gaseous and

volatile compounds can be used due to their ability to release/evaporate into the headspace,

condensate onto product surfaces dictated by solubility, and diffuse from the surface to the food

matrix. Due to their low tendency to evaporate, non-volatile active compounds can be applied in

AP applications through their direct addition to the packaging materials (e.g., polymer films),

where they can diffuse/desorb afterward to the food in a controlled manner. Various non-volatile

active compounds, including synthetic and natural compounds, and their controlled release

systems have been investigated by many researchers and are subjects of comprehensive reviews

in several papers (Almasi et al., 2020; Chen et al., 2018; Khaneghah et al., 2018; Vilela et al.,

2018).

Active gases and volatiles are considered more effective than non-volatile actives, especially

in applications where a relatively large package headspace is present. Due to their high mobility,

10

actives in their gas/vapor phase can quickly diffuse into the headspace and reach the surface of the

product through condensation and absorption phenomena. However, degradation and mass transfer

losses, as well as toxicity are significant concerns for their processing and application. For

example, 1-Methylcyclopropene (1-MCP) is a plant growth regulator that can block ethylene

receptor sites on plant materials (i.e., fresh produce, ornamentals, and cut flowers) to effectively

delay ripening and senescence (Antunes et al., 2010; Asil et al., 2013; Cao et al., 2012; Han et al.,

2015; Pongprasert & Srilaong, 2014; Seglie et al., 2011; Watkins, 2006). Its high activity at low

concentrations, minimal residues in treated produce, and low toxicity have resulted in its

widespread use as an approved agrochemical for the preservation of fresh produce (Watkins,

2015). However, under normal environmental conditions, 1-MCP is a chemically unstable gas and

tends to react with oxygen and/or other organic compounds or self-polymerized. Also, when the

gas is compressed, it poses an explosive hazard (Neoh et al., 2007). Ethylene is a fruit ripening

phytohormone; usually received by its receptor sites on fruits and involved in many aspects of fruit

maturation and senescence (Barry & Giovannoni, 2007; Tucker et al., 2017). It is often used to

promote ripening, de-greening, and enhanced color development in many fruits, especially the

climacterics (Ho et al., 2011b). For commercial uses, ethylene is usually stored in compressed gas

cylinders. This delivery system however is inconvenient for postharvest and plant growth

regulation applications due to safety concerns. Chlorine dioxide (ClO2), sulfur dioxide (SO2), and

ethanol are widely used in the food industry (including food packaging application) as

antimicrobial agents against pathogenic and spoilage microorganisms in perishable products, such

as meat, poultry, fish, and bakery products (Chen et al., 2020; Kaliyan et al., 2007; Latou et al.,

2010; Xu et al., 2011). However, these gases/volatiles are chemically unstable due to their high

11

reactivity, flammability, explosiveness, or corrosiveness at high concentrations. Other active

volatile compounds utilized in AP applications are essential oils (EOs) and their constituents. EOs

are mixtures of organic volatile compounds normally derived from herbs and spices, many of

which exhibit antimicrobial/antioxidant activities. These compounds represent a complex blend of

compounds such as aldehydes, ketones, esters, alcohols, terpenoids and acids, were up to 85 %

wt., of the oil consists of the bioactive constituents and the remaining fraction consists of other

minor components (Ribeiro-Santos et al., 2017). The main components of EOs are usually

responsible for their biological properties, but the minor compounds can also synergistically

contribute to the biological effects (Burt, 2004; Pavela, 2015). Although the applications of EOs

and their constituents are promising in laboratory and controlled settings, some challenges hurdle

their industrial scale-up, such as their premature evaporation losses during processing as well as

their oxidative degradation during storage before intended use.

Stabilization of gaseous/volatile active compounds is necessary for the design of delivery

carriers for AP applications. One strategy is to convert these compounds into solid-state materials

that are chemically stable and have low vapor pressure. This strategy will facilitate the controlled

release of gases/volatiles, which is essential in AP applications. Also, controlled release systems

will be beneficial to optimize the concentration of the active agents in the packaging system, which

preventing undesirable sensorial and toxicity issues (Mastromatteo et al., 2010). In the next

sections, various approaches for stabilizing and controlling the release of gaseous/volatile

compounds are discussed. Examples of active gaseous/volatile compounds used in food

applications are summarised in Table 2.1, which are the focus of this review chapter.

12

Table 2.1: Active gaseous/volatile compounds for active food packaging applications.

Gas/volatile Structural formula Bioactivity References

1-MCP

Delays ripening and

senescence in plant

materials

Watkins,

(2006)

Ethylene

Fruit ripening trigger

phytohormone

Ho et al.,

(2016)

Ethanol

Antimicrobial agent Latou et al.,

(2010)

Carbon dioxide

Antimicrobial agent Kaliyan et

al., (2007)

Chlorine dioxide

Antimicrobial agent Chen et al.,

(2020)

Sulfur dioxide

Antimicrobial agent Xu et al.,

(2011)

Allyl

isothiocyanate

Antimicrobial agent Dai & Lim,

(2014)

Hexanal

Antimicrobial agent.

Phospholipase D

Inhibitor

Jash & Lim,

(2018)

13

Benzaldehyde

Antimicrobial agent Jash et al.,

(2018)

Carvone

Antimicrobial agent.

Sprout inhibitor of

potato tuber

Costa E Silva

et al., (2007)

Cinnamaldehyde

Antimicrobial agent Cui et al.,

(2020)

Thymol

Antimicrobial/antioxi

dant agent

Ramos et al.,

(2012),

(2014)

14

Carvacrol

Antimicrobial/

antioxidant agent

Altan et al.,

(2018)

Menthol

Antimicrobial agent Piran et al.,

(2017)

Eugenol

Antimicrobial/antioxi

dant agent

Garrido-

Miranda et

al., (2018);

Requena et

al., (2017)

2.4 Controlled and triggered release of active gases/volatiles through

encapsulation

Encapsulation is a versatile technique to enclose active compounds in solid, liquid, or

gaseous forms (core materials) within liquid, semisolid, or solid matrices (encapsulants) of

protective materials, often referred to as a carrier, wall, or shell (Mourtzinos & Biliaderis, 2017;

Nedovic et al., 2011; Ray et al., 2016). Encapsulation offer means to: (1) protect sensitive active

components from harsh environments (e.g., light, oxygen, water) that causes losses in their

structural integrity and bioactivity; (2) facilitate delivery by transforming liquids or gases into

15

easily handled solid materials; (3) enhance efficacy by improving solubility and stability; and (4)

provide controlled release kinetics (i.e., temporal/spatial release and/or triggered release) of core

materials (Kitts & Liu, 2015; Mishra, 2015; Nedovic et al., 2011). Encapsulation is beneficial to

protect bioactive compounds (e.g., flavors, vitamins, probiotics, colors, sweeteners, enzymes,

antioxidants/antimicrobial agents, nutrients) in food applications (Đorđević et al., 2014; Zhong et

al., 2015).

Encapsulation techniques can be based on physical (e.g., spray-drying, spray chilling, spray

coating, electrospinning/electrospraying, extrusion, centrifugation, fluidized bed, supercritical

fluids, co-crystallization, and lyophilization), physicochemical (e.g., simple or complex

coacervation, micelles, liposomes, emulsions, lipid matrices, and solvent evaporation), and/or

chemical methods (e.g., interfacial polymerization and molecular inclusion). Detailed information

on these methods can be found in several comprehensive reviews (Bhushani &

Anandharamakrishnan, 2014; Comunian & Favaro-Trindade, 2016; Đorđević et al., 2014; Fang &

Bhandari, 2012; Farjami & Madadlou, 2017; Fu et al., 2016; Ghorani & Tucker, 2015; Loong Tak

Lim, 2015; Mao et al., 2017; Santiago & Castro, 2016; Shewan & Stokes, 2013; Vemmer & Patel,

2013). Different cores and encapsulants can be manufactured to form capsules, particles,

aggregates, coatings, films, and fibers of various sizes, shapes, morphological properties, and

release kinetics (Assadpour & Jafari, 2019; Drosou et al., 2017; Ghayempour & Montazer, 2016;

Saifullah et al., 2019; Tapia-Hernández et al., 2015). Since active components are different in

molecular weight, volatility, polarity, stability, etc., the optimal encapsulation methods should be

applied to meet the particular physicochemical and molecular requirements. For gases/volatiles,

the correct choice of the wall material (encapsulant) with optimal physical and chemical properties

16

is crucial for their stability and encapsulation efficiency. An ideal encapsulant should: (1) exhibit

appropriate solubility, interfacial activity, and rheological properties for processing; (2) provide

maximum protection to the core materials against deleterious factors (e.g., oxygen, heat, light,

humidity); (5) provide stability and controlled release attributes (i.e., temporal/spatial release

and/or triggered release) of core materials during end-use applications; and ( 6) suitable for

industrial use (e.g., non-toxic, cost-effective, etc.) (Mourtzinos & Biliaderis, 2017; Ray et al.,

2016; Zuidam & Shimoni, 2010). In the following sections, selected approaches to encapsulate

and controlled release of gaseous/volatile active compounds will be discussed focusing on the

encapsulation mechanisms and release kinetics of the bioactive compounds involved.

2.4.1 Cyclodextrins

2.4.1.1 Molecular structure and properties of cyclodextrins

The inclusion of small molecules within structural cavities associated together by

intermolecular forces is a field known as supramolecular chemistry (Andrade et al., 2015). One

example of such cavities is cyclodextrins (CDs), which are naturally occurring cyclic

oligosaccharides produced from the enzymatic modification of starch molecules. Typical CDs

consist of six, seven, or eight glucose residues linked by α(1—4) glycoside bonds, denominated

α-, β-, and γ-CD, respectively (Fig. 2.1) (Andrade et al., 2015; Kfoury et al., 2016). Some

physicochemical properties of CDs are outlined in Table 2.2 (Ho et al., 2014; Marques, 2010). The

molecular structure of CD is shaped as a hollow truncated cone with a hydrophilic exterior surface,

due to the presence of primary and secondary hydroxyl groups at the edges of the cone, as well as

a hydrophobic interior cavity formed by the apolar hydrogens and ether-like oxygen at the inside

face of the cone (Del Valle, 2004). CDs can be considered as empty capsules of different molecular

17

sizes, in which guest molecules can be included in their nonpolar cavity, forming inclusion

complexes. The inclusion complex is usually characterized by one guest molecule engulf in one

CD molecule, although more than one guest molecule, especially compounds of low-molecular-

weight, can fit into the CD cavity. Also, more than one CD molecule can bind to one guest

molecule in the case of some high-molecular-weight compounds (Đorđević et al., 2014).

CDs are non-toxic, biodegradable, and biocompatible compounds with generally

recognized as safe (GRAS) status. Toxicological studies on CDs have shown that their oral

ingestion is non-toxic, where they lack absorption in the upper gastrointestinal tract and completely

metabolized by the colon microflora (Irie & Uekama, 1997; Loftsson & Duchêne, 2007). The

unique properties of CDs make them one of the most commonly used materials for the

encapsulation of active compounds, including gases/volatiles, to increase their solubility or

stability in food, pharmaceutical, and agricultural applications (Dos Santos et al., 2017; Hu et al.,

2014; Loftsson & Duchêne, 2007; Marques, 2010; Matencio et al., 2020).

Fig. 2.1: The chemical structure of cyclodextrins (CDs) and their geometrical dimensions.

18

Table 2.2: Some important properties of cyclodextrins (Ho et al., 2014; Marques, 2010).

Properties α-cyclodextrins β-cyclodextrins γ-cyclodextrins

Molecular formula C36H60O30 C42H70O35 C48H80O40

No. of glucose units 6 7 8

Molecular weight (Da) 972 1135 1297

Height of torus (nm) 0.8 0.8 0.8

Outer diameter (nm) 1.4 – 1.5 1.5 – 1.6 1.7 – 1.8

Cavity diameter (nm) 0.5 – 0.6 0.6 – 0.8 0.8 – 1.0

Melting onset (°C) ~275 ~280 ~275

No. of water molecules in cavity 6 11 17

Water solubility at 298.15 K

(%w/v)

14.5 1.85 23.2

Cavity volume of 1 g (mL) 0.10 0.14 0.20

2.4.1.2 Encapsulation of gases/volatiles into cyclodextrins

Cyclodextrins can form soluble, reversible inclusion complexes with active compounds,

resulting in compound stabilization. CD inclusion complexes alter the physical state of

gases/volatiles, allowing increased shelf stability and ease of material handling (Andrade et al.,

2015). The possibility of molecular encapsulation of gases into α-CD was first reported by Cramer

and Henglein (1956). Aqueous α-CD was found to entrap gaseous hydrocarbons (e.g., propane,

methane, ethane, ethylene), Cl2, Kr, Xe, O2, and CO2 to form clathrates. In general, CDs interact

with hydrophobic compounds after they substitute the water molecules that occupy the apolar CD

cavities to form the inclusion complexes. Many factors influence the encapsulation efficiency and

the interactions between guest molecules and CDs. These factors including the type of CD (α-, β-

19

and γ- CD), CD intramolecular water content, inclusion complex preparation method, and guest

molecule type, geometry, hydrophobicity, and concentration (Da Rocha Neto et al., 2018; Cevallos

et al., 2010; Zhang et al., 2015). The interactive forces involved between host and guest could be

dipole-dipole bonds, hydrophobic interaction, apolar–polar interactions, hydrophobic associations,

van der Waals, dispersion forces, and reduction of conformational strain; no evidence for forming

or breaking covalent bonds (Liu & Guo, 2002; Mura, 2014). Depending on a particular inclusion

complex, some interactions are more critical than others; however, electrostatic and hydrogen

bonding significantly affect the complexes shape and structure (Ho et al., 2014). Selected examples

of CD inclusion complexes for various gaseous/volatile compounds are presented in Table 2.3.

Neoh et al., (2007) explored the encapsulation of 1-MCP gas into aqueous α-CD under

different encapsulation parameters (i.e., initial 1-MCP concentration, α-CD concentration,

encapsulation time, temperature, and agitation rate) using a closed agitated vessel with a flat gas–

liquid interface. Their results showed that the effect of the initial 1-MCP concentration on the

inclusion ratio was negligible. Encapsulation amount increased with increasing α-CD

concentration and agitation speed enhanced the diffusion of 1-MCP into the solution, but decreased

with increasing the temperature due to the increase of both the solubility of the inclusion complex

in the solution and the self-polymerization between the dissolved 1-MCP molecules. The

optimized parameters for encapsulation are 80,000 µL/L, 5 % (w/w), 9 h, 15 °C, and 200 rpm, for

initial 1-MCP concentration, α-CD concentration, encapsulation time, temperature, and agitation

rate, respectively, which resulted in an inclusion ratio of 0.95 mole 1-MCP/mole α-CD (5.02 g/100

g). In another study by Ho et al., (2011a), they encapsulated ethylene in aqueous α-CD under

different pressures of ethylene using the liquid preparation method. They reported that the pressure

20

did not significantly affect the inclusion ratio, although the yield (amount of precipitates) increased

significantly as the pressure and time increased. The concentration of ethylene in the complexes

ranged from 0.98 to 1.03 mole ethylene/ mole α-CD (Ho et al., 2011a). The encapsulation of a

guest molecule into CDs, using the liquid preparation method, involves a simultaneous two-step

reaction. The guest molecule first dissolved into the aqueous solution followed by its absorption

into the core of α-CD. The rate-limiting step of encapsulation was the gas absorption, which

followed a pseudo-first-order reaction (Neoh et al., 2007). In aqueous solution, the apolar cavities

of α-CD can interact with two water molecules via energetically unfavorable polar–apolar

interactions, which can be easily displaced by the less polar guest molecules. Therefore, the main

driving force for forming inclusion complexes is substituting the high enthalpy water molecules in

CD cavities with the relatively more hydrophobic guest molecules (Astray et al., 2009; Szejtli,

1989).

In the above reports, inclusion complexes were produced by compressing the gas (i.e., 1-

MCP and ethylene) into saturated α-CD aqueous solutions at a certain pressure and time. This

method is known as the liquid encapsulation method. The major drawback of this method is that

it is a batch process which can take up to several days to proceed through crystallization,

precipitation, filtration, and dehydration steps. Also, it has a low complex yields (Ho et al., 2011a;

Neoh et al., 2007). One alternate technique is called the solid encapsulation method which involves

directly injection of the gaseous molecules into α-CD powder in dry state. Neoh et al., (2006)

compared the encapsulation of CO2 into solid-state α-CD (with different initial moisture contents

(M.C) 2, 10, and 30 % w/w) and saturated α-CD solution under CO2 pressure up to 3 MPa. For

solid-state α-CD, increasing the pressure significantly increased the inclusion ratio, while the effect

21

of initial moisture content of α-CD was not consistent. The highest encapsulation capacity (5.92

g/100 g) was observed at 10 % M.C, 3 MPa, and 10 h. Also, 100 % yield of the complex was

obtained. Compared with the complex yield based on the liquid encapsulation method, the yield

was only 56 % and the encapsulation capacity was lower (4.79 g/100 g) at 3 MPa and 120 h.

Although the solid encapsulation approach resulted in considerably higher yields in shorter

encapsulation times than the saturated α-CD solution method, the latter produced guest-host

complex crystals that are more stable during subsequent storage under various humidity conditions

(up to 75 % RH). By using the solid encapsulation technique, Ho et al., (2015) encapsulated

ethylene in amorphous and crystalline α-CD. The crystalline α-CD had lower encapsulation

capacity than the amorphous counterpart at low pressure and short time (0.4 – 0.8 MPa and 4 – 24

h). However, the encapsulation capacity was enhanced when the pressure and time increased up

to 1.6 MPa for 96 h. The equilibrium encapsulation capacity was achieved after 48 h for the

crystalline powder, while 8 h was observed for the amorphous powder. One possible reason for

the longer time taken to reach equilibrium for the crystalline α-CD powder is that it has an orderly

and densely packed molecular crystalline structure, thus restricting the diffusion of gases to the

cavity. In contrast, the amorphous α-CD powder had a loose and random molecular arrangement

with more accessible cavity to the guest molecules for complexation. However, the researchers

reported that the host-guest complex prepared from the amorphous α-CD powder was unstable;

rapid evaporative loses were observed during the depressurization step (Ho et al., 2015).

In an attempt to combine the advantages of both gas encapsulation methods (i.e., liquid and

solid methods), Ho and his co-workers studied the effect of crystallization of amorphous

complexed powders during the encapsulation of CO2 (Ho et al., 2016a; Ho et al., 2016b). They

22

added water to the amorphous α-CD powder prior to encapsulation to increase the initial moisture

content from 5.51 to 13, 15, and 17 % (w/w); close to or higher than the level of crystallization.

The results showed that the added water facilitated the crystallization of the amorphous CO2-α-

CD complex, thereby locking CO2 into the cavity. This resulted in a considerably higher inclusion

ratio as compared to amorphous α-CD without the addition of water or crystalline α-CD powders

under the same encapsulation conditions (Ho et al., 2016a).

Among the three types of CDs (α-, β- and γ- CD), α-CD is the most suitable one to form

complexes with hydrophobic low molecular weight gases. It has the smallest cavity (0.5–0.6 nm)

that offers more interaction and better binding force between the guest molecule and cavity walls

(Hedges et al., 1995). For volatile EO compounds, the selection of CD type depends on the

geometry and the molecular weight of the guest molecule. For example, the complexation of allyl

isothiocyanate (AITC) with aqueous α- and β-CD was investigated by Li et al., (2007). The AITC

inclusion capacity increased with increasing molar ratio of AITC:CD. Almost 100 % inclusion

capacity was achieved at the AITC:CD molar ratio of 2:1 using a coprecipitation method. The

inclusion efficiencies, represented by the percentage of the complexed CD to the total CD added,

were 85.6% for AITC-β-CD complex powder and 88.4% for AITC-α-CD complex powder. AITC

was accommodated with the best fit into the smallest cavity of α-CD (Li et al. 2007). By contrast,

the inclusion complexes of acetaldehyde-β-CD was 25-fold higher than that of α-CD and γ-CD

complex under the same encapsulation conditions (Almenar et al., 2007). In a study that

investigated the complexes of ethyl butyrate and hexanal with β-CD and γ-CD, Zhang et al., (2015)

found that the inclusion ratio of the ethyl butyrate-γ-CD was higher than that of the ethyl butyrate

-β-CD, while the opposite result was observed for hexanal. Moreover, the inclusion ratios of the

23

ethyl butyrate complexes were 2.6 – 3.7 times greater than those of the hexanal complexes (Zhang

et al., 2015).

Recently, the chlorine dioxide gas was encapsulated into α-CD using the co-precipitation

method (Chen et al., 2020). An appropriate ClO2 headspace concentration was needed to achieve

a high inclusion ratio. Low ClO2 headspace concentration (1 mole ClO2:1 mole α-CD) resulted in

low loading and yield of ClO2, while high ClO2 concentration (10 mole ClO2:1 mole α-CD) caused

oxidation of α-CD, which negatively affected the encapsulation. When ClO2 in the headspace to

α-CD was 6:1, an inclusion capacity of 6.44 % (w/w) was achieved, representing a 1:1 guest to

host ratio. Compared to other gases, such as CO2 and ethylene, ClO2 required less headspace

concentration to reach 1:1 molar ratio. For example, Ho et al., (2011a) reported that ethylene

concentration of 0.2 MPa was needed to get 0.98–0.99 mole of ethylene/mole of α-CD, whereas

only around 0.07 MPa of ClO2 was required to achieve 1 mol of CO2/mole of α-CD (Chen et al.,

2020). The high encapsulation efficiency of ClO2 was attributed to its high solubility in water,

leading to efficient contact of ClO2 with α-CD. This study opens up more avenues for the

encapsulation of hydrophilic compounds into CDs.

2.4.1.3 Release properties of gases/volatiles from cyclodextrin complexes

Inclusion complexes are useful to enhance the stability of gaseous/volatile active

compounds during storage, whist allowing their triggered release by dissolving the inclusion

complexes in water or exposure to evaluated humidity. The release of gest molecules from CDs is

affected by various factors including temperature, relative humidity, pH, enzymes, and the

physicochemical properties of guests, with temperature and moisture being the most dominant

factors (Goubet et al., 1998). Neoh et al. investigated the kinetics of heat induced dissociation of

24

1-MCP-α-CD inclusion complex to provide an insight into its thermal stability (Neon et al., 2008).

They concluded that the thermal stability of the complex is dependent on the initial inclusion ratio

of the inclusion complex. The activation energy of dissociation tended to increase with decreasing

inclusion ratio. When heated to 120°C, the inclusion ratio decreased; meanwhile, the inclusion

complex's moisture content increased with increasing heating time. The release of one 1-MCP

molecule from the CD cavity due to heat treatment was accompanied by adsorption of 2.8 water

molecules, implying the critical role of water molecules to induce the dissociation of 1-MCP-α-

CD inclusion complex (Neon et al., 2008). This finding can be useful for the applications involve

elevated temperature such as melt extrusion of thermoplastic films. Neoh et al., (2010) also studied

the dissociation of 1-MCP-α-CD inclusion complex in response to linearly increasing humidity

from 10 to 90 % RH at a rate of 10 % RH/h. They reported that increasing RH generally prompted

the complex dissociation. Rapid dissociation was observed between 60 and 70 % RH, but the

dissociation rate was greatly retarded at 80 % RH, due to the collapse of the crystalline structures.

Interestingly, an increase in dissociation rate was observed at 90 % RH due to the dissolution of

the complex (Neoh et al., 2010). Similarly, Ariyanto and Yoshii, (2019) reported that the release

of 1-MCP from the inclusion complex powders increased with stepwise increase of humidity from

20 to 80 % RH and temperature from 40 to 60°C (Ariyanto & Yoshii, 2019).

The release kinetics of ethylene gas from an inclusion complex prepared using aqueous α-

CD solution was studied by Ho et al., (2011a), (2011b). Their inclusion complexes can be

dissociated in water to release ethylene gas. Also, increased storage humidity accelerated the

release of ethylene from the complex. The inclusion complex was stable at 53 % RH even after 28

d of storage, but storage at 94 % RH resulted the release of the majority of ethylene in 14 d. When

25

stored in dry condition (<10 % RH), increasing temperature accelerated ethylene gas liberation

from the inclusion complex. Moreover, mixing ethylene-α-CD complex with deliquescent salts

(e.g., CaCl2, MgCl2) enhanced the release of ethylene gas and achieved the maximum release after

24 h under 76 and 94 % RH at 18°C (Ho et al., 2015). Compared with the crystalline ethylene-α-

CD complex formed by the liquid encapsulation method, faster ethylene release was observed from

the amorphous ethylene-α-CD complex formed by the solid encapsulation method (Ho &

Bhandari, 2016). Also, the amorphous complex released ethylene immediately after formation

even under low humidity conditions (11 % RH) and the release accelerated markedly with

increasing RH. By contrast, the crystalline inclusion complexes can retain the guests stronger than

the amorphous inclusion complexes. Although the high level of moisture in amorphous CD had a

negative effect on complexation, the exposure of controlled amount of moisture can promote the

crystallization of the resulting inclusion complex, thereby enhancing the entrapment of guest

molecules and improving their storage stability (Ho & Bhandari, 2016). The release mechanism

of guest molecules from CDs inclusion complex is moisture activated diffusion-controlled in

which the release rate can be accelerated at elevated RH and temperature. Researchers have

modeled the release rate using Avrami's and power-law models with various degrees of success

(Chen et al., 2020; Ho et al., 2011b, 2015; Ho & Bhandari, 2016; Neoh et al., 2006). Ho et al.,

(2011b) found that the Avrami's equation predicted the release rate of ethylene from ethylene-α-

CD complex more accurately than the power-law equation.

2.4.1.4 Modified cyclodextrins

Cyclodextrins can be modified to further manipulate their physiochemical properties for

specific applications. Synthetic modifications of CDs have been conducted by researchers via

26

different chemical paths including alkylation, hydroxyalkylation, and acetylation of the hydroxyl

group of CDs (Marques, 2010). The most widely used modified CD derivatives are 2-

hydroxypropyl-β-CD, 2-hydroxypropyl-γ-CD, triacetyl‐β‐CD (Andrade et al., 2015; Shin et al.,

2019). These CD derivatives have been used to encapsulate volatiles such as AITC, thymol,

carvacrol, eugenol (Celebioglu et al., 2018a, 2018b; Kamimura et al., 2014; Shin et al., 2019). Shin

et al., (2019) found that triacetyl‐β‐CD provided more sustained release of AITC than unmodified

β‐CD. Another modification is via synthesizing CDs polymers using cross-linkers to build a 3-D

network. In this technique, several CDs molecules are cross-linked together by linking units to

form CDs “nanosponges” with highly porous nanostructures (Sherje et al., 2017). CD nanosponges

with a wide range of physical and chemical properties can be synthesized using various cross-

linker such as carbonyldiimidazole, diphenyl carbonate and pyromellitic anhydride (Andrade et

al., 2015). CD nanosponges can be used as an encapsulating material to improve encapsulation

capacity of the active compounds. For example, Zhang, (2016) encapsulated 1-MCP into CD

nanosponge synthesized by reacting β‐CD with carbonyldiimidazole as a cross-linker. He found

that the chemical modification of β-CD effectively increased 1-MCP loading level through

creating inter-molecule cavities in the gap between β‐CD molecules and cross-linkers. Trotta et

al., (2011) encapsulated 1-MCP and CO2 into β-CD based carbonate nanosponge. The resulting

inclusion complexes had higher gas retention and more sustained release than the unmodified β-

CD.

27

Table 2.3: Inclusion complexes of various active gaseous/volatile compounds into cyclodextrins.

Gas/volatile CD Encapsulation conditions* Encapsulation yields Key findings Reference

1-MCP α-CD

• Aqueous α-CD at 2.9 – 8.25

% (w/w)

• 1-MCP concentration =

80000 µL/L

• P = 0.101 MPa

• T = 15 – 30°C

• t = 9 h

• Agitation up to 300 rpm

• Inclusion ratio up to

0.95 mole/1-

MCP/mole α-CD

• Encapsulation

capacity up to 5.02

g/100 g

• Effect of the initial 1-MCP

headspace concentration was

negligible

• Encapsulation increased with

increasing CD concentration

and agitation speed, but

decreased with increasing

temperature

Neoh et

al., (2007)

Ethylene α-CD

• Aqueous α-CD at 11.27 %

(w/w)

• P = 0.2 – 1.5 MPa

• T = 25°C

• t = 12 – 120 h

• No agitation

• Inclusion ratio =

0.98 – 1.03 mole

ethylene/mole α-

CD

• Encapsulation

capacity = 2.74 –

2.88 g/100 g

• Pressure and time did not

increase ethylene

concentrations in the

complexes, but did yield

higher amounts of the

inclusion complex

Ho et al.,

(2011a)

Ethylene α-CD

• Solid amorphous α-CD at 4.1

and 7.5 % (w/w) M.C

• P = 1.0 – 1.5 MPa

• T = 25°C

• t = 24 – 120 h


0.42 – 0.87 mole

ethylene/mole α-

CD

• Encapsulation

capacity = 1.17 –

2.43 g/100 g

• Amorphous α-CD with lower

M.C showed higher

encapsulation capacity

Ho &

Bhandari,

(2016)

28

Carbon

dioxide α-CD

• Saturated α-CD solution and

Solid α-CD at 2 – 30 %

(w/w) M.C

• P = 1.0 – 3.0 MPa

• T = 25°C

• t = 12 – 120 h


0.97 – 1.41 mole

CO2/mole α-CD

• Encapsulation

capacity = 4.20 –

6.10 g/100 g

• Initial moisture content of α-

CD had no consistent effect

• Saturated α-CD solution

produced more stable

complex crystals than the

solid counterpart

• Increasing pressure and time

increased the maximum

inclusion ratio

Neoh et

al., (2006)

Carbon

dioxide α-CD

• Solid amorphous and

crystalline α-CD

• P = 0.4 – 1.6 MPa

• T = 25°C

• t = 1 – 96 h


0.55 – 1.45 mole

CO2/mole α-CD

• Encapsulation

capacity = 2.38 –

6.27 g/100 g

• Encapsulation capacity of

crystalline α-CD was higher

than that of amorphous α-

CD, but time to reach

equilibrium was longer for

crystalline α-CD

Ho et al.,

(2015)

Chlorine

dioxide α-CD

• Saturated α-CD solution

• ClO2 to α-CD molar ratio of

1:1 – 10:1

• Co-precipitation method

• Two hours at 22°C then four

hours at 4 °C


0.74 – 1 mole

ClO2/mole α-CD

• Encapsulation

capacity = 4.73 –

6.44 g/100 g

• Low ClO2 headspace

concentrations resulted in a

low inclusion ratio, while

high concentrations caused

oxidation of α-CD and

negatively affected

encapsulation

• Optimal ClO2 to α-CD molar

ratio was 6:1

Chen et

al., (2020)

29

AITC α- and

β-CD

• Aqueous α-CD at 12.5 %

(w/w)

• Aqueous β-CD at 2.33 %

(w/w)

• AITC/β-CD molar ratio of

0.4:1 – 2.4:1

• AITC/α-CD molar ratio of

2:1


• T = 40°C

• t = 3 h


0.85 mole

AITC/mole β-CD

• Inclusion ratio = 1

mole AITC/mole α-

CD

• Encapsulation

capacity = 6.87

g/100 g for AITC-

β-CD and 8.17

g/100 g for AITC-

α-CD

• AITC bound more strongly

with α-CD than with β-CD

due to the matched molecule

size of AITC with the cavity

of α-CD

Li et al.,

(2007)

Hexanal β- and

γ-CD

• Aqueous β-CD at 2.9 %

(w/w)

• Aqueous γ-CD at 9.1 %

(w/w)

• Hexanal/CD molar ratio of

3:1


• T = 70°C

• Treated by ultrasonic for 30

min

• Encapsulation

capacity = 4.41

g/100 g for

hexanal-β-CD and

3.33 g/100 g for

hexanal-γ-CD

• β-CD possessed more

optimal cavity size than γ-

CD for the encapsulation of

hexanal

Zhang et

al., (2015)

Acetaldehyd

e

α-, β-

and γ-

CD

• Aqueous CD solution at 1:1

molar ratio

• Acetaldehyde concentration

of 70, 140, or 280 µL

• T = 23°C

• Centrifuge at 1600 rpm for

40 min

- • β-CD had higher capacity to

form inclusion complex with

acetaldehyde than α- and γ-

CDs Almenar

et al.,

(2007)

30

Cinnamalde

hyde β-CD

• Various concentrations (0.3 –

1.8 % w/v) of β-CD in

aqueous ethanol

• β-CD/CIN molar ratio 1:0.25

– 1:3

• T = 55°C

• Shaken for 4 h

• All complexes

exhibited > 90%

encapsulation

efficiency

• Encapsulation efficiency

decreased with increasing

CIN concentration

• The increase in β-CD and

CIN concentrations induced

aggregation and resulted in a

larger particle size

Jo et al.,

(2015)

Thymol β-CD

• Saturated β-CD solution and

β-CD slurry

• Thymol/β-CD molar ratio of

1:1

• T = 25°C

• t = 48 h

• Encapsulation

capacity = 9.00 –

10.5 g/100 g

• Encapsulation

efficiency = 73.73 –

82.5 %

• Aqueous β-CD solution

resulted in a higher inclusion

complex compared to β-CD

slurry Tao et al.,

(2014)

Carvacrol β-CD

• Saturated β-CD solution and

β-CD slurry

• Carvacrol/β-CD molar ratio

of 1:1

• T = 25°C

• t = 48 h

• Encapsulation

efficiency = 83.8 –

91.3 %

• Higher encapsulation

efficiency was achieved

when using an aqueous β-CD

solution than β-CD slurry Santos et

al., (2015)

*P, T, and t are the process pressure, temperature, and time, respectively.

31

2.4.2 Metal-organic frameworks

2.4.2.1 Molecular structure and properties of metal-organic frameworks

Metal-organic frameworks (MOFs) have emerged as an important class of crystalline

materials consisting of metal ions or clusters linked by organic bridging ligands through covalent

interactions to form 1-, 2-, or 3-D network structures (Ruiz et al., 2018). The structure formation

of the 3-D MOFs is shown in Fig. 2.2. These frameworks have ultrahigh porosity (void volumes

up to 90 %) with extremely high internal surface areas (> 6000 m2/g), which is greater than those

of zeolites and activated carbons (Furukawa et al., 2010). For zeolites, most of their mass is

inactive aluminosilicate matrix. By contrast, the metal in MOF is the matrix itself and each metal

site is available for adsorption, resulting in a higher metal/mass ratio and a higher adsorption

capacity for MOFs than zeolites (Chopra et al., 2017). These properties, along with the exceptional

degree of variability for both the inorganic metal cores (e.g., Cu+, Cu2+, Ag+, Cd2+, Zn2+, Co2+, Li+,

Mg2+, Ni2+) and organic linkers (e.g., polycarboxylates, phosphonates, sulfonates, imidazolates,

amines, acids, pyridyl, phenolates) in their structures, make MOFs of particular interest for

potential applications in membranes, catalysis, and biomedical imaging, sensor devices, or gas

storage and separation (Horcajada et al., 2012; Li et al., 2009; Zhou et al., 2012). Since MOFs was

first reported in 1998 (Hoskins & Robson, 1989), thousands of MOF materials have been

synthesized using different methods such as hydrothermal, solvothermal, and microwave assisted,

sonochemical, electrochemical, mechanochemical, ionothermal, dry gel conversion, and

microfluidic synthesis methods (Lee et al., 2013). Moreover, many post-synthetic modifications

were carried out by researchers to introduce metal ions or desired functional groups into the MOF

32

materials to amend their structure for specific purposes (Kalaj & Cohen, 2020; Tanabe & Cohen,

2011; Wang & Cohen, 2009).

The biological toxicity of MOFs is not well established; however, the existence of organic

ligands functional group, metal ions, the solvent used during synthesis, and crystal size could be

the sources of MOFs toxicity (Sharanyakanth & Radhakrishnan, 2020). The solubility and

degradation of MOF in a biological system depend on MOF composition, pH, ionic strength, and

peristaltic movements of the biological system. Most organic and inorganic nanomaterials are not

completely soluble in the biological system, thus leads to their accumulation which may results in

cellular stresses which form the basis of their toxicity (Sajid, 2016). Since most MOFs are toxic

and unsafe in food and medical applications, a series of bio-based nano-porous nontoxic MOFs

were recently prepared through recrystallization of α-, β-, and γ-CD in the presence of alkali metal

cations (CD-MOFs). These green materials have a great potential for applications in the fields of

gas adsorption/separation and controlled release application of active compounds (He et al., 2019;

Li et al., 2020; Sharanyakanth & Radhakrishnan, 2020).

Fig. 2.2: A schematic illustration of 3-D metal-organic framework (MOF).

33

2.4.2.2 Encapsulation of gases/volatiles into metal-organic frameworks

Metal-organic frameworks are potentially useful in selective adsorptive binding and release

of gases. Unlike encapsulation into CDs which depend on the stereometric compatibility between

the active compound (guest) and the encapsulant (host), adsorption of gases/volatiles in MOFs

depends on the pore size, porosity, and surface area of the adsorbent. The pore size determines the

type of gas that can be adsorbed. The surface area and porosity determine the adsorption capacity

(Chopra et al., 2017). The metal ions in MOFs are tightly held in the pores by covalent interactions

with organic linkers or solvent molecules during their synthesis. Activation of MOFs by heat

removes solvent molecules (Dietzel et al., 2008). This results in a structure with open or

coordinatively unsaturated metal sites. These sites are essential for the adsorption of gas molecules

(Ma & Zhou, 2010). A review of literature from selected papers and their key findings on the

adsorption of active gaseous/volatile compounds by MOF solid matrices are summarized in Table

2.4.

Factors, such as MOF type, gases/volatiles, atmosphere gas composition, and temperature

are important in affecting the adsorption rate and capacity of gases/volatiles by MOFs. Chopra et

al., (2017) reported that the adsorption of ethylene by Basolite C300 (copper-based MOF with a

trimesic acid linker group) was 30 % higher in a 100 % N2 environment than in a mixed

environment of 85 % N2 and 15 % CO2. CO2 did not affect ethylene sorption by Basolite A520

(aluminum-based MOF with a fumaric acid linker group). Also, Basolite C300 had higher

adsorption capacity than Basolite A520 (Chopra et al., 2017). These observations were attributed

to the selective adsorption caused by the host-guest affinity of the metal sites (Li et al., 2012).

Water vapor added to the N2 or N2/CO2 environments was found to interfere with the adsorption

34

of ethylene, since water destabilized the MOF structure and competed with ethylene adsorption

for the binding sites. Water molecules adsorbed faster than ethylene because of their smaller

kinetic diameter (2.65 Å) than 3.9 Å of ethylene (Chopra et al., 2017). Compared to ethylene, the

adsorption capacities of 1-MCP by Basolite C300 and A520 were low, at 17.6 and 2 % w/w,

respectively (Chopra et al., 2017). Immediate release of gust compounds after loading is one of

the limitations of using MOFs for controlled release applications. Guan et al., (2019) developed a

system consists of a MOF core and an alginate-based shell to entrap ethylene. The MOF comprises

a coordination complex of Al and 1,3,5-benzenetricarboxylate ligands. After charging with

ethylene, the MOF was further entrapped in a close knit bead formed with alginate-Fe(III) matrix,

which degraded upon the exposure to sodium citrate aqueous solution, thereby triggering a release

of ethylene (Guan et al., 2019). Here, the ethylene complex can be stored under ambient conditions

without evaporative losses of ethylene.

In another study, Li et al., (2020) developed MOF based on α‐CD (α-CD-MOF-Na and α-

CD-MOF-K) to increase the adsorption capacity and storage stability for ethylene gas. The

ethylene encapsulation capacity of α-CD-MOF-Na and α-CD-MOF-K was 47.4 and 52.9 % (w/w),

respectively, which was significantly higher than that of α-CD (3.6 g/100 g) (Li et al., 2020), Cu-

MOF (1.56 g/100 g) (Zhang et al., 2016), and Al-MOF (4.83 g/100 g) (Guan et al., 2019). Kathuria

et al., (2019) prepared bio-based cyclodextrins and alkali metal ion-based MOFs as a non-toxic

adsorbent material to encapsulate ethanol. The CD-MOF adsorbed ethanol up to 20 g/100 g. The

complex was thermally stable up to 200 °C and can stabilize ethanol at elevated temperature due

to the formation of an inclusion complex between CD-MOF and ethanol. Also, γ-CD-MOF was

35

found to have high selectivity for CO2 absorption at low pressures (< 0.1 MPa) and allowed its

reversible release at room temperature (Gassensmith et al., 2011).

Recently, MOFs were investigated as carriers for volatile EOs (Table 2.4). Wu et al., (2019)

reported the adsorption of thymol by the porous Zn-MOF through noncovalent interactions

(adsorbed amount is 3.96 g/100 g). Also, Lashkari et al., (2017) encapsulated AITC into the

cavities of three MOF materials, namely HKUST-1, MOF-74(Zn), and RPM6-Zn, with loading

capacities of 42, 27, and 14 % (w/w), respectively. MOFs with high internal surface area provide

more affinity sites for adsorption of guest molecules. Other factors that affect the uptake of volatile

guest molecules by these MOFs are cage-like pores and interpenetrating frameworks (Lashkari et

al., 2017). Compared with CDs, CD-MOFs had a higher menthol loading due to their high ordered

structure with greater crystallinity as well as their very high porous structure with a large internal

surface area (Hu et al., 2021).

2.4.2.3 Release properties of gases/volatiles from metal-organic framework complexes

Although many studies have been conducted to study the separation and storage of gases

by MOFs, a limited number of studies investigated their controlled release under mild conditions.

The release of guest compounds from MOFs is typically triggered by the exposure of the complex

to moisture, elevated temperature, and low pressure (≤ 1 atm). The release rate of guest molecules

depends considerably on the size of cavities, type of metal ions, rigidity, and solvent solubility of

MOFs (Ho et al., 2014; McKinlay et al., 2008). Chopra et al., (2017) found that the release of

ethylene from Basolite C300 depends on the atmosphere gas composition. In dry N2, it desorbed

26 % (w/w) of the adsorbed ethylene over a period of 2 d. Upon the exposure to free water, 75.5

% of the ethylene remaining in the MOF was desorbed due to the high affinity of water to Basolite

36

C300 that displaced the bounded ethylene. However, in the dry N2/CO2 environment, 47 % of

ethylene was desorbed but 94% of the ethylene was released upon the exposure to free water vapor.

Also, ethylene desorption was dependent on the type of MOF materials, where Basolite A520

desorbed a negligible amount of ethylene (13 %) in dry conditions and there was no appreciable

additional desorption with the addition of free water. A negligible amount of the adsorbed 1-MCP

was desorbed from Basolite C300 and Basolite A520 (0.02 and 0.75 % w/w, respectively), which

could be due to 1-MCP either degraded or polymerized inside the MOF structure and was not free

to be released (Chopra et al., 2017). One other limitation of gases-loaded MOF is the immediate

release after loading into MOFs. Almost 96 % of total ethylene adsorbed by CuTPA MOF was

released immediately after its loading under atmospheric conditions (Zhang et al., 2016). One

possible solution is to apply a coating layer onto the MOF surface. Guan et al., (2019) developed

a system consisting of a MOF core and an alginate-based shell to entrap ethylene. Ethylene was

released gradually when the Al-MOF-alginate-Fe(III) beads were immersed in 200 mM sodium

citrate solution; released 0.41–0.46 mg/L per mg Al-MOF of ethylene in 3 h. The release of

ethylene was higher than that when the Al-MOF-alginate-Fe(III) beads were immersed in water

(0.13–0.20 mg/L per mg Al-MOF) (Guan et al., 2019). Britt et al., (2009) reported that CO2

adsorbed by Mg-MOF-74 could be fully regenerated at 80°C. Lashkari et al., (2017) found that

MOFs (HKUST-1, MOF-74(Zn), and RPM6-Zn) were able to encapsulate and retain more than

90 % of encapsulated AITC molecules within their pores at low RH storage conditions. However,

when exposed to high RH (95–100 %), the release of AITC vapor was triggered. As expected, the

release rate was affected by temperature, MOF type, and its 3-D network structure. Although MOF

complexes exhibited poor storage stability, α-CD-MOF complexes have high stability, being more

37

stable than α-CD complexes under ambient conditions. Hence, α-CD-MOF had higher ethylene

storage stability and encapsulation capacity when compared to MOFs and CDs (Li et al., 2020).

38

Table 2.4: Adsorption of various active gaseous/volatile compounds by metal-organic framework.

Gas/

volatile

MOF Adsorption

conditions*

Adsorption

capacity (g/100 g)

Key findings Reference

Ethylene

• Basolite A520

(Al(III)/fumaric

acid)

• Basolite C300

(Cu(II)/benzene-

1,3,5-

tricarboxylate)

• N2 and N2/CO2

environments

• P = 0.101 MPa

• T = 23°C

• t = 6 d

• A520 = 0.004

• C300 = 0.013 –

4.22

• Atmosphere gas composition

affected the adsorption of

guests.

• C300 adsorbed more ethylene

than A520

• Increasing the initial headspace

concentration of ethylene

increased the amount of

ethylene adsorbed by MOF

Chopra et

al., (2017)

Ethylene

• CuTPA (CuAc/ terephthalic acid)

• P = 0.101 MPa

• T = 25°C

• t = 2 h

• 1.56 • 96% of the total ethylene

adsorbed was recovered Zhang et

al., (2016)

Ethylene

• Al-MOF (Al/1,3,5-

benzenetricarboxyl

ate)

• P = 0.101 MPa

• T = 23°C

• t = 3 h

• 4.83

• Entrapping the MOF complex

into an alginate-based shell

enhanced the stability and

controlled the release of

ethylene

Guan et

al., (2019)

1-MCP

• Basolite A520

(Al(III)/fumaric

acid)

• Basolite C300

• P = 0.101 MPa

• T = 23°C

• t = 4 h

• A520 = 2.04

• C300 = 17.6

• Maximum adsorption was

achieved within the 0.5 h upon

exposure with C300 being the

fastest

• No release of 1-MCP achieved

Chopra et

al., (2017)

39

(Cu(II)/benzene-1,3,5-

tricarboxylate)

Ethanol

• CD-MOF (K/γ-CD) • P = 0.101 MPa

• T = 23°C

• t = 48 h

• 20 • CD-MOF was thermally stable

up to 200°C and retained a

certain degree of ethanol up to

150°C.

Kathuria

et al.,

(2019)

Carbon

dioxide

• Mg-MOF-74

(Mg/2,5

dioxidoterephthalat

e)

• P = 0.101 MPa

• T = 25°C

• 8.9 • Mg-MOF-74 provided optimal

balance between

separation/adsorption capacity

and ease of regeneration

Britt et al.,

(2009)

Carbon

dioxide

• CD-MOF-2 (Rb/ γ-

CD)

• P = 0.101 MPa

• T = 0 – 25°C

• t = 10 min

• 11.8 – 15.1

• CD-MOF-2 was selective for

the ab/desorption of CO2 at

low pressures

Gassensmi

th et al.,

(2011)

AITC

• HKUST-1 (Copper

benzene

tricarboxylate)

• MOF-74(Zn/ 2,5

dihydroxyterephtha

lic acid)

• RPM6-Zn

(Zn/biphenyl-4,40

dicarboxylate)

• AITC vapor

brought into the

cavities of

MOFs using

nitrogen as a

carrier gas

• T = 18°C

• HKUST-1 = 42

• MOF-74(Zn) =

14

• RPM6-Zn = 27

• The high internal surface area

of the MOFs provided high

affinity sites for adsorption of

gas molecules Lashkari

et al.,

(2017)

40

Thymol • Zn-MOF (Zn/2-

aminoterephthalic

acid)

• Access amount

of thymol in

chloroform

• Overnight at

room

temperature

• 3.96

• Adsorbed thymol was stable

but evaporated at an elevated

temperature between 100 and

250°C.

Wu et al.,

(2019)

Menthol • CD-MOF (K/α-, β-,

and γ-CD)

• Menthol/CD-

MOF molar

ratio of 1:1

• T = 75°C

• t = 60 min

• α-CD-MOF = 5

• β-CD-MOF =

22

• γ-CD-MOF =

12

• β-CD-MOF had a higher

affinity for menthol molecules

than the other two CD-MOFs

Hu et al.,

(2021)

Carvacrol • MIL-53(Al/

terephthalic acid)

• Mg-MOF-74

(Mg/2,5-

dihydroxyterephtha

lic acid)

• Supercritical

CO2

encapsulation

• T = 25°C

• t = 14 h

• MIL-53(Al) =

34.4

• Mg-MOF-74 =

30.1

• The SC-CO2 encapsulation

was more effective than the

typical liquid phase

encapsulation

Monteagud

o-Olivan et

al., (2019)

*P, T, and t are the process pressure, temperature, and time, respectively.

41

2.4.3 Other supramolecular assemblies

Many other solid matrices have been studied for gases/volatiles encapsulation, ranging from

inorganic (e.g., activated carbons, carbon nanotubes, zeolites, silicate materials, montmorillonite,

and halloysite nanotubes) to organic (e.g., cucurbit[n]urils, V-type starches, maltodextrins, gums,

and proteins) matrices (Table 2.5). Inorganic solid matrices are mostly used as adsorbent materials

to separate and purify mixtures, remove hazardous, and store gases (Ackley et al., 2003; Ibrahim,

2013; Sircar et al., 1996; Yao et al., 2017). A limited number of articles reported the desorption of

gases from those matrices for controlled release applications. This is due to either the immediate

desorption of gases after the adsorption or the harsh conditions needed to retrieve the adsorbed

gases, where a combination of increase in temperature and/or reduction in pressure is used, which

are opposite conditions of adsorption (Grande, 2012; Siriwardane et al., 2001). However, in some

cases, treatment with water or acid can decapsulate the adsorbed gases (Gesser et al., 1984).

For example, 98 % of ethanol adsorbed by activated carbon (7.4 g/100g) can be recovered by

passing an air flow at room temperature (Silvestre-Albero et al., 2009), while no release of ClO2

was observed from activated carbon at room temperature and a minimal release was noticed at

60°C (Wood et al., 2010). Lee et al., (2006) reported that activated carbon did not release 1-MCP

when exposed to elevated RH conditions due to their strong affinity. However, 1-MCP adsorbed

by silica gel released substantially when exposed to both dry and humidified conditions (Lee et

al., 2006). Chopra et al., (2017) observed only 10 % release of ethylene from zeolite 13X at dry

conditions and when exposed to free water, no increase in ethylene release was detected. Also, the

desorption of 1-MCP from zeolite was negligible in dry as well as humid conditions. They also

found that the encapsulation capacity of zeolite was lower than that of MOFs (Basolite C300 and

42

Basolite A520). Zeolites with different extra framework cations (Na+, Ca2+, or La3+) were

examined as potential carriers for fragrance (limonene and linalool) entrapment and delivery (Li

et al., 2020). The retention of fragrances in matrices increases in the order Na < Ca < La for either

limonene or linalool. Zeolites were able to prolong the release of fragrance for up to 30 d as

compared to 2 d for free fragrances. Ethanol has been absorbed onto silicon dioxide powder to

prepare ethanol emitter. It is commercially available as Ethicap™. When placed in humidified

conditions, the silicon dioxide absorbed the moisture, thereby triggering the release of ethanol

(Nayik & Muzaffar, 2014). Halloysite nanotubes have been found to be effective matrices for

encapsulation and controlled release of EOs and their derivatives (Alkan Tas et al., 2019; Biddeci

et al., 2016; Jang et al., 2017; Maruthupandy & Seo, 2019).

On the other hand, biopolymers such as starches, maltodextrins, and cucurbit[n]urils have been

used as organic matrices for encapsulation and controlled release of gaseous/volatile active

compounds. For example, encapsulation of 1-MCP into cucurbit[6]uril, a macrocyclic molecule

with a cavity similar to α-CD, was reported by Zhang et al., (2011). They obtained different release

profiles of 1-MCP when the complex was dissolved in different solutions (i.e., sodium bicarbonate,

benzoic acid, and distilled water). Ethylene and CO2 were successfully encapsulated into V-type

starches. Release of these gases can be achieved by dissolving the inclusion complexes in water or

by exposing them to elevated RH, which was accelerated by increasing temperature (Li et al.,

2019; Shi et al., 2020; Shi et al., 2017). Other types of starches (e.g., porous and modified starches)

and their derived products (e.g., maltodextrins) were used to encapsulate volatile compounds as

well (Fang et al., 2020; Glenn et al., 2010; Mourtzinos et al., 2008; Shahidi Noghabi & Molaveisi,

43

2020; Shi et al., 2017; Tampau et al., 2017; Ulloa et al., 2017). In general, they have low retention

of volatiles.

Table 2.5: Various solid matrices used to encapsulate active gaseous/volatile compounds.

Encapsulant Gas/volatile References

Activated carbons 1-MCP, ethylene, CO2, ethanol,

and ClO2

Al-Muhtaseb, (2010); Balsamo et

al., (2013); Lee et al., (2006);

Preslar & Mouat, (2018); Silvestre-

Albero et al., (2009); Wood et al.,

(2010)

Carbon nanotubes CO2, AITC, and oregano EO

Babu et al., (2013); Dias et al.,

(2013); Prodana et al., (2015);

Wang et al., (2016)

Zeolites 1-MCP, ethylene, CO2,

limonene, and linalool

Cavenati et al., (2004); Chopra et

al., (2017); Li et al., (2020);

Siriwardane et al., (2001); Yoon

and Huh, (1994)

Silica and silicate

materials

1-MCP, ClO2, ethanol,

carvacrol, cinnamaldehyde,

eugenol, thymol, and others

Bernardos et al., (2015); Himed et

al., (2019); Jobdeedamrong et al.,

(2018); Lee et al., (2006); Lee et

al., (2012); Smith et al., (1987)

Halloysite

nanotubes

Thyme and peppermint EOs,

carvacrol, AITC, and ethylene

Alkan Tas et al., (2019); Biddeci et

al., (2016); Gaikwad et al., (2018);

44

Hendessi et al., (2016); Jang et al.,

(2017); Lee & Park, (2015);

Maruthupandy & Seo, (2019)

Montmorillonite

Carvacrol, AITC,

cinnamaldehyde, eugenol, and

thymol

Bernardos et al., (2019); Scaffaro et

al., (2020)

Cucurbit[n]urils 1-MCP, ethylene, and CO2 Pan et al., (2016); Shi et al., (2019);

Zhang et al., (2011)

V-type starch Ethylene and CO2 Li et al., (2019); Shi et al., (2020);

Shi et al., (2017)

Maltodextrin Thymol, carvacrol, and

cinnamaldehyde

Shahidi Noghabi & Molaveisi,

(2020); Tackenberg et al., (2014);

Ulloa et al., (2017)

2.4.4 Encapsulation in polymer films

To control their release profile, active compounds are often incorporated directly into

polymeric film/coating matrices derived from natural and synthetic polymers for food packaging

applications. However, since stability of active compounds is a major requirement for any

controlled release systems, this approach is not optimal, especially for gaseous and volatile

compounds. Pre-encapsulation is needed by protecting them in another carrier followed by the

dispersion into the film/coating matrix to prevent evaporative losses during the manufacturing

45

process and control their release during end-use applications. Table 2.6 summarized selected

polymer-based films containing active gaseous/volatile compounds with/without prior

encapsulation.

Direct mixing into packaging material is the most commonly studied method of EOs

encapsulation, which can be accomplished by two methods. In the first method, EOs or their

derivatives can be added directly to polymer-based films, made by extrusion, UV-curing, or

solvent casting technologies (Mohamad et al., 2020; Monedero et al., 2010; Raouche et al., 2011).

In this case, the material properties of the packaging film will be affected by the volatile additives.

For example, Kuorwel et al., (2014) found that the incorporation of different EO components (i.e.,

carvacrol, linalool and thymol) into a thermoplastic starch film resulted in a decrease in the tensile

strength of the film with increasing EOs concentrations, but not the thermal properties, water vapor

permeability and transparency of the starch films. Gao et al., (2017) developed an antimicrobial

poly(lactic acid) (PLA) film for controlled release of AITC vapor. The antimicrobial film had

lower strength and stiffness, but higher flexibility as compared to the neat PLA film. Moreover,

lower O2 and CO2 transmission rate, and higher UV absorption rate were observed for the AITC-

containing films than the neat counterpart (Gao et al., 2017). The evaporation losses, thermal

degradation of the active substance, uneven distribution, poor mechanical properties of films are

typical drawbacks of this approach (Lukic et al., 2020). The second approach is based on

impregnating or coating/spraying EOs onto the preformed films. For instance, Requena et al.,

(2017) developed poly(hydroxybutyrate-co-hydroxyvalerate) monolayer films using melt

blending and compression-moulding, followed by spraying it with carvacrol or eugenol. Another

monolayer was placed over the EOs layer and compressed using the hydraulic press. Only 7 %

46

(w/w) of the EOs were lost during the thermal compression. Goñi et al., (2018) impregnated

carvone into commercial LDPE films using supercritical CO2 technology. The impregnation

process had no significant effect on the mechanical properties of the film. In general, the

compatibility between EOs and film materials, the mechanical properties of film materials, and the

release rate of EOs should be considered when preparing EOs-based packaging materials.

Direct incorporation of EOs into polymer films, such as those prepared using melt extrusion

process, is inefficient due to evaporation and thermal degradation losses. In order to retain the

maximal efficacy and allow for controlled release of the volatile active components, pre-

encapsulation of the EOs within an optimal carrier solid matrix is needed before loading them into

polymer films. This approach can be useful for reducing the flavor of EOs, protecting them from

reacting with other components, and enhancing EOs storage stability (Ju et al., 2019). Various

methods have been reported in the literature. For example, Wang et al., (2017) encapsulated AITC

into β-CD before dispersing it to PLA films in a two-step extrusion method. The addition of the

inclusion complex significantly enhanced the flexibility and thermal stability of films. Also, the

release of AITC from the film can be triggered by relative humidity and temperature. Wu et al.,

(2021) found that soybean protein films containing thymol adsorbed by diatomite had higher

tensile strength than films containing thymol alone and had higher elongation at break than films

containing diatomite alone. When compared to films with thymol alone, diatomite was found to

reduce the loss of thymol during the film preparation and storage, as well as a sustained release of

thymol was observed (Wu et al., 2021). The pre-encapsulation of cinnamaldehyde into modified

montmorillonite highly enhanced its retention/release in chitosan films (Cui et al., 2020). Also, the

film had increased rigidity, ductility, and barrier properties to water vapor, as well as improved

47

UV resistance as compared to films with/without unencapsulated cinnamaldehyde. The elevated

temperature condition encountered during PLA film formation, using extrusion and

thermoforming, dramatically decreased the carvacrol and AITC content in the film (Raouche et

al., 2011). By pre-encapsulating them into β-CD, the volatile compounds were protected against

evaporation losses and thermal degradation.

In the aforementioned systems, the main release mechanism of the active compounds from

polymer films is a passive diffusion process, in which the active compounds, driven by a

concentration gradient, diffuse through the polymer matrix towards the surface then desorbed into

the headspace air or the food material. The matrix may interact with a triggering agent, such as

moisture, to induce swelling and/or phase transition that accelerate the release of the bioactive

compounds (Almasi et al., 2020).

Table 2.6: Polymer-based films containing active gaseous/volatile compounds with/without prior

encapsulation.

Gas/volatile Film structure* Method References

1-MCP LDPE, PVC, PP, PS, and

EVOH – with α-CD

Heat-pressing;

solvent-casting

Hotchkiss et al.,

(2007)

1-MCP LDPE – with α-CD

grafted onto polyolefins

Coextrusion Wood et al., (2017)

Ethylene PEGDA – with α-CD UV-curing Capozzi et al., (2016);

Pisano et al., (2015)

AITC PLA Solvent-casting Gao et al., (2017)

AITC LDPE – with β-CD Extrusion Shin et al., (2019)

48

Thymol/

carvacrol

PLA/PCL Solvent-casting;

impregnation using

supercritical CO2

Lukic et al., (2020)

Thymol Soybean protein isolate –

with/without diatomite

Solvent-casting Wu et al., (2021)

Thymol/kesum/

curry

PLA Solvent-casting Mohamad et al.,

(2020)

Thymol Starch/chitosan and

starch/chitosan/zeolite

Solvent-casting

Impregnation using

supercritical CO2

Pajnik et al. (2020)

Carvacrol PVA Solvent-casting Andrade et al., (2020)

Thyme oil

Whey protein concentrate Thermal

crosslinking;

combined with

mild-thermal high-

pressure

denaturation

Bleoanca et al.,

(2020)

AITC /carvacrol PLA/PEG – with/without

β-CD

Melt processing

(extrusion and

thermoforming)

Raouche et al., (2011)

Carvacrol PLA/polyester – with

MMT nanoclay

Extrusion Scaffaro et al., (2020)

Carvacrol Thermoplastic starch –

with layered silicate

Extrusion Campos-Requena et

al., (2018)

Cinnamaldehyde PLA – with β-CD Solvent-casting Zhang et al., (2020)

Cinnamaldehyde/

Carvacrol

LDPE – with β-CD Melting process Canales et al., (2019)

Cinnamon oil Chitosan/gum arabic Solvent-casting Xu et al., (2018)

Cinnamaldehyde Chitosan Solvent-casting Cui et al., (2020)

Hexenal Soy protein isolate Solvent-casting Monedero et al.,

(2010)

49

Eugenol Starch – with/without

maltodextrin

Solvent-casting Cheng et al., (2019);

Talón et al., (2019)

Eugenol/

carvacrol

PHBV bilayers Melt blending and

compression-

moulding;

bioactives are

sprayed in-between

layers

Requena et al., (2017)

Carvone LDPE Impregnation using

supercritical CO2

Goñi et al., (2018)

Limonene Wheat gluten and

i-carrageenans

Solvent-casting Marcuzzo et al.,

(2012)

Oregano oil

Whey protein isolate and

triticale protein

Solvent-casting Aguirre et al., (2013);

Seydim & Sarikus,

(2006)

Clove oil

CMC/PVA Solvent-casting Muppalla et al.,

(2014)

*Low-density polyethylene (LDPE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene

(PS), ethylene vinyl alcohol (EVOH), polyethylene glycol (PEG), polyethylene glycol diacrylate

(PEGDA), polylactic acid (PLA), poly(ε-caprolactone) (PCL), polyvinyl alcohol (PVA),

poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), montmorillonite (MMT) and carboxymethyl

cellulose (CMC).

2.4.5 Encapsulation in electrospun nonwovens

Another innovative technology to encapsulate active compounds is electrospinning. It is a

fiber-forming technology for producing fibers with diameters ranging from hundreds of

nanometers to tens of micrometers, using electrostatic force (Reneker et al., 2007; Yu et al., 2017).

A typical electrospinning setup is showed in Fig. 2.3A. The infusion pump delivers the polymer

solution to the syringe equipped with a spinneret attached to the positive electrode. When the

critical voltage (5-30 kV DC) is supplied to the spinneret, the polymer droplet at the spinneret tip

50

overcomes its surface tension, and a jet of ultrafine fiber ejects to the collector, which is attached

to the ground electrode (Loong Tak Lim, 2015; Vega-Lugo & Lim, 2012). The single needle

electrospinner has a relatively low production rate. Although multiple-needle setup can increase

production throughput, this approach can increase the complexity of the setup (Zhou et al., 2009).

To address this issue, free surface electrospinning processes have been developed, involving

different spin electrode configurations (Brettmann et al., 2012). Fig. 2.3B showed a high

productivity free surface electrospinner approach with a stretched conductive wire as the spinning

electrode. Unlike the single spinneret system, in this approach the polymer solution is being loaded

into the carriage which glided along a positively charged wire electrode back and forth to evenly

coat the wire electrode with the spin dope solution. High voltage (30-60 kV DC) is being applied

to electrify the spin dope solution, resulting in the formation of polymer multiple jets ejecting from

wire toward the substrate positioned in between the spinning and grounded electrodes (Xiao &

Lim, 2018; Zaitoon & Lim, 2020). In general, the electrospinning process and the fiber

characteristics (e.g., morphology, size) are governed by processing (e.g., voltage, spinneret-

substrate distance or distance between electrodes, flow rate), solution (e.g., surface tension,

electrical conductivity, viscosity, solvent vapor pressure), and ambient (e.g., temperature, relative

humidity) parameters (Ray et al., 2016; Reneker et al., 2007; Zaitoon & Lim, 2020).

51

Fig. 2.3: Schematic representation of conventional single needle electrospinner (A) and free

surface wire electrospinner (B).

Various synthetic and biopolymers, as well as composite blends, have been electrospun into

ultrathin fibers of different properties. The nonwovens produced from these fibers are versatile

materials due to their high surface-to-volume ratio, lightweight, porous, and tunable morphology

(Persano et al., 2013; Yu et al., 2017). They are widely used in many applications such as wound

dressing, tissue engineering, encapsulation, biosensors, membrane distillation, nanofiltration

processes, removal of contaminants, and protective clothing (Castro Coelho et al., 2021; Huang et

al., 2006; Li et al., 2002; Liu et al., 2020; Ray et al., 2016). Since electrospinning is a non-thermal

process, it is ideal for the encapsulation of heat-sensitive nutraceuticals, drugs, volatile actives,

and so on (Zaitoon & Lim, 2020). Over the past decade, researchers have been exploring various

electrospun nonwovens as encapsulants for active compounds as well as a carrier for delivering

encapsulated active compounds. Table 2.7 listed some recent studies on encapsulation of volatile

52

active compounds into electrospun nonwovens based on synthetic and natural polymers. The

findings of these research works indicate that the retention of active compounds in the electrospun

nonwovens and their release behavior depends essentially on the polymer and solvents properties

as well as the experimental procedure. Altan et al. (2018) found that the release amount of

carvacrol from zein fibers at equilibrium was lower than that released from PLA fibers. This

difference was correlated with the low capacity of zein fibers to entrap carvacrol than PLA fibers.

Also, zein fibers, with and without carvacrol, had ribbon morphologies with smooth surfaces,

while carvacrol-loaded PLA fibers had wrinkles and deep pits on the surfaces that provided a much

greater surface area for enhancing the entrapment and release of carvacrol (Altan et al., 2018).

Solid carriers (e.g., CDs, silica) can be incorporated to enhance the retention of volatiles in the

electrospun nonwovens as well as to provide sustained release behaviors (Aytac et al., 2014;

Melendez-Rodriguez et al., 2019). The solid carriers can be incorporated either by pre-

encapsulating volatiles prior to electrospinning (Aytac et al., 2017) or adding as a formulation

ingredient of the spin-dope solution prior to electrospinning (Wen et al., 2016).

Additionally, coaxial electrospinning could be used to provide greater control and retention of

volatile active compounds (Li et al., 2020; Yao et al., 2017; Zhang et al., 2019). The coaxial setup

is similar to the single needle setup except that a compound spinneret is used wherein the two tubes

are arranged in concentric fashion within the spinneret, each of which contains a spin dope solution

delivers by a separate pump. Typically, the core solution consists of the active substance and shell

solution composed of a spinnable polymeric material. It results in ultrathin fiber structures with

high quality and improved functionality (Koushki et al., 2018). Free surface electrospinning is

another attractive technique for encapsulating active compounds due to its high process

53

throughput, which is more conducive for commercial production scale than the typical spinneret

approach. However, studies on controlled release of volatile actives using free surface electrospun

fibers are relatively limited as compared to those produced by spinneret method, probably due to

the more elaborate electrode and collector setups in the former (Ahenkorah et al., 2020; Huang,

2016). Similar to polymer-based films, the release of active compounds from electrospun fibers is

diffusion-controlled (Altan et al., 2018).

Table 2.7: Recent studies on the encapsulation of volatile active compounds into electrospun

nonwovens.

Gas/volatile Nonwoven

structure* Solvents Setup References

1-MCP PS – with α-CD Chloroform/toluene

(6:4, v/v)

Single

needle

Neoh et al.,

(2017)

Carvacrol Zein and PLA Aqueous ethanol

(80%, v/v) for zein;

chloroform/ DMF

(9:1, v/v) for PLA

Single

needle

Altan et al.,

(2018)

Eugenol PHBV – with

mesoporous silica

nanoparticles

2,2,2-trifluoroethanol Single

needle

Melendez-

Rodriguez et

al., (2019)

Eugenol PVP – with shellac Ethanol Coaxial Li et al.,

(2020)

Hexanal Zein/PEO Aqueous ethanol Single

needle

Ranjan et al.,

(2020)

Cinnamon EO PVA – with/without

β-CD

water Single

needle

Wen et al.,

(2016)

Peppermint and

chamomile EOs

Gelatin Acetic acid and water

(22:3, v/v)

Single

needle

Tang et al.,

(2019)

Thymol Zein – with/without

γ-CD

Dimethylformamide Single

needle

Aytac et al.,

(2017)

54

Thymol PLG Hexafluoroisopropanol

and ethanol

Coaxial Zhang et al.,

(2019)

Thymol/ eugenol

Hydroxypropyl-β-

CD,

hydroxypropyl-γ-

CD, or methyl-β-

CD

Water Single

needle

Celebioglu et

al., (2018b),

(2018a)

Thymol and

carvacrol

EC/PEO and

CAC/PEO

Aqueous ethanol for

EC/PEO; acetic

acid:acetone for

CAC/PEO

Free surface Huang,

(2016)

Cinnamaldehyde Zein Aqueous ethanol

(80%, v/v)

Single

needle

Cerqueira et

al., (2016)

AITC PLA Dichloromethane Single

needle

Kara et al.,

(2016)

AITC PVA – with/without

γ-CD

Water Single

needle

Aytac et al.,

(2014)

AITC SPI/PEO and PLA

– with/without β-

CD

1% (w/w) NaOH

solution for SPI/PEO;

chloroform:DMF for

PLA

Single

needle

Vega-Lugo &

Lim, (2009)

Carvone PVA and PCL Water Single

needle

Ramamoorthy

& Rajiv,

(2014)

Thyme EO PCL and PVA DMF/chloroform (1:1,

v/v) for PCL; ethanol

for PVA

Coaxial Koushki et

al., (2018)

Orange EO Zein Aqueous ethanol

(80%, v/v)

Coaxial Yao et al.,

(2017)

Orange EO Zein – with/without

β-CD

Aqueous ethanol

(70%, v/v)

Single

needle

Kringel et al.,

(2020)

*Polystyrene (PS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polylactic acid

(PLA), polyvinyl pyrrolidone (PVP), polyethylene oxide (PEO), poly(lactide-co-glycolide)

(PLG), ethyl cellulose (EC), cellulose acetate (CAC), soy protein isolate (SPI), polyvinyl alcohol

(PVA), dimethylformamide (DMF), and poly(ε-caprolactone) (PCL).

55

2.5 Controlled and triggered release of active gases/volatiles through

Precursor compounds

Encapsulation of active gaseous/volatile compounds is challenging. Direct dispersion of these

compounds into an encapsulant material is inefficient due to evaporation loss. One approach to

overcome this challenge is to substitute the gaseous/volatile compounds with their chemical

precursors with enhanced stability. The release of gases/volatiles from their precursors can be

triggered by different mechanisms, including moisture, pH, temperature, light, enzyme and so on.

The precursor is isolated from the triggering factors before the intended application by means of a

barrier structure, during which the precursor remains stable for an extended time. The enhanced

stability not only would greatly facilitate the encapsulation process (i.e., development of

formulations and compositions) but is also desirable for achieving optimal storage/durable life and

allowing the development of a carrier that provide a predictable release starting time. Table 2.8

summarised various precursors of active gaseous/volatile compounds and their activation

mechanisms.

One of the most well-documented precursor systems is that based on the simple reaction

between bicarbonates and acids to generate carbon dioxide gas (Eq. 2.1). The reaction produces a

salt and carbonic acid, which readily decomposes to CO2 and water. Several CO2 emitters have

been reported in the literature based on sodium bicarbonate and organic acids (e.g., citric, ascorbic

acids), which react in the presence of water to release CO2 (Hansen et al., 2016; Holck et al., 2014;

Véronique, 2008). Another less common CO2 emitter system is based on ferrous carbonate and

metal halide catalyst. Similarly, chlorine dioxide is released after the acidification of chlorite,

wherein the metal chlorite (e.g., sodium chlorite) and the acids are both served as the precursors

56

(Eqs. 2.2 and 2.3). When in contact with water/moisture, chlorous acid (HClO2) is formed, which

via disproportionation reaction, resulted in the formation of ClO2 (Zhou et al., 2018a; Zhou et al.,

2018b). Bisulfite salts (e.g., sodium metabisulfite) can be used as a precursor for sulfur dioxide.

Upon contact with water/moisture, bisulfite can be converted into SO2 according to Eq. 2.4

(Lichter et al., 2008; Xing et al., 2011; Xu et al., 2011). Also, calcium sulfite can release SO2 under

acidic conditions (Xu et al., 2011).

𝐻+ + 𝑁𝑎𝐻𝐶𝑂3 → 𝐻2𝑂 + 𝐶𝑂2 + 𝑁𝑎+ (Eq. 2.1)

𝐻+ + 𝑁𝑎𝐶𝑙𝑂2 → 𝐻𝐶𝑙𝑂2 + 𝑁𝑎+ (Eq. 2.2)

5𝐻𝐶𝑙𝑂2 → 2𝐻2𝑂 + 4𝐶𝑙𝑂2 + 𝐻𝐶𝑙 (Eq. 2.3)

𝑆2𝑂52− → 2𝐻+ + 2𝑆𝑂2 + 𝐻2𝑂 (Eq. 2.4)

Precursors for organic volatile compounds can be synthesized by reacting the target volatile

compound with a non-volatile substrate through the formation of labile covalent bonds (Fig. 2.4).

The resulting precursors are non-volatile and remained stable during storage/handling, and yet

labile enough for triggered release of the original volatile through selective cleavage of covalent

bonds by means of hydrolysis, enzymatic, or oxidative reactions (Herrmann, 2007; Levrand et al.,

2007). The precursor compounds, so-called profragrances, are extensively being exploited in

cosmetic and perfume applications to control the release of bioactive volatile compounds (e.g.,

aldehydes, ketones, alcohols, esters), thereby enhancing their longevity and olfactory performance

(Berton et al., 2018; Gautschi et al., 2001; Herrmann, 2007, 2010, 2012, 2017; Levrand et al.,

2007; Starkenmann et al., 2008; Trachsel et al., 2013). For example, the controlled release of

57

bioactive volatile compounds (i.e., fragrances) can be achieved through the use of reversible Schiff

bases (Herrmann, 2009), hydrazone (Levrand et al., 2007), aminal (Levrand et al., 2011), enamine

(Brannock et al., 1964) formations, by reacting the volatiles (e.g. carbonyl compounds) with amine

derivatives. This procedure results in the formation of non-volatile and stable profragrances.

However, once exposed to moisture or acidic conditions, hydrolysis takes place, thereby releasing

the volatile fragrances.

Fig. 2.4: Concept of the formation of precursors for volatile compounds and their subsequent

release through selective bond cleavage by a triggering agent.

58

Many volatiles used in AP applications belong to alcohols, aldehydes, ketones, and esters

compounds. Researchers have explored different precursors to control the release of these classes

of bioactive agents (Table 2.4). For example, Buchs et al., (2012) stabilized alcohols by reacting

them with hemiacetal derivatives of pyridine-2-carbaldehyde in the presence of metal cations

(Zn2+) in organic solvents, to form alcohol precursors. Sustainable release of alcohol can be

initiated by the hydrolysis of the precursors. Yang et al., (2003) produced β-amino alcohol

derivatives of fragrant ketones and aldehydes (e.g., lauryl aldehyde, lilial, citronellal,

benzaldehyde, anisaldehyde, and menthone), by reacting them with trimethylsilyl cyanide,

followed by a reduction reaction with lithium aluminium hydride. These amino alcohol precursors

are non-volatile and stable precursors compounds, yet can be oxidized by sodium periodate or

sodium bismuthate to release the bioactive volatiles readily (Yang et al., 2003). The reversible

nucleophilic reaction between the aldehydes/ketones and an N,N’-disubstituted-1,2-diamine have

also been investigated to generate non-volatile imidazolidine precursors. These imidazolidines can

be hydrolyzed under mild acidic conditions, releasing the aldehydes/ketones ( Levrand et al., 2011;

Godin et al., 2010; Morinaga et al., 2010).

Recently, a precursor approach to achieve triggered releases of aldehydes (e.g., hexenal,

benzaldehyde, and salicylaldehyde) was developed by converting the volatile aldehydes into stable

imidazolidine precursors via reacting them with N,N’-dibenzylethane-1,2-diamine. These

aldehyde precursors are stable under dry conditions, yet they can be hydrolyzed under mild acidic

conditions to release the aldehydes readily (Jash & Lim, 2018; Jash et al., 2018; Shi et al., 2021).

Other methods involved the coupling of salicylaldehyde and hexanal to branched polyethylenimine

having hydrolyzable imidazolidine moieties (Dulvi, 2019) and covalently attaching hexanal to the

59

biodegradable polymer chitosan to form iminated N-hexylimine-chitosan film through Schiff base

reaction (Fadida et al., 2015). Releases of these aldehydes from the polymeric precursors can be

achieved by acid hydrolysis of the imine bonds.

In the above reports, precursors were synthesized through the reaction of volatiles with

substrates (Fig. 2.4). Another synthesis approach is via using the volatiles’ correspondings as a

reactant instead of the volatiles themselves. Robles and Bochet, (2005) generated a photo-labile α-

acetoxy ether precursor for aldehydes (methional, (R)-citronellal, and phenylacetaldehyde). The

precursors were synthesized through the reduction reaction of the aldehydes’ corresponding ester

by di-isobutylaluminium hydride, followed by quenching of the intermediate aluminum

hemiacetal by acetic anhydride. Upon the exposure to UV irradiation, the precursors release the

aldehydes. Additionally, Sarker and his coworkers developed a series of boron derivatives of

methylenecyclopropane as precursors for 1-MCP gas. Boronized-MCP compounds were

developed by reacting boron derivatives (dicyclohexyl-chloroborane, dihexyl-chloroborane, boron

tri-chloride, and diphenyl-chloroborane) with lithiated methylene cyclopropane (Sarker et al.,

2015, 2020). These compounds are stable at ambient conditions and capable of releasing 1-MCP

gradually when hydrolyzed with water or moisture. The rate of release of 1-MCP depends on the

ambient temperature and the structure of the compounds.

Naturally occurring precursors, such as those derived from herbs and spices, have been

exploited for AP applications. For example, ground mustard seeds have been investigated as the

natural precursor for AITC, which is a potent antimicrobial volatile (Bahmid et al., 2020; Dai &

Lim, 2014). AITC vapor can be released from mustard seed meal powder when glucosinolate

60

(sinigrin) in the tissues is hydrolyzed in a myrosinase-catalyzed reaction. The release rate can be

manipulated by controlling the accessibility to water.

To facilitate the end-use deployment of these precursors in AP applications, they are often

being incorporated within a carrier system via various approaches, such as adding to liquid

absorber pads (Hansen et al., 2016; Holck et al., 2014), enclosure in sachets/pouches (Sun et al.,

2017; Wang et al., 2019; Zhou et al., 2018b), adsorption onto mineral adsorbents (e.g., silicates,

zeolites) (Khanna et al., 1998; Lovely, 1968; Speronello et al., 1997), dispersion in polymer films

(Bai et al., 2016; Ray et al., 2013; Wellinghoff, 1993; Zhou et al., 2020), encapsulation in

electrospun fibers (Ahenkorah et al., 2020; Jash & Lim, 2018; Zhou et al., 2018a), and so on.

Besides protecting the precursors before activation, these delivery carrier systems are important

for controlling the release of the active gases/volatiles from the precursor compounds. By

controlling the mass transfers of the triggering species (e.g., water, acid) and the regenerated active

compound in the carrier matrix, the release kinetics can be optimized for specific AP applications.

For example, Jash & Lim, (2018) reported a higher release rate of hexenal from its imidazolidine

precursor when loaded into PLA fibers than that in ethyl cellulose fibers due to the higher solubility

of hexanal in the PLA polymer. Ray et al., (2013) observed a slow release rate of ClO2 from its

precursors when incorporated in PLA films. The ClO2 release rate decreased as the amount of the

polymer increased, making the reactants (distributed in the films) difficult to fully participate in

the reaction.

61

Table 2.8: Precursor compounds used to stabilize and control the release of gaseous/volatile

active compounds.

Gas/volatile Precursor Triggering agent References

Carbon dioxide Sodium bicarbonate + acid Moisture

Coma, (2008);

Hansen et al.,

(2016)

Carbon dioxide Ferrous carbonate + metal

halide catalyst Moisture

Restuccia et al.,

(2010);

Sivertsvik, (2003)

Chlorine dioxide Sodium chlorite + acid Moisture

Zhou et al.,

(2020); Zhou et

al., (2018a),

(2018b)

Sulfur dioxide Sulfite/bisulfite salts Moisture or

acidic solution

Lichter et al.,

(2008); Xing et

al., (2011)

1-MCP Boron derivatives of

methylenecyclopropane Hydrolysis

Sarker et al.,

(2020), (2015)

Alcohol Hemiacetal complexes Hydrolysis Buchs et al.,

(2012)

Aldehydes/ketones Amino alcohol derivatives Oxidation Yang et al.,

(2003)

Aldehydes/ketones Aldoxane derivatives Temperature (60

– 80°C)

Womack et al.,

(2004)

Aldehydes/ketones Photo-labile α-acetoxy

ethers

UV radiation Robles & Bochet,

(2005)

Aldehydes/ketones Hydrazones Hydrolysis under

acidic condition

Levrand et al.,

(2007)

62

Aldehydes/ketones Imidazolidines Hydrolysis under

acidic condition

Godin et al.,

(2010); Jash &

Lim, (2018); Jash

et al., (2018); Shi

et al., (2021)

Aldehydes Poly(ethylene imine) with

imidazolidine moieties

Hydrolysis under

acidic condition

Dulvi, (2019);

Morinaga et al.,

(2010)

Allyl isothiocyanate Ground mustard seeds Enzymatic

hydrolysis

Bahmid et al.,

(2020); Dai &

Lim, (2014)

Hexenal N-hexylimine-chitosan Hydrolysis with

HCl

Fadida et al.,

(2015)

2.6 Applications of gases/volatiles controlled release systems in active food

packaging

Although studies on the controlled release of active gaseous/volatiles compounds are many,

commercially products are limited. Table 2.9 summarized some of the commercially available

products for modified atmosphere and AP applications. Other emerging controlled release systems

that are being investigated by researchers are discussed in Section 2.4 and 2.5, many of which are

promising for AP applications. For example, Ariyanto et al., (2019) developed a delivery system

for 1-MCP gas based on coating 1-MCP-α-CD inclusion complex on a cellulose paper. The

moisture in the package headspace can trigger the release of 1-MCP gas. The system could inhibit

ethylene production and extended the shelf life of apples. In another study, Ortiz et al., (2013)

generated 1-MCP releasing pads. These pads contain 1-MCP-α-CD complex sandwiched between

63

two layers of solvent casted soy protein films. The developed pads were effective in extending the

shelf life of tomatoes. Unlike 1-MCP, ethylene is used to accelerate ripping in fruit. A controlled

release system for ethylene based on its encapsulation into α-CD was developed by Ho et al.,

(2016). Ethylene treated mango using their system showed more quick and uniform ripening with

attractive skin color than the untreated fruit. Another system was developed by Chopra and his

coworker, in which MOF (Basolite C300) and Tyvek® sachet were identified as ethylene adsorbent

and delivery system, respectively (Chopra et al., 2017). Ethylene release from the sachet can

induce fast and uniform ripening in banana.

Although encapsulation of CO2 into α-CD was earlier patented in Japan in 1987 (Trotta et al.,

2011), no commercial applications were found based on this technology, which could be attributed

to the limited storage capacity achieved. Recently, Ho et al., (2018) found that mixing CO2-α-CD

inclusion complex with cottage cheese curds extended their shelf life for up to 6 weeks. CO2 was

slowly released from the complex, solubilized into the curds, and diffused in the container

headspace, inhibiting mold and yeast growth. Other controlled release systems for CO2 are based

on its precursors. CO2 pads containing sodium bicarbonate and acid were able to inhibit the

microbial growth on meat (Hansen & Mielnik, 2014), chicken (Holck et al., 2014), fish products

(Hansen et al., 2009; Hansen et al., 2016). Ethanol released from its emitters (sachets containing

ethanol adsorbed by silicon dioxide powder) was able to extend the shelf life of pre-baked buns

and sliced wheat bread (Franke et al., 2002; Latou et al., 2010). Zhou and his colleagues developed

three ClO2 generating systems based on its precursor. The first system is a sachet filled with

sodium chlorite. When it is placed in fresh produce packages, CO2 and moisture naturally released

from produce during respiration react with sodium chlorite, generating ClO2 (Zhou et al., 2018b).

64

In the second system, they encapsulated sodium chlorite into polyethylene oxide electrospun

nonwovens (Zhou et al., 2018a). The third system is a polylactic acid film containing sodium

chlorite and glucono delta-lactone (GDL). Upon containing with moisture, GDL can be partially

hydrolyzed to become a weak acid (gluconic acid) that reacts with sodium chlorite to release ClO2

gas (Zhou et al., 2020). The three systems were used successfully to inactivate the microbial

growth on fresh tomato without impacting its sensory qualities.

For EOs and their constituents, many reviews discussed their uses as antimicrobial and

antioxidant agents in Ap applications (Domínguez et al., 2018; Ju et al., 2019; Maisanaba et al.,

2017; Sanches-Silva et al., 2014). These compounds can be incorporated in food packaging using

different methods such as direct mixing, adsorbing, or coating into packaging materials;

encapsulation followed by adding to packaging materials; or gaseous form addition to packaging

(Ju et al., 2019). Another novel approach for using volatile EOs in food packaging is via converting

them into stable chemical precursors (Dulvi, 2019; Jash et al., 2018; Jash & Lim, 2018; Shi et al.,

2021). Recently, Ahenkorah et al., (2020) developed ethylcellulose/poly(ethylene oxide)

nonwovens containing hexanal precursor (1,3-dibenzylethane-2-pentyl imidazolidine) and citric

acid. The absorption of moisture by the nonwovens formed citric acid solution which when came

in contact with the precursor, hydrolyzed the latter, and activated the release of hexanal vapor. The

nonwoven effectively extended the shelf life of the highly perishable papaya fruits. The EOs

precursors can be useful for the controlled release of volatile EOs used in AP applications.

65

Table 2.9: Some commercially available controlled release systems for active gaseous/volatile

compounds.

Gas/

volatile Trade name Format References

1-MCP EthylblocTM Powder of 1-MCP

encapsulated into α-CD

AgroFresh, (n.d.)

SmartFreshTM Powder and tablet of 1-MCP


AgroFresh, (n.d.)

TruPickTM 1-MCP adsorbed by MOF Editorial, (2016)

HazelTM 1-MCP adsorbed by activated

carbon

Hazel

Technologies,

(n.d.)

FYSIUMTM Cartridge for generating 1-

MCP

Pace International,

(n.d.)

Ethylene RipeStuffTM Powder of ethylene


UniQuest, (n.d.)

Easy-Ripe Catalytic generator Catalytic-

Generators, (n.d.)

ARCO & ULTRA IV Catalytic generator American-Ripener,

(n.d.)

Ethanol Antimold-MildTM Ethanol absorbed onto silicon

dioxide powder

FREUND, (n.d.)

NegamoldTM Ethanol adsorbed onto silicon

dioxide powder mixed with

iron powder

FREUND, (n.d.)

CO2 CO2 FreshpadTM Emitter based on sodium

bicarbonate and citric acid

CO2

TECHNOLOGY,

(n.d.)

SuperFreshTM Based on sodium bicarbonate

and citric acid

VdP International,

(n.d.)

66

Active CO2 padTM Based on sodium bicarbonate

and citric acid

CELLCOMB,

(n.d.)

UltraZap® XtendaPak - NOVIPAX, (n.d.)

McAirlaid’s CO2

PadTM

- McAirlaid’s, (n.d.)

ClO2 MicrosphereTM Powder of sodium chlorite and

acid

Lelah et al., (2008)

MicrogardeTM Film containing sodium

chlorite and acid

Lelah et al., (2008)

SO2 UVASYSTM Multilayers bad containing

sodium metabisulfite TESSARA, (n.d.)

MATESATM Multilayers bad containing

sodium metabisulfite

BIOPAC, (n.d.)

DECCOGRAPAGETM Multilayers bad containing

sodium metabisulfite

DECCO ITALIA

Srl, (n.d.)

SO2 Pads - PPS Packaging

Company, (n.d.)

AITC WasaouroTM Allyl mustard oils Mitsubishi-

Chemical, (n.d.)

2.7 Conclusion

In recent years, there has been a growing interest in developing controlled release technologies

for gaseous/volatile active compounds to facilitate their uses in AP applications of agri-food

products. Stabilization of gases/volatiles is a major requirement to control their release, prevent

degradation and mass transfer losses, and design of controlled release packaging. Various

approaches are being explored to stabilize and control the release of gaseous/volatile active

67

compounds, including encapsulation (e.g., into solid matrices, polymer-based films, electrospun

nonwovens) and triggered release systems involving precursor technology. For the encapsulation

approach, it is critical to choose the optimal encapsulant for the active compounds since

encapsulants are extremely different in their composition and properties, which greatly affect the

encapsulation capacity and the release kinetics, as well as the physical and chemical properties of

the resultant complexes. Encapsulation of gaseous/volatile active compounds is always

challenging due to their evaporation loss during manufacturing. One approach to overcome this

challenge is to substitute the gases/volatiles with their chemical precursors with enhanced stability.

The release of gases/volatiles from their precursors can be triggered by different mechanisms,

including moisture, pH, temperature, light, enzyme and so on. Although precursors have been

developed and applied in cosmetic and perfume industries for many volatile compounds, their uses

in AP applications are relatively limited. Controlled release systems based on encapsulation and

precursor approaches are promising for AP applications. Delivery systems (e.g., pads, sachets,

films, nonwovens, and so on) are widely used to facilitate the ultimate applications of the

complexes or precursors as well as impart additional controlled release attributes to the active

compounds.

Although there are many studies in the literature focusing on controlled release technologies

of gaseous/volatile active compounds and/or testing the efficacy of the developed technologies,

there is a lack of research on investigating the stability, retention, and release kinetics of active

compounds under typical end-use conditions. This area of research needs further investigation,

especially to improve the understanding of the physical/chemical interactions between the active

compounds, carrier, and food materials. Since the solid forms (i.e., complexes, precursors) are

68

often safer and easier to deliver the optimum concentrations of active compounds to the food

packaging than the gases/volatiles themselves, research studies are also needed to develop

controlled release systems with release profiles that match the optimum concentration profiles of

the food materials, considering the food deterioration kinetics. Additionally, most of the studies

reported in the literature are focused on the controlled release of antimicrobial/antioxidant active

compounds. Another research area that not explored yet is the controlled release of

gaseous/volatile pesticidal compounds for in-package fumigation of fresh produce as opposed to

the batch fumigation process being done today in the industry, which delays shipment and

shortening the available shelf life of the products.

Finally, the gap between commercialization and research related to the use of active

gaseous/volatile compounds in food packaging has not been fully occupied. Research efforts from

multidisciplinary teams, including food and material scientists as well as packaging technologists,

are recommended.

69

Chapter 3: Justification, hypothesis, and objectives

3.1 Ethyl formate as a fumigant

Ethyl formate (EF) is an FDA-approved food flavoring agent naturally occurring in soil, water,

vegetation, and many food products (e.g., beer, wine, grapes, wheat, barley, raisin, rice, cheese)

(Desmarchelier, 1999; Desmarchelier et al., 1999; Malanca et al., 2009; Ren & Desmarchelier,

2002). For more than two decades, many studies have shown that EF is an effective insecticide

and antimicrobial which is efficacious for the fumigation of fresh produce and grain commodities

(Bessi et al., 2016; Bessi et al., 2015; Bolin et al., 1972; Learmonth et al., 2012; Lee et al., 2018;

Ren & Mahon, 2006; Simpson et al., 2004; Simpson et al., 2007; Smit et al., 2020; Stewart & Mon,

1984; Utama et al., 2002). These studies also found that EF had a minimal to no effect on product

quality.

While EF solution is an effective pesticidal and antimicrobial agent, it is highly volatile (200

mmHg at 20°C), flammable, and susceptible to hydrolytic degradation in the presence of moisture,

producing formic acid and ethanol (Ren & Mahon, 2006; Ryan & Bishop, 2003). These properties

make its end-use handling and application very challenging. To suppress flammability, EF has

been commercially mixed with carbon dioxide in compressed cylinders at a concentration of 16.7

% wt., for fumigation of food commodities. This product is commercially available as

Vapormate®, which exploits the synergistic effects of carbon dioxide and EF against pests (Bessi

et al., 2016; Ryan & Bishop, 2003). Also, Liquid EF has been mixed with nitrogen for safe in-

transit fumigation of shipping containers ( Coetzee et al., 2019, 2020). These approaches are

attractive for large enclosed space fumigation but inefficient to provide controlled release

70

attributes for the gases. Moreover, this technology requires significant spaces for secure storage of

compressed gas cylinders as well as pressure regulation and metering devices to ensure safety and

accurate dosage of the fumigant. Therefore, a method to stabilize and control the release of EF is

essential. Such a method can be used to trigger and manipulate the release of EF for active

packaging (AP) and insect fumigation applications of fresh produce. The innovative packaging-

based fumigation system will drastically simplify the existing fumigation approach based on

conventional batch fumigation systems.

3.2 Hypothesis

On the basis of findings from Chapter 2, regarding the controlled release of active

gaseous/volatile compounds for AP applications, EF can be stabilized through its conversion into

a chemical precursor that is stable during storage/handling, and yet labile enough for triggered

release of the original volatile through selective cleavage of covalent bonds, by means of

hydrolysis/enzymatic/oxidative reactions (Levrand et al., 2007). To the best of our knowledge, EF

precursors (EFPs) have not been reported in the literature. While the reactions between carboxylic

acid hydrazides and triethyl orthoformate (Eq. 3.1) have been investigated by researchers for the

synthesis of oxadiazole derivatives, the intermediates of this reaction, i.e., the formohydrazonate

derivatives could be promising as EFPs (Ainsworth, 1955, 1965; Ainsworth & Hackler, 1966). We

hypothesize that the formohydrazonate derivatives could be hydrolyzed under mild acidic

conditions to trigger the release of EF vapor (Eq. 3.2).

R–CONHNH2 + (C2H5O)3CH → R–CONHN=CHOC2H5 + 2C2H5OH (Eq. 3.1)

R–CONHN=CHOC2H5 + H2O → R–CONHNH2 + HCOOC2H5 (Eq. 3.2)

71

where R represents a hydrogen atom, an alkyl, or aryl group.

It is further hypothesized that the encapsulation of the synthesized precursors is required to

facilitate their end-use deployment in packaging applications and to prevent premature release

during storage. Electrospun nonwovens, generated from electrospinning process, can be used to

encapsulate EFP to facilitate the delivery of EF for in-package fumigation of fresh produce.

Converting the highly volatile EF into a solid-state precursor, in conjunction with the activated

release strategy, can be useful for AP applications of fresh produce to mitigate insect pest

problems, as well as to inhibit the proliferation of spoilage microorganisms.

3.3 Objectives

In accordance with the hypotheses, the objectives of this research are to:

1) synthesize a stable EFP that can be hydrolyzed readily to trigger the release of EF vapor

(Chapter 4).

2) develop bead-free ethyl cellulose/poly(ethylene oxide) (EC-PEO) nonwovens using a free

surface wire electrospinning method (Chapter 5).

3) encapsulate EFP in the EC-PEO electrospun nonwovens in order to facilitate its end-use

applications (Chapter 6).

4) evaluated EFP for controlled release of EF vapor for in-package fumigation of blueberries

to control an invasive insect pest – spotted-wing drosophila, and study the effect of EF

fumigation on fruit quality (Chapter 7).

72

Chapter 4: Synthesis and Characterization of Ethyl Formate Precursor for

Activated Release Application#

#Content of this chapter has been published as a research paper: Zaitoon, A., Lim, L. T., & Scott-

Dupree, C. (2019). Synthesis and Characterization of Ethyl Formate Precursor for Activated

Release Application. Journal of Agricultural and Food Chemistry, 67(50), 13914-13921.

https://doi.org/10.1021/acs.jafc.9b06335.

#Based on the data presented in this chapter, a provisional patent application was filled: Zaitoon,

A., Lim, L. T., and Scott-Dupree, C. 2020. Precursor compounds of ester compounds. International

Patent application PCT/CA2020/050725.

73

4.1 Abstract

Ethyl formate (EF) is a generally-recognized-as-safe flavoring agent commonly used in the

food industry. It is a naturally-occurring volatile with insecticidal and antimicrobial properties,

promising as an alternate fumigant to methyl bromide which is undesirable due to its ozone

depletion in the stratosphere and toxic properties. However, EF is highly volatile, flammable, and

susceptible to hydrolytic degradation. These properties present considerable end-use challenges.

In this study, EF precursor (EFP) was synthesized via the condensation reaction of adipic acid

dihydrazide with triethyl orthoformate to form diethyl N,N'-adipoyldiformohydrazonate, as

confirmed by Fourier transformed infrared and solid-state nuclear magnetic resonance

spectroscopies. Differential scanning calorimetry analysis showed that the precursor had a melting

point of 174°C. The physical properties of the precursor were studied using scanning electron

microscopy and dynamic light scattering analysis, which showed that the precursor made up of

agglomerated particulates with irregular shapes and sizes. The EFP was non-volatile and remained

stable under dry conditions but could be hydrolyzed readily to trigger the release of EF vapor. The

release behaviors of EF from the precursor were evaluated by citric acid-catalyzed hydrolysis,

showing that 0.38 ± 0.008 mg EF/mg EFP was released at 2 h at 25°C, representing about 98 % of

the theoretical release. Both EF release rate and its total release amount decreased significantly (p

< 0.05) with decreasing temperature and relative humidity. EFP can be useful for controlled release

of EF for fumigation and other applications in destroying insect pests and inhibiting the

proliferation of spoilage microorganisms.

74

4.2 Introduction

Fruit and vegetable wastage remains a worldwide issue that has gained considerable

attention. Approximately 50 % of the global production of fruits and vegetables are lost or wasted

due to suboptimal postharvest procedures leading to microbial growth, insect infestation, and

quality losses (FAO, 2011, 2015; Porat et al., 2018). This wastage not only causes environmental

impacts, resources depletion, and waste management issues, but also results in economic losses.

As food producers and policy makers are striving to improve food security and sustainable

production, optimizing postharvest preservation to maintain the quality of fresh produce and

maximize shelf-life is essential.

Packaging is critical to protect the products from the undesirable environmental factors

during transportation and storage. To enhance the protective function, active packaging (AP)

systems are equipped with one or more components that interact with the product and/or the

package headspace, thereby activating a mechanism to protect the product (Kerry et al., 2006; Lim,

2011; Mihindukulasuriya & Lim, 2014; Wilson et al., 2017). For example, antimicrobial AP

systems are capable of releasing volatile or non-volatile antimicrobial compounds to inhibit the

growth of spoilage and pathogenic microorganisms (Appendini & Hotchkiss, 2002; Floros et al.,

1997; Nayik & Muzaffar, 2014; Ozdemir & Floros, 2004). In commercial applications, ethanol

and allyl isothiocyanate emitter systems leverage moisture from the package headspace to trigger

their release from solid carriers, thereby delaying microbial spoilage in packaged food products

(Day, 2008; Lim, 2011; Nayik & Muzaffar, 2014). A similar concept can be applied to deliver

fumigant compounds to address insect pest issues in fresh produce during distribution.

75

Ethyl formate (EF) is an FDA approved food-flavoring agent with a generally-recognized-as-

safe (GRAS) status. It is a naturally-occurring volatile present in fruits, vegetables, rice, barley,

grapes, wine, beer, and cheese (Desmarchelier, 1999; Ren & Desmarchelier, 2002; Simpson et al.,

2004). Researchers have shown that EF is effective as a fumigant for destroying insect pests in

various crops and fresh produce (Bessi et al., 2016; Bessi et al., 2015; Learmonth et al., 2012;

Simpson et al., 2004; Simpson et al., 2007; Stewart & Mon, 1984). Stewart & Mon, (1984)

achieved a 98 % mortality of green peach aphids (Myzus persicae) after using EF vapor on film-

wrapped lettuce. They used vacuum fumigation at 60 mmHg for 1 h with EF dosage of 0.5 %

(vol.). Simpson et al. reported that EF fumigation of strawberries and grapes with concentrations

ranging from 0.04 to 4.7 % (vol.) were effective to induce various degrees of mortality in western

flower thrips (Frankliniella occidentalis), two-spotted spider mites (Tetranychus urticae),

mealybugs (Pseudococcidae), and omnivorous leafroller (Platynota stultana) (Simpson et al.,

2004; Simpson et al., 2007). They also found that the exposure of the fruits to EF did not affect

their quality. Utama et al., (2002) studied the antimicrobial activities of EF vapor on selected fruit

and vegetable spoilage microorganisms grown in an agar medium. They reported that EF with a

concentration of 6.5-11.5 mmol per Petri dish (d = 9 cm) was germicidal against the growth of R.

stolonifer, C. musae, E. carotoVora, and P. aeruginosa, but did not show complete inhibition of

P. digitatum growth. Bolin et al., (1972) reported that EF at 6.6 mL/kg inhibited the growth of

Saccharomyces rouxii and Saccharomyces mellis on Deglet Noor dates. These studies

demonstrated that EF elicited potent insecticidal and antimicrobial properties which could be

exploited for AP applications.

76

However, EF solution has a relatively high vapor pressure (200 mmHg at 20°C) and high

flammability (Ren & Mahon, 2006). Moreover, it is susceptible to hydrolytic degradation in the

presence of water to produce formic acid and ethanol (Ryan & Bishop, 2003). These properties

make the end-use handling and application of EF challenging. The development of solid-state

fumigation technology, by converting EF into a stabilized precursor, can be useful for controlled

fumigation of fruit and vegetable within their packages during distribution, thereby eliminating the

limitations of the conventional batch fumigation, avoiding the handling of large quantities of

fumigant in enclosed spaces, making the treatment more cost-effective, and reducing unwanted

release of EF into the environment.

Because precursors are more stable than their corresponding volatiles, they are often utilized

in cosmetic and perfume applications to control the release of volatile compounds, in order to

enhance their longevity and olfactory performance. The precursor synthesis often involves reacting

the target volatile compound with a less volatile substrate to form stable covalent bonds that can

be cleaved readily to release the volatiles, via triggering conditions such as light, moisture, pH,

and temperature (Herrmann, 2007, 2012; Levrand et al., 2007). A similar triggered release

mechanism for volatile antimicrobial aldehydes (hexenal and benzaldehyde) was developed by

Jash et al., (2018) and Jash & Lim, (2018). They converted the volatile aldehydes into stable

imidazolidine precursors via reacting them with N,N’-dibenzylethane-1,2-diamine. The resulting

aldehyde precursors are stable under dry environmental conditions, yet they can be hydrolyzed

under mild acidic conditions to release the aldehydes.

77

To our knowledge, EFPs have not been reported in the literature. While the reactions between

carboxylic acid hydrazides and triethyl orthoformate (Eq. 4.1) have been investigated by

researchers for the synthesis of oxadiazole derivatives, the intermediates of this reaction, i.e., the

formohydrazonate derivatives could be promising as EFPs (Ainsworth, 1955, 1965; Ainsworth &

Hackler, 1966). We hypothesize that the formohydrazonate derivatives could be hydrolyzed under

mild acidic conditions to trigger the release of EF vapor (Eq. 4.2).

R–CONHNH2 + (C2H5O)3CH → R–CONHN=CHOC2H5 + 2C2H5OH (Eq. 4.1)

R–CONHN=CHOC2H5 + H2O → R–CONHNH2 + HCOOC2H5 (Eq. 4.2)

where R represents a hydrogen atom, an alkyl, or aryl group.

The objectives of this research are to: (1) synthesize a stable precursor that can be hydrolyzed

readily to trigger the release of EF; (2) study the release kinetics of EF vapor from the precursor;

and (3) evaluate the stability of the precursor under different storage conditions.

4.3 Materials and Methods

4.3.1 Materials

Triethyl orthoformate (reagent grade, 98 %), adipic acid dihydrazide (98 %), and ethyl formate

(reagent grade, 97 %) were purchased from Sigma-Aldrich (Oakville, ON, Canada). Anhydrous

citric acid, 2-propanol, sodium chloride, potassium nitrate, and magnesium nitrate were bought

from Fisher Scientific (Ottawa, ON, Canada). Anhydrous ethanol was supplied by Commercial

Alcohol (Brampton, ON, Canada).

78

4.3.2 Synthesizing of ethyl formate precursor

To synthesize EFP, two methods were adopted. In method (A), a suspension of adipic acid

dihydrazide (500 mg), excess of triethyl orthoformate (5 mL), and anhydrous ethanol (20 mL) in

a 50 mL round-bottom flask were heated under reflux at 80°C with stirring in an oil bath for 6 h.

The solution was then stored overnight at 4°C to form precipitates. The suspension was filtered,

and the particles were air dried to yield the precursor product. The filtrate was vaporized by

vacuum drying at 40°C to give an additional amount of the product. In method (B), a suspension

of adipic acid dihydrazide (500 mg) and triethyl orthoformate (20 mL) was prepared in a 50 mL

round-bottom flask and refluxed at 110°C, with stirring, in an oil bath for 30 h. After cooling to

room temperature, the mixture was filtered, and the residue was air dried.

4.3.3 Fourier transformed infrared analysis

Infrared spectra of triethyl orthoformate, adipic acid dihydrazide, and products (A and B) were

analyzed using Fourier transformed infrared (FTIR) spectrometer (IRPrestige21, Shimadzu Corp.,

Kyoto, Japan) equipped with an attenuated total reflection (ATR) accessory (Pike Tech, Madison,

WI, USA). Each sample was mounted on the ATR diamond crystal, compressed, and scanned 40

times in the region from 600 to 4000 cm-1 at a resolution of 4 cm-1. FTIR spectra were analyzed

using the IR Solution software (Shimadzu Corp., Kyoto, Japan).

4.3.4 Nuclear magnetic resonance spectroscopy

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was utilized to determine the

molecular structure of the synthesized precursor. All solid-state NMR experiments were

conducted at 298 K using a Bruker 500 MHz spectrometer (Avance II WB, Bruker Corporation,

79

Billerica, MA, USA), operating at 11.74 T (13C Larmor frequency of 125 MHz). Dry samples

were packed into 4 mm zirconia rotors and spun at 7 kHz at the magic angle spinning

(MAS). Standard cross-polarization pulse sequence with total suppression of sidebands

(CPTOSS) from the Bruker library was employed for the experiments. A 1H 90° pulse length of

2.95 µs, a contact time of 2 ms, and a recycle delay of 7.2 s were used. A total of 1000 scans were

collected, and spectra were processed with 50 Hz line broadening. The analysis of each spectrum

was performed using TopSpinTM Software (Version 3.5pl7, Bruker Corporation, Billerica, MA,

USA).

4.3.5 Differential scanning calorimetry

The thermal properties of adipic acid dihydrazide and the precursors were studied using a

differential scanning calorimeter (DSC; Q2000, TA Instruments, New Castle, DE, USA). Nitrogen

with a flow rate of 18 mL/min was used as the purging gas. About 1.5–3.5 mg of the samples were

accurately weighted into DSC aluminum pans and hermetically sealed with lids. An empty sealed

pan was used as a reference. Samples were equilibrated at 20°C, then heated to 250°C at a heating

rate of 15°C/min. Thermograms were analyzed using TA Universal Analysis Software (TA

Instruments, New Castle, DE, USA).

4.3.6 Particle size analyzer

Particle size distribution of the precursor was determined with a dynamic light scattering (DLS)

particle size analyzer (Nanotrac Flex-180° DLS; Microtrac Inc., Montgomeryville, PA, USA).

Samples were diluted in 2-propanol at a concentration of 0.5 mg/mL. Measurements were

conducted at room temperature and with a refractive index of 1.37 and 1.50 for fluid and particles,

80

respectively. The data acquisition and analysis were done by Microtrac Flex software (Version

11.1.0.4, Microtrac Inc., Montgomeryville, PA, USA).

4.3.7 Scanning electron microscopy

The morphological characteristics were examined using scanning electron microscope (SEM;

Quanta FEG 250, FEI Company, Hillsboro, OR, USA), at an accelerating voltage of 10 kV.

Measurements were conducted for both the particles and dried dilution which was prepared by

dispersing the dilution in Section 7.3.6 on a layer of aluminum foil and air drying. Samples were

mounted on metal stubs using double-sided adhesive carbon tape and coated with 20 nm

conductive layer of gold on its surface using a sputter coater (Desk V TSC, Denton Vacuum,

Moorestown, NJ, USA). Image analysis software (Image Pro-Premier 9.2, Media Cybernetics Inc.,

Rockville, MD, USA) was used to analyze the micrographs.

4.3.8 Headspace analysis

The cumulative release of EF from the precursor was determined using an automatic headspace

analysis system (Fig. 4.1). The system comprising of an environmental chamber (MLR-350H,

Sanyo Electric Co., Ltd. Japan), a gas chromatograph (GC 6890, Agilent Technologies Inc., Santa

Clara, CA, USA) equipped with a flame ionization detector (FID), stream selection valves

(EMTCA-CE, VICI Valco Inst., Houston, TX, USA), 1/16 stainless steel tubing, and a control

board (SRI Instruments Inc., Las Vegas, Nev., USA). The capillary column used with the GC was

Agilent J&W DB-624 (Agilent Technologies Inc., Santa Clara, CA, USA) with 30 m length, 0.53

mm I.D, and 3 µm film thickness. The temperatures of the detector and the oven were 200°C and

40°C, respectively. The flow rates of N2, H2, and O2 were 30, 50, and 200 mL/min, respectively.

81

Fig. 4.1: Schematic representation of the automatic headspace analysis system for studying EF

release kinetics. GSV; gas sampling valve. SV0 and SV1; stream selection valves.

The calibration curve was prepared by measuring standard headspace concentration of a known

amount of EF. Chromatograms were analyzed using the Peak Simple software (SRI Instruments,

CA, USA). At any given sampling point, the total amount of EF released into the headspace (Mt,

µL) was determined according to Eqs. 4.3 to 4.5 by the addition of the recorded amount (Mr, µL)

and the accumulated loss (Ml, µL) of all the previous sampling points up to that point.

82

𝑀𝑟 = 𝐶𝑟𝑉𝑟 (Eq. 4.3)

𝑀𝑙 = ∑ (𝐶𝑟−𝑖𝑉𝑒)𝑟−1𝑖=1 (Eq. 4.4)

𝑀𝑡 = 𝑀𝑟 + 𝑀𝑙 (Eq. 4.5)

where Cr is the recorded EF concentration at that point (µL/L), which calculated based on

calibration constants. Vr and Ve represent the volume of the jar (L) and the volume of headspace

gas extracted from the jar (L), respectively.

4.3.9 Ethyl formate release studies

To activate the release of EF, two approaches were evaluated. In the first approach, 1-2 mg of

the precursor was placed in a 10 mL glass beaker inside a hermetically sealed 1L glass jar as shown

in Fig. 4.1. The headspace gas was extracted through a septum attached to the jar lid at

predetermined time intervals. EF release was triggered by distributing 1 mL of 0.1 N citric acid on

the precursor particles using a pipette (Fisher Scientific, Ottawa, ON, Canada) just before closing

the jar and attaching it to the sampling needle. The release of EF from the precursor was studied

at 5, 15, and 25°C.

In the second study, release of EF was evaluated using an acidified substrate which was

prepared by impregnating 0.3 mL of 5% (w/v) citric acid/anhydrous ethanol solution into a 3 cm

× 3 cm spun-bound polypropylene nonwoven, followed by drying it overnight at 40°C. The

precursor (1-2 mg; method B) was then spread on top of the acidified substrate and exposed to the

test relative humidity maintained in a hermetically 1 L glass jar using silica gel (0 % RH) or

saturated salt solutions (magnesium nitrate, 53 % RH; sodium chloride, 75 % RH; potassium

83

nitrate, 94 % RH) (Fig. 4.1) (ASTM Standard E104-02, 2012; Greenspan, 1977). EF release

expressed in mg/mg.L (milligram of EF per milligram of EFP per litre of headspace air) was

determined using the headspace analysis system described in Section 4.3.8.

4.3.10 Stability study

The stability of the precursor was studied over a period of 30 d. The precursor was stored at

25°C under 0, 60, and 100 % RH. An environmental chamber (MLR-350H, Sanyo Electric Co.,

Ltd. Japan) was used to control the humidity levels and temperature. Samples from each condition

were tested for EF release at 1, 15, and 30th day at 25°C using 0.1 N citric acid solution as described

in Section 4.3.9.

4.3.11 Data analysis

Differences between treatments were analyzed using the SAS® University Edition software

package (SAS Institute Inc., Cary, NC, USA), using PROC GLIMMIX with one-way ANOVA.

The means were compared using Tukey’s honest significance difference test. P-values <0.05 were

considered statistically significant. All treatments were triplicated, and results were expressed as

the mean values ± standard error.

The release kinetics of EF from its precursor were modeled using an empirical pseudo-first

order reaction kinetic model (Eq. 4.6):

𝐶𝑒−𝐶

𝐶𝑒−𝐶0= 𝑒−𝑘𝑡 (Eq. 4.6)

84

where Ce is the concentration of EF in the headspace of the 1 L test jar at an infinite time; C is the

EF released at time, t; C0 is the EF initial concentration (which was equal to zero in our case); and

k is the release rate constants. Non-linear regression analyses were conducted to fit the release data

and determine the model parameters.

4.4 Results and discussion

4.4.1 Ethyl formate precursor formation

The EFP was synthesized through the condensation between adipic acid dihydrazide and

triethyl orthoformate. At elevated temperature, the ethanol moiety of triethyl orthoformate was

eliminated, followed by nucleophilic addition to the amino groups of adipic acid dihydrazide to

form diethyl N,N'-adipoyldiformohydrazonate (Fig. 4.2). During the synthesis reaction with

method (A), the initial white suspension turned into a colorless-clear solution after 3.5 h. The FTIR

spectrum of the product at 3.5 h (Fig. 4.3І) showed that the reaction was incomplete, as shown by

the existence of N–H stretching at 3306–3290 cm-1 and N–H bending at 1533 cm-1 of the –NH2

moiety of the dihydrazide. The three absorbance peaks at 1660, 1571, and 1242 cm-1, which were

absent for adipic acid dihydrazide and triethyl orthoformate spectra, could be attributed to C=N

stretching, N–H bending in –NH–, and C–O–C stretching of the EFP, respectively. The intensity

of these peaks increased as the reaction progressed to 5 h due to the formation of the precursor.

Moreover, at 5 h, the –NH2 bands (stretching at 3306–3290 cm-1 and bending at 1533 cm-1) were

diminished, due to the depletion of the dihydrazide substrate. However, these bands were still

noticeable on the spectrum, indicating that the substrate was not totally exhausted. Additionally,

the absorbance peaks at 750–630 cm-1 could be related to out-of-plane N–H bending or CH2

85

wagging. Out-of-plane C–H bending could be responsible for the peaks around 962 cm-1.

Stretching vibration of C–N and C=O were found at 1013 and 1616 cm-1, respectively. The peaks

around 2820–3113, and 3128–3250 cm-1 could be attributed to C–H and N–H stretching,

respectively (Günzler & Gremlich, 2002; Pavia et al., 2013). At 6 h, the product showed further

increase in intensity for C=N stretching at 1660 cm-1, –NH– bending at 1571 cm-1, and C–O–C

stretching at 1242 cm-1 of the precursor, with concomitant diminishing of –NH2 bands from the

dihydrazide. These observations suggested that the adipic acid dihydrazide substrate was depleted

during the reaction. The reaction based on this approach resulted in ~94 % yield of the precursor

product.

By contrast, in method (B), the white suspension persisted throughout the entire synthesis

process. The FTIR spectra of the product (Fig. 4.3П) collected at 10 and 24 h showed the existence

of adipic acid dihydrazide, as evidenced by the presence of –NH2 absorbance bands (N–H

stretching at 3310–3288 cm-1 and N–H bending at 1531 cm-1). The spectra also showed C=N

stretching at 1662 cm-1, –NH– bending at 1568 cm-1, and C–O–C stretching at 1242 cm-1 of the

precursor, as in product (A) spectra, indicating the formation of the precursor. Similar to the spectra

from Method (A), the broad peak at 716 cm-1 could be caused by the out-of-plane N–H bending

or CH2 wagging, while the band at 970 cm-1 could be related to the out-of-plane bending for C–H.

The peaks at 1013-1033, 1616, 2820–3113, and 3128–3250 cm-1 correspond to the stretching

vibration of C–N, C=O, C–H, and –NH–, respectively (Günzler & Gremlich, 2002; Pavia et al.,

2013). At 30 h, the intensity of the characteristic bands of the precursor compound, i.e., C=N

stretching (1662 cm-1), –NH– bending (1568 cm-1), and C–O–C stretching (1242 cm-1) became

86

more prominent with concomitant disappearance of the –NH2 bands, suggesting the completion of

the reaction. At 30 h, method (B) resulted in the precursor precipitates giving ~91% yield.

Solid-state 13C NMR spectroscopy confirmed the molecular structure of the synthesized

precursor products (from Methods A and B) to be diethyl N,N'-adipoyldiformohydrazonate (Fig.

4.4): 13C NMR (500 MHZ, ẟ in ppm): ẟ = 167 (–C=O); 152 (HC=N–N); 61 (H2C–O–CH); 33

(H2C–C=O); 23 (H2C–CH2–CH2); 13 (H2C–CH3). The precursor spectrum from Method (A)

exhibited additional peaks at 140 and 174 ppm that were absent from that of Method (B). The

origins of these two peaks are unclear, although they are indicative of the presence of other

chemical species, possibly 1,4-di(1,3,4-oxadiazol-2-yl)butane as reported by Ainsworth, (1955)

and Ainsworth & Hackler, (1966).

Fig. 4.2: Formation of ethyl formate precursor, diethyl N,N'-adipoyldiformohydrazonate, via

condensation of adipic acid dihydrazide and triethyl orthoformate. The hydrolysis of the

precursor, catalyzed by an acid, releases EF.

87

Fig. 4.3: Fourier transformed infrared (FTIR) spectra of triethyl orthoformate, adipic acid

dihydrazide, and precursors derived from method (Ӏ) and method (П) (see Section 4.3.2) as a

function of reaction time.

88

Fig. 4.4: Solid-state 13C nuclear magnetic resonance (NMR) spectra for precursors derived from

methods (A) and (B) (see Section 4.3.2).

4.4.2 Thermal analysis

In accordance with the FTIR and NMR results presented above, the DSC thermogram of the

precursor from Method (A) showed a small shoulder at 163.6 ± 0.9°C before the maximum peak

at 174.9 ± 1.1°C (Fig. 4.5). The two overlapped endothermic peaks suggested the melting points

of two compounds in a physical mixture. The peak at lower melting point was likely due to the

presence of impurity, while the larger peak was attributed to the precursor. On the other hand, the

precursor from method (B) showed only one melting peak with a melting temperature of 173.9 ±

0.9°C, which was not significantly different (P > 0.05) from method (A). Moreover, the enthalpies

of melting were 207.9 ± 7.3 and 204.7 ± 3.5 J/g for methods (A) and (B), respectively, suggesting

89

that the impurity did not affect the melting properties of the precursor. Both precursors from

methods (A) and (B) started to decompose above the melting point, at around 194°C, which could

be related to the decomposition of –C=N– and C–O–C that have medium to low decomposition

energy (Grewer, 1991). The perturbation of the thermograms at the decomposition regions

(indicated by arrows in Fig. 4.5) could be attributed to the volatilization of gases from the

decomposed samples. Similar behavior was reported by Mathkar et al., (2009) for pharmaceutical

compounds (azatadine maleate and labetalol hydrochloride). The thermal decomposition region

was not observed in the thermogram for adipic acid dihydrazide, which showed a sharp melting

peak at 182.5°C indicating that it was relatively thermally more stable than the precursor product

within the temperature range investigated.

Fig. 4.5: Representative differential scanning calorimetry (DSC) thermograms of adipic acid

dihydrazide and precursors from Methods (A) and (B) (see Section 4.3.2).

90

4.4.3 Particle size distribution and morphology

The SEM micrographs showed that the precursor from method (B) was made up of

agglomerated particulates with irregular shapes and sizes (Fig. 4.6І). In preliminary studies,

attempts were made to disperse the precursor particulates in water; however, the agglomerates

persisted even after 5 h of stirring, indicating its limited solubility in water. However, the

particulates could be dispersed in 2-propanol to form a milky suspension. The size distribution of

the particles was measured using DLS. As shown in Fig. 4.7, a relatively wide size distribution

(range of 145-4620 nm) was observed, with two peaks at 467 nm of 67.8 % volume percentage

and 1796 nm of 32.2 %. The microstructure of the particles from the dried dilution proved that the

agglomerates were dispersed into fine particles in 2-propanol suspension, which formed a

continuous layer upon drying (Fig. 4.6П), although small aggregates were visible at higher

magnifications.

Fig. 4.6: Scanning electron microscope (SEM) micrographs of precursor (B). (І); for particles,

and (П); for dried dilution.

91

Fig. 4.7: Particle size distribution of precursor (B).

4.4.4 Activated release of ethyl formate

The release of EF vapor was achieved through the hydrolysis of the C=N bond on diethyl N,N'-

adipoyldiformohydrazonate, as illustrated in Fig. 4.2. To this end, two approaches were taken. In

the first approach, the release was activated using 0.1 N citric acid solution at 5, 15, and 25°C to

simulate different storage temperatures for fruits and vegetables. As shown in Fig. 4.8, increasing

the temperature significantly (p < 0.05) increased the release rate of EF. At 25°C, rapid releases

with the first 60 min were observed for the precursors from both methods, followed by slower

release profiles. The accumulative EF releases of precursors from methods (A) and (B), after 120

min, were not significantly different (P > 0.05), at 0.35 ± 0.006 and 0.38 ± 0.002 mg/mg.L, which

represent about 89 and 98 % of the theoretical release, respectively (Table 4.1). At 15 and 5°C,

slower release trends were observed with accumulative releases of 0.32 ± 0.013 and 0.26 ± 0.011

mg/mg.L, respectively, at 120 min. Similar behaviors were reported by Jash and Lim on activated

release of hexanal from an imidazolidine precursor, as temperature increased from 5 to 45°C (Jash

& Lim, 2018).

92

The release data were fitted satisfactory using the pseudo-first-order kinetic model (Eq. 4.6)

with R2 around 0.99 (Table 4.1). The release rate constant (k) increased significantly (p < 0.05)

with increasing temperature. Moreover, differences in Ce values were not significant (P > 0.05)

between precursors prepared from the two methods.

In the second approach, the release of EF from its precursor, prepared using method (B), was

achieved by exposing the precursor particulates, which were pre-deposited onto a citric acid

impregnated nonwoven substrate, to humidified air (Section 4.3.9). Here, the absorption of

moisture by the citric acid formed free acidic solution which when came in contact with the

precursor, hydrolyzed the latter and triggered the release of EF vapor. As shown in Fig. 4.9,

increasing RH in the headspace significantly enhanced EF release rate. At 94 % RH, 0.32 ± 0.014

mg/mg.L (80.8 % release) was released at 120 min. In comparison, at 75 % RH, the amount of EF

released was significantly (p < 0.05) reduced by half (Table 4.2). At 53 % RH, only 9.6 % of EF

was released, while under dry condition (0 % RH), no release was detected. A pseudo-first order

kinetic equation (Eq. 4.6) was fitted to the release data. The estimated parameters are summarized

in Table 4.2. As shown, the k value increased significantly as RH increased from 0 to 75 % RH,

although the difference in k value was not significant (p > 0.05) between 75 and 94 % RH.

Similarly, Ce increased significantly (p < 0.05) with increasing RH. This RH-dependent release

behaviour can be beneficial in end-use applications. For example, in packaging application, an

increase in RH of package headspace could be used as an activator to trigger the release of EF

from the EFP, such as packages for fruits and vegetables (Lee et al., 1995; Petersen et al., 1999;

Ragaert et al., 2007).

93

Fig. 4.8: Ethyl formate (EF) release of precursors from methods (A) and (B) at different

temperatures, activated using 0.1 N citric acid solution.

Fig. 4.9: Ethyl formate (EF) released from the precursor (B) at 25°C using different RH with

acidified substrate; or using 0.1N citric acid solution.

94

Table 4.1: Ethyl formate (EF) released from the precursor (A and B) at different temperatures

using 0.1N citric acid, and the fitted model parameters.

Precursor Temp EF released at 120 min Ce k R2

°C mg/mg.L % mg/mg.L min-1

A 25 0.348 ± 0.006a,b 89.0 ± 1.5 0.350 ± 0.005a 0.037 ± 0.000a 0.999

B 25 0.382 ± 0.008a 97.9 ± 2.0 0.389 ± 0.007a,b 0.030 ± 0.001b 0.998

B 15 0.316 ± 0.013b 81.1 ± 3.2 0.456 ± 0.014a,b 0.010 ± 0.000c 0.998

B 5 0.261 ± 0.011c 66.8 ± 2.9 0.792 ± 0.004c 0.004 ± 0.001d 0.999

Different alphabets (a–d) indicate statistical significant difference (p < 0.05) within each column.

Table 4.2: Ethyl formate (EF) released from the precursor (B) at 25°C using different RH with

acidified substrate, and the fitted model parameters.

RH EF released at 120 min Ce k R2

% mg/mg.L % mg/mg.L min-1

94 0.315 ± 0.014a 80.8 ± 3.5 0.333 ± 0.019a 0.026 ± 0.002a 0.990

75 0.178 ± 0.028b 45.7 ± 7.1 0.200 ± 0.037b 0.024 ± 0.004a 0.994

53 0.037 ± 0.007c 9.6 ± 1.8 0.072 ± 0.013c 0.006 ± 0.001b 0.979

Different alphabets (a–c) indicate statistical significant difference (p < 0.05) with each column.

95

4.4.5 Stability of the precursor

The storage stability of the EFP was evaluated for up to 30 d at 25°C under 0, 60, and 100 %

RH conditions. The precursor showed high stability at 0 % RH, where no significant (p > 0.05)

changes were detected in the amount of EF released for days 1, 15, and 30 (Fig. 4.10). However,

there was an 18 and 45 % reduction in EF release from the samples stored at 60 % RH after 15 and

30 d, respectively. At 100 % RH, the extents of reduction increased to 62 and 81 % for 15 and 30

d, respectively. The decreased stability of the precursor with increasing RH can be attributed to

the auto-hydrolysis of the precursor during prolonged storage, which can be effectively arrested

by storing the precursor under 0 % RH environment to maximize the availability of EF.

Fig. 4.10: Ethyl formate (EF) released from the precursor stored at 25°C under different RH.

Different alphabets (a–e) indicate statistical significant difference (p < 0.05).

96

4.5 Conclusion

In this study, an EFP was synthesized through the condensation reaction between adipic acid

dihydrazide and triethyl orthoformate. FTIR and solid-state 13C NMR spectroscopies confirmed

the molecular structure of the synthesized precursor – diethyl N,N'-adipoyldiformohydrazonate.

DSC thermogram showed that the precursor melting temperature and enthalpy are 174°C and

204.7 J/g, respectively. SEM micrographs showed that the precursor consisted of aggregated

particles with irregular shapes and sizes. DLS profile showed a wide particle size distribution with

two peaks at 467 and 1796 nm. The release of EF from the precursor could be effectively triggered

using 0.1 N citric acid. Increasing temperature resulted in a higher EF release rate, with up to 98

% of the theoretical available EF being released at 25°C. Negligible EF was detected at 0 % RH,

but a substantial activation of EF release was observed when RH increased beyond 53 % RH. The

precursor remained stable during storage under dry environmental conditions for up to 30 d. The

EFP developed here could be useful in AP of fresh produce to destroy insect pests and inhibit the

proliferation of spoilage/pathogenic microorganisms.

97

Chapter 5: Encapsulation of the ethyl formate precursor into electrospun

nonwovens Part 1: Effect of Poly(ethylene oxide) on the Electrospinning

Behavior and Characteristics of Ethyl Cellulose Composite Fibers#

#Content of this chapter has been published as a research paper: Zaitoon, A., & Lim, L. T. (2020).

Effect of poly (ethylene oxide) on the electrospinning behavior and characteristics of ethyl

cellulose composite fibers. Materialia, 10, 100649. https://doi.org/10.1016/j.mtla.2020.100649.

98

5.1 Abstract

Ethylcellulose (EC) is a non-toxic polymer widely used in food, pharmaceutical, and

biomedical applications. Previous studies showed that the electrospinning of EC is challenging

due to the formation of beads rather than continuous fibers. To overcome this limitation, this study

evaluated poly(ethylene oxide) (PEO) as a process aid to enhance the electrospinning of

ethylcellulose (EC) solutions. Fibers were electrospun using a free surface electrospinning

technique which is more conducive for scale-up production than the typical spinneret approach.

The effects of PEO content, PEO molecular weight, and aqueous ethanol concentration on EC

solutions parameters, spinnability, and fiber morphology were studied. Among the spin-dope

solutions, neat EC (10 %, w/w) solution had the highest conductivity (38.28 ± 0.466 µS/cm) and

lowest surface tension (42.3 ± 0.06 mN/m) and viscosity (0.331 ± 0.006 Pa.s). These properties

led to the formation of irregular particulates interweaved with ultrafine fibers of an average

diameter of 0.075 ± 0.003 µm. The incorporation of 1 % (w/w) PEO (Mw = 100 kDa) in EC

solution enhanced its spinnability, where stable polymer jets were observed during the

electrospinning process, producing fibers of significantly (p < 0.05) larger average diameter (0.297

± 0.007 µm). Interactions between EC and PEO in the electrospun were elucidated using Fourier

transformed infrared spectroscopy. This study contributed to the understanding of the effect of

PEO on the electrospinnability of EC solutions, which can be useful during the development of

EC-PEO nonwovens for specific end-use applications.

99

5.2 Introduction

Electrospinning is a fiber-forming technology for producing fibers with diameters ranging

from hundreds of nanometers to tens of micrometers, by stretching the spinning polymer solution

into jets using electrostatic force (Hohman et al., 2001; Reneker et al., 2007). Nonwovens derived

from electrospun fibers are very versatile materials due to their high surface-to-volume ratio,

lightweight, porous, and tunable morphology. They are widely used in membrane distillation,

nanofiltration processes, encapsulation, removal of contaminants, wound dressing, tissue

engineering, and protective clothing (Huang et al., 2006; Li et al., 2002; Ray et al., 2016; Shao et

al., 2018). Since electrospinning is a non-thermal process, it is ideal for the encapsulation of heat-

sensitive nutraceuticals, drugs, volatile bioactives, and so on. Many synthetic (e.g., poly(lactic

acid), poly(ethylene oxide), nylon, polyvinylpyrrolidone, polystyrene, etc.) and natural (e.g.,

polysaccharides, proteins, DNA, etc.) polymers, as well as composite blends, have been

electrospun into ultrathin fibers of different properties (Bhardwaj & Kundu, 2010; Hu et al., 2014;

Huang et al., 2003; Lim et al., 2019).

Ethylcellulose (EC) is a non-toxic and environmentally friendly polymer derived from

cellulose. EC has repeated anhydroglucose (ring) units, each of which has three reactive hydroxyl

groups partially or fully converted into ether end groups (Davidovich-Pinhas et al., 2015; Mahnaj

et al., 2013). The degree of substitution (DS) has a strong effect on the physical properties of EC.

EC polymers with DS in the range of 1.0 –1.5 are water-soluble, while those with DS in the range

of 2.4 – 2.5 are soluble in polar and nonpolar organic solvents (Koch, 1937). Due to their desirable

physical properties and neutral characteristics (tasteless, odorless, noncaloric, and physiologically

100

inert) (Gravelle et al., 2018), EC polymers are widely being used in food, pharmaceutical, and

biomedical applications (Desai et al., 2006; Iqbal et al., 2002; Sannino et al., 2009).

Several studies evaluated the fiber-forming behavior of EC during electrospinning. In general,

the electrospinnability of EC dissolved in different solvents (e.g., tetrahydrofuran,

dimethylacetamide, dimethylformamide, acetone, acetic acid, and anhydrous/aqueous ethanol) is

challenging due to the formation of morphological defects (e.g., beaded- fibers, beads, inconsistent

fiber thickness) (Ahmad et al., 2013; Ahn et al., 2012; Crabbe-Mann et al., 2018; Jash & Lim,

2018; Liu et al., 2018; Liu, Li, et al., 2018; Wang et al., 2013; Wu et al., 2005; Yu et al., 2014).

To overcome these issues, a second polymer is often added as a process aid to promote the

formation of uniform ultrathin fibers. Poly(ethylene oxide) (PEO) is biodegradable,

biocompatible, and water-soluble polymer (Surov et al., 2018). It is a versatile process aid for the

electrospinning of various polymers, including poly(lactic acid) (Dai & Lim, 2015), alginate/pectin

blend (Alborzi et al., 2010, 2013), soy protein isolate (Vega-Lugo & Lim, 2008, 2009), whey

protein isolate, microalgae protein concentrate (Moreira et al., 2019; Verdugo et al., 2014),

chitosan (Deng et al., 2018; Duan et al., 2004), sodium alginate (Nie et al., 2009), and cellulose

diacetate (Korehei & Kadla, 2014).

In a typical electrospinning setup, the spin-dope solution is being pumped through a single-

or multiple-spinneret electrode to electrify the solution. By contrast, free surface electrospinning

involves spinning the spin-dope solution from a continuous surface, such as a wire, cylinder, or

plate. The advantages of the free surface electrospinner are simple operation, high throughput, high

fiber uniformity, large area coverage, and high packing density (Brettmann et al., 2012). In general,

101

the electrospinning phenomena are governed by processing (e.g., voltage, type of the spinning

electrode, distance between electrodes, carriage speed), solution (e.g., surface tension, electrical

conductivity, viscosity, solvent vapor pressure), and ambient (e.g., temperature and relative

humidity) parameters (Lim et al., 2019; Ray et al., 2016; Reneker et al., 2007). Spin-dope solutions

with high viscosity tend to require elevated electrostatic force to initiate jetting, while those with

low viscosity tend to form unstable polymer jets that break up into beads. Polymer solutions with

elevated surface tension have a tendency to form beads or ultrathin fibers embedded with beads,

while those with low surface tension tend to produce uniform and continuous electrospun fibers.

Electrically conductive spin-dope solutions allow the migration of charge to the surface, which is

essential to establish the repulsion force needed for jetting. However, the electrospinning of highly

conductive solutions can be challenging due to charge slippage along the polymer jet (Fong et al.,

1999; Ramakrishna et al., 2005; Vega-Lugo & Lim, 2008). Therefore, the polymer solution

parameters are critical for establishing a stable electrospinning process and for controlling the

morphology of the fiber.

In this study, EC-PEO nonwovens were produced by using a free surface wire

electrospinning method, by dissolving EC and PEO in an ethanol-water binary solvent. The effects

of solvent ratio, PEO molecular weight and content on the properties (viscosity, electrical

conductivity, and surface tension) of the spin-dope solutions, electrospinning behaviors, and fibers

morphology were investigated.

102


5.3.1 Materials

EC (viscosity 22 cP, 48 % ethoxyl content), and PEO (PEO100, Mw = 100 kDa; PEO300,

Mw = 300 kDa; and PEO900, Mw = 900 kDa) were purchased from Sigma-Aldrich (Oakville,

ON, Canada). Anhydrous ethanol was obtained from Commercial Alcohol (Brampton, ON,

Canada). Milli-Q® water was used for preparing the aqueous ethanol solutions.

5.3.2 Spin-dope solutions and rheological measurements

To prepare spin-dope solutions for electrospinning, various EC (10%, w/w) solutions with

different PEO concentrations were prepared in aqueous ethanol solutions of different ethanol-

water blend ratios (Table 5.1). Polymer solutions were prepared in 20 mL capped vials to prevent

solvent evaporation, heated at 60°C for 30 h, with stirring, to completely dissolve the polymers.

The resulting spin-dope solutions were cooled to 22 ± 1.5°C before testing. The rheological

properties of the polymeric solutions were determined by Rheometer (A1000, TA Instruments,

New Castle, DE, USA) equipped with a flat-plate accessory (steel plate, diameter 30 mm, gap 1

mm). Measurements were taken at a controlled shear rate ranging from 100 and 1000 s-1 at 25°C.

The apparent viscosity was plotted against shear rates. Shear stress versus shear rate data was fitted

to the power-law equation (Eq. 5.1):

𝜏 = 𝑘�̇�𝑛 (Eq. 5.1)

103

where τ is the shear stress (Pa), k is the consistency coefficient (Pa.sn), γ ̇ is the shear rate (s-1), n

is the flow behavior index. Data were analyzed using TA Rheology Advantage Data Analysis

Software (TA Instruments, New Castle, DE, USA).

5.3.3 Electrical conductivity and surface tension measurements

The electrical conductivity of polymer solutions was measured using a conductivity meter

(Accumet® XL20, Fisher Scientific, Ottawa, ON, Canada). The dynamic surface tension of

polymer solution was determined using a bubble pressure tensiometer (SITA pro line f10, SITA

Messtechnik GmbH, Dresden, Germany), equipped with a needle through which air was bubbled

continuously into the solutions at a fixed bubbling frequency of 1 Hz at 22 ± 1.5°C.

5.3.4 Electrospinning process

Polymer solutions were electrospun using a free surface electrospinner (NS LAB, Elmarco,

Czech Republic) (Fig. 5.1). About 5 g of the polymer solution was loaded into a 10 mL reservoir

attached to a carriage. The carriage speed was set at 100 mm/s. A voltage of 45 kV was applied to

the positive wire. The vertical distance between the positive and ground wires was 230 mm. The

spinning process was conducted at 22 ± 1.5°C and 50 % RH.

104

Table 5.1: Polymer solutions with different EC-PEO blend ratios in different aqueous ethanol prepared for electrospinning. Electrical

conductivity, surface tension, consistency coefficient (k), flow behavior index (n), and apparent viscosity (µ) at 1000 s-1 were

indicated.

Code EC

(%,

w/w)

PEO

(%, w/w)

Aqueous

ethanol

(%)

Conductivity

(µS/cm)

S. tension

(mN/m)

k

(Pa.sn)

n R2 µ

(Pa.s)

F1 10 0% PEO 90 a38.28 ± 0.47 42.3 ± 0.1a 0.45 ± 0.13a 0.959 ± 0.005a 0.999 0.331 ± 0.006a,e

F2 10 1% PEO100 70 42.48 ± 0.23b 62.0 ± 0.4e,c 1.43 ± 0.03b 0.861 ± 0.002b 0.998 0.525 ± 0.016b,g

F3 10 1% PEO100 80 40.01 ± 0.35a,b 55.6 ± 0.3e 0.94 ± 0.05a,b 0.895 ± 0.005c 0.998 0.436 ± 0.021b,c,f

F4 10 1% PEO100 90 29.59 ± 0.48c,e 54.9 ± 0.5e 0.61 ± 0.01a 0.938 ± 0.005d 0.999 0.385 ± 0.011a,c,e

F5 10 1% PEO100 95 22.03 ± 0.13f 25.9 ± 0.3b 0.48 ± 0.01a 0.941 ± 0.004d,a 0.999 0.299 ± 0.011e

F6 10 2% PEO100 90 26.62 ± 0.81e,d 61.6 ± 0.4c 0.74 ± 0.03a 0.927 ± 0.007d 0.998 0.443 ± 0.007b,c,f

F7 10

3% PEO100 90 24.78 ± 1.24d,f 68.7 ± 2.4d 0.93 ± 0.03a,b 0.896 ± 0.004c 0.998 0.499 ± 0.018b,f,g

F8 10 1% PEO300 90 30.34 ± 0.82c 55.1 ± 1.5e,c 0.78 ± 0.02c 0.839 ± 0.001e 0.997 0.405 ± 0.017a,c,f

F9 10 1% PEO900 90 30.40 ± 0.06c 55.9 ± 0.5e 2.22 ± 0.57d 0.802 ± 0.003f 0.997 0.538 ± 0.009g

Different alphabets (a–g) indicate a statistical significant difference (p < 0.05) within each column.

105

Fig. 5.1: Schematic diagram of the free surface electrospinning for forming EC-PEO nonwovens.

The polymer solution was loaded into the carriage which glided back-and-forth along a

positively charged wire electrode to evenly coat it with the spin dope solution. When the critical

voltage was supplied to the wire electrode, the solution ejected as multiple jets toward the

collector substrate positioned in between the spinning and grounded electrodes.

5.3.5 Microstructural and infrared analyses

The morphology of the fiber was examined using scanning electron microscope (SEM;

Quanta FEG 250, FEI Company, Hillsboro, OR, USA), at an accelerating voltage of 10 kV. Fiber

samples, randomly selected from three different spots, were mounted on metal stubs using double-

sided adhesive carbon tape and coated with 20 nm conductive layer of gold on its surface using a

sputter coater (Desk V TSC, Denton Vacuum, Moorestown, NJ, USA). Image analysis software

(Image Pro-Premier 9.2, Media Cybernetics Inc., Rockville, MD, USA) was used to determine the

average diameter of the fibers by analyzing approximately 200 fiber threads.

106

The infrared spectra of the nonwovens were analyzed using a Fourier transformed infrared

(FTIR) spectrometer (IRPrestige21, Shimadzu Corp., Kyoto, Japan), equipped with an attenuated

total reflection (ATR) accessory (Pike Tech, Madison, WI, USA). Samples were scanned 40 times

in the region from 600 to 4000 cm-1 at a resolution of 4 cm-1. Spectra were analyzed for the

interaction between polymers blend using the IR Solution software (Shimadzu Corp., Kyoto,

Japan).

5.3.6 Data analysis

Statistical analysis was conducted using SAS® University Edition software package (SAS

Institute Inc., Cary, NC, USA). All data were analyzed using PROC GLIMMIX to determine the

differences between the factors. Means were compared by Tukey’s honest significance difference

test at p<0.05 considered statistically significant. All analytical determinations were measured in

triplicate and results were expressed as the mean values ± standard error.


5.4.1 Polymer solution parameters

5.4.1.1 Apparent viscosity

The rheological properties of the spin-dope solutions, as affected by the solvent (F2 to F5),

PEO content (F1, F4, F6, and F7), and molecular weight (F4, F8, and F9), are presented in Table

5.1 and Fig. 5.2. All solutions have power-law index (n) values of less than one, which are

indicative of their non-Newtonian shear-thinning behaviors. With the presence of 1% (w/w) of

PEO100, the apparent viscosity and the consistency coefficient (k) values of the spin-dope solution

107

increased with increasing water content in the aqueous ethanol solvent (Fig. 5.2A). Moreover,

spin-dope solutions prepared from solvent with higher water content tended to exhibit a greater

shear-thinning than those with lower water content. This phenomenon can be attributed to the

increased availability of water that enhanced the interaction of PEO with the solvent through the

formation of hydrogen bonds between PEO ether oxygen and water molecules (Aray et al., 2004).

The increase in viscosity of spin-dope solution with higher water contents might have been a result

of enhanced solvent/polymer interaction, leading to expanded PEO chain morphology and chain-

chain entanglement in the solution.

The addition of PEO100 (1 to 3 %, w/w) into EC solutions (10 %, w/w) dissolved in 90 %

ethanol resulted in a considerable increase in the solution viscosity (Fig. 5.2B). EC solution

without PEO (F1) has the lowest viscosity values and exhibited near-Newtonian behavior (n =

0.95). However, an increase in shear-thinning behavior was observed as PEO content increased.

The molecular weight of PEO also has a significant (p < 0.05) effect on solutions viscosity, where

increasing PEO molecular weight from 100 to 900 kDa increased the viscosity of the solutions and

the solution became more shear-thinning (Fig. 5.2C). Similar observations were reported by

Korehei & Kadla, (2014) for PEO/cellulose diacetate solutions, where the solution viscosity

increased with increasing PEO content and PEO molecular weight. The influence of PEO on the

solution viscosity can be attributed to the intermolecular interactions between the ether groups of

PEO with the hydroxyl groups of EC (Alborzi et al., 2010; Nie et al., 2009; Son et al., 2004; Vega-

Lugo & Lim, 2012). This enhancement increased with increasing PEO content and molecular

weight. Also, the shear-thinning behaviors of the EC-PEO solutions indicated that the chain-chain

interactions were shear-sensitive.

108

Fig. 5.2: Effects of solvent (A), PEO content (B), and PEO molecular weight (C) on the apparent

viscosity values of EC solutions at different shear rates. Sample codes (F1−F9) are described in

Table 5.1.

5.4.1.2 Electrical conductivity

Electrical conductivity determines the ability of charge to migrate to the surface of the spin

dope solution, which is critical for establishing electrostatic repulsion force needed to initiate

jetting during the electrospinning process. The effects of ethanol-water ratio (F2 to F5), PEO

content (F1, F4, F6, and F7), and molecular weight (F4, F8, and F9) on the electrical conductivity

of EC solution are summarized in Table 5.1. The electrical conductivity decreased significantly (p

< 0.05) from 42.48 ± 0.23, 40.01 ± 0.35, 29.59 ± 0.48, to 22.03 ± 0.12 µS/cm as the ethanol content

increased from 70, 80, 90, to 95 %, respectively. The electrical conductivity decreased slightly as

109

ethanol content increased from 70 to 80 %. However, a sharp decrease in electrical conductivity

was observed when the alcohol content increased from 80, 90, to 95 % levels. This trend is

consistent with the lower electrical conductivity of anhydrous ethanol (0.3 µS/cm) than Milli-Q

grade water (26.4 µS/cm) used for preparing the binary solvents (Rilo et al., 2013). While ethanol

is barely conductive, the addition of water increased the electrical conductivity of the resulting

solutions. Since the relationship between ethanol content and conductivity is nonlinear, complex

intermolecular interactions existed between ethanol and water as the component ratio changed

(Personna et al., 2013; Rilo et al., 2013). Similar results were observed by Fong and his coworkers

for PEO solutions prepared in aqueous ethanol solvent. They reported that as ethanol content

increased from 0 to 40%, the electrical resistivity (inverse of conductivity) increased from 110 to

386 m, respectively (Fong et al., 1999).

Although EC solution without PEO (i.e., F1) had a relatively high conductivity (38.28 ±

0.47 µS/cm), the incorporation of PEO100, at 1, 2, and 3 % (w/w) levels, resulted in spin-dope

solutions with significantly (p < 0.05) lower conductivity values (29.59 ± 0.48, 26.62 ± 0.81, and

24.78 ± 1.24 µS/cm, respectively). This phenomenon can be interpreted as due to the PEO

solvation of the dissolved ions in the solution, reducing the concentration of the free ions necessary

for carrying the electrical charges throughout the solution, thus reducing its electrical conductivity

(Sartori et al., 1990). On the other hand, the effects of PEO molecular weight on electrical

conductivity were insignificant (p > 0.05), where increasing PEO molecular weight from 100, 300

to 900 kDa slightly increased the electrical conductivity of the solutions from 29.59 ± 0.482, 30.34

± 0.816, to 30.40 ± 0.061 µS/cm. The minimal effect of molecular weight on conductivity was

previously reported by Bordi et al. (1988) and attributed to that the changes in the polymer

110

flexibility are confined to polymers having relatively small molecular weights (below 103 Da),

where end-group effects are relevant. Whereas, the contribution of end-group effects diminishes

as the polymer chain increases (Bordi et al., 1988). Also, the results are in accordance with findings

from other studies on the effect of PEO addition to sodium alginate-pectin, Botryococcus braunii

protein concentrate, and whey protein isolate (Alborzi et al., 2010; Vega-Lugo & Lim, 2012;

Verdugo et al., 2014).

5.4.1.3 Surface tension

During electrospinning, the electrostatic force applied to the spin-dope solution must

overcome its surface tension in order to initiate the jetting phenomenon. Thus, a reduction in

surface tension in spin-dope solution would favor electrospinning. Table 5.1 summarized the effect

of ethanol concentration (F2 to F5), PEO content (F1, F4, F6, and F7) and molecular weight (F4,

F8, and F9) on the surface tension of the spin dope solutions. As shown, there is a significant (p <

0.05) decreasing trend of surface tension from 62.0 ± 0.4 to 55.6 ± 0.3, 54.9 ± 0.52, to 25.9 ± 0.31

mN/m as ethanol concentration increased from 70, 80, 90, to 95 %, respectively. Theron et al.,

(2004) reported that 95 % aqueous ethanol solvent has a surface tension of 22.3 mN/m, which is

close to what obtained in our study for polymer solution dissolved 95 % aqueous ethanol solvent.

Therefore, the decrease in surface tension of the solutions with increasing ethanol concentration

can be attributed to the low surface tension of ethanol (21.2 ± 0.13 mN/m) as compared to water

(72.4 ± 0.22 mN/m) in the binary solvent. Fong and his coworkers reported similar findings for

PEO solutions prepared using ethanol and water co-solvents, where surface tension decreased with

increasing ethanol content (Fong et al., 1999). Similarly, Zhang et al., (2005) reported that

111

changing the solvent of poly(vinyl alcohol) from water to a mixture of water and ethanol had

decreased the surface tension of the solutions as the ethanol content increased.

Comparing with EC solution without PEO (F1; 42.3 ± 0.06 mN/m), the addition of PEO

significantly (p < 0.05) increased the surface tension from 54.9 ± 0.52, 61.6 ± 0.45 to 68.7 ± 2.41

mN/m as PEO100 content increased from 1, 2 to 3 %, respectively. Also, within the molecular

weight range investigated, the PEO molecular weight had a negligible effect on the surface tension

of the spin-dope solutions (54.9 ± 0.52, 55.1 ± 1.50, to 55.9 ± 0.55 mN/m for 100, 300, to 900 kDa

PEO, respectively). These results imply that the addition of PEO had resulted in an enhanced

polymer chains entanglement at the air-liquid interface through EC-PEO hydrogen bonds and

hydrophobic interactions. Similar PEO-induced surface tension enhancement effects have been

reported by other researchers on sodium alginate and whey protein isolate solutions (Lu et al.,

2006; Vega-Lugo & Lim, 2012).

5.4.2 Infrared vibrational analysis of electrospun nonwovens

To elucidate the interaction between EC and PEO, FTIR analysis was conducted on the

nonwoven samples (Fig. 5.3A). The spectrum of neat EC nonwoven (F1) showed stretching bands

at 3300 – 3600 cm-1 and 2700 – 3000 cm-1 of the unsubstituted O–H and the symmetric and

asymmetric C–H stretching from the EC monomer, respectively. The absorbance band at 1053 cm-

1 was attributed to the characteristic C–O–C stretching of EC cyclic ether (Desai et al., 2006;

Ravindra et al., 1997). The FTIR spectra of EC-PEO nonwovens showed peaks that are

characteristic of the pristine EC and PEO polymers. Since the polymer solutions contained mainly

of EC (10% w/w) and a trace amount of PEO (up to 3% w/w), the overall spectral features of the

112

generated nonwovens resembled those of EC polymer. However, with the increasing of PEO

content to 2 or 3% (F6 or F7) levels, the characteristic bands of PEO were detected at 1101, 964,

and 843 cm-1 due to the C–O–C asymmetric stretch, the rocking and twisting of C–H2, and bending

of C–H, respectively (Dai & Lim, 2015; Oliveira et al., 2013). Comparing with the same peaks in

pristine PEO (1093, 960, and 841 cm-1, respectively), these peaks appeared at higher wavenumbers

(Fig. 5.3B). Moreover, the addition of PEO resulted in low intensity of the stretching band at 3300

– 3600 cm-1 (Fig. 5.3A). These results suggested the presence of intermolecular interactions

between the ether groups of PEO with the hydroxyl groups of EC, which explained the changes in

the polymer solutions parameters discussed above.

113

Fig. 5.3: Fourier transformed infrared (FTIR) spectra for PEO powder, neat EC fiber (F1), and

EC-PEO fibers, with 1 % PEO (F4), 2 % PEO (F6), and 3 % PEO (F7). A: In the region from

600 to 4000 cm-1. B: In the region from 700 to 1300 cm-1, indicating the shift in PEO

characteristic bands of EC-PEO fiber.

114

5.4.3 Spinnability and morphology of the electrospun fiber

The effects of ethanol concentration (F2−F5), PEO content (F1, F4, F6, and F7), and PEO

molecular weight (F4, F8, and F9) on the electrospinnability of EC-PEO polymer solutions and

fiber morphology are summarized in Table 5.2 and Figs. 5.4−5.7. With EC solution (10 % w/w)

alone, at 100 mm/s carriage speed and 45 kV applied voltage, jetting was not observed. Instead,

fine particulates were sprayed toward the substrate. SEM micrograph (Fig. 5.4, F1) showed

irregular particulates interweaved with ultrafine fibers of an average diameter of 0.075 ± 0.003

µm. When the carriage speed was increased to 150 mm/s and the voltage to 65 kV, irregular

particulates persisted with an increase in average diameter to 0.120 ± 0.004 µm. Similar

observations were reported by Jash and Lim for 10 % (w/w) EC solution dissolved in anhydrous

ethanol. They found that their spin-dope solution was electrosprayed into irregular particulates

instead of forming fibers (Jash & Lim, 2018). Increasing EC content to 20 % (w/w) prevented both

jetting and spraying due to its low charge density, where the electrostatic repulsion generated was

insufficient to overcome the increased viscosity and surface tension of the EC solutions at elevated

concentrations. However, the incorporation of 1 % (w/w) PEO100 into 10 % (w/w) EC solution in

90 % aqueous ethanol (Table 5.1, F4) resulted in spin-dope parameters (29.59 ± 0.482 µS/cm, 54.9

± 0.52 mN/m, and 0.385 ± 0.011 Pa.s) favorable for electrospinning. As shown in SEM micrograph

(Fig. 5.4, F4), continuous fibers with cylindrical-like morphology were formed with an average

diameter of 0.297 ± 0.007 µm. At high magnification, the fibers appeared to have longitudinal

wrinkles on the surface (Fig. 5.5, F4). Due to its higher volatility, ethanol tended to evaporate first

as the fiber took flight towards the collector, forming a solidified skin layer. As the residual water

115

continued to evaporate, the collapse of the skin layer might have created the longitudinal wrinkles

(Koombhongse et al., 2001).

Table 5.2: Spinnability of the polymer solutions and the average fiber diameter. Sample codes

(F1−F9) are described in Table 5.1.

Sample code Spinnability Fiber diameter (µm)

F1 - 0.075 ± 0.003a

F2 - 0.102 ± 0.006a

F3 +++ 0.316 ± 0.009b,d

F4 +++ 0.297 ± 0.007b

F5 + 0.219 ± 0.007c

F6 +++ 0.365 ± 0.006d,e

F7 ++ 0.416 ± 0.008e

F8 ++ 1.033 ± 0.05f

F9 + 1.915 ± 0.09g

-: Difficult electrospinning (jetting absence; spraying), +: Electrospinning with unstable jets, ++:

Electrospinning with semi-stable jets, and +++: Stable electrospinning process.

At 70 % ethanol concentration, the spinnability was difficult and a small amount of

ultrathin fibers (0.102 ± 0.006 µm) and beads were observed (Fig. 5.4, F2). Increasing the

concentration of ethanol to 80 % enhanced the spinnability and stable polymer jets were observed.

Cylindrical fibers with an average diameter of 0.316 ± 0.009 µm were collected after solvent

116

evaporation (Fig. 5.4, F3). A further increase of ethanol to 95 % resulted in unstable jets during

the electrospinning process, forming fluffy materials deposited on different spots of the collector.

SEM micrograph (Fig. 5.4, F5) showed that the fibers (0.219 ± 0.007 µm) were imbedded with

irregular beads and had a broad thickness distribution due to the co-existence of ultrathin fibers.

At higher magnification (Fig. 5.5), F2 fibers revealed smooth fibers with branch points. F3 fibers

had a larger diameter and more longitudinal wrinkles in the skin as compared to F4 fibers, while

the wrinkles were less dominant in F5, indicating that the longitudinal wrinkles were more

prevalent with increased water content in the solvent. Among the four formulations (F2 to F5),

only F3 and F4 gave stable electrospinning processes. Comparing their solution parameters (Table

5.1), it can be seen that electrical conductivity and viscosity changed significantly (p < 0.05)

among the solutions. While the surface tension values for F3 and F4 were comparable (55.6 ± 0.34

and 54.9 ± 0.52 mN/m, respectively), F2 had the highest and F5 the lowest values (62.0 ± 0.4 and

25.9 ± 0.3 mN/m, respectively). Thus, it can be concluded that surface tension was the determining

factor affecting the electrospinning process here. This is consistent with the results from other

studies that surface tension of polymer solution is mainly determined by the solvent(s) while less

dependent on the variation of the polymer(s) concentration (Theron et al., 2004).

In terms of PEO concentration effect (Figs. 5.4 and 5.7), increasing PEO100 concentration

from 1 to 2 % (w/w) in EC (10 % w/w) solutions (90 % ethanol; F4 to F6) did not result in a

noticeable change in the electrospinning process; stable polymer jets were observed in all cases.

Further increase of PEO100 content to 3 % (F7), however, reduced the electrospinning stability of

the solution, as reflected by the formation of beads embedded ultrathin fibers. The average fiber

diameter increased significantly (p < 0.05) from 0.297 ± 0.007 to 0.416 ± 0.008 µm as the PEO

117

content increased from 1 to 3 % (w/w). Moreover, the molecular weight of PEO significantly (p <

0.05) affected the solutions spinnability. At 90 % ethanol concentration, PEO300 resulted in a

fluffy electrospun nonwoven (Fig. 5.6, F8) with an average fibers diameter of 1.033 ± 0.05 µm,

which was considerably larger than those of PEO100 (0.297 ± 0.007 µm) at the same

concentration. Increasing PEO molecule weight to 900 kDa resulted in unstable jets, producing a

fluffy nonwoven with fused fibers, with an average diameter of 1.915 ± 0.09 µm (Fig. 5.6, F9). It

is worth mentioning that reducing the ethanol concentration to 80 % level for PEO 300 and

PEO900 did not enhance the electrospinnability of the solutions. Here, more fused fibers were

observed with increased fiber diameter to 1.899 ± 0.051 and 2.207 ± 0.064 µm, respectively (Fig.

5.7, A and B).

The observed differences in spinnability and fibers morphology were correlated with

changes in solution viscosity, surface tension, and conductivity with the addition of PEO. Without

PEO (F1), high conductivity (38.28 ± 0.466 µS/cm), low surface tension (42.3 ± 0.06 mN/m), and

low viscosity (0.331 ± 0.006 Pa.s) favored the formation of beads rather than continuous fibers

(Table 5.1), due to lack of chain entanglement required to stabilize the polymer jet. The addition

of 1 % (w/w) of PEO100 increased surface tension to 54.9 ± 0.52 mN/m and viscosity to 0.385 ±

0.011 Pa.s, while decreased the conductively significantly to 29.59 ± 0.482 µS/cm. This condition

improved the jetting phenomenon, favored the formation of continuous fibers, confirming the

importance of chain entanglement to prevent bead formation. Increasing PEO100 to 3 % (w/w),

however, resulted in a solution nonoptimal for electrospinning (24.78 ± 1.242 µS/cm, 68.7 ± 2.41

mN/m, and 0.499 ± 0.018 Pa.s; Table 5.1). The positive effect of PEO on promoting the

electrospinning of EC in aqueous ethanol can be attributed to the intermolecular interactions

118

between the ether groups of PEO with the hydroxyl groups of EC, which might have enhanced the

PEO-EC chain interactions and chain-chain entanglement to stabilize the polymer jet during the

electrospinning process. Similar observations were reported by Vega-Lugo and Lim for the

electrospinning of whey protein isolate solutions, which could not be electrospun unless a trace

amount of PEO was added to the solutions (Vega-Lugo & Lim, 2012). Nie and his coworker

reported that a trace amount of high molecular weight PEO (0.175 % w/v, 1000 kDa) enhanced

the spinnability of aqueous sodium alginate solution, but not with low molecular weight PEO (20

kDa), even when added in a large quantity (35 % w/v) (Nie et al., 2009). Results from these studies

are in agreement with our findings. Also, the increase in fiber diameter with the addition of PEO

was observed for the electrospinning of sodium alginate-pectin, chitosan, and cellulose diacetate

(Alborzi et al., 2010; Duan et al., 2004; Korehei & Kadla, 2014). In summary, the findings suggest

that PEO content had a significant effect (p < 0.05) on the solution parameters. At 10 % (w/w) EC

level in aqueous ethanol of 80–90 % concentration, the ranges of conductivity, surface tension,

and apparent viscosity (at 1000 s-1) that provided stable electrospinning processes for forming

fibers were 40–27 μS/cm, 55–62 mN/m, and 0.38–0.44 Pa.s, respectively.

119


distribution of neat EC nonwoven (F1) and EC-PEO nonwovens generated using 1 % PEO100 in

70 % (F2), 80 % (F3), 90 % (F4), and 95 % (F5) aqueous ethanol as a solvent. Sample codes

120

(F1−F5) are described in Table 5.1. Electrospinning process conducted at a carriage speed of 100

mm/s and voltage of 45 kV.

Fig. 5.5: Scanning electron microscope (SEM) micrographs for EC-PEO nonwovens, generated

using 70, 80, 90, and 95 % aqueous ethanol (F2, F3, F4, and F5) at 30000x magnification,

showed the skin wrinkles. The underlying fibers were also wrinkled, but they are out of focus in

this image

121


distribution of EC-PEO nonwovens with 2 and 3 % PEO 100 kDa (F6, and F7), 1 % PEO 300

kDa (F8), and 1 % PEO 900 kDa (F9) in 90 % aqueous ethanol. Sample codes (F6−F9) are

described in Table 5.1. Electrospinning process conducted at a carriage speed of 100 mm/s and

voltage of 45 kV.

122


distribution of EC-PEO nonwovens with 1 % (w/w) PEO 300 kDa (A) and 1 % (w/w) PEO 900

kDa (B) in 80 % aqueous ethanol. Electrospinning process conducted at a carriage speed of 100

mm/s and voltage of 45 kV.

5.5 Conclusion

In this study, the incorporation of PEO, with different concentrations and molecular

weights, in EC solution was studied. Moreover, different aqueous ethanol (70 –95 %) were used

as solvents. The electrospinning of 10 % (w/w) EC, when dissolved in 90 % aqueous ethanol,

resulted in irregular particulates interweaved with ultrafine fibers (0.075 ± 0.003 μm), instead of

forming continuous fibers. Both aqueous ethanol and PEO addition had significant (p < 0.05)

effects on the solution parameters, spinnability, and corresponding fiber morphology. While 70

and 95 % aqueous ethanol solvents were not suitable for electrospinning of EC-PEO solutions, 80

and 90 % aqueous ethanol solvents were optimal for forming stable polymer jets. When changing

the solvent in the solution, the surface tension was the most critical parameter among others. The

123

addition of 1 % PEO (100 kDa) in EC solution enhanced its spinnability with stable polymer jets.

However, the spinnability was hampered when using PEO with higher molecular weight (300 and

900 kDa), where viscosity effect was more dominant than other solution parameters. The FTIR

analysis revealed interactions between PEO and EC, which might have enhanced PEO-EC chain

interactions and chain-chain entanglement to stabilize the polymer jet during the electrospinning

process. Due to the higher electrospinning production throughput, the free surface electrospinning

technique is more conducive for scale-up production than the typical spinneret approach reported

in the literature. This study provided a basic understanding of the effects of PEO on the

electrospinnability of EC solutions in order to improve the properties and the performance of EC-

PEO nonwovens. Since aqueous ethanol is less toxic than other organic solvents, the EC-PEO

composite fibers produced can potentially find applications in bioactive encapsulation, high-

performance filtration, tissue scaffolding, and active food packaging.

124

Chapter 6: Encapsulation of the ethyl formate precursor into electrospun

nonwovens Part 2: Activated release of ethyl formate vapor from its precursor

encapsulated in ethyl Cellulose/Poly(Ethylene oxide) electrospun nonwovens

intended for active packaging of fresh produce#

#Content of this chapter has been published as a research paper: Zaitoon, A., Lim, L. T., & Scott-

Dupree, C. (2021). Activated release of ethyl formate vapor from its precursor encapsulated in

ethyl Cellulose/Poly (Ethylene oxide) electrospun nonwovens intended for active packaging of

fresh produce. Food Hydrocolloids, 112, 106313. https://doi.org/10.1016/j.foodhyd.2020.106313.

125

6.1 Abstract

Ethyl formate (EF) is a naturally occurring insecticidal and antimicrobial volatile compound

promising as an alternative to synthetic fumigants. In this work, the EF precursor (EFP) was

encapsulated in free-surface electrospun ethylcellulose/poly(ethylene oxide) (EC-PEO)

nonwovens and the release of EF from the precursor was evaluated using gas chromatography.

The effect of EFP (up to 100 % w/w, polymer content basis) on EC-PEO solution parameters and

fiber morphology was studied. Scanning electron microscopy revealed that EFP particles were

entrapped within the electrospun EC-PEO fibers. Fourier transform infrared spectroscopy did not

reveal any specific interaction between EFP and the polymers in the nonwovens. When exposed

to 0.1 N citric acid (CA) solution for 2 h at 25°C, the nonwoven loaded with 10 % EFP (w/w)

released 0.037 mg EF/mg nonwoven (96 % of the theoretical release). The EF release decreased

significantly (p < 0.05) with decreasing either the CA concentration or the temperature.

Preliminary studies showed that the release of EF from the EFP-loaded nonwovens delayed the

growth of spoilage microorganisms on strawberries for up to 10 d at 5°C, suggesting the feasibility

of the EFP-loaded nonwoven for active packaging (AP) applications.

126

6.2 Introduction

Ethyl formate (EF) is a generally regarded as safe compound present naturally in many

agricultural products that imparts them with unique flavor and aroma (Coetzee et al., 2019;

Desmarchelier, 1999; Ren & Desmarchelier, 2002; Kuchi & Sharavani, 2019). Previous studies

have shown that EF is a potent insecticidal and antimicrobial agent promising as an alternative to

toxic fumigants (e.g., methyl bromide and phosphine) that are being used in quarantine treatments

of fruits/vegetables (Bessi et al., 2016; Bessi et al., 2015; Bolin et al., 1972; Learmonth et al., 2012;

Lee et al., 2018; Simpson et al., 2007; Simpson et al., 2004; Stewart & Mon, 1984; Utama et al.,

2002). Quarantine and pre-shipment treatments of fruits/vegetables are essential to prevent

microbial growth, quality losses, outbreaks of foodborne pathogens, and infestation/invasion by

insect pests that can be detrimental to both products and environment (Deng et al., 2019; Feliziani

et al., 2016b, 2016a; Mahajan et al., 2014). Hence, many countries established rigorous regulations

on imported fruits/vegetables to ensure that they are free of insect pests and pathogenic

microorganisms.

Fumigation treatments of fruits/vegetable typically are carried out in enclosed chambers filled

with gaseous fumigant to destroy insect pests and inhibit the growth of spoilage/pathogenic

microorganisms (Ding & Lee, 2019; Fan et al., 2018; Kumar et al., 2017; Oh & Liu, 2020; Sun et

al., 2019; Toffano et al., 2017). The fumigation quarantine can take from several hours up to a day.

This extra handling step not only can potentially inflict product injuries but also delay product

shipment, thereby shortening the available product shelf-life at the retail level. By contrast, in-

transit fumigation of shipping containers can be beneficial to eliminate the holding of products in

the fumigation area, thereby streamlining the supply chain and reducing cost (Coetzee et al., 2019).

127

To enable in-transit product fumigation, EF may be delivered to the headspace of the individual

packaging unit by using an inert carrier. However, EF is highly volatile, flammable, and

susceptible to hydrolytic degradation in the air (Lee et al., 2018; Kuchi & Sharavani, 2019). One

promising approach to overcome these challenges is via converting EF into a chemical precursor

which is stable during storage/handling, and yet labile enough for triggered release of the original

volatile through selective cleavage of covalent bonds, by means of hydrolysis/ enzymatic/oxidative

reactions (Levrand et al., 2007). In Chapter 4, an EFP compound was developed via reacting adipic

acid dihydrazide with triethyl orthoformate, to produce diethyl N,N'-adipoyldiformohydrazonate.

EFP is non-volatile and remained stable under dry conditions but could be hydrolyzed readily in

the presence of moisture and acid to trigger the release of EF vapor, suggesting that the precursor

is promising for controlled release and AP applications.

To facilitate the end-use deployment, the EFP powder needs to be encapsulated in an optimal

delivery carrier. Encapsulation is a versatile technique to enclose bioactive compounds (core

materials) within liquid, semisolid, or solid matrices (encapsulants) (Mourtzinos & Biliaderis,

2017; Nedovic et al., 2011). Encapsulation protects the sensitive bioactives from harsh

environments, facilitate delivery, enhance efficacy, and provide controlled release kinetics of core

materials (Kitts & Liu, 2015; Mishra, 2015; Nedovic et al., 2011). Over the past decade,

researchers have been exploring various electrospun nonwovens as encapsulants for bioactive

compounds. These nonwovens are produced using non-thermal electrospinning process, in which

submicron fibers are generated by stretching the spin-dope solution into polymer jets using

electrostatic force (Hohman et al., 2001; Lim et al., 2019; Ramakrishna et al., 2005). Due to their

high surface-to-volume ratio, electrospun fibers are ideal for the encapsulation of bioactive

128

compounds where surface activation is important. For example, Altan et al., (2018) encapsulated

carvacrol within zein and poly(lactic acid) (PLA) nonwovens using electrospinning process. The

encapsulated carvacrol (20 % w/w) showed a sustained diffusion controlled release behavior and

were able to inhibit 99.6 and 91.3 % of mold and yeast growth on bread samples for zein and PLA

nonwovens, respectively. Ranjan et al., (2020) developed zein-PEO electrospun nonwoven

containing hexanal at zein:PEO:hexanal 2:5:1 (w/w) ratio. The sustained diffusion of hexanal

vapor from the nonwoven extended the shelf life of peach by 4 d compared to the untreated control

samples. Also, gallic acid loaded hydroxypropyl methylcellulose electrospun fibers were

developed by Aydogdu et al., (2019). The nonwoven had the ability to decrease the oxidation of

walnut during storage. These studies show that the electrospinning technique is very versatile for

the encapsulation of active compounds in AP applications.

Since EFP is a novel compound that was developed to stabilize and control the release of EF

(Chapter 4), its encapsulation in polymer carriers is also a novel approach and has not been done

in previous works. The objectives of this study are to encapsulate EFP in EC-PEO electrospun

nonwovens in order to facilitate its end-use application, and to study the release kinetics of EF

vapor from its precursor-loaded nonwovens.


6.3.1 Materials

Triethyl orthoformate (reagent grade, 98 %), adipic acid dihydrazide (98 %), citric acid

monohydrate (CA; reagent grade, 99 %), ethyl cellulose (EC; 22 cP, 48 % ethoxy content),

129

poly(ethylene oxide) (PEO; 100 kDa molecular weight), and ethyl formate (EF; reagent grade, 97

%) were purchased from Sigma-Aldrich (Oakville, ON, Canada). Anhydrous ethanol was supplied

by Commercial Alcohol (Brampton, ON, Canada). Water used was Milli-Q grade.

6.3.2 Ethyl formate precursor formation

The synthesis of EFP followed the procedure presented in Chapter 4. A suspension of 500 mg

adipic acid dihydrazide and 20 mL triethyl orthoformate was prepared in a 50 mL round-bottom

flask. The suspension was heated under reflux in an oil bath, with stirring, at 110°C for 30 h. The

mixture was then filtered to obtain the residue, which was then air-dried to yield the EFP white

particles.

6.3.3 Spin-dope solution preparation

EC-PEO polymer solution containing 10 and 1 % (w/w) EC and PEO, respectively, was

prepared by dissolving the polymers in 90 % (v/v) aqueous ethanol. These polymer concentrations

were chosen based on the findings from Chapter 5 to obtain bead-free electrospun fibers. The EC-

PEO solution was stirred at 60°C for 0.5 h to completely dissolve the polymers and then allowed

to cool to 22 ± 1.5°C with continuous stirring for another 1 h. Separately, the EFP powder was

dispersed in 90 % (v/v) aqueous ethanol at 10, 30, 50, 70, and 100 % (w/w) concentration, with

respect to the total polymer content. The mixtures were stirred at 22 ± 1.5°C for 2 h to form milky

EFP suspensions. The suspensions were then added to the EC-PEO solution and stirred at 22 ±

1.5°C for 2 h to create homogeneous spin-dope solutions for electrospinning. Spin-dope solutions

containing 0, 10, 30, 50, 70, and 100 % (w/w) EFP were abbreviated as 0% EFP, 10% EFP, 30%

EFP, 50% EFP, 70% EFP, and 100% EFP, respectively.

130

6.3.4 Spin-dope solution properties

The rheological properties of spin-dope solutions were measured by a rheometer (A1000, TA

Instruments, New Castle, DE, USA), equipped with a flat-plate accessory (steel plate; diameter 30

mm, gap 1 mm) at controlled shear rates between 100 and 1000 s-1 at 25°C. Flow data, shear stress

(𝜏, Pa) versus shear rate (�̇�, s-1), was modeled by the power-law equation: (Eq. 6.1).

𝜏 = 𝐾�̇�𝑛 (Eq. 6.1)

where K is the consistency coefficient (Pa.sn) and n is the flow behavior index. The apparent

viscosities were calculated as 𝜏/�̇�. Data were analyzed using TA Rheology Advantage Data

Analysis Software (TA Instruments, New Castle, DE, USA).

The electrical conductivity of spin-dope solutions was determined by a conductivity meter

(Accumet® XL20, Fisher Scientific, Ottawa, ON, Canada) and the dynamic surface tension was

measured by a bubble pressure tensiometer (SITA pro line f10, SITA Messtechnik GmbH,

Dresden, Germany), where air was bubbled continuously into solutions at a fixed bubbling

frequency of 1 Hz at 22 ± 1.5°C.

6.3.5 Electrospinning process

Spin-dope solutions were electrospun using a free surface electrospinner (NS LAB, Elmarco,

Czech Republic) (Fig. 6.1). About 5 – 10 mg of the polymer solution was loaded into the carriage

which glided back-and-forth along a positively charged wire electrode to evenly coat it with the

spin-dope solution. The carriage speed was set at 100 mm/s. The applied voltage to the wire

electrode was 45 kV. The distance between the wire electrode and collection substrate, which was

131

positioned in between the spinning and grounded electrodes, was fixed at 230 mm. The

electrospinning process was conducted at 23 ± 1.5°C and 50 % RH.

Fig. 6.1: Schematic diagram of the free surface electrospinning setup based on a stretched wire

as a spinning electrode

6.3.6 Scanning electron microscopy analysis

The morphology of EFP-loaded nonwovens was examined using a scanning electron

microscope (SEM; Quanta FEG 250, FEI Company, Hillsboro, OR, USA), at an accelerating

voltage of 10 kV. Samples were cut from three different spots of the nonwovens, attached to metal

stubs using double-sided adhesive carbon tape, and then coated with 20 nm conductive

gold/palladium layer using a sputter coater (Desk V TSC, Denton Vacuum, Moorestown, NJ,

132

USA). The fibers diameter was determined from SEM images by analyzing approximately 200

fibers using the image analysis software (Image Pro-Premier 9.2, Media Cybernetics Inc.,

Rockville, MD, USA).

6.3.7 Differential scanning calorimetry and Fourier transformed infrared spectrometry

The thermal properties of pristine EFP and EFP-loaded nonwovens were studied using

differential scanning calorimeter (DSC; model Q2000, TA Instruments, New Castle, DE, USA).

Samples (5 ± 2 mg) sealed in hermetic alodined DSC pans were equilibrated at 20°C, and then

heated to 250 °C at a heating rate of 10°C/min. Dry nitrogen was used as the purging gas with a

flow rate of 18 mL/min. Thermograms were analyzed using TA Universal Analysis Software (TA

Instruments, New Castle, DE, USA).

The infrared spectra of pristine EFP and EFP-loaded nonwovens were analyzed using an

Fourier transformed infrared (FTIR) spectrometer (IRPrestige21, Shimadzu Corp., Kyoto, Japan).

Samples were pressed on a diamond internal reflection element (Pike Tech, Madison, WI, USA)

and scanned in the 600 to 3300 cm-1 wavenumber range at a resolution of 4 cm-1. An average of

40 scans was taken for each spectrum. Spectra were analyzed using IR Solution software

(Shimadzu Corp., Kyoto, Japan).

6.3.8 Ethyl formate release from encapsulated precursor

The cumulative release of EF vapor from its precursor-loaded nonwovens was measured using

a gas chromatograph (GC 6890, Agilent Technologies Inc., Santa Clara, CA, USA) integrated with

an automatic headspace sampling system (Chapter 4). The gas chromatograph was equipped with

a flame ionization detector (FID) and a capillary column DB-624 with 30 m length, 0.53 mm I.D,

133

and 3 µm film thickness (Agilent Technologies Inc., Santa Clara, CA, USA). The temperatures of

the detector and the oven were 200 and 40°C, respectively. The flow rates of N2 carrier, and H2

and O2 to the FID were 30, 50, and 200 mL/min, respectively.

To study the release of EF from the encapsulated precursor, a 3×3 cm piece of EFP-loaded

nonwovens was placed in a sealed 1 L glass jar. The release of EF was triggered by adding 0.3

mL of citric acid (CA) solution of different concentrations (0.001, 0.01, 0.1, and 1 N) to the EFP-

loaded nonwoven using a pipette just before closing the jar. The headspace gas was extracted

through a septum attached to the jar lid at predetermined time intervals. The release of EF was

studied at 5, 15, and 25°C in an environmental chamber (MLR-350H, Sanyo Electric Co., Ltd.

Japan). The calibration of the FID was prepared by measuring standard headspace concentration

of known amounts of EF. Peak Simple software (SRI Instruments, CA, USA) was used to analyze

the chromatograms. The total amount of EF released into the headspace at any sampling point (Mt,

µL) was calculated by adding the recorded amount (Mr, µL) to the accumulated loss (Ml, µL) of

all the previous sampling points, according to eqs 6.2 to 6.4:

𝑀𝑟 = 𝐶𝑟𝑉𝑟 (Eq. 6.2)

𝑀𝑙 = ∑ (𝐶𝑟−𝑖𝑉𝑒)𝑟−1𝑖=1 (Eq. 6.3)

𝑀𝑡 = 𝑀𝑟 + 𝑀𝑙 (Eq. 6.4)

where Cr represents the EF concentration at the sampling point (µL/L), derived from the calibration

curve. Vr is the total volume of the jar (L), while Ve is the volume of headspace gas extracted from

134

the jar (L). EF release was expressed in mg/mg·L (milligram of EF vapor per milligram of

nonwoven per liter of headspace air).

6.3.9 Stability study

EC-PEO nonwovens loaded with 10 % EFP were stored for 30 days under different relative

humidity levels (0, 60, or 100 % RH) at 25°C in an environmental chamber (MLR-350H, Sanyo

Electric Co., Ltd. Japan) to evaluate their storage stability. The releases of EF from the nonwovens

were tested on 1st, 15th, and 30th day using 0.1 N CA solution at 25°C as described in Section 6.3.8.

6.3.10 Preliminary studies on preservation of strawberries

Strawberries were purchased from a local grocery store. Samples were sorted and only sound

fruits were used for testing. Each group of four fruits (~ 80 g) were placed in thermoformed

poly(ethylene terephthalate) (PET) container (16 oz, Hinged Deli Container, Genpak, NY, USA).

Nonwovens loaded with 70 % EFP were cut into 3 × 3 cm pieces (8 ± 1 mg). One or two pieces of

the nonwoven squares were placed on polystyrene petri dish ( 6 cm × 1.5 cm, Fisher Scientific,

Ottawa, ON, Canada), having four holes (0.5 cm dia) punctured on its lid, to release 1 and 2 mg

of EF vapor, respectively, into the headspace of the PET container (Fig. 6.2). The release of EF

vapor was activated by adding 0.3 mL of 0.1 N CA solution to the EFP-loaded nonwoven just

before closing the PET container. The setup was kept at 5°C for 10 d in an environmental chamber

(MLR-350H, Sanyo Electric Co., Ltd. Japan). Photographs of the fruits were recorded by a digital

camera (DMC-FZ50, Panasonic, Kadoma, Osaka, Japan) on the 1st, 8th, and 10th day to visually

evaluate the microbial spoilage. Oxygen and carbon dioxide concentrations in the headspace of

the packages were determined using a headspace gas analyser (Gaspace Advance Micro, Systech

135

Illinois, Thame, UK), where the headspace was extracted every 2 days through the septum. The

release of EF in the packages was evaluated as described in Section 6.3.8. The percentage weight

loss of the fruit ( Wl, %) was determined using Eq. 6.5:

𝑊𝑙 =𝑊𝑖−𝑊𝑑

𝑊𝑖 (Eq. 6.5)

where Wi is the initial weight of the fruit and Wd is the weight of the fruit on the 8th, and 10th day.

Fig. 6.2: Schematic diagram of the setup used for studying the efficacy of ethyl formate (EF)

vapor, released from its precursor-loaded nonwovens, on extending the shelf-life of strawberries.

6.3.11 Data analysis

Statistical analysis was conducted using SAS® software package (University Edition, SAS

Institute Inc., Cary, NC, USA). One way ANOVA was carried out to determine the significant of

differences between treatments at 95 % confidence interval. All treatments were triplicated, and

results were expressed as the means ± standard error. Mean values were compared by Tukey’s

136

honest significance difference test. The release kinetics of EF from its precursor-loaded

nonwovens were modeled using a pseudo-first order reaction (Eq. 6.6):

𝐶𝑒−𝐶

𝐶𝑒−𝐶0= 𝑒−𝑘𝑡 (Eq. 6.6)

where Ce and C are the equilibrium EF concentration in the headspace at infinite time and the EF

concentration at time, t, respectively. C0 is the intial concentration of EF which is equal to zero. k

is the diffusion rate constant. Ce and k values were estimated from non-linear regression analyses

by fitting Eq. 6 to the release data.


6.4.1 Effect of ethyl formate precursor loading on polymer solution properties

Since polymer solution properties greatly affect the electrospinnability and the fiber

morphology, the effect of EFP on viscosity, electrical conductivity, and surface tension of EC-

PEO solutions were studied (Table 6.1). The power-law model (Eq. 6.1) fitted the shear stress

versus shear rate data well, with coefficient of determination (R2) values greater than 0.99. All the

solutions had n values of less than one, indicating non-Newtonian shear-thinning behaviors. This

shear-thinning behavior is generally an indication of chain interactions and chain-chain

entanglement of EC and PEO polymers in the solutions, which are prerequisites to enable

electrospinning, as shown in Chapter 4. EC-PEO solution (0 % EFP) had the lowest apparent

viscosity values. The addition of EFP into EC-PEO solutions resulted in a significant (p < 0.05)

increase in k and apparent viscosity values. For example, the apparent viscosity of EC-PEO

solutions at a shear rate of 1000 s-1 increased significantly (p < 0.05) from 0.387 ± 0.011 to 0.571

137

± 0.031 Pa.s when EFP content increased from 0 to 70 %, respectively. Increasing EFP to 100 %

of the polymer content did not further result in significant change in apparent viscosity (Table 6.1).

Similarly, the addition of up to 30 % EFP significantly (p < 0.05) increased the surface tension of

the spin-dope solutions to 75.63 ± 1.85 mN/m, as compared with the unloaded EC-PEO solution

(55.10 ± 0.43 mN/m). However, further increase in EFP content to 50, 70, and 100 % EFP levels

resulted in a decrease in surface tension to 52.43 ± 1.92, 49.80 ± 0.93, and 48.36 ± 1.11 mN/m,

respectively. On the other hand, the addition of EFP at elevated amounts resulted in significant (p

< 0.05) decrease in electrical conductivity, reaching 23.24 ± 0.80 µS/cm at 100 % EFP, as

compared to the neat EC-PEO solution (31.26 ± 0.5 µS/cm) (Table 6.1).

The effects of EFP on EC-PEO solution parameters could be attributed to the suspended EFP

particles in the polymer solution. The rheological properties of suspensions and their flow

behaviors are primarily dictated by the particle-liquid and particle-particle interfacial interactions,

in addition to many other factors such as the particle volume fraction, particle size, particle size

distribution, particle shape, and medium viscosity (Nutan & Reddy, 2010). With increasing EFP

content in the suspension, the mean distance between particles decreases, which leads to increased

friction and thus an increase in viscosity. The increase in surface tension observed at lower EFP

content could be related to the capability of EFP particles to migrate to the surface and resist

external forces. However, at higher EFP content, overcrowding of particles at the air-liquid

interfaces interfered with the liquid phase, weakening the cohesive force between the solvent

molecules, thereby lowering the surface tension. On the other hand, the negative effect of EFP on

the electrical conductivity of polymer solutions could be attributed to the charge-counter acting

effect of EFP upon polymers. Although EFP formed a suspension in 90 % (v/v) aqueous ethanol,

138

a small amount of EFP might have been dissolved in the solvent, affecting the EC-PEO solution

parameters as well. Similar findings were observed by Chu et al., (2006) for the effect of adding

TiO2 and SiO2 particles into aqueous polyvinyl alcohol solutions. They found that increasing TiO2

up to 30 % (w/w) and SiO2 up to 7 % (w/w) resulted in a significant increase in viscosity and

surface tension of the suspensions.

6.4.2 Characterization of the electrospun nonwovens

The effect of EFP on the morphological structures of EC-PEO electrospun fibers and their

diameters distribution were shown in Fig. 6.3. The SEM micrograph showed that neat EC-PEO

nonwoven (0% EFP) had continuous bead-free fibers with cylindrical-like morphology. The

fibers’ surface had longitudinal wrinkles as a result of the preferential evaporation of ethanol from

the aqueous ethanol solvent (Chapter 5). The exposure of spin dope to the air as it was being

ejected from the wire electrode towards the collector caused ethanol to evaporate rapidly at the

beginning, forming cylindrical fibers. The followed evaporation of the residual water collapsed

the fibers, creating longitudinal wrinkles (Koombhongse et al., 2001). Diameters of the fiber

showed a normal distribution and had a narrow diameter range with an average of 0.215 ± 0.053

µm (Table 6.1). The addition of EFP resulted in the appearance of irregular entities along the length

of the fibers which were the embedded EFP particles having the particle size distribution ranged

from 0.15 to 4.62 µm (Chapter 4). As expected, the SEM micrographs showed that the counts of

the irregular entities increased with increasing EFP content. At low magnification, the EFP

particles appeared to distribute evenly throughout the nonwoven specimens. These results suggest

that EC-PEO fibers are suitable matrices for entrapping EFP particles.

139

Table 6.1: Electrical conductivity, surface tension, flow behavior index (n), consistency coefficient (k), coefficient of determination

(R2) for fitting shear stress versus shear rate data using Eq. 6.1, and apparent viscosity (µ) at a shear rate of 1000 s-1 of EC-PEO

solutions loaded with different ethyl formate precursor (EFP) contents. The average fiber diameters of resulting electrospun fibers

were also indicated.

Polymer

solution

Conductivity

(µS/cm)

S. tension

(mN/m) n

k

(Pa.sn) R2

µ,

(Pa.s)

Diameter

(µm)

0% EFP 31.26 ± 0.51a 55.10 ± 0.43a 0.938 ± 0.005a 0.606 ± 0.003a 0.999 0.387 ± 0.011a 0.215 ± 0.053a

10% EFP 31.22 ± 1.01a 65.43 ± 0.99b 0.937 ± 0.003a 0.643 ± 0.003a 0.999 0.405 ± 0.012a,b 0.336 ± 0.007b

30% EFP 30.53 ± 0.85a,b 75.63 ± 1.85c 0.925 ± 0.002a,b 0.802 ± 0.007b 0.998 0.462 ± 0.007a,b,d 0.367 ± 0.095c

50% EFP 27.96 ± 0.38b,c 52.43 ± 1.92a,d 0.916 ± 0.003b 0.906 ± 0.026c 0.997 0.488 ± 0.028b,d,c 0.287 ± 0.004d

70% EFP 26. 74 ± 0.52c 49.80 ± 0.93a,d 0.889 ± 0.002c 1.28 ± 0.041d 0.997 0.571 ± 0.031c 0.281 ± 0.006d

100% EFP 23.24 ± 0.80d 48.36 ± 1.11d 0.925 ± 0.004a,b 0.934 ± 0.012c 0.999 0.549 ± 0.004d,c 0.276 ± 0.011d

Different alphabets (a–d) indicate a statistical significant difference (p<0.05) within each column.

140

Compared to the unloaded EC-PEO nonwoven, the addition of up to 30 % EFP resulted in a

significant (p < 0.05) increase in the average fiber diameter to 0.367 ± 0.095 µm, respectively.

This increasing fiber diameter trend correlated positively with the surface tension, but no specific

trends were observed for electrical conductivity and apparent viscosity. Further addition of EFP

however decreased the average fiber diameters and widened the diameter distributions (Table 6.1).

These observations could be attributed to the increase in the net stretching force on the polymer

jets as they ejected from the spinning electrode, as a result of the extra mass carried by the

embedded EFP particles with increasing loading. Similar observations were reported by Dai and

Lim for the encapsulation of mustard seed meal powder in electrospun PLA-PEO nonwovens,

where fiber diameter decreased with the addition of the powder (Dai & Lim, 2015). Similarly,

Araújo and his co-workers reported a decrease in fiber diameters when different ratios of ZnO and

TiO2 were entrapped in poly(methacrylic acid-co-methyl methacrylate) electrospun fibers (Araújo

et al., 2015).

In order to evaluate the nature of interaction between EFP, EC, and PEO, infrared spectra of

freshly prepared EC-PEO nonwovens loaded with 0, 10, 30, 50, 70, and 100 % EFP were analyzed

using FTIR and compared with the pristine EFP spectrum (Fig. 6.4). For EC-PEO nonwoven (0 %

EFP), the overall spectral features resembled those of EC polymer, since the cellulose derivative

is the majority component (~91 % w/w) of the blend (Chapter 5). The spectrum showed stretching

bands at 2850 – 3120 cm-1 of the symmetric and asymmetric C–H stretching. The characteristic

C–O–C stretching band of EC cyclic ether was observed at 1055 cm-1 (Pavia et al., 2008). By

adding EFP at elevated concentrations, its characteristic bands were detected at 1664, 1618, 1568,

and 1244 cm-1 of C=N, C=O, N–H, and C–O–C, respectively. Comparing with the same bands of

141

pristine EFP, no noticeable wavenumber shifts were observed for those peaks, although their

intensity increased with increasing EFP content. The ratio of the peak height of EFP at 1618 cm−1

to that of EC at 1055 cm−1 were 0, 0.10, 0.23, 0.35, 0.45, and 0.62 for nonwovens loaded with 0,

10, 30, 50, 70, and 100% EFP, respectively. The EFP particles likely were physically entrapped

within the polymer matrices. Also, the wavenumber of EC’s characteristic peak remained

unaffected with the incorporation of EFP. These results suggest that there were no specific

interactions between IR-active covalent bonds in EFP particles and the polymers present in the

nonwoven. Similar findings were observed by Jash & Lim, (2018) for the encapsulation of hexanal

precursor compound in electrosprayed EC spheres and electrospun PLA nonwovens, where no

significant shift was detected in the major absorbance bands of EC and PLA after the incorporation

of the precursor.

To better understand the component interaction of the nonwoven, the effect of EFP on the

thermal properties of EC-PEO nonwovens was evaluated (Fig. 6.5). The thermograph of 0% EFP

EC-PEO nonwoven showed an endothermic melting peak of EC at 237.74 ± 0.96°C, whereas the

thermal properties of PEO were not noticeable, due to the relatively low content of the latter (~9

% w/w) as compared to EC (~91 % w/w). Overall, the thermograph is resembling those of EC

polymer reported in the literature (Álvarez et al., 2019; Barboza et al., 2009; Chan et al., 2005).

The onset of melting point of EFP lowered significantly (p < 0.05) as it was added to EC-PEO

nonwovens (Table 6.2). With increasing EFP loading from 10 to 100 % (w/w), the endothermic

melting peak of the precursor progressively became more noticeable, with a concomitant shift to

higher temperatures (from 163.66 ± 0.39 to 171.82 ± 0.26°C), approach to that of the pristine EFP

particles (173.76 ± 0.74°C) (Table 2). This could be a result of the difference in the environments

142

in which the free and encapsulated EFP were located, where electrospun fibers have a very large

surface area as compared to that of the EFP particles. Also, the re-crystallization of EFP fraction

that dissolved in aqueous ethanol solvent might have resulted in imperfections in the crystal

structures and thus leading to the lowering of the melting temperatures for EFP (Weeks, 1963).

Besides the melting temperatures, an increase in melting enthalpy of EFP from 15.18 ± 0.18 to

80.06 ± 2.23 J/g was observed with increasing EFP content from 10 to 100 % (Table 6.2),

suggesting its physical embedment within the nonwoven matrices.

143


distribution of EC-PEO nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w,

polymer content basis).

144

Fig. 6.4: Fourier transformed infrared (FTIR) spectra of pristine EFP particles and EC-PEO

electrospun nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w, polymer content

basis).

Fig. 6.5: Differential scanning calorimeter (DSC) thermographs of pristine EFP particles and EC-

PEO electrospun nonwovens loaded with EFP at 0, 10, 30, 50, 70, and 100 % (w/w).

145

Table 6.2: Onset of melting (To) and melting peak (Tm) temperatures, and enthalpies of melting

(Hm) of pristine ethyl formate precursor (EFP) particles and that encapsulated in EC-PEO

electrospun nonwovens at 10, 30, 50, 70, and 100 % (w/w, polymer content basis).

Sample To (°C) Tm (°C) Hm (J/g)

Pristine EFP 168.74 ± 0.92a 173.76 ± 0.74a 202.37 ± 1.23a

10% EFP nonwoven 150.39 ± 1.41b 163.66 ± 0.39b 15.18 ± 0.18b

30% EFP nonwoven 154.28 ± 0.55c 167.39 ± 0.37c 37.92 ± 0.46c

50% EFP nonwoven 155.83 ± 0.15c,d 168.50 ± 0.21c 55.52 ± 0.23d

70% EFP nonwoven 158.15 ± 0.61c,d 171.28 ± 0.16d 67.94 ± 0.57e

100% EFP nonwoven 158.27 ± 0.36d 171.82 ± 0.26d 80.06 ± 2.23f

Different alphabets (a–f) indicate a statistical significant difference (p<0.05) within each column.

6.4.3 Activated release of ethyl formate from its precursor-loaded nonwovens

The synthetized EFP remained stable under dry conditions but susceptible to hydrolysis in

the presence of a mild acid to activate the release of EF vapor (Fig. 6.6). The release of EF vapor

from its precursor embedded in EC-PEO electrospun fibers was triggered by the hydrolysis of the

C=N bond on diethyl N,N'-adipoyldiformohydrazonate molecules, catalyzed by the presence of

CA (Chapter 4). Several phenomena were involved in triggering the release of EF vapor: (1)

permeation of the CA solution into the surface of nonwoven; (2) diffusion of the solution through

the polymer matrices reaching the EFP particles; (3) hydrolysis of the EFP, generating EF vapor;

(4) diffusion of EF vapor through the polymer matrices towards the nonwoven surface; and (5)

desorption of EF vapor into the headspace air. Therefore, the hydrophilicity and barrier properties

of the polymer carrier are critical parameters to control the release kinetic of EF vapor. In the

present study, although EC is insoluble in water, it is hydrophilic in nature due to the free hydroxyl

146

groups on its backbone (Upadhye & Rajabi-Siahboomi, 2013; Wellons & Stannett, 1966), thereby

it was used as a polymer carrier in releasing EF vapor from EFP particles.

To study the effect of CA concentration, the release kinetic of EF from EC-PEO nonwoven

loaded with 10% EFP was tested using 0.001, 0.01, 0.1, and 1 N CA solutions at 25°C. As shown

in Fig. 6.7A, a rapid release was observed upon activating the nonwovens, followed by a slow

release and then reaching a plateau for release profiles under 0.01, 0.1, and 1 N. However, a slow

release trend was observed for CA solution at 0.001 N. Beyond 120 min of activation, the

accumulative release of EF was roughly the same (0.037 mg/mg.L) for 0.1 and 1 N. However,

decreasing CA concentration to 0.01 and 0.001 N resulted in a significant (p < 0.05) reduction in

EF release to 0.035 ± 0.001 and 0.029 ± 0.001 mg/mg.L, respectively (Table 6.3). The pseudo-

first-order kinetic equation (Eq. 6.6) fitted the release data well, with R2 value of above 0.98. The

estimated model parameters (Ce and k) from regression analysis were presented in Table 6.3. The

differences in Ce values were not significant, while k values increased significantly (p < 0.05) with

increasing CA concentration, which correlated positively with the high release rate at high

normality. No comparable differences were noted between the EF release rate/amount under 0.1

and 1 N, thereby 0.1 N CA solution was used in the rest of the study.

The effect of temperature on the EF release profile was evaluated at 5, 15, and 25°C by adding

0.1 N CA solution to 10% EFP-loaded nonwovens. As shown in Fig. 6.7B, the rapid release period

was observed within the first 60, 30, and 20 min upon activation at 5, 15, and 25°C, respectively.

Moreover, the accumulative release at 120 min significantly (p < 0.05) increased from 0.029 ±

0.001 to 0.037 ± 0.001 mg/mg.L, as the temperature increased from 5 to 25°C, respectively (Table

147

6.3). Comparing the release profiles of EF from the precursor-loaded nonwovens with those from

the precursor particles reported in Chapter 4, the former showed faster release rate due to the high

surface area-to-volume ratio of the electrospun fibers as compared to that of the EFP particles. The

release profiles were fitted to pseudo-first order kinetic equation (Eq. 6.6). As summarized in Table

3, Ce values significantly (p < 0.05) increased with increasing the temperature, indicating higher

accumulative EF release at higher temperatures. Also, the k values increased with increasing

temperature, which can be attributed to the enhanced precursor hydrolysis reaction as well as

increased diffusion of CA solution and EF vapor as temperature increased. Similar phenomenon

was reported by Jash and Lim who observed an increase in the release of hexanal, from its

imidazolidine precursor encapsulated in electrosprayed EC spheres and electrospun PLA

nonwovens, as temperature increased (Jash & Lim, 2018). Also, Mu et al., (2017) reported an

enhance in the release rate of ethanol from its emitter, prepared by a gelatification reaction between

ethanol and sodium stearate.

The EF release profiles from EC-PEO nonwovens loaded with 10, 30, 50, 70, and 100 % EFP

were studied using 0.1 N CA solution at 25°C (Fig. 6.7C). As expected, higher EF release was

achieved from higher EFP loading. After 120 min upon activation, the accumulative release

increased from 0.037 ± 0.001 to 0.146 ± 0.005 mg/mg.L, as the EFP loading increased from 10 to

100 % (w/w), respectively. The release data were fitted satisfactory using Eq. 6.6, and Ce and k

values were presented in Table 6.3. No significant (p > 0.05) differences were observed in k values

as EFP loading increased. The EF release from 10% EFP nonwoven represented 96 % of the

theoretical load, which revealed that the majority of the loaded EFP was effectively encapsulated

within the fiber matrix (Table 6.3). This release percentage was 91, 88, 83, and 75 % for 30, 50,

148

70, and 100% EFP-loaded nonwovens, respectively. This decreasing trend indicated that the

encapsulation efficiency decreased with increasing EFP loading capacity.

To evaluate the storage stability of EFP-loaded nonwovens, the 10% EFP nonwoven was

stored for up to 30 d at 25°C under different RH environments. The nonwoven showed higher

stability at dry condition (0 % RH) than at humidified conditions (60 and 100 % RH). As shown

in Fig. 6.8, there were no significant (p > 0.05) differences in the EF release for days 1, 15, and 30

at 0 % RH. However, at 60 % RH, 33 and 56 % reductions in EF release were observed for days

15 and 30, respectively. When the samples were stored at 100 % RH, substantial reductions of EF

released by 64 and 86 % were detected on days 15 and 30, respectively. Interestingly, the EC-PEO

encapsulated EFP particles were less stable than the unencapsulated precursor when tested under

the same storage conditions as reported in Chapter 4. The reduced storage stability observed for

the encapsulated EFP could be due to the enlarged surface area of the precursor particles as they

were being dispersed throughout the electrospun fibers, while the unencapsulated counterpart

tended to exist as aggregates of particulates. Taken all together, these observations suggested that

the EFP embedded in the nonwovens could be degraded during prolonged storage when exposed

to moisture. Storage under dry condition is essential to ensure maximal EF release during the end-

use application

149

Table 6.3: The EF released from its precursor embedded in EC-PEO electrospun fibers at 120

min and the fitted model parameters as affected by CA concentration, temperature, and EFP

loading capacity.

Nonwoven Citric

acid

Temp EF released at 120 min Ce k R2

°C mg/mg.L % mg/mg.L min-1

10% EFP 1 N 25 0.037 ± 0.001a 95.9 0.037 ± 0.000a 0.109 ± 0.01a 0.99

10% EFP 0.1 N 25 0.037 ± 0.000a,c 96.0 0.037 ± 0.001a,c 0.085 ± 0.00a,b 0.99

10% EFP 0.01

N 25 0.035 ± 0.001a 89.3 0.034 ± 0.001a 0.055 ± 0.01b,c 0.98

10% EFP 0.001

N 25 0.029 ± 0.001b 74.9 0.042± 0.003b 0.011 ± 0.00c 0.98

10% EFP 0.1 N 25 0.037 ± 0.000a,c 96.0 0.037 ± 0.001a,c 0.085 ± 0.01a,b 0.99

10% EFP 0.1 N 15 0.034 ± 0.002c,d 87.1 0.033 ± 0.002c,d 0.063 ± 0.01d 0.99

10% EFP 0.1 N 5 0.029 ± 0.001d 76.0 0.030 ±0.001d 0.031± 0.00e 0.9

10% EFP 0.1 N 25 0.037 ± 0.000a,c 96.0 0.037 ± 0.001a,c 0.085 ± 0.01a,b 0.99

30% EFP 0.1 N 25 0.081 ± 0.002e 91.1 0.082 ± 0.003e 0.071 ± 0.01f,g 0.98

50% EFP 0.1 N 25 0.115 ± 0.001f 87.8 0.116 ± 0.002f 0.065 ± 0.00f,g 0.99

70% EFP 0.1 N 25 0.136 ± 0.002g 83.2 0.135 ± 0.002g 0.051 ± 0.01g 0.99

100% EFP 0.1 N 25 0.146 ± 0.005g 74.8 0.145 ± 0.004g 0.046 ± 0.00g 0.99

Different alphabets (a–g) indicate statistical significant difference (p<0.05) within each column

for each effect (i.e., CA concentration, temperature, and EFP loading).

150

Fig. 6.6: Ethyl formate precursor, diethyl N,N'-adipoyldiformohydrazonate, and its acid

hydrolysis, releasing ethyl formate vapor.

Fig. 6.7: The release of ethyl formate from its precursor (EFP) embedded in EC-PEO electrospun

fibers as affected by the CA concentration (A), temperature (B), and EFP loading capacity (C).

151

Fig. 6.8: Ethyl formate (EF) released from the 10% EFP nonwovens stored for 30 days at 25°C

under 0, 60, and 100 % RH. Different alphabets (a–d) indicate statistical significant difference

(p < 0.05).

6.4.4 The efficacy of ethyl formate precursor for delaying spoilage in strawberries

Preliminary tests were conducted to evaluate the EFP-loaded nonwovens for delaying the

spoilage of strawberries. Fig. 6.9 showed the appearance of strawberries on days 1, 8, 10 of storage

at 5°C. The EF vapor released from the nonwovens showed high ability to delay microbial growth

on strawberries. No microbial growth was observed visually for up to 10 d on strawberries treated

2 mg EF vapor. Strawberries treated with 1 mg EF vapor started to spoil on day 10. In comparison,

spoilage was evident in untreated samples (control) on day 8 and extensively deteriorated on day

10 (arrows in Fig. 6.9). To better understand how the PET containers affect the respiration of the

fruit, headspace O2 and CO2 concentrations were determined during storage period. As shown in

Fig. 6.10A, the initial headspace O2 level was 20.9 % for all treatments. The O2 concentration

declined gradually during storage, reaching 15.5 ± 1 % at day 10 for packages treated with 2 mg

EF. No significant differences were observed between treatments. By contrast, the headspace CO2

152

concentration increased to 6.8 ± 0.4 % at the end of the experiment for packages treated with 2 mg

EF (Fig. 6.10B), which corroborated with the depletion of O2 concentration profile. The buildup

of CO2 level inside the packages was higher for treated samples compared to control samples. The

depletion of O2 and buildup of CO2 levels were attributed to the respiration of strawberries.

Besides visual assessment, strawberries weight losses were evaluated which is associated with

respiration and moisture evaporation through the fruit skin. As shown in Fig. 6.10C, samples

treated with 2 mg EF had the lowest weight loss; 1 and 1.5 % on the 8 and 10 day of storage,

respectively. There were no significant (p > 0.05) differences in weight loss between the two

treatments (1 and 2 mg EF). However, the control sample suffered the greatest weight loss.

Approximately 1.5 and 4.5 % of their weight was lost on the 8 and 10 day, respectively, which

attributed to the increased moisture loss due to the physiological injuries caused by the microbial

growth. Furthermore, the release of EF vapor inside the package’s headspace (with/without fruits)

was measured for up to 10 d to study its release/depletion profiles (Fig. 6.10D). For package

without fruit, the release of EF rapidly increased to reach a maximum (0.11 ± .01 mg/mg.L) after

2 h upon activating the nonwoven. The maximum level of EF remained for up to 24 h before

starting to deplete. At day 10, the EF level inside the package declined to 0.05 ± 0.01 mg/mg.L. In

the presence of fruit, the release of EF reached a peak of 0.09 ± 0.01 mg/mg.L after 1 h up on

activation then followed by a rapid decrease to reach 0.01 ± 0.00 mg/mg.L after 24 h, and then

stabilized at this level till the end of the experiment. The depletion of EF concentration could be

ascribed to its permeability through PET as well as its hydrolytic degradation in the presence of

moisture, producing formic acid and ethanol, both of them are naturally antimicrobial agents (Ryan

& Bishop, 2003). The differences in the EF release/depletion profiles between packages

153

with/without fruits was attributed to the solubility of EF and its rapid degradation in strawberry

(Satish Kuchi & Sai Ratna Sharavani, 2019). Simpson and his coworkers fumigated strawberries

with EF vapor to exterminate insect pests. After EF treatment, they observed no significant

differences in strawberry conditions between EF treated and untreated fruit (Simpson, Bikoba and

Mitcham, 2004). These observations suggested that EF might have be degraded in the fruit to

biogenic levels, which is desirable to prevent physiological damage to the fruit and residues in

food products. The EFP-loaded nonwovens developed in this study could be promising for treating

fresh horticultural produce to inactivate microbial growth and extend its shelf-life.

Fig. 6.9: Photographs of strawberries fumigated with 1 and 2 mg ethyl formate (EF) released

from EC-PEO nonwoven loaded with 70 % (w/w) EF precursor. Samples were stored in PET

packages for 10 days at 5°C, showing delayed mold growth for treated samples compared to the

control.

154

Fig. 6.10: Headspace concentrations of O2 (A) and CO2 (B), fruit weight loss percentages (C),

and ethyl formate (EF) release/depletion profiles inside PET packages for 10 d.

6.5 Conclusion

In this study, EFP was encapsulated in EC-PEO electrospun fibers using free surface

electrospinning. Increasing EFP content in EC-PEO solutions resulted in an increase in their

apparent viscosity and a decrease in their electrical conductivity, whereas their surface tension

increased by adding up to 30% EFP and then decreased again when increasing EFP content to

100%. The largest fiber diameter was observed in EC-PEO nonwovens loaded with 30% EFP.

Although some of EFP particles were larger than the electrospun fibers, they were physically

entrapped and uniformly dispersed within the EC-PEO nonwovens, as revealed by SEM analysis.

155

FTIR analysis did not reveal any specific interactions between IR-active covalent bonds in EFP

EFP, EC and PEO. The release of EF from its precursor-loaded nonwovens could be effectively

triggered using 0.1 N CA solution, whereas decreasing the normality caused a decline in EF release

rate. Increasing temperature resulted in higher EF release rate. Up to 96 % of the theoretical

available EF was released from 10% EFP nonwoven at 25°C. The precursor-loaded nonwovens

remained stable during storage under dry environmental conditions (0 % RH) for up to 30 d. The

EF vapor released from the EFP-loaded nonwovens delayed spoilage and extended shelf-life of

strawberries for up to 10 days. The EFP nonwovens developed here could be useful in AP

applications of fresh produce to destroy the insect pests and to inhibit the proliferation of

spoilage/pathogenic microorganisms.

156

Chapter 7: In-Package Fumigation of Blueberries Using Ethyl Formate Vapor

Released from a Solid-State Precursor: Effects on Spotted-Wing Drosophila

(Drosophila suzukii Matsumura) Mortality and Fruit Quality #

# Content of this chapter was submitted for publication as a research paper: Zaitoon, A., Jabeen,

A., Ahenkorah, C., & Scott-Dupree, C., & Lim, L. T. (2021). In-Package Fumigation of

Blueberries Using Ethyl Formate Vapor Released from a Solid-State Precursor: Effects on

Spotted-Wing Drosophila (Drosophila suzukii Matsumura) Mortality and Fruit Quality. Food

Packaging and Shelf Life (under revision).

157

7.1 Abstract

Ethyl formate (EF) is a food-flavoring agent with a generally regarded as safe (GRAS) status.

It is a naturally-occurring insecticidal volatile compound promising as an alternative to synthetic

insect pest fumigants that are toxic and/or undesirable to the environment. In this study, the EF

precursor (EFP) was investigated as an activated EF release system for in-package fumigation of

spotted-wing drosophila (SWD) in blueberries. EF vapor released from the EFP after hydrolysis

in the presence of citric acid solution, controlled all life stages of SWD in blueberries. There was

a negative correlation between the cumulative EF exposure and the fruit loading factor in the

containers. Complete control of SWD adults was observed after exposing the inoculated fruits to

a package headspace containing 6 mg/L of EF for 4 h at 3 and 6 % (w/v) fruit loading factors, and

4 mg/L for 2 h at 30 % load factor (22 ± 1°C). SWD eggs, larvae, and pupae were more tolerant

toward EF vapor than the adults, in which 16 mg/L for 24 h was effective for their complete

control. There were no significant (p > 0.05) differences in blueberry quality parameters between

EF treated and untreated berries. Results from this study suggested that in-package EF fumigation

via the EFP system has a potential as a postharvest treatment for SWD control in blueberries. This

active packaging (AP) approach can be beneficial for streamlining the existing fumigation

approach, thereby increasing the available shelf-life of fresh produce.

158

7.2 Introduction

Ethyl formate (EF) is an FDA-approved food flavoring agent that is naturally occurring in

atmosphere, soil, water, vegetation, and many food products (e.g., beer, wine, grapes, wheat,

barley, raisin, rice, cheese) (Desmarchelier, 1999; Desmarchelier et al., 1999; Malanca et al., 2009;

Ren & Desmarchelier, 2002). For more than two decades, many studies have shown that EF is an

effective insecticide that is efficacious for the fumigation of fresh produce and grain commodities.

For example, Stewart and Mon achieved a 98 % mortality of green peach aphids (Myzus persicae)

after exposing to EF vapor in air at 4.5 mg/L concentration level for 1 h on film-wrapped lettuce

by using vacuum fumigation treatment in a chamber at 60 mmHg and 2.5°C (Stewart & Mon,

1984). Ren and Mahon (2006) evaluated the toxicity of EF against Tribolium castaneumn,

Callosobruchus phaseol, Rhyzopertha dominica, and Sitophilus oryzae in stored split fava beans,

sorghum, and wheat by a double fumigant exposure treatment (first dose of 85 mg/kg for 4 h

followed by another dose of 85 mg/kg). They reported high levels of control of all life stages of

insects in the stored crops. Simpson and his co-workers found that the fumigation of strawberries

and grapes with EF dosages ranging from 0.4 to 43 mg/L in air for 2 h at 24°C were effective to

induce various degrees of mortality in western flower thrips (Frankliniella occidentalis), two-

spotted spider mites (Tetranychus urticae), mealybugs (Pseudococcidae), and omnivorous

leafroller (Platynota stultana), without affecting fruit quality (Simpson et al., 2004; Simpson et

al., 2007). Complete control of grain chinch bug, Macchiademus diplopterus was achieved after

fumigating various stone and pome fruit cultivars with EF at 50 mg/L in air for 1 h, without

inducing internal and external phytotoxic damage (Smit et al., 2020). The antimicrobial activities

of EF vapor against selected fruit and vegetable spoilage microorganisms were studied in vitro by

159

Utama et al. (2002). They found that EF concentration of 11.5 mmol per Petri dish (9 cm diameter)

was germicidal against the growth of Rhizopus stolonifera, Colletotrichum musae, Erwinia

carotovora, and Pseudomonas aeruginosa but did not completely inhibit the growth of Penicillium

digitatum (Utama et al., 2002). Complete inhibition of Saccharomyces rouxii and Saccharomyces

mellis growth on Deglet Noor dates was achieved after exposing to EF vapor in air at the level of

6.6 mL/kg of fruit (Bolin et al., 1972). Also, In Chapter 6, EF vapor at a concentration of 4 mg/L

in air, released from the EFP-loaded electrospun nonwovens, delayed the growth of spoilage

microorganisms on strawberries for up to 10 d at 5°C.

Although EF is an effective pesticidal and antimicrobial agent, in the solution phase, it is highly

volatile (200 mmHg at 20°C), flammable, and susceptible to hydrolytic degradation in the presence

of moisture, producing formic acid and ethanol (Ren & Mahon, 2006; Ryan & Bishop, 2003).

These properties make its end-use handling and application very challenging. To suppress

flammability, EF has been commercially mixed with carbon dioxide in compressed cylinders at a

concentration of 16.7 % wt., for fumigation of food commodities. This product is commercially

available as Vapormate®, which exploits the synergistic effects of carbon dioxide and EF against

insect pests (Bessi et al., 2016; Ryan & Bishop, 2003). Also, liquid EF has been mixed with

nitrogen for safe in-transit fumigation of shipping containers (Coetzee et al., 2019; Coetzee et al.,

2020). These approaches are attractive for large enclosed space fumigation but inefficient to

provide controlled release attributes of the gases for in-package fumigation. Moreover, this

approach requires bulky spaces for secure storage of compressed gas cylinders as well as pressure

regulation and metering devices to ensure safety and accurate dosage of the fumigant. These issues

can be addressed by converting EF into a non-volatile precursor compound with long-term storage

160

stability, but can be activated to release the active volatile under mild conditions triggered by

hydrolytic, enzymatic, or oxidative reactions (Levrand et al., 2007).

In Chapter 4, we developed a precursor compound for EF through a condensation reaction

between adipic acid dihydrazide with triethyl orthoformate to form the EFP, diethyl N,N′ -

adipoyldiformohydrazonate. The resulting EFP powder is non-volatile and remained stable under

dry conditions, but could be hydrolyzed to trigger the release of EF vapor in the presence of a

mild-acid catalyst. The EFP approach could be useful in facilitating the end-use deployment for

fumigation purposes by simplifying transportation, handling, and packaging procedures. The EFP

potentially can also be exploited for the development of innovative AP systems involving in-

package fumigation of fresh produce to mitigate insect pest risks during distribution. This approach

is beneficial for overcoming the limitations of the existing bulk fumigation methods that delay the

products’ shipment and reduce their available shelf-life, preventing the handling of vast amounts

of fumigant in confined spaces, making the treatment more cost-effective, and reducing the

unintended release of EF into the atmosphere.

Spotted wing drosophila (SWD), Drosophila suzukii (Matsumura) (Diptera: Drosophilidae),

was first recorded in Eastern Asia but has become a serious invasive pest of fresh produce (i.e.,

berries, cherries, peaches, plums, apricots) in Canada, USA, and Europe (Asplen et al., 2015;

Biondi et al., 2016; Lee et al., 2011). Females oviposit eggs into the skin of ripening fresh fruit.

When eggs hatch, the larvae feed on the fruit, causing damage that renders the fruit unmarketable.

Also, the oviposition holes predispose the fruit to pathogens and insect pests resulting in further

injury. Considering the potential fruit damage and the significant economic impact by SWD, it

161

was used in this study as a surrogate insect pest to elucidate the efficacy of the EF activated release

fumigation system, based on the EFP compound, developed in Chapter 4.

In this research, we proposed an innovative approach for fumigating blueberries at an

individual packaging level through incorporating an EFP within the package to control the release

of EF vapor. The objectives of this research are to: (1) study the release of EF vapor from the EFP

and determine the cumulative EF exposure in thermoformed poly(ethylene terephthalate) (PET)

containers loaded with blueberries; (2) study the effect of EF vapor on the mortality of SWD eggs,

larvae, pupae, adults in blueberries; and (3) evaluate the effect of EF vapor on fruit quality

parameters and fruit volatiles content (i.e., ethanol and EF).


7.3.1 Materials

Triethyl orthoformate (reagent grade, 98 %), adipic acid dihydrazide (98 %), citric acid

monohydrate (CA, reagent grade, 99 %), and ethyl formate (EF, reagent grade, 97 %) were

purchased from Sigma-Aldrich (Oakville, ON, Canada). Fresh organic blueberries were obtained

from local grocery stores. Blueberries were visually inspected to remove defective berries. Prior

to use, blueberries were washed and dried to remove any surface contaminant. Berries with similar

size, color, and texture were used for experiments. All tests were conducted in 0.5 L non-perforated

thermoformed poly(ethylene terephthalate) (PET) containers (16 oz, Clear Hinged Deli Container,

Genpak, NY, USA).

162

7.3.2 Ethyl formate precursor formation and capsules preparation

Ethyl formate was synthesized as described in Chapter 4. A suspension of adipic acid

dihydrazide (500 mg) and triethyl orthoformate (20 mL) was placed in a 50 mL round-bottom flask

and heated (110°C) under reflux with stirring for 30 h. The suspension was then filtered and the

residue was air-dried to yield the EFP, which appeared as white powders (~91 % yield). In Chapter

4, we observed that 1 mg of EFP, upon activation by 0.1 N citric acid solution, resulted in the

release of 0.38 mg of EF vapor.

To evaluate the efficacy of EF against the insect pest, different amounts of EFP were used

during the fumigation treatment, as summarized in Table 7.1. As depicted in Fig. 7.1, EFP powder

was weighed and rolled in spun-bond polypropylene (PP) nonwoven, placed in a PET blister (0.6

× 1.4 × 2.5 cm3), and then sealed by an adhesive foil (Small unit cold seal blister pack system, Ezy

Dose, Burnsville, MN, USA). Five holes (0.2 cm diameter each) were created on the foil to allow

for the release of EF vapor from the EFP-containing capsule. The release of EF vapor was activated

by adding 500 µL of 0.1 N citric acid solution to the capsules through one of the holes using a

pipette (Fisher Scientific, Ottawa, ON, Canada).

163

Table 7.1: Quantities of ethyl formate precursor (EFP, mg) incorporated inside the 0.5 L

thermoformed PET container and EF vapor (mg) released for fumigation and fruit quality

experiments. Corresponding EF dosages (mg/L) were calculated based on the empty volume of

the container.

EFP (mg) EF vapor released (mg) Corresponding EF

dosage (mg/L)

1.32 0.5 1

2.64 1 2

5.28 2 4

7.92 3 6

13.2 5 10

21.12 8 16

Fig. 7.1: Schematic representation of: [A] the ethyl formate (EF) capsule and its activation by

adding citric acid solution to release EF vapor; and [B] the experimental setup for fumigation of

blueberries in thermoformed PET container.

164

7.3.3 Headspace concentration of ethyl formate

The headspace EF concentration was determined using a gas chromatograph (GC 6890,

Agilent Technologies Inc., Santa Clara, CA, USA) integrated with an automatic headspace

sampling system, as shown in Chapter 4. EF capsules containing 1.32 mg of EFP were activated

to release EF vapor as described in Section 7.3.2. The activated capsules were then placed inside

the 0.5 L thermoformed PET containers, having 0, 15, 30, 100, 150 g of blueberries (loading factor

of 0, 3, 6, 20, and 30 % w/v, respectively) at 22°C (Fig. 7.1 B). Headspace EF concentration was

measured for 4 h (at 10 min intervals) and at 24 h. The GC was equipped with a flame ionization

detector (FID) set at 200°C with respective flows of 30 mL/min N2, 50 mL/min H2, and 200

mL/min O2. The oven temperature was 40°C. The calibration of the FID was prepared by

measuring standard headspace concentration of known amounts of EF. Peak Simple software (SRI

Instruments, CA, USA) was used to analyze the chromatograms. The headspace concentration of

EF at any sampling point (Mt, μL) was calculated by adding the recorded amount to the

accumulated loss of all the previous sampling points, according to eq (7.1):

𝑀𝑡 = 𝐶𝑟𝑉𝑟 + ∑ 𝐶𝑟−1𝑉𝑒𝑟−1𝑖=1 Eq (7.1)

where Cr is the recorded EF concentration at the sampling point (μL/L); Vr and Ve are the volume

of the container (L) and the volume of headspace gas extracted from the container (L), respectively.

Headspace EF concentration was expressed in mg/mg.L (milligram of EF vapor per milligram of

EFP per liter of headspace air). Cumulative EF exposure (mg.h/mg.L), i.e., EF concentration

(mg/mg.L) × time (h) products, was calculated by integrating the concentration across the duration

of fumigation, according to Monro (1971).

165

7.3.4 Spotted-wing drosophila rearing conditions

The SWD colony was established in August 2020 at the University of Guelph from lab-reared

SWD obtained from Dr. Brent Sinclair (Western University, London, Ontario). Rearing was done

in ventilated Plexiglass cages (30 cm3). The SWD was provided with an artificial cornmeal diet

prepared in the lab according to the recipe of Dalton et al. (2011). This food source was also served

as an oviposition substrate. Artificial diet (36 g) was placed on 10 cm Petri dishes and provided to

SWD adults in rearing cages. Cotton plugs (2.0 cm height × 1.0 cm diameter), moistened with

distilled water, were placed in rearing cages as a moisture source for the adults. Petri dishes with

diet were replaced every other day. Old Petri dishes with SWD eggs were removed from the rearing

cage, covered with lids, and placed in a separate cage, where most of the eggs hatched in 1 to 2 d.

The newly emerged larvae feed on the diet forming tunnels, completing all life stages in 12 to 16

d. Newly emerged adults were transferred back to the insect cages to maintain the mass population

to be used in bioassays. The colony was kept in a growth chamber at 22 ± 1°C, 60 ± 5 % RH, and

16:8 light:dark photoperiod.

7.3.5 Spotted-wing drosophila eggs, larvae, and pupae

To obtain eggs from blueberries for the larval emergence bioassays, 40 – 45 blueberries in a

single layer were placed in rearing cages with SWD mated females. After 6 h, blueberries were

removed and were examined under a microscope to count SWD eggs in each blueberry by counting

the oviposition punctures with protruding egg filaments. Five blueberries containing 12 – 14 eggs

each were placed in a thermoformed PET container, larvae emerging from these eggs were used

in larval emergence bioassays. These larvae were also used to obtain the SWD pupae assessed in

the pupal emergence bioassay. PET containers containing infested blueberries with SWD eggs (5

166

berries having 12 – 14 eggs) were kept in a growth chamber. At 3 d, when larvae were emerged

the numbers were tabulated, those blueberries with 12 – 14 larvae were used in pupal emergence

bioassay. Likewise, to obtain SWD pupae, infested blueberries (5 berries having 12 – 14 eggs)

were kept in the growth chamber for 10 d, by that time eggs were hatched, newly emerged larvae

completed all instars and changed to pupae to be used in adult emergence bioassays.

7.3.6 Fumigant toxicity bioassays

Adult toxicity bioassays were performed in PET containers with 0, 3, 6, 20, and 30 % (w/v)

loading of blueberries. EF capsules with different amounts of EFP were activated as described in

Section 7.3.2 to release EF vapor at dosages of 1, 2, 4, and 6 mg/L inside the PET containers (Table

7.1). Ten SWD adults (3 – 5 d old) were aspirated from the rearing cage and transferred to these

containers which were then closed and stored at 22 ± 1°C. The PET containers with SWD adults

and blueberries were used as the control treatment. There were 5 replications per treatment and the

containers were arranged in a complete randomized design. Mortality data were assessed at 2 and

4 h. For treatments with 0, 15, 30 g of blueberries, containers were kept closed for 2 h and mortality

of SWD adults was determined by lightly shaking the containers to determine if the insects were

alive. At 4 h post-treatment, containers were opened and the SWD adults were evaluated. Those

did not fly or exhibit appendage movement were considered dead. For treatments with 100 and

150 g berries, due to the high loading capacity of berries, mortality assessment at 2 h by visual

assessment through the container was challenging. Therefore, mortality data were assessed by

opening the containers. Another set of containers were used to count the mortality at 4 h post-

treatment.

167

To perform the larval, pupal, and adult emergence bioassays, PET containers with five

blueberries infested with SWD eggs, larval, and pupae, respectively, were used as described in

Section 7.3.5. The containers were exposed to different EF doses (2, 4, 6, 10, and 16 mg/L; Table

7.1) released from EF capsules. Infested fruits in the control containers were untreated. All

treatments were replicated five times. At 24 h of EF exposure at 22 ± 1°C, the containers were

opened, and EF capsules were removed. Containers were aerated for 30 min then closed again and

kept at the same conditions for 3, 7, and 6 d for larval, pupal, and adult emergence bioassays,

respectively. After that, berries were removed from the containers, gently cut open using a

stainless-steel double-ended spatula. The number of larvae (dead or alive) in each blueberry was

counted using a dissecting microscope (10× magnification) for larval emergence bioassays. Also,

the larvae were carefully removed and transferred to a 30 mL plastic cup filled halfway with water.

Larvae in water were counted again to confirm the numbers of larvae emerged from treated eggs.

The number of pupae in each blueberry was counted using the dissecting microscope (10×

magnification) for pupal emergence bioassays. However, for adult emergence bioassays, SWD

adults emerged from treated pupae were assessed simply by counting the SWD adults in the

containers.

7.3.7 Fruit quality evaluation

Fruit quality parameters including firmness, total soluble solids (TSS), pH, titratable acidity

(TA), and volatiles content were measured before treatment, after 24 h of exposure to EF vapor

(24 h post-treatment), and then after another 24 h of aeration (48 h post-treatment), as described

below. PET containers, having 10 blueberries (15 g) each, were fumigated with EF vapor at

dosages of 2, 4, 6, 10, and 16 mg/L (Table 7.1) for 24 h by activating the EF capsules (Section

168

7.3.2). Unfumigated PET containers with berries were used as the control. All containers were

placed in an environmental chamber at 22 ± 1°C and 85 % RH. After 24 h of EF exposure, some

containers were used to evaluate the fruit quality (i.e., 24 h post-treatment), and the remaining

containers were opened and kept at the same conditions for another 24 h to be used for fruit quality

assessment after aeration (i.e., 48 h post-treatment). All treatments were replicated five times.

Firmness (N) of the fruit was measured using a texture analyzer (Model TA-HD plus, Texture

Technologies Corp., Scarsdale, NY, USA) equipped with a 5 kg load cell. The samples were

deformed to 50% of the original height using a crosshead speed of 1 mm/s and a 35 mm diameter

cylinder stainless flat probe. Each sample was subjected to a two-cycle compression with 5 s

between cycles. All measurements were made on 20 berries for each treatment. Data were analyzed

using Exponent software (Model TA-HD plus, Texture Technologies Corp., Scarsdale, NY, USA).

Total soluble solids, expressed as %, of the fruit juice was determined using an optical portable

refractometer (Fisher Scientific, FS1394620, MA, USA). A pH meter (Fisher Scientific, Accumet

XL20 pH meter, Ottawa, ON, Canada) was used to measure the pH of the juice as well as the TA

(%) through titration of the juice with 0.1 N NaOH to a pH endpoint of 8.2. Fermentative volatiles

content (i.e., ethanol and EF) was determined as described by Simpson et al. (2004). An aliquot of

3 g of the juice was placed in a 10 mL glass beaker, sealed with a rubber septum, and stored at -

30°C. For subsequent analysis, beakers were opened and placed inside hermetically sealed 1 L

glass jars. Samples in the jars were thawed at 22 ± 1°C for 1 h, then held for another 1 h at 65°C.

The headspace gas was then analyzed for ethanol and EF using the gas chromatography method

as described in Section 7.3.3.

169

7.3.8 Data analysis

Headspace concentration, mortality data for each life stage of SWD, and fruit quality data for

all trials were analyzed using SAS® University Edition software package (SAS Institute Inc., Cary,

NC, USA), using PROC GLIMMIX with ANOVA, to determine the significant differences

between treatments at 95 % confidence interval. Mean values were compared by Tukey’s honest

significance difference test. Data were expressed as the means ± standard errors.


7.4.1 Headspace concentration and cumulative EF exposure

Unlike EF which is a highly volatile and flammable compound, EFP is non-volatile and

remained stable under dry environmental conditions. However, it could be hydrolyzed under mild

acidic conditions to activate the release of EF vapor, through the hydrolysis of the C=N bond on

the hydrazonate moiety of the EFP molecule, catalyzed by citric acid (Chapter 4). As shown in

Fig. 7.2, rapid EF releases were observed within the first 60 min, followed by slower release rate

profiles. In empty containers (Fig. 7.2 A), a progressive increase in EF headspace concentration

was observed with increasing time, reaching a maximal level of 0.37 ± 0.04 mg/mg.L after

approximately 4 h. As the fruit loading factor increased to 3, 6, 20, 30 % (w/v) levels, the maximal

concentrations decreased significantly (p < 0.05) to 0.32 ± 0.02, 0.28 ± 0.09, 0.23 ± 0.02, 0.18 ±

0.03 mg/mg.L, respectively. Moreover, at 4 h, the EF concentrations decreased to 0.31 ± 0.02, 0.23

± 0.03, 0.09 ± 0.01, and 0.06 ± 0.01 mg/mg.L for the same loading factors, respectively (Figs. 7.2

B – E). The EF concentration build-up rates, as calculated from the slope of the linear portion of

the curves, decreased with increasing the fruit loading factor (0.32 ± 0.03, 0.30 ± 0.06, 0.29 ± 0.04,

170

0.27 ± 0.02, and 0.20 ± 0.03 mg/mg.h.L for 0, 3, 6, 20, and 30 % w/v loading factor, respectively).

In Fig. 7.2, the corresponding cumulative EF exposure values were plotted on the secondary Y-

axis, showing increasing trends with increasing treatment time. The final cumulative exposure

value at 4 h was the highest for the empty container, while the cumulative value for the fruit-loaded

container decreased with increasing loading factor (Figs. 2). It is worth mentioning that at 24 h, a

further decrease in EF headspace concentration was observed in all containers, reaching 0.28 ±

0.03, 0.04 ± 0.02, 0.00 ± 0.01, 0.00 ± 0.01, 0.00 ± 0.00 mg/mg.L for 0, 3, 6, 20, and 30 % w/v

loading factor, respectively. The depletion of EF in the headspace can be attributed to the

absorption by the fruits, permeability loss through the PET containers, and hydrolytic degradation.

In the presence of moisture, EF is susceptible to hydrolysis, producing formic acid and ethanol,

both of which exhibit antimicrobial properties (Ryan & Bishop, 2003). The negative correlation

between EF sorption and fruit loading factor was consistent with observations reported by other

researchers on EF treatment for apple and banana (Agarwal et al., 2015; Park et al., 2020).

Since the headspace concentration of EF in the container varied considerably with time, the

effect of fruit loading factor was evaluated based on cumulative exposure to better reflect the extent

of fumigation treatment. As shown in Fig. 7.3, linear relations (R2 > 0.90) were observed between

cumulative exposure and fruit loading factor:

At 2 h:

Cumulative EF exposure (mg.h/mg.L) = - 0.0234 × fruit load factor (%, w/v) + 1.094 (Eq. 7.2)

At 4 h:

Cumulative EF exposure (mg.h/mg.L) = - 0.0052 × fruit load factor (%, w/v) + 0.4206 (Eq. 7.3)

These equations could be used to predict the cumulative EF exposure corresponding to any fruit

loading factor (0 – 30 %, w/v) at 2 and 4 h of EF treatment. Furthermore, the concentration and

171

exposure profiles (Fig. 7.2) could be used to estimate the headspace concentration and cumulative

fumigant exposure at any given applied dosage. For example, if the quantity of EFP in the EF

capsule increases from 1.32 to 2.64 mg (Table 7.1), the concentration and exposure levels are

expected to double in the same volume of the headspace air.

Fig. 7.2: Headspace concentration of ethyl formate (EF) (mg/mg.L; milligram of EF vapor per

milligram of EFP per liter of headspace air) and cumulative EF exposure (concentration (mg/mg.L)

× time (h) product) in PET containers with blueberries at 0 [A], 3 [B], 6 [C], 20 [D], and 30 %

(w/v) [E] loading factor. EF capsules containing 1.32 mg of EFP were used (Section 7.3.3).

172

Fig. 7.3: Effect of fruit loading factor (%, w/v) on the cumulative ethyl formate (EF) exposure

(mg.h/mg.L) at 2 and 4h.

7.4.2 Mortality of spotted-wing drosophila adults exposed to ethyl formate vapor

The toxicity of EF vapor on SWD adults was evaluated over a dosage range of 0 – 6 mg/L

(Table 7.1). EF vapor, exposure time, and fruit loading factor had significant (p < 0.05) effects on

the mortality of SWD adults (Fig. 7.4). Toxicity of higher than > 95 % was achieved after the

SWD adults were exposed to 6 mg/L EF for 2 h or 4 mg/L EF for 4 h (Fig. 7.4 A and B). The EF

fumigant toxicity to SWD decreased with increasing fruit loading factor from 0 to 6 % (w/v). On

the other hand, the toxicity increased when loading factor increased to 20 and 30 % (w/v). The

different trends observed here could be due to the solubility of EF and its degradation in

blueberries, thereby lowering the available EF concentration in the headspace (Fig. 7.2) and

resulting in reduced SWD mortality in containers loaded with 3 and 6 % (w/v) fruit levels (Fig.

7.4 A and B). However, at elevated fruit loading factor, at 20 and 30 %, (w/v) levels, the decreased

headspace in the containers elevated the EF concentration at the same level of EF dosage, thereby

173

inflicting higher mortality to the insect (Fig. 7.4 C and D). Since solubility and degradation of EF

in fruits increased with increasing both fruit loading factor and exposure time, SWD adult mortality

enhanced in low fruit loading containers when increasing exposure time to 4 h as compared to high

fruit loading containers. For example, when exposed to 2 mg/L EF for 2 h, SWD adult mortality

was 38, 16, 10, 42, and 70 % for containers with 0, 3, 6, 20, and 30 % (w/v) berries, while at 4 h,

mortality reached 84, 74, 68, 70, 86 % for the same fruit load factors, respectively. Additionally,

comparable toxicity was obtained following the exposure of SWD adults at an equivalent

cumulative EF exposure. As shown in Fig. 7.5, the level of SWD mortality at 2 mg/L EF for 2 h

(i.e., 2×2) was close to that at 1 mg/L EF for 4 h (i.e., 1×4). Similarly, 4×2 and 2×4 treatments

resulted in comparable mortality levels at various fruit loading factors. The cumulative EF

exposure values tended to be higher for low-concentration/long-time treatments (i.e., 1×4 and 2×4)

than those of high-concentration/short-time treatments (i.e., 2×2 and 4×2) at low fruit loading

factors (0 – 6 %, w/v). On the other hand, an opposite trend was observed at higher loading factors

(20 and 30 %, w/v) (Fig. 7.5). This explained the slight differences in SWD adult mortality between

low-concentration/long-time and high-concentration/short-time treatments at the same fruit

loading factor.

The toxicity of EF and its modes of action can be ascribed to it being rapidly metabolized

to formic acid in the stored product pest, in which formic acid is capable of inhibiting

mitochondrial cytochrome c oxidase (Nicholls, 1975; Petersen, 1977). Haritos and Dojchinov

(2003) reported that the formic acid formed via the hydrolysis of EF in beetle (Sitophilus oryzae)

tissues was an effective cytochrome c oxidase inhibitor at 10 –100 mM levels (Haritos &

Dojchinov, 2003). Similar observations were reported by Song and Scharf (2008) for two

174

Drosophila strains (Canton-S and Hikone-R), where formic acid was both a dominant hydrolytic

metabolite of EF, and that it was neurologically active at physiologically relevant concentrations.

Also, formic acid (5 mM) resulted in significant neuroexcitatory effects on the nervous system of

house fly (Musca domestica) larvae (Song & Scharf, 2008). Previous studies have shown the

insecticidal toxicity of EF against a wide range of insect pest in various crops and fresh produce

(Bessi et al., 2016; Park et al., 2020; Simpson et al., 2004; Simpson et al., 2007; Smit et al., 2020;

Stewart & Mon, 1984; Xin et al., 2008). For example, Simpson et al. (2004) reported that EF

fumigation of strawberries (15 %, w/v, load factor) with doses ranging from 7 to 30 mg/L for 2 h

were effective in causing varying amounts of mortality in western flower thrips (Frankliniella

occidentalis) and two-spotted spider mites (Tetranychus urticae). Complete control of western

flower thrips was achieved at 22 mg/L for 1 h, while two-spotted spider mites were less susceptible

to EF, resulting in 66 % mortality at the same dose. Smit et al. (2020) assessed the toxicity of EF

against grain chinch bug, Macchiademus diplopterus, in stone and pome fruit using EF

concentrations of 15 – 50 mg/L. A minimum concentration of 50 mg/L EF for 1 h was required to

obtain 100% mortality of the grain chinch bug. Recently, Kwon et al. (2021) found that EF vapor

released from liquid EF injected on filter paper was able to completely control SWD adults using

5 mg/L EF for 4 h at 21 °C (Kwon et al., 2021). Their results were close to that observed in our

study for empty containers (Fig. 7.4); however, we used EFP as a stable fumigation system to

control the release of EF vapor.

175

Fig. 7.4: Mortality of spotted-wing drosophila (SWD) adults at 22 ± 1 °C as affected by EF

dosage and fruit loading factor: At 2 h [A] and 4 h [B] for 0 – 6 % (w/v) load factor, and at 2 h

[C] and 4 h [D] for 20 – 30 % (w/v) load factor. Different alphabets (a–k) indicate statistical

significant difference (p < 0.05).

176

Fig. 7.5: Mortality of spotted-wing drosophila (SWD) adults after exposure to similar cumulative

ethyl formate (EF) exposures (mg.h/L) for load factors 0 – 30 % (w/v). Cumulative EF exposure

values (concentration “C” × time “T” products) were generated from Figs. 7.2 and 7.3 for the

different dosages (Table 7.1).

7.4.3 Mortality of spotted-wing drosophila eggs, larvae, and pupae exposed to

ethyl formate vapor

Five doses of EF (2, 4, 6, 10 and 16 mg/L; Table 7.1) were tested against SWD eggs, larvae,

and pupae in blueberries, for 24 h at 22 ± 1°C, to evaluate their toxicity effects on larval, pupal,

and adult emergences, respectively (Fig. 7.6). Compared to the untreated control, the EF

concentration had a significant (p < 0.05) effect on larvae, pupae, and adult emergence in

blueberries. Complete control of SWD eggs, larvae, and pupae was achieved when infested

blueberries were exposed to 16 mg/L EF for 24 h. At EF dose of 10 mg/L, larvae, pupae, and adults

emerged were 21.4, 1.6, and 11.7 %, respectively. Lower EF concentrations resulted in higher

emergences. There were no significant (p > 0.05) differences between the emergence of larvae,

177

pupae, and adults. These results indicated that SWD eggs, larvae, and pupae were more tolerant to

EF vapor than SWD adults, with eggs being the most tolerant life stage at high concentration (Figs.

7.4 and 7.6). Similarly, Kwon et al. (2021) reported that SWD eggs were the most tolerant life

stage to EF fumigation and that treatment at 48 mg/L for 4 h at 21°C was effective for complete

control of SWD eggs in blueberries. Also, Walse et al. (2012) observed that SWD eggs and larvae

were the most methyl bromide tolerant when found in strawberries. These results were consistent

with studies conducted on other insect species (Bessi et al., 2016; Jamieson et al., 2015; Park et

al., 2020; Ren & Mahon, 2006).

Fig. 7.6: Spotted-wing drosophila larvae, pupae, and adults emerged from eggs, larvae, and

pupae, respectively, in blueberries exposed to ethyl formate (EF) vapor at 22 ± 1°C for a 24 h

period. Different alphabets (a–e) indicate statistical significant difference (p < 0.05).

7.4.4 Effect of ethyl formate fumigation on fruit quality

The effects of EF fumigation on firmness, TSS, pH, TA, and volatile content are

summarized in Tables 7.2 and 7.3. Before the EF fumigation treatment, firmness, TSS, pH, and

178

TA values were 7.28 ± 0.56 N, 13.71 ± 0.41 %, 2.38 ± 0.02, 0.76 ± 0.01 %, respectively. There

were no significant (p > 0.05) differences in fruit quality parameters between EF treated and

untreated berries (Table 7.2). Fermentative volatiles in blueberries before treatment were 44 ± 3.21

%, and 0.68 ± 0.14 mg/kg for ethanol and EF, respectively. Exposing blueberries to EF vapor for

24 h at elevated doses without aeration significantly (p < 0.05) increased ethanol and EF content

in berries, reaching 109.45 ± 6.48 and 9.49 ± 0.73 mg/kg, respectively, at EF dosage of 16 mg/L

(Table 7.3). Samples treated for 24 h were then aerated for 24 h (48 h post-treatment). After

aeration, volatile concentrations in blueberries decreased to the levels similar to that of the

untreated samples, except for 16 mg/L EF dose where ethanol and EF decreased to 76.27 ± 4.32

and 1.04 ± 0.29 mg/kg, respectively. Simpson et al. (2004) reported no significant differences in

strawberry conditions between treated and untreated fruit after EF fumigation at dosages up to 22

mg/L. However, increased levels of ethanol, acetaldehyde, ethyl acetate, and EF were detected in

EF treated fruit (Simpson et al., 2004). Table grape exposed to 45 mg/L EF for 1 or 2 h was found

to be well tolerated except for increased rachis browning; however, after 2 d storage, the rachis of

untreated fruit was similarly browned (Simpson et al., 2007). Agarwal et al. (2015) observed no

morphological changes in color, texture, and firmness in apples treated with 5 – 80 mg/L EF for 1

– 4 d compared to untreated samples even after 1, 2, and 3 weeks of treatment. They noticed higher

EF residues in treated apples. The residue of EF declined to natural levels after one day of aeration.

No internal or external phytotoxic damage was observed on various pome and stone fruit cultivars

after treatment with a dose range of 50 –150 mg/L EF for 1 h (Smit et al., 2020). When blueberries

were continuously stored at 5 °C for 14 d after exposing to 70 mg/L EF for 4 h, no significant

179

differences in blueberries appearance (i.e., soft spot or berry shrivel) were observed between EF

treated and untreated samples (Kwon et al., 2021).

180

Table 7.2: Effect of exposure to ethyl formate (EF) vapor at different doses at 22 ± 1 °C on fruit quality parameters. Firmness (N),

total soluble solids (TSS, %), pH, and titratable acidity (TA, %) were measured after 24 h of exposure to EF vapor, and after another

24 h of aeration (48 h post-treatment).

EF dosage

(mg/L)

Firmness (N) TSS (%) pH TA (%)

24 h 48 h 24 h 48 h 24 h 48 h 24 h 48 h

0 6.22 ± 0.68 5.93 ± 0.59 14.02 ± 0.64 14.50 ± 0.54 2.63 ± 0.11 2.59 ± 0.09 0.66 ± 0.07 0.64 ± 0.08

2 7.06 ± 1.67 5.06 ± 0.85 14.28 ± 0.16 14.37 ± 0.75 2.53 ± 0.05 2.68 ± 0.06 0.66 ± 0.05 0.53 ± 0.04

4 5.84 ± 0.61 5.45 ± 0.83 14.23 ± 0.39 14.48 ± 0.31 2.37 ± 0.03 2.56 ± 0.08 0.95 ± 0.01 0.61 ± 0.03

6 6.55 ± 0.74 5.82 ± 0.69 13.55 ± 0.21 14.52 ± 0.30 2.44 ± 0.04 2.35 ± 0.02 0.83 ± 0.04 0.91 ± 0.07

10 6.79 ± 0.64 5.43 ± 0.94 13.10 ± 0.33 14.64 ± 0.17 2.51 ± 0.06 2.60 ± 0.02 0.70 ± 0.08 0.69 ± 0.02

16 5.09 ± 0.44 5.68 ± 0.88 12.65 ± 0.32 13.35 ± 0.34 2.31 ± 0.02 2.46 ± 0.03 1.09 ± 0.07 0.78 ± 0.02

Table 7.3: Effect of exposure to ethyl formate (EF) vapor at different doses at 22 ± 1°C on fruit volatiles content (i.e., ethanol and EF,

mg/kg). Volatiles were measured after 24 h of exposure to EF vapor, and after another 24 h of aeration (48 h post-treatment).

EF dosage

(mg/L)

Ethanol content (mg/kg) EF content (mg/kg)

24 h 48 h 24 h 48 h

0 47.41 ± 3.04 51.21 ± 8.82 0.74 ± 0.11 0.70 ± 0.42

2 46.55 ± 7.24 53.22 ± 3.54 0.95 ± 0.07 0.72 ± .027

4 52.83 ± 7.58 47.31 ± 4.89 1.17 ± 0.18 0.75 ± 0.05

6 57.63 ± 3.27 51.64 ± 9.05 2.38 ± 0.13 0.75 ± 0.10

10 76.52 ± 5.71 53.92 ± 6.68 4.81 ± 0.54 0.78 ± 0.21

16 109.49 ± 6.48 76.27 ± 4.32 9.45 ± 0.73 1.04 ± 0.29

181

7.5 Conclusion

In this study, EFP was evaluated for controlled release of EF vapor for in-package fumigation of

blueberries, through a capsule delivery system. To activate the release of EF vapor, 0.1 N citric

acid solution was added to the capsules. Fumigation of blueberries was conducted at 22 ± 1°C in

commercial thermoformed PET containers (0.5 L) with different loading factors 0 – 30 % (w/v)

for 2 and 4 h. A negative correlation was observed between the cumulative EF exposure and the

fruit loading factor in the containers. EF vapor controlled all life stages of SWD in blueberries –

eggs, larvae, pupae, and adults. Complete control of SWD adult was observed after the exposure

to 6 mg/L EF for 4 h at 3 and 6 % (w/v) fruit loading factors, while 4 mg/L for 2 h was sufficient

to control SWD adult at 30 % loading factor. There were no significant (p > 0.05) differences

between larval, pupal, and adult emergences after the exposure to 2 – 16 mg/L EF for 24 h.

Treatment at 16 mg/L for 24 h was effective to achieve complete control of SWD eggs, larvae,

pupae. These results indicated that SWD eggs, larvae, and pupae were more tolerant toward EF

vapor than SWD adults. No significant (p > 0.05) differences in blueberries quality parameters

(i.e., firmness, TSS, pH, and TA) were observed between EF treated and untreated berries.

However, high levels of ethanol and EF were detected in treated berries. These levels decreased to

natural levels after 24 h of aeration. The results from this study indicated that the EFP compound

could be used safely in AP applications of fresh produce to destroy insect pests.

182

Chapter 8: Conclusion and future work

8.1 Overall conclusion

In this thesis, an ethyl formate precursor (EFP) was developed to address the high vapor

pressure and chemical stability issues of the volatile ethyl formate (EF). The precursor was

synthesized through the condensation reaction between adipic acid dihydrazide and triethyl

orthoformate. FTIR and solid-state 13C NMR spectroscopies confirmed the molecular structure of

the synthesized precursor – diethyl N,N'-adipoyldiformohydrazonate. It was a stable compound

and had a melting temperature of 174°C. EFP consisted of aggregated particles with irregular

shapes and sizes, having a broad particle size distribution with two peaks at 467 and 1796 nm. The

release of EF could be effectively triggered through the hydrolysis of EFP under mid acidic

conditions (e.g., using 0.1 N citric acid (CA)). Increasing temperature resulted in a higher EF

release rate, with up to 98 % of the theoretical available EF being released at 25°C. The precursor

remained stable during storage under 0 % RH for up to 30 d, thus it should be stored at dry

environmental conditions to ensure maximal EF release during end-use application.

Additionally, EFP was encapsulated in ethylcellulose/poly(ethylene oxide) (EC-PEO)

electrospun fibers using free surface electrospinning. First, the polymer concentrations were

optimized to obtained bead-free electrospun fibers. The electrospinning of neat 10 % (w/w) EC,

when dissolved in 90 % aqueous ethanol, resulted in irregular particulates interweaved with

ultrafine fibers. The addition of 1 % PEO in EC solution enhanced its spinnability and produced

bead-free fibers. These polymer concentrations were chosen for the encapsulation of EFP particles

at elevated concentrations. Although some of EFP particles were larger than the electrospun fibers,

183

they were physically entrapped and uniformly dispersed within the EC-PEO nonwovens. FTIR

analysis did not reveal any specific interactions between IR-active covalent bonds in EFP, EC and

PEO. When the EFP-nonwoven exposed to 0.1 N CA, up to 96 % of the available theoretical EF

was released after 2 h at 25°C. Decreasing the acid concentration caused a decline in EF release

rate, while increasing temperature resulted in a higher EF release rate. The EF vapor released from

the EFP-loaded nonwovens delayed spoilage and extended the shelf-life of strawberries for up to

10 d.

Furthermore, EFP was used as an EF activated release system for in-packaging fumigation of

blueberries. A capsule made of PET blister was used as a delivery system of EFP powder. To

activate the release of EF vapor, 0.1 N CA solution was added to the capsules. Different quantities

of EFP were used to achieve different concentrations of EF vapor inside the test containers. There

was a negative correlation between the EF concentration/cumulative EF exposure and the loading

ratio of blueberries in the containers. EF vapor was able to control all life stages of spotted-wing

drosophila (SWD) in blueberries. The results indicated that SWD eggs, larvae, and pupae were

more tolerant toward EF vapor than SWD adults. No significant (p < 0.05) differences in

blueberries quality were observed between EF treated and untreated berries.

The results from this thesis indicated that both EFP powder and EFP-loaded nonwovens could

be used safely in active packaging applications of fresh produce to mitigate insect pest problems

and inhibit the proliferation of spoilage/pathogenic microorganisms. The envisaged innovative

packaging-based fumigation system will drastically simplify the existing fumigation approach

involving compressed gas cylinders that are inconvenient to transport and bulky for storage. This

innovative approach could have a significant competitive edge against the existing treatments.

184

8.2 Future works

Based on research observations and results of this study, recommendations for future work are

outlined as follows:

• Since a facile synthesis method to produce EFP was developed in this study, through

the condensation reaction between adipic acid dihydrazide with triethyl orthoformate,

further research is needed to investigate the possibility of using other

hydrazines/hydrazides and ortho esters for the formation of precursors for various

active esters (e.g., methyl formate and ethyl acetate). [Currently ongoing]

• The substrate used for EFP synthesis (i.e., adipic acid dihydrazide) is not a food-grade

compound. After the hydrolysis of the precursor, the substrate may migrate and pose a

risk of food contamination. Although the encapsulation of EFP compound could limit

the mass transport phenomenon, exploring the use of non-migratable, non-toxic, and

food-grade substrates (e.g., polymers with hydrazide or amine moieties such as

polyethylenimine) is needed.

• In this study, EFP was encapsulated into electrospun nonwovens and the release of

EF was activated by distributing a CA solution on the EFP-loaded nonwovens.

Although a high release rate/amount of EF vapor was achieved, it presents some end-

use challenging for active packaging applications. One promising approach is via

encapsulating both EFP and the acid into bilayer nonwovens. During the end-use

applications, the moisture in the fruit/vegetable headspace air diffuses through the

nonwoven, solubilizes the acid, and forms a free acidic solution which when diffuses

185

into the precursor layer, hydrolyzes the latter, and thereby triggering the release of EF

vapor. [Currently ongoing]

• Since beneficial synergistic effects have been reported in the literature when EF is

mixed with CO2 in compressed gas cylinders, at 16.7 % wt., for fumigation of fruits,

vegetables, and grains (commercially available product as Vapormate® by Linde

Group), the encapsulation of EFP, sodium bicarbonate, and citric acid into multilayer

nonwovens may result in a safer and easier system than other commercially available

delivery systems. During application, the absorption of moisture by the citric acid

forms a free acidic solution which diffuses through the nonwovens, hydrolyzing the

precursor and reacting with sodium bicarbonate to trigger a simultaneous release of

both EF vapor and CO2. The EF/CO2 ratio can be manipulated by controlling the

concentrations of EFP, sodium bicarbonate, and citric acid in the nonwovens.

[Currently ongoing]

• One problem with the formation of polymer-based films containing EFP and its acid

activator is that the acid will actively react with EFP, leading to a premature release of

EF. This problem could be solved by incorporating acid anhydrides (e.g., benzoic

anhydride and glucono delta-lactone), instead of an acid in the films. The anhydrides

can be hydrolyzed to form an acid under chemical equilibrium upon dissolution in

water. When this acid comes into contact with EFP encapsulated in the same film, it

can then triggers the release of EF vapor. [Currently ongoing]

186

• The retention of EF vapor inside the package headspace would determine its

insecticidal/antimicrobial effectiveness. Permeation of EF vapor through packaging

materials as well as its degradation would reduce its efficacy. Further research effort is

needed to develop a better understanding of the effects of packaging materials and

storage conditions on the effectiveness of the developed EF activated release systems.

• Further studies are needed to develop functional prototypes for EFP integrated with its

activation mechanism and investigate the effect of EF vapor on mortality of various

insect species, inhibition of spoilage microorganisms, and fruit quality during storage

at temperatures recommended to maintain the cold chain. Optimization of EF doses at

low temperatures will be required, since the release of EF vapor from its precursor is

expected to be temperature-dependent.

187

Chapter 9: References

Ackley, M. W., Rege, S. U., & Saxena, H. (2003). Application of natural zeolites in the

purification and separation of gases. Microporous and Mesoporous Materials, 61(1–3), 25–

42. https://doi.org/10.1016/S1387-1811(03)00353-6

Agarwal, M., Ren, Y., Newman, J., & Learmonth, S. (2015). Ethyl Formate: A Potential

Disinfestation Treatment for Eucalyptus Weevil (Gonipterus platensis) (Coleoptera:

Curculionidae) in Apples. Journal of Economic Entomology, 108(6), 2566–2571.

https://doi.org/10.1093/jee/tov242

AgroFresh. (n.d.). Expanding your potential with a growing lineup of technologies. Retrieved

January 20, 2021, from https://www.agrofresh.com/technologies/

Aguirre, A., Borneo, R., & Leon, A. E. (2013). Antimicrobial, mechanical and barrier properties

of triticale protein films incorporated with oregano essential oil. Food Bioscience, 1, 2–9.

https://doi.org/10.1016/j.fbio.2012.12.001

Ahenkorah, C. K., Zaitoon, A., Apalangya, V. A., Afrane, G., & Lim, L. T. (2020). Moisture-

activated release of hexanal from imidazolidine precursor encapsulated in

ethylcellulose/poly(ethylene oxide) nonwoven for shelf-life extension of papaya. Food

Packaging and Shelf Life, 25, 100532. https://doi.org/10.1016/j.fpsl.2020.100532

Ahmad, B., Stoyanov, S., Pelan, E., Stride, E., & Edirisinghe, M. (2013). Electrospinning of

ethyl cellulose fibres with glass and steel needle configurations. Food Research

International, 54(2), 1761–1772. https://doi.org/10.1016/j.foodres.2013.09.021

Ahn, Y., Hu, D. H., Hong, J. H., Lee, S. H., Kim, H. J., & Kim, H. (2012). Effect of co-solvent

on the spinnability and properties of electrospun cellulose nanofiber. Carbohydrate

Polymers, 89(2), 340–345. https://doi.org/10.1016/j.carbpol.2012.03.006

Ainsworth, C. (1955). The Condensation of Aryl Carboxylic Acid Hydrazides with Orthoesters.

Journal of the American Chemical Society, 77(5), 1148–1150.

https://doi.org/10.1021/ja01610a019

Ainsworth, C. (1965). 1,3,4-Oxadiazole. Journal of the American Chemical Society, 87(24),

5800–5801. https://doi.org/10.1021/ja00952a056

Ainsworth, C., & Hackler, R. E. (1966). Alkyl-1,3,4-oxadiazoles. 31(10), 3442–3444.

Al-Muhtaseb, S. A. (2010). Adsorption and desorption equilibria of nitrogen, methane, ethane,

and ethylene on date-pit activated carbon. Journal of Chemical and Engineering Data,

55(1), 313–319. https://doi.org/10.1021/je900350k

188

Alborzi, S., Lim, L. T., & Kakuda, Y. (2010). Electrospinning of sodium alginate-pectin ultrafine

fibers. Journal of Food Science, 75(1), 100–107. https://doi.org/10.1111/j.1750-

3841.2009.01437.x

Alborzi, S., Lim, L. T., & Kakuda, Y. (2013). Encapsulation of folic acid and its stability in

sodium alginate-pectin-poly(ethylene oxide) electrospun fibres. Journal of

Microencapsulation, 30(1), 64–71. https://doi.org/10.3109/02652048.2012.696153

Alkan Tas, B., Sehit, E., Erdinc Tas, C., Unal, S., Cebeci, F. C., Menceloglu, Y. Z., & Unal, H.

(2019). Carvacrol loaded halloysite coatings for antimicrobial food packaging applications.

Food Packaging and Shelf Life, 20, 100300. https://doi.org/10.1016/j.fpsl.2019.01.004

Almasi, H., Jahanbakhsh Oskouie, M., & Saleh, A. (2020). A review on techniques utilized for

design of controlled release food active packaging. In Critical Reviews in Food Science and

Nutrition (pp. 1–21). https://doi.org/10.1080/10408398.2020.1783199

Almenar, E., Auras, R., Wharton, P., Rubino, M., & Harte, B. (2007). Release of acetaldehyde

from β-cyclodextrins inhibits postharvest decay fungi in vitro. Journal of Agricultural and

Food Chemistry, 55(17), 7205–7212. https://doi.org/10.1021/jf071603y

Altan, A., Aytac, Z., & Uyar, T. (2018). Carvacrol loaded electrospun fibrous films from zein

and poly(lactic acid) for active food packaging. Food Hydrocolloids, 81, 48–59.

https://doi.org/10.1016/j.foodhyd.2018.02.028

Álvarez, C. M., Restrepo-Uribe, L., López, J. A., Estrada, O. A., & Noriega, M. D. P. (2019).

Improvement of stability and release of (-)-epicatechin by hot melt extrusion. Journal of

Polymer Engineering, 40(1), 75–85. https://doi.org/10.1515/polyeng-2019-0140

American-Ripener. (n.d.). Ethylene Generators. Retrieved January 20, 2021, from

https://www.ripening.com/ethylene-generators/ethylene-generators.php

Andrade, B., Song, Z., Li, J., Zimmerman, S. C., Cheng, J., Moore, J. S., Harris, K., & Katz, J. S.

(2015). New frontiers for encapsulation in the chemical industry. ACS Applied Materials

and Interfaces, 7(12), 6359–6368. https://doi.org/10.1021/acsami.5b00484

Andrade, J., González-Martínez, C., & Chiralt, A. (2020). Effect of carvacrol in the properties of

films based on poly (vinyl alcohol) with different molecular characteristics. Polymer

Degradation and Stability, 179, 109282.

https://doi.org/10.1016/j.polymdegradstab.2020.109282

Antunes, M. D. C., Dandlen, S., Cavaco, A. M., & Miguel, G. (2010). Effects of postharvest

application of 1-MCP and postcutting dip treatment on the quality and nutritional properties

of fresh-cut kiwifruit. Journal of Agricultural and Food Chemistry, 58(10), 6173–6181.

https://doi.org/10.1021/jf904540m

Anu Bhushani, J., & Anandharamakrishnan, C. (2014). Electrospinning and electrospraying

189

techniques: Potential food based applications. Trends in Food Science and Technology,

38(1), 21–33. https://doi.org/10.1016/j.tifs.2014.03.004

Appendini, P., & Hotchkiss, J. H. (2002). Review of antimicrobial food packaging. Innovative

Food Science and Emerging Technologies, 3(2), 113–126. https://doi.org/10.1016/S1466-

8564(02)00012-7

Araújo, E. S., Libardi, J., Faia, P. M., & De Oliveira, H. P. (2015). Hybrid ZnO/TiOLoaded in

Electrospun Polymeric Fibers as Photocatalyst. Journal of Chemistry, 2015.

https://doi.org/10.1155/2015/476472

Aray, Y., Marquez, M., Rodríguez, J., Vega, D., Simón-Manso, Y., Coll, S., Gonzalez, C., &

Weitz, D. A. (2004). Electrostatics for exploring the nature of the hydrogen bonding in

polyethylene oxide hydration. Journal of Physical Chemistry B, 108(7), 2418–2424.

https://doi.org/10.1021/jp036921o

Ariyanto, H. D., Chiba, M., Oguma, K., Tatsuki, M., & Yoshii, H. (2019). Release behavior of 1-

methylcylopropene coated paper-based shellac solution in response to stepwise humidity

changes to develop novel functional packaging for fruit. Packaging Technology and

Science, 32(10), 523–533. https://doi.org/10.1002/pts.2468

Ariyanto, H. D., & Yoshii, H. (2019). Effect of stepwise humidity change on the release rate

constant of 1-methylcyclopropene (1-MCP)in a cyclodextrin inclusion complex powder.

Food Packaging and Shelf Life, 21, 100322. https://doi.org/10.1016/j.fpsl.2019.100322

Asil, M. H., Karimi, M., & Zakizadeh, H. (2013). 1-MCP improves the postharvest quality of cut

spray carnation (Dianthus caryophyllus L.) “Optima” flowers. Horticulture Environment

and Biotechnology, 54(1), 58–62. https://doi.org/10.1007/s13580-013-0044-8

Asplen, M. K., Anfora, G., Biondi, A., Choi, D.-S., Chu, D., Daane, K. M., Zappalà, L., Jiang,

Z.-L., Ponti, L., Isaacs, R., Pascual, M., Kimura, M. T., Hutchison, W. D., Vogt, H., Vétek,

G., Gibert, P., Plantamp, C., Kárpáti, Z., Philips, C. R., … Hoelmer, K. A. (2015). Invasion

biology of spotted wing Drosophila (Drosophila suzukii): a global perspective and future

priorities. Journal of Pest Science, 88(3), 469–494.

Assadpour, E., & Jafari, S. M. (2019). Advances in Spray-Drying Encapsulation of Food

Bioactive Ingredients: From Microcapsules to Nanocapsules. Annual Review of Food

Science and Technology, 10, 103–131. https://doi.org/10.1146/annurev-food-032818-

121641

ASTM Standard E104-02. (2012). Standard Practice for Maintaining Constant Relative Humidity

by Means of Aqueous Solutions. Www.Astm.Org. https://doi.org/10.1520/E0104-02R12.2

Astray, G., Gonzalez-Barreiro, C., Mejuto, J. C., Rial-Otero, R., & Simal-Gándara, J. (2009). A

review on the use of cyclodextrins in foods. Food Hydrocolloids, 23(7), 1631–1640.


190

Aydogdu, A., Sumnu, G., & Sahin, S. (2019). Fabrication of gallic acid loaded Hydroxypropyl

methylcellulose nanofibers by electrospinning technique as active packaging material.

Carbohydrate Polymers, 241–250. https://doi.org/10.1016/j.carbpol.2018.12.065

Aytac, Z., Dogan, S. Y., Tekinay, T., & Uyar, T. (2014). Release and antibacterial activity of

allyl isothiocyanate/β-cyclodextrin complex encapsulated in electrospun nanofibers.

Colloids and Surfaces B: Biointerfaces, 120, 125–131.

https://doi.org/10.1016/j.colsurfb.2014.04.006

Aytac, Z., Ipek, S., Durgun, E., Tekinay, T., & Uyar, T. (2017). Antibacterial electrospun zein

nanofibrous web encapsulating thymol/cyclodextrin-inclusion complex for food packaging.

Food Chemistry, 233, 117–124. https://doi.org/10.1016/j.foodchem.2017.04.095

Babu, D. J., Lange, M., Cherkashinin, G., Issanin, A., Staudt, R., & Schneider, J. J. (2013). Gas

adsorption studies of CO2 and N2 in spatially aligned double-walled carbon nanotube

arrays. Carbon, 61, 616–623. https://doi.org/10.1016/j.carbon.2013.05.045

Bahmid, N. A., Pepping, L., Dekker, M., Fogliano, V., & Heising, J. (2020). Using particle size

and fat content to control the release of Allyl isothiocyanate from ground mustard seeds for

its application in antimicrobial packaging. Food Chemistry, 308, 12573.

https://doi.org/10.1016/j.foodchem.2019.125573

Bai, Z., Cristancho, D. E., Rachford, A. A., Reder, A. L., Williamson, A., & Grzesiak, A. L.

(2016). Controlled Release of Antimicrobial ClO2 Gas from a Two-Layer Polymeric Film

System. Journal of Agricultural and Food Chemistry, 64(45), 8647–8652.

https://doi.org/10.1021/acs.jafc.6b03875

Balsamo, M., Budinova, T., Erto, A., Lancia, A., Petrova, B., Petrov, N., & Tsyntsarski, B.

(2013). CO2 adsorption onto synthetic activated carbon: Kinetic, thermodynamic and

regeneration studies. Separation and Purification Technology, 116, 214–221.

https://doi.org/10.1016/j.seppur.2013.05.041

Barboza, F., Vecchia, D. D., Tagliari, M. P., Silva, M. A. S., & Stulzer, H. K. (2009).

Differential scanning calorimetry as a screening technique in compatibility studies of

acyclovir extended release formulations. Pharmaceutical Chemistry Journal, 43(6), 363.

https://doi.org/10.1007/s11094-009-0304-1

Barry, C. S., & Giovannoni, J. J. (2007). Ethylene and fruit ripening. Journal of Plant Growth

Regulation, 26(2), 143–159. https://doi.org/10.1007/s00344-007-9002-y

Bernardos, A., Bozik, M., Alvarez, S., Saskova, M., Perez-Esteve, E., Kloucek, P., Lhotka, M.,

Frankova, A., & Martinez-Manez, R. (2019). The efficacy of essential oil components

loaded into montmorillonite against Aspergillus niger and Staphylococcus aureus. Flavour

and Fragrance Journal, 34(3), 151–162. https://doi.org/10.1002/ffj.3488

Bernardos, A., Marina, T., Žáček, P., Pérez-Esteve, É., Martínez-Mañez, R., Lhotka, M.,

191

Kouřimská, L., Pulkrábek, J., & Klouček, P. (2015). Antifungal effect of essential oil

components against Aspergillus niger when loaded into silica mesoporous supports. Journal

of the Science of Food and Agriculture, 95(14), 2824–2831.

https://doi.org/10.1002/jsfa.7022

Berton, P., Shamshina, J. L., Bica, K., & Rogers, R. D. (2018). Ionic Liquids as Fragrance

Precursors: Smart Delivery Systems for Volatile Compounds. Industrial and Engineering

Chemistry Research, 57(47), 16069–16076. https://doi.org/10.1021/acs.iecr.8b02903

Bessi, H., Ferchichi, C., Yousfi, S., Guido, F., Issaoui, M., Bikoba, V., Mitcham, E., Grissa, K.,

& Bellagha, S. (2016). Determining Effect of Ethyl Formate and Vapormate® on

Disinfestation Efficiency and Organoleptic Quality of Date Fruits. Tunisian Journal of

Plant Protection, 11, 51–62.

Bessi, Haithem, Bellagha, S., Lebdi, K. G., Bikoba, V., & Mitcham, E. J. (2015). Ethyl Formate

Fumigation of Dry and Semidry Date Fruits: Experimental Kinetics, Modeling, and Lethal

Effect on Carob Moth. Journal of Economic Entomology, 108(3), 993–999.

https://doi.org/10.1093/jee/tov032

Bhardwaj, N., & Kundu, S. C. (2010). Electrospinning: A fascinating fiber fabrication technique.

Biotechnology Advances, 28(3), 325–347. https://doi.org/10.1016/j.biotechadv.2010.01.004

Biddeci, G., Cavallaro, G., Di Blasi, F., Lazzara, G., Massaro, M., Milioto, S., Parisi, F., Riela,

S., & Spinelli, G. (2016). Halloysite nanotubes loaded with peppermint essential oil as filler

for functional biopolymer film. Carbohydrate Polymers, 152, 548–557.

https://doi.org/10.1016/j.carbpol.2016.07.041

Biji, K. B., Ravishankar, C. N., Mohan, C. O., & Srinivasa Gopal, T. K. (2015). Smart packaging

systems for food applications: a review. In Journal of Food Science and Technology.

https://doi.org/10.1007/s13197-015-1766-7

Biondi, A., Traugott, M., & Desneux, N. (2016). Special issue on Drosophila suzukii: from

global invasion to sustainable control. In Journal of Pest Science (pp. 603–604).

https://doi.org/10.1007/s10340-016-0787-y

BIOPAC. (n.d.). SO2 Pad Grape. Retrieved January 20, 2021, from

https://www.biopac.com.au/so2-pad-table-grapes-lychees-matesa/

Bleoanca, I., Enachi, E., & Borda, D. (2020). Thyme antimicrobial effect in edible films with

high pressure thermally treated whey protein concentrate. Foods, 9(7), 855.

https://doi.org/10.3390/foods9070855

Bolin, H., King Jr, A., Stanley, W., & Jurd, L. (1972). Antimicrobial Protection of Moisturized

Deglet Noor Dates. Applied and Environmental Microbiology, 23(4), 799–802.

Bordi, F., Cametti, C., & Di Biasio, A. (1988). Electrical conductivity behavior of poly(ethylene

192

oxide) in aqueous electrolyte solutions. Journal of Physical Chemistry, 92(16), 4772–4777.

https://doi.org/10.1021/j100327a042

Brannock, K. C., Bell, A., Burpitt, R. D., & Kelly, C. A. (1964). Enamine Chemistry. IV.

Cycloaddition Reactions of Enamines Derived from Aldehydes and Acyclic Ketones.

Journal of Organic Chemistry, 29(4), 801–812. https://doi.org/10.1021/jo01027a009

Brettmann, B. K., Tsang, S., Forward, K. M., Rutledge, G. C., Myerson, A. S., & Trout, B. L.

(2012). Free surface electrospinning of fibers containing microparticles. Langmuir, 28(25),

259714–259721. https://doi.org/10.1021/la301422x

Britt, D., Furukawa, H., Wang, B., Glover, T. G., & Yaghi, O. M. (2009). Highly efficient

separation of carbon dioxide by a metal-organic framework replete with open metal sites.

Proceedings of the National Academy of Sciences of the United States of America, 106(49),

20637–20640. https://doi.org/10.1073/pnas.0909718106

Buchs, B., Fieber, W., Drahoňovský, D., Lehn, J. M., & Herrmann, A. (2012). Stabilized

hemiacetal complexes as precursors for the controlled release of bioactive volatile alcohols.

Chemistry and Biodiversity, 9(4), 689–701. https://doi.org/10.1002/cbdv.201100383

Buchsnée Levrand, B., Godin, G., Trachsel, A., De Saint Laumer, J. Y., Lehn, J. M., &

Herrmann, A. (2011). Reversible aminal formation: Controlling the evaporation of bioactive

volatiles by dynamic combinatorial/covalent chemistry. European Journal of Organic

Chemistry, 681–695. https://doi.org/10.1002/ejoc.201001433

Burt, S. (2004). Essential oils: Their antibacterial properties and potential applications in foods -

A review. International Journal of Food Microbiology, 94(3), 223–253.

https://doi.org/10.1016/j.ijfoodmicro.2004.03.022

Campos-Requena, V. H., Rivas, B. L., Pérez, M. A., Garrido-Miranda, K. A., & Pereira, E. D.

(2018). Release of essential oil constituent from thermoplastic starch/layered silicate

bionanocomposite film as a potential active packaging material. European Polymer Journal,

109, 64–71. https://doi.org/10.1016/j.eurpolymj.2018.08.055

Canales, D., Montoille, L., Rivas, L. M., Andrés Ortiz, J., Yañez-S, M., Rabagliati, F. M., Ulloa,

M. T., Alvarez, E., & Zapata, P. A. (2019). Fungicides films of low-density polyethylene

(LDPE)/inclusion complexes (Carvacrol and Cinnamaldehyde) against botrytis cinerea.

Coatings, 9(12), 795. https://doi.org/10.3390/coatings9120795

Cao, S., Yang, Z., & Zheng, Y. (2012). Effect of 1-methylcyclopene on senescence and quality

maintenance of green bell pepper fruit during storage at 20°C. Postharvest Biology and

Technology, 70, 1–6. https://doi.org/10.1016/j.postharvbio.2012.03.005

Capozzi, L. C., Bazzano, M., Cavallero, M. C., Barolo, C., Buscaino, R., Ferri, A., Sangermano,

M., Vallauri, D., & Pisano, R. (2016). Polymeric supports for controlled release of ethylene

for food industry. International Polymer Processing, 31(5), 570–576.

193

https://doi.org/10.3139/217.3233

Castro Coelho, S., Nogueiro Estevinho, B., & Rocha, F. (2021). Encapsulation in food industry

with emerging electrohydrodynamic techniques: Electrospinning and electrospraying – A

review. In Food Chemistry (p. 127850). https://doi.org/10.1016/j.foodchem.2020.127850

Catalytic-Generators. (n.d.). EASY-RIPE® ethylene generator. Retrieved January 20, 2021, from

https://www.catalyticgenerators.com/products/easy-ripe-ethylene-generator/

Cavenati, S., Grande, C. A., & Rodrigues, A. E. (2004). Adsorption Equilibrium of Methane,

Carbon Dioxide, and Nitrogen on Zeolite 13X at High Pressures. Journal of Chemical and

Engineering Data, 49(4), 1095–1101. https://doi.org/10.1021/je0498917

Celebioglu, A., Yildiz, Z. I., & Uyar, T. (2018a). Fabrication of Electrospun

Eugenol/Cyclodextrin Inclusion Complex Nanofibrous Webs for Enhanced Antioxidant

Property, Water Solubility, and High Temperature Stability. Journal of Agricultural and

Food Chemistry, 66(2), 457–466. https://doi.org/10.1021/acs.jafc.7b04312

Celebioglu, A., Yildiz, Z. I., & Uyar, T. (2018b). Thymol/cyclodextrin inclusion complex

nanofibrous webs: Enhanced water solubility, high thermal stability and antioxidant

property of thymol. Food Research International, 106, 280=290.

https://doi.org/10.1016/j.foodres.2017.12.062

CELLCOMB. (n.d.). Cellsorb – Active. Retrieved January 20, 2021, from

http://www.cellcomb.com/en/food-absorbers-2/co2-pad/

Cerqueira, M. A., Fabra, M. J., Castro-Mayorga, J. L., Bourbon, A. I., Pastrana, L. M., Vicente,

A. A., & Lagaron, J. M. (2016). Use of Electrospinning to Develop Antimicrobial

Biodegradable Multilayer Systems: Encapsulation of Cinnamaldehyde and Their

Physicochemical Characterization. Food and Bioprocess Technology, 9(11), 1874–1884.

https://doi.org/10.1007/s11947-016-1772-4

Chan, L. W., Ong, K. T., & Heng, P. W. S. (2005). Novel film modifiers to alter the physical

properties of composite ethylcellulose films. Pharmaceutical Research, 22(3), 476–489.

https://doi.org/10.1007/s11095-004-1886-7

Chen, M., Chen, X., & Yam, K. (2020). Encapsulation complex of chlorine dioxide in α-

cyclodextrin: Structure characterization and release property. Food Control, 107, 1066783.

https://doi.org/10.1016/j.foodcont.2019.106783

Chen, X., Chen, M., Xu, C., & Yam, K. L. (2018). Critical review of controlled release

packaging to improve food safety and quality. Critical Reviews in Food Science and

Nutrition, 59(15), 2386–2399. https://doi.org/10.1080/10408398.2018.1453778

Cheng, J., Wang, H., Kang, S., Xia, L., Jiang, S., Chen, M., & Jiang, S. (2019). An active

packaging film based on yam starch with eugenol and its application for pork preservation.

194

Food Hydrocolloids, 96, 546–554. https://doi.org/10.1016/j.foodhyd.2019.06.007

Chopra, S., Dhumal, S., Abeli, P., Beaudry, R., & Almenar, E. (2017). Metal-organic

frameworks have utility in adsorption and release of ethylene and 1-methylcyclopropene in

fresh produce packaging. Postharvest Biology and Technology, 130, 48–55.

https://doi.org/10.1016/j.postharvbio.2017.04.001

Chu, W. B., Yang, J. W., Wang, Y. C., Liu, T. J., Tiu, C., & Guo, J. (2006). The effect of

inorganic particles on slot die coating of poly(vinyl alcohol) solutions. Journal of Colloid

and Interface Science, 297(1), 215–225. https://doi.org/10.1016/j.jcis.2005.10.056

CO2 TECHNOLOGY. (n.d.). Solutions and Applications for Seafood. Retrieved January 20,

2021, from https://www.co2technologies.com/seafood_products.html

Coetzee, E. Marco, Newman, J., Coupland, G. T., Thomas, M., van der Merwe, J., Ren, Y. L., &

McKirdy, S. J. (2019). Commercial trials evaluating the novel use of ethyl formate for in-

transit fumigation of shipping containers. Journal of Environmental Science and Health -

Part B Pesticides, Food Contaminants, and Agricultural Wastes, 54(7), 717–727.

https://doi.org/10.1080/03601234.2019.1631101

Coetzee, Eugene M., Du, X., Thomas, M. L., Ren, Y. L., & McKirdy, S. J. (2020). In-transit

fumigation of shipping containers with ethyl formate + nitrogen on road and continued

journey on sea. Journal of Environmental Science and Health - Part B Pesticides, Food

Contaminants, and Agricultural Wastes, 55(9), 820–826.

https://doi.org/10.1080/03601234.2020.1786328

Comunian, T. A., & Favaro-Trindade, C. S. (2016). Microencapsulation using biopolymers as an

alternative to produce food enhanced with phytosterols and omega-3 fatty acids: A review.

Food Hydrocolloids, 61, 442–457. https://doi.org/10.1016/j.foodhyd.2016.06.003

Costa E Silva, M., Galhano, C. I. C., & Moreira Da Silva, A. M. G. (2007). A new sprout

inhibitor of potato tuber based on carvone/β-cyclodextrin inclusion compound. Journal of

Inclusion Phenomena and Macrocyclic Chemistry, 57(1–4), 121–124.

https://doi.org/10.1007/s10847-006-9210-2

Crabbe-Mann, M., Tsaoulidis, D., Parhizkar, M., & Edirisinghe, M. (2018). Ethyl cellulose,

cellulose acetate and carboxymethyl cellulose microstructures prepared using

electrohydrodynamics and green solvents. Cellulose, 25(3), 1687–1703.

https://doi.org/10.1007/s10570-018-1673-y

Cramer, F., & Henglein, F. M. (1956). Einschlußverbindungen der Cyclodextrine mit Gasen.

Angewandte Chemie, 68(20), 649–649. https://doi.org/10.1002/ange.19560682008

Cui, R., Yan, J., Cao, J., Qin, Y., Yuan, M., & Li, L. (2020). Release properties of

cinnamaldehyde loaded by montmorillonite in chitosan-based antibacterial food packaging.

International Journal of Food Science and Technology. https://doi.org/10.1111/ijfs.14912

195

Da Rocha Neto, A. C., de Oliveira da Rocha, A. B., Maraschin, M., Di Piero, R. M., & Almenar,

E. (2018). Factors affecting the entrapment efficiency of β-cyclodextrins and their effects on

the formation of inclusion complexes containing essential oils. Food Hydrocolloids,

77(509–523). https://doi.org/10.1016/j.foodhyd.2017.10.029

Dai, R., & Lim, L. T. (2014). Release of allyl isothiocyanate from mustard seed meal powder.

Journal of Food Science, 79(1), 47–53. https://doi.org/10.1111/1750-3841.12322

Dai, R., & Lim, L. T. (2015). Release of allyl isothiocyanate from mustard seed meal powder

entrapped in electrospun PLA-PEO nonwovens. Food Research International, 77(467),

475. https://doi.org/10.1016/j.foodres.2015.08.029

Dalton, D. T., Walton, V. M., Shearer, P. W., Walsh, D. B., Caprile, J., & Isaacs, R. (2011).

Laboratory survival of Drosophila suzukii under simulated winter conditions of the Pacific

Northwest and seasonal field trapping in five primary regions of small and stone fruit

production in the United States. Pest Management Science, 67(11), 1368–1374.

Davidovich-Pinhas, M., Barbut, S., & Marangoni, A. G. (2015). The gelation of oil using ethyl

cellulose. Carbohydrate Polymers, 117(869–878).


Day, B. P. . (2008). Active packaging of food. In J. Kerry & B. Paul (Eds.), Smart Packaging

Technologies for Fast Moving Consumer Goods. New York, NY: Wiley.

DECCO ITALIA Srl. (n.d.). Dual Release SO2 pad for table grape post harvest conservation.

Retrieved January 20, 2021, from http://www.deccoitalia.it/wp-

content/uploads/2015/04/TDS_DECCOGRAPAGE_en.pdf

Del Valle, E. M. M. (2004). Cyclodextrins and their uses: A review. Process Biochemistry,

39(9), 1033–1046. https://doi.org/10.1016/S0032-9592(03)00258-9

Deng, L., Taxipalati, M., Zhang, A., Que, F., Wei, H., Feng, F., & Zhang, H. (2018). Electrospun

Chitosan/Poly(ethylene oxide)/Lauric Arginate Nanofibrous Film with Enhanced

Antimicrobial Activity. Journal of Agricultural and Food Chemistry, 66(24), 6219–6226.


Deng, L. Z., Mujumdar, A. S., Pan, Z., Vidyarthi, S. K., Xu, J., Zielinska, M., & Xiao, H. W.

(2019). Emerging chemical and physical disinfection technologies of fruits and vegetables:

a comprehensive review. Critical Reviews in Food Science and Nutrition, 60(15), 2481–

2508. https://doi.org/10.1080/10408398.2019.1649633

Desai, J., Alexander, K., & Riga, A. (2006). Characterization of polymeric dispersions of

dimenhydrinate in ethyl cellulose for controlled release. International Journal of

Pharmaceutics, 308(1–2), 115–123. https://doi.org/10.1016/j.ijpharm.2005.10.034

Desmarchelier, J. M. (1999). Ethyl formate and formic acid: occurrence and environmental fate.

196

Postharvest News and Information, 10, 7–12.

Desmarchelier, J. M., Johnston, F. M., & Le Trang Vu. (1999). Ethyl formate, formic acid and

ethanol in air, wheat, barley and sultanas: Analysis of natural levels and fumigant residues.

Pesticide Science, 55(8), 815–824. https://doi.org/10.1002/ps.2780550808

Dias, M. V., De Fátima F. Soares, N., Borges, S. V., De Sousa, M. M., Nunes, C. A., De

Oliveira, I. R. N., & Medeiros, E. A. A. (2013). Use of allyl isothiocyanate and carbon

nanotubes in an antimicrobial film to package shredded, cooked chicken meat. Food

Chemistry, 141(3), 3160–3166. https://doi.org/10.1016/j.foodchem.2013.05.148

Dietzel, P. D. C., Johnsen, R. E., Fjellvåg, H., Bordiga, S., Groppo, E., Chavan, S., & Blom, R.

(2008). Adsorption properties and structure of CO2 adsorbed on open coordination sites of

metal-organic framework Ni2(dhtp) from gas adsorption, IR spectroscopy and X-ray

diffraction. Chemical Communications, 41, 5125–5127. https://doi.org/10.1039/b810574j

Ding, P., & Lee, Y. L. (2019). Use of essential oils for prolonging postharvest life of fresh fruits

and vegetables. International Food Research Journal, 26(2), 363–366.

Domínguez, R., Barba, F. J., Gómez, B., Putnik, P., Bursać Kovačević, D., Pateiro, M., Santos,

E. M., & Lorenzo, J. M. (2018). Active packaging films with natural antioxidants to be used

in meat industry: A review. Food Research International, 113, 93–101.

https://doi.org/10.1016/j.foodres.2018.06.073

Đorđević, V., Balanč, B., Belščak-Cvitanović, A., Lević, S., Trifković, K., Kalušević, A., Kostić,

I., Komes, D., Bugarski, B., & Nedović, V. (2014). Trends in Encapsulation Technologies

for Delivery of Food Bioactive Compounds. Food Engineering Reviews, 7(4), 452–490.

https://doi.org/10.1007/s12393-014-9106-7

Dos Santos, C., Buera, P., & Mazzobre, F. (2017). Novel trends in cyclodextrins encapsulation.

Applications in food science. Current Opinion in Food Science, 16, 106–113.

https://doi.org/10.1016/j.cofs.2017.09.002

Drosou, C. G., Krokida, M. K., & Biliaderis, C. G. (2017). Encapsulation of bioactive

compounds through electrospinning/electrospraying and spray drying: A comparative

assessment of food-related applications. Drying Technology, 35(2), 139–162.

https://doi.org/10.1080/07373937.2016.1162797

Duan, B., Dong, C., Yuan, X., & Yao, K. (2004). Electrospinning of chitosan solutions in acetic

acid with poly(ethylene oxide). Journal of Biomaterials Science, Polymer Edition, 15(6),

898–711. https://doi.org/10.1163/156856204774196171

Dulvi, T. (2019). Activated Release of Salicylaldehyde and Hexanal from Branched

Polyethylenimine Polymeric Precursor System Encapsulated in Ethyl Cellulose-

Poly(ethylene oxide) Nonwovens. University of Guelph.

197

Editorial. (2016). Frameworks for commercial success. Nature Chemistry, 8(11), 987.

https://doi.org/10.1038/nchem.2661

Fadida, T., Selilat-Weiss, A., & Poverenov, E. (2015). N-hexylimine-chitosan, a biodegradable

and covalently stabilized source of volatile, antimicrobial hexanal. Next generation

controlled-release system. Food Hydrocolloids, 48, 213–219.


Fan, W., Cao, Y., Ren, H., Wang, Y., & Wang, Q. G. (2018). Effects of ethanol fumigation on

inhibiting fresh-cut yam enzymatic browning and microbial growth. Journal of Food

Processing and Preservation, 42(2), e13434. https://doi.org/10.1111/jfpp.13434

Fang, Y., Fu, J., Liu, P., & Cu, B. (2020). Morphology and characteristics of 3D nanonetwork

porous starch-based nanomaterial via a simple sacrifice template approach for clove

essential oil encapsulation. Industrial Crops and Products, 143, 111939.

https://doi.org/10.1016/j.indcrop.2019.111939

Fang, Z., & Bhandari, B. (2012). Encapsulation Techniques for Food Ingredient Systems. In

Food Materials Science and Engineering (pp. 320–348).

https://doi.org/10.1002/9781118373903.ch12

FAO. (2011). Global food losses and food waste - extent, causes and prevention: Study

conducted for the International Congress SAVE FOOD! at Interpack2011. In Interpack.

https://doi.org/10.1016/j.appet.2017.05.013

FAO. (2015). Food loss and waste facts. In

Http://Www.Fao.Org/Resources/Infographics/Infographics-Details/En/C/317265/.

Farjami, T., & Madadlou, A. (2017). Fabrication methods of biopolymeric microgels and

microgel-based hydrogels. Food Hydrocolloids, 62, 262–272.


Feliziani, E., Lichter, A., Smilanick, J. L., & Ippolito, A. (2016a). Disinfecting agents for

controlling fruit and vegetable diseases after harvest. Postharvest Biology and Technology,

122(53), 69. https://doi.org/10.1016/j.postharvbio.2016.04.016

Feliziani, E., Lichter, A., Smilanick, J. L., & Ippolito, A. (2016b). Physical treatments to control

postharvest diseases of fresh fruits and vegetables. Postharvest Biology and Technology,

122(30), 40. https://doi.org/10.1016/j.postharvbio.2016.04.016

Floros, J. D., Dock, L. L., & Han, J. H. (1997). Active packaging technologies and applications.

Food Cosmet Drug Packag.

Fong, H., Chun, I., & Reneker, D. H. (1999). Beaded nanofibers formed during electrospinning.

Polymer, 40(16), 4585–4592. https://doi.org/10.1016/S0032-3861(99)00068-3

198

Franke, I., Wijma, E., & Bouma, K. (2002). Shelf life extension of pre-baked buns by an active

packaging ethanol emitter. Food Additives and Contaminants, 19(3), 314–322.

https://doi.org/10.1080/02652030110072704

FREUND. (n.d.). Antimold-Mild®. Retrieved January 20, 2021, from

http://www.freund.co.jp/english/chemical/preservation/antimoldmild.html

Fu, Y., Sarkar, P., Bhunia, A. K., & Yao, Y. (2016). Delivery systems of antimicrobial

compounds to food. Trends in Food Science and Technology, 57, 165–177.

https://doi.org/10.1016/j.tifs.2016.09.013

Furukawa, H., Ko, N., Go, Y. B., Aratani, N., Choi, S. B., Choi, E., Yazaydin, A. Ö., Snurr, R.

Q., O’Keeffe, M., Kim, J., & Yaghi, O. M. (2010). Ultrahigh porosity in metal-organic

frameworks. Science, 329(5990), 424–428. https://doi.org/10.1126/science.1192160

Gaikwad, K. K., Singh, S., & Lee, Y. S. (2018). High adsorption of ethylene by alkali-treated

halloysite nanotubes for food-packaging applications. Environmental Chemistry Letters,

16(3), 1055–1062. https://doi.org/10.1007/s10311-018-0718-7

Gao, H., Fang, X., Chen, H., Qin, Y., Xu, F., & Jin, T. Z. (2017). Physiochemical properties and

food application of antimicrobial PLA film. Food Control, 73, 1522–1531.

https://doi.org/10.1016/j.foodcont.2016.11.017

Garrido-Miranda, K. A., Rivas, B. L., Pérez-Rivera, M. A., Sanfuentes, E. A., & Peña-Farfal, C.

(2018). Antioxidant and antifungal effects of eugenol incorporated in bionanocomposites of

poly(3-hydroxybutyrate)-thermoplastic starch. LWT, 98, 260–267.

https://doi.org/10.1016/j.lwt.2018.08.046

Gassensmith, J. J., Furukawa, H., Smaldone, R. A., Forgan, R. S., Botros, Y. Y., Yaghi, O. M., &

Stoddart, J. F. (2011). Strong and reversible binding of carbon dioxide in a green metal-

organic framework. Journal of the American Chemical Society, 133(39), 15312–15315.

https://doi.org/10.1021/ja206525x

Gautschi, M., Bajgrowicz, J. A., & Kraft, P. (2001). Fragrance chemistry - Milestones and

perspectives. Chimia, 55(5), 379–387.

Gesser, H. D., Rochon, A., Lemire, A. E., Masters, K. J., & Raudsepp, M. (1984). Pressure

dependence of methane encapsulation in type 3A zeolites. Zeolites, 4(1), 22–24.

https://doi.org/10.1016/0144-2449(84)90067-8

Ghayempour, S., & Montazer, M. (2016). Micro/nanoencapsulation of essential oils and

fragrances: Focus on perfumed, antimicrobial, mosquito-repellent and medical textiles.

Journal of Microencapsulation, 33(6), 497–510.

https://doi.org/10.1080/02652048.2016.1216187

Ghorani, B., & Tucker, N. (2015). Fundamentals of electrospinning as a novel delivery vehicle

199

for bioactive compounds in food nanotechnology. Food Hydrocolloids, 51, 227–240.


Glenn, G. M., Klamczynski, A. P., Woods, D. F., Chiou, B., Orts, W. J., & Imam, S. H. (2010).

Encapsulation of plant oils in porous starch microspheres. Journal of Agricultural and Food

Chemistry, 58(7), 4180–4184. https://doi.org/10.1021/jf9037826

Godin, G., Levrand, B., Trachsel, A., Lehn, J. M., & Herrmann, A. (2010). Reversible formation

of aminals: A new strategy to control the release of bioactive volatiles from dynamic

mixtures. Chemical Communications, 46(18), 3125–3127. https://doi.org/10.1039/c002302g

Goñi, M. L., Gañán, N. A., Martini, R. E., & Andreatta, A. E. (2018). Carvone-loaded LDPE

films for active packaging: Effect of supercritical CO2-assisted impregnation on loading,

mechanical and transport properties of the films. Journal of Supercritical Fluids, 133, 278–

290. https://doi.org/10.1016/j.supflu.2017.10.019

Goubet, I., Le Quere, J. L., & Voilley, A. J. (1998). Retention of Aroma Compounds by

Carbohydrates: Influence of Their Physicochemical Characteristics and of Their Physical

State. A Review. Journal of Agricultural and Food Chemistry, 46(5), 1981–1990.

https://doi.org/10.1021/jf970709y

Grande, C. A. (2012). Advances in Pressure Swing Adsorption for Gas Separation. ISRN

Chemical Engineering. https://doi.org/10.5402/2012/982934

Gravelle, A. J., Marangoni, A. G., & Davidovich-Pinhas, M. (2018). Ethylcellulose Oleogels. In

Edible Oleogels. https://doi.org/10.1016/b978-0-12-814270-7.00014-9

Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of

Research of the National Bureau of Standards Section A: Physics and Chemistry, 81(1), 89–

96. https://doi.org/10.6028/jres.081A.011

Grewer, T. (1991). The influence of chemical-structure on exothermic decomposition.

Thermochimica Acta, 187, 133–149.

Guan, Y., Teng, Z., Mei, L., Zhang, J., Wang, Q., & Luo, Y. (2019). An entrapped metal-organic

framework system for controlled release of ethylene. Journal of Colloid and Interface

Science, 533, 207–215. https://doi.org/10.1016/j.jcis.2018.08.057

Günzler, H., & Gremlich, H.-U. (2002). IR spectroscopy: an introduction. Wiley-VCH,

Weinheim.

Hammouda, B., Ho, D. L., & Kline, S. (2004). Insight into clustering in poly(ethylene oxide)

solutions. Macromolecules, 37(18), 6932–6937. https://doi.org/10.1021/ma049623d

Han, C., Zuo, J., Wang, Q., Xu, L., Wang, Z., Dong, H., & Gao, L. (2015). Effects of 1-MCP on

postharvest physiology and quality of bitter melon (Momordica charantia L.). Scientia

200

Horticulturae, 182, 86–91. https://doi.org/10.1016/j.scienta.2014.07.024

Hansen, A. Å., & Mielnik, M. (2014). Effect of Different Packaging Methods on Quality and

Shelf Life of Fresh Reindeer Meat. Packaging Technology and Science, 27(12), 987–997.

https://doi.org/10.1002/pts.2075

Hansen, A. Å., Moen, B., Rødbotten, M., Berget, I., & Pettersen, M. K. (2016). Effect of vacuum

or modified atmosphere packaging (MAP) in combination with a CO2 emitter on quality

parameters of cod loins (Gadus morhua). Food Packaging and Shelf Life, 9, 29–37.

https://doi.org/10.1016/j.fpsl.2016.05.005

Hansen, A. A., Mørkøre, T., Rudi, K., Rødbotten, M., Bjerke, F., & Eie, T. (2009). Quality

changes of prerigor filleted atlantic salmon (salmo salar L.) packaged in modified

atmosphere using CO2 emitter, traditional MAP, and vacuum. Journal of Food Science,

74(6), 242–249. https://doi.org/10.1111/j.1750-3841.2009.01233.x

Haritos, V. S., & Dojchinov, G. (2003). Cytochrome c oxidase inhibition in the rice weevil

Sitophilus oryzae (L.) by formate, the toxic metabolite of volatile alkyl formates.

Comparative Biochemistry and Physiology - C Toxicology and Pharmacology, 136(2), 135–

143.

Hazel Technologies. (n.d.). Reduce Your Waste. Increase Your Sales. Retrieved January 20,

2021, from https://www.hazeltechnologies.com/

He, Y., Zhang, W., Guo, T., Zhang, G., Qin, W., Zhang, L., Wang, C., Zhu, W., Yang, M., Hu,

X., Singh, V., Wu, L., Gref, R., & Zhang, J. (2019). Drug nanoclusters formed in confined

nano-cages of CD-MOF: dramatic enhancement of solubility and bioavailability of

azilsartan. Acta Pharmaceutica Sinica B, 9(1), 97–106.

https://doi.org/10.1016/j.apsb.2018.09.003

Hedges, A. R., Shieh, W. J., & Sikorski, C. T. (1995). Use of Cyclodextrins for Encapsulation in

the Use and Treatment of Food Products (pp. 60–71). https://doi.org/10.1021/bk-1995-

0590.ch006

Hendessi, S., Sevinis, E. B., Unal, S., Cebeci, F. C., Menceloglu, Y. Z., & Unal, H. (2016).

Antibacterial sustained-release coatings from halloysite nanotubes/waterborne

polyurethanes. Progress in Organic Coatings, 101, 253–261.

https://doi.org/10.1016/j.porgcoat.2016.09.005

Herrmann, A. (2007). Controlled release of volatiles under mild reaction conditions: From nature

to everyday products. Angewandte Chemie - International Edition, 46(31), 5836–5863.

https://doi.org/10.1002/anie.200700264

Herrmann, A. (2009). Dynamic mixtures and combinatorial libraries: Imines as probes for

molecular evolution at the interface between chemistry and biology. Organic and

Biomolecular Chemistry, 7(16), 3195–3204. https://doi.org/10.1039/b908098h

201

Herrmann, A. (2010). The Chemistry and Biology of Volatiles. No. 612.0157 C4. Chichester:

Wiley. https://doi.org/10.1002/9780470669532

Herrmann, A. (2012). Dynamic mixtures: Challenges and opportunities for the amplification and

sensing of scents. Chemistry - A European Journal, 18(28), 8568–8577.

https://doi.org/10.1002/chem.201200668

Herrmann, A. (2017). Profragrance chemistry as an interdisciplinary research area and key

technology for fragrance delivery. Chimia, 71(7–8), 414–419.

https://doi.org/10.2533/chimia.2017.414

Himed, L., Merniz, S., Monteagudo-Olivan, R., Barkat, M., & Coronas, J. (2019). Antioxidant

activity of the essential oil of citrus limon before and after its encapsulation in amorphous

SiO2. Scientific African, 6, e00181. https://doi.org/10.1016/j.sciaf.2019.e00181

Ho, B. T., & Bhandari, B. R. (2016). Novel solid encapsulation of ethylene gas using amorphous

α-cyclodextrin and the release characteristics. Journal of Agricultural and Food Chemistry,

64(17), 3318–3323. https://doi.org/10.1021/acs.jafc.5b06037

Ho, B. T., Hofman, P. J., Joyce, D. C., & Bhandari, B. R. (2016). Uses of an innovative ethylene-

α-cyclodextrin inclusion complex powder for ripening of mango fruit. Postharvest Biology

and Technology, 113, 77–86. https://doi.org/10.1016/j.postharvbio.2015.11.005

Ho, B. T., Joyce, D. C., & Bhandari, B. R. (2011a). Encapsulation of ethylene gas into α-

cyclodextrin and characterisation of the inclusion complexes. Food Chemistry, 127(2), 572–

580. https://doi.org/10.1016/j.foodchem.2011.01.043

Ho, B. T., Joyce, D. C., & Bhandari, B. R. (2011b). Release kinetics of ethylene gas from

ethylene-α-cyclodextrin inclusion complexes. Food Chemistry, 129(2), 259–266.

https://doi.org/10.1016/j.foodchem.2011.04.035

Ho, B. T., Yuwono, T. D., Joyce, D. C., & Bhandari, B. R. (2015). Controlled release of ethylene

gas from the ethylene-α-cyclodextrin inclusion complex powder with deliquescent salts.

Journal of Inclusion Phenomena and Macrocyclic Chemistry, 83(3), 281–288.

https://doi.org/10.1007/s10847-015-0563-2

Ho, T. M., Howes, T., & Bhandari, B. R. (2014). Encapsulation of gases in powder solid

matrices and their applications: A review. Powder Technology, 259, 87–108.

https://doi.org/10.1016/j.powtec.2014.03.054

Ho, T.M., Howes, T., & Bhandari, B. R. (2016a). Encapsulation of CO2 into amorphous alpha-

cyclodextrin powder at different moisture contents - Part 1: Encapsulation capacity and

stability of inclusion complexes. Food Chemistry, 203, 348–355.


Ho, T.M., Howes, T., Jack, K. S., & Bhandari, B. R. (2016b). Encapsulation of CO2 into

202

amorphous alpha-cyclodextrin powder at different moisture contents - Part 2:

Characterization of complexed powders and determination of crystalline structure. Food

Chemistry, 206, 92–101. https://doi.org/10.1016/j.foodchem.2016.03.044

Ho, Thao M., Howes, T., & Bhandari, B. R. (2015). Encapsulation of CO2 into amorphous and

crystalline α-cyclodextrin powders and the characterization of the complexes formed. Food


Ho, Thao M., Howes, T., & Bhandari, B. R. (2018). An innovative approach to extend the shelf

life of cottage cheese curds using food grade CO2-α-cyclodextrin complex powder: A

preliminary study. Journal of Food Processing and Preservation, 42(2), e13514.

https://doi.org/10.1111/jfpp.13514

Hohman, M. M., Shin, M., Rutledge, G., & Brenner, M. P. (2001). Electrospinning and

electrically forced jets. II. Applications. Physics of Fluids, 13(8), 2221–2236.

https://doi.org/10.1063/1.1384013

Holck, A. L., Pettersen, M. K., Moen, M. H., & Sørheim, O. (2014). Prolonged shelf life and

reduced drip loss of chicken filets by the use of carbon dioxide emitters and modified

atmosphere packaging. Journal of Food Protection, 77(7), 1133–1141.

https://doi.org/10.4315/0362-028X.JFP-13-428

Horcajada, P., Gref, R., Baati, T., Allan, P. K., Maurin, G., Couvreur, P., Férey, G., Morris, R.

E., & Serre, C. (2012). Metal-organic frameworks in biomedicine. Chemical Reviews,

112(2), 1232–1268. https://doi.org/10.1021/cr200256v

Hoskins, B. F., & Robson, R. (1989). Infinite Polymeric Frameworks Consisting of Three

Dimensionally Linked Rod-like Segments. Journal of the American Chemical Society,

111(15), 5962–5962. https://doi.org/10.1021/ja00197a079

Hotchkiss, J. H., Watkins, C. B., & Sanchez, D. G. (2007). Release of 1-methylcyclopropene

from heat-pressed polymer films. Journal of Food Science, 72(5), 330–334.

https://doi.org/10.1111/j.1750-3841.2007.00391.x

Hu, Q. Da, Tang, G. P., & Chu, P. K. (2014). Cyclodextrin-based host-guest supramolecular

nanoparticles for delivery: From design to applications. Accounts of Chemical Research,

47(7), 2017–2025. https://doi.org/10.1021/ar500055s

Hu, X., Liu, S., Zhou, G., Huang, Y., Xie, Z., & Jing, X. (2014). Electrospinning of polymeric

nanofibers for drug delivery applications. Journal of Controlled Release, 185, 12–21.

https://doi.org/10.1016/j.jconrel.2014.04.018

Hu, Z., Li, S., Wang, S., Zhang, B., & Huang, Q. (2021). Encapsulation of menthol into

cyclodextrin metal-organic frameworks: Preparation, structure characterization and

evaluation of complexing capacity. Food Chemistry, 338, 127839.

https://doi.org/10.1016/j.foodchem.2020.127839

203

Huang, W. (2016). Electrospinning of Ethyl Cellulose - Poly(ethylene oxide) and Cellulose

Acetate – Poly(ethylene oxide) Nonwovens for the Encapsulation and Release of Thymol

and Carvacrol. The University of Guelph.

Huang, Z. M., He, C. L., Yang, A., Zhang, Y., Han, X. J., Yin, J., & Wu, Q. (2006).

Encapsulating drugs in biodegradable ultrafine fibers through co-axial electrospinning.

Journal of Biomedical Materials Research - Part A, 77(1), 169–179.

https://doi.org/10.1002/jbm.a.30564

Huang, Z. M., Zhang, Y. Z., Kotaki, M., & Ramakrishna, S. (2003). A review on polymer

nanofibers by electrospinning and their applications in nanocomposites. Composites Science

and Technology, 63(15), 2223–2253. https://doi.org/10.1016/S0266-3538(03)00178-7

Ibrahim, K. S. (2013). Carbon nanotubes-properties and applications: a review. Carbon Letters,

14(3), 131–144. https://doi.org/10.5714/cl.2013.14.3.131

Iqbal, Z., Babar, A., & Ashraf, M. (2002). Controlled-release naproxen using micronized ethyl

cellulose by wet-granulation and solid-dispersion method. Drug Development and

Industrial Pharmacy, 28(2), 129–134. https://doi.org/10.1081/DDC-120002445

Irie, T., & Uekama, K. (1997). Pharmaceutical applications of cyclodextrins. III. Toxicological

issues and safety evaluation. Journal of Pharmaceutical Sciences, 86(2), 147–162.

https://doi.org/10.1021/js960213f

Jamieson, L. E., Griffin, M. J., Page-Weir, N. E. M., Redpath, S. P., Chhagan, A., Connolly, P.

G., & Woolf, A. B. (2015). The tolerance of tomato potato psyllid life stages to ethyl

formate. New Zealand Plant Protection, 68, 91–97.

https://doi.org/10.30843/nzpp.2015.68.5872

Jang, S. H., Jang, S. R., Lee, G. M., Ryu, J. H., Park, S. Il, & Park, N. H. (2017). Halloysite

Nanocapsules Containing Thyme Essential Oil: Preparation, Characterization, and

Application in Packaging Materials. Journal of Food Science, 82(9), 2113–2120.

https://doi.org/10.1111/1750-3841.13835

Jash, A., & Lim, L. T. (2018). Triggered release of hexanal from an imidazolidine precursor

encapsulated in poly(lactic acid) and ethylcellulose carriers. Journal of Materials Science,

53(3), 2221–2235. https://doi.org/10.1007/s10853-017-1635-z

Jash, A., Paliyath, G., & Lim, L. T. (2018). Activated release of bioactive aldehydes from their

precursors embedded in electrospun poly(lactic acid) nonwovens. RSC Advances, 8(36),

19930–19938. https://doi.org/10.1039/c8ra03137a

Jo, Y. J., Cho, H. S., Choi, M. J., Min, S. G., & Chun, J. Y. (2015). Effect of various

concentration of β-cyclodextrin inclusion complexes containing trans-cinnamaldehyde by

molecular self-assembly. International Journal of Food Engineering, 11(5), 619–627.

https://doi.org/10.1515/ijfe-2015-0030

204

Jobdeedamrong, A., Jenjob, R., & Crespy, D. (2018). Encapsulation and Release of Essential

Oils in Functional Silica Nanocontainers. Langmuir, 34(44), 13235–13243.

https://doi.org/10.1021/acs.langmuir.8b01652

Ju, J., Chen, X., Xie, Y., Yu, H., Guo, Y., Cheng, Y., Qian, H., & Yao, W. (2019). Application

of essential oil as a sustained release preparation in food packaging. Trends in Food Science

and Technology, 92, 22–32. https://doi.org/10.1016/j.tifs.2019.08.005

Kalaj, M., & Cohen, S. M. (2020). Postsynthetic Modification: An Enabling Technology for the

Advancement of Metal-Organic Frameworks. ACS Central Science, 6(7), 10646–11057.

https://doi.org/10.1021/acscentsci.0c00690

Kaliyan, N., Morey, R. V., Wilcke, W. F., Alagusundaram, K., & Gayathri, P. (2007).

Applications of Carbon Dioxide in Food and Processing Industries: Current Status and

Future Thrusts. ASAE Annual Meeting., 1. https://doi.org/10.13031/2013.23553

Kamimura, J. A., Santos, E. H., Hill, L. E., & Gomes, C. L. (2014). Antimicrobial and

antioxidant activities of carvacrol microencapsulated in hydroxypropyl-beta-cyclodextrin.

LWT - Food Science and Technology, 57(2), 701–709.


Kara, H. H., Xiao, F., Sarker, M., Jin, T. Z., Sousa, A. M. M., Liu, C. K., Tomasula, P. M., &

Liu, L. (2016). Antibacterial poly(lactic acid) (PLA) films grafted with electrospun

PLA/allyl isothiocyanate fibers for food packaging. Journal of Applied Polymer Science,

133(2). https://doi.org/10.1002/app.42475

Kathuria, A., Pauwels, A. K., Buntinx, M., Shin, J., & Harding, T. (2019). Inclusion of ethanol in

a nano-porous, bio-based metal organic framework. Journal of Inclusion Phenomena and

Macrocyclic Chemistry, 95(1), 91–98. https://doi.org/10.1007/s10847-019-00920-y

Kerry, J. P., O’Grady, M. N., & Hogan, S. A. (2006). Past, current and potential utilisation of

active and intelligent packaging systems for meat and muscle-based products: A review.

Meat Science, 74(1), 113–130. https://doi.org/10.1016/j.meatsci.2006.04.024

Kfoury, M., Hădărugă, N. G., Hădărugă, D. I., & Fourmentin, S. (2016). Cyclodextrins as

encapsulation material for flavors and aroma. In Encapsulations. Academic Press.

https://doi.org/10.1016/b978-0-12-804307-3.00004-1

Khanna, N., Head, T. D., & Lowery, B. D. (1998). Method for producing chlorine dioxide using

acidified expanded amorphous aluminum silicate impregnated with chlorite (Patent No.

U.S. Patent No. 6,132,748). U.S. Patent No. 6,132,748.

Kitts, D. D., & Liu, Y. (2015). Encapsulation strategies to stabilize a natural folate, L-5-

methyltetrahydrofolic acid, for food fortification practices. In Nanotechnology and

Functional Foods: Effective Delivery of Bioactive Ingredients (p. 142).

https://doi.org/10.1002/9781118462157.ch9

205

Koch, W. (1937). Properties and Uses of Ethylcellulose. Industrial and Engineering Chemistry,

29(6), 687–690. https://doi.org/10.1021/ie50330a020

Koombhongse, S., Liu, W., & Reneker, D. H. (2001). Flat polymer ribbons and other shapes by

electrospinning. Journal of Polymer Science, Part B: Polymer Physics, 39(21), 2598–2606.

https://doi.org/10.1002/polb.10015

Korehei, R., & Kadla, J. F. (2014). Encapsulation of T4 bacteriophage in electrospun

poly(ethylene oxide)/cellulose diacetate fibers. Carbohydrate Polymers, 100, 150–157.


Koushki, P., Bahrami, S. H., & Ranjbar-Mohammadi, M. (2018). Coaxial nanofibers from

poly(caprolactone)/ poly(vinyl alcohol)/Thyme and their antibacterial properties. Journal of

Industrial Textiles, 47(5), 834–852. https://doi.org/10.1177/1528083716674906

Kriegel, C., Kit, K. M., McClements, D. J., & Weiss, J. (2009). Electrospinning of chitosan-

poly(ethylene oxide) blend nanofibers in the presence of micellar surfactant solutions.

Polymer, 50(1), 189–200. https://doi.org/10.1016/j.polymer.2008.09.041

Kringel, D. H., da Silva, W. M. F., Biduski, B., Waller, S. B., Lim, L. T., Dias, A. R. G., &

Zavareze, E. da R. (2020). Free and encapsulated orange essential oil into a β-cyclodextrin

inclusion complex and zein to delay fungal spoilage in cakes. Journal of Food Processing

and Preservation, 44(5), e14411. https://doi.org/10.1111/jfpp.14411

Kumar, S., Mohapatra, D., Kotwaliwale, N., & Singh, K. K. (2017). Vacuum Hermetic

Fumigation: A review. Journal of Stored Products Research, 71, 47–56.

https://doi.org/10.1016/j.jspr.2017.01.002

Kuorwel, K. K., Cran, M. J., Sonneveld, K., Miltz, J., & Bigger, S. W. (2014). Physico-

mechanical properties of starch-based films containing naturally derived antimicrobial

agents. Packaging Technology and Science. https://doi.org/10.1002/pts.2015

Kwon, T. H., Park, C. G., Lee, B. H., Zarders, D., Roh, G. H., Kendra, P. E., & Cha, D. H.

(2021). Ethyl formate fumigation and ethyl formate plus cold treatment combination as

potential phytosanitary quarantine treatments of Drosophila suzukii in blueberries. Journal

of Asia-Pacific Entomology, 24(1), 129–135.

Lashkari, E., Wang, H., Liu, L., Li, J., & Yam, K. (2017). Innovative application of metal-

organic frameworks for encapsulation and controlled release of allyl isothiocyanate. Food


Latou, E., Mexis, S. F., Badeka, A. V., & Kontominas, M. G. (2010). Shelf life extension of

sliced wheat bread using either an ethanol emitter or an ethanol emitter combined with an

oxygen absorber as alternatives to chemical preservatives. Journal of Cereal Science, 52(3),

457–465. https://doi.org/10.1016/j.jcs.2010.07.011

206

Learmonth, S., Ren, Y., Agarwal, M., Newman, J., Cheng, H., & Sutton, J. (2012). Evaluation of

ethyl formate & nitrogen for the disinfestation of eucalyptus weevils on export apples.

Department of Agriculture & Food, Western Australia,Murdoch University, Horticulture

Australia Ltd, PN#AP09045.

Lee, J. C., Bruck, D. J., Curry, H., Edwards, D., Haviland, D. R., Van Steenwyk, R. A., &

Yorgey, B. M. (2011). The susceptibility of small fruits and cherries to the spotted-wing

drosophila, Drosophila suzukii. Pest Management Science, 67(11), 1358–1367.

https://doi.org/10.1002/ps.2225

Lee, J. S., Kim, H. K., Kyung, Y., Park, G. H., Lee, B. H., Yang, J. O., Koo, H. N., & Kim, G. H.

(2018). Fumigation activity of ethyl formate and phosphine against tetranychus urticae

(acari: Tetranychidae) on imported sweet pumpkin. Journal of Economic Entomology,

111(4), 1625–1632. https://doi.org/10.1093/jee/toy090

Lee, L., Arul, J., Lencki, R., & Castaigne, F. (1995). A review on modified atmosphere

packaging and preservation of fresh fruits and vegetables: Physiological basis and practical

aspects—Part I. Packaging Technology and Science, 9(1), 1–17.

https://doi.org/10.1002/pts.2770080605

Lee, M. H., & Park, H. J. (2015). Preparation of halloysite nanotubes coated with Eudragit for a

controlled release of thyme essential oil. Journal of Applied Polymer Science, 132(46).

https://doi.org/10.1002/app.42771

Lee, Y., Ahn, B., & Kim, Y. (2012). Chlorine dioxide gas releasing sachet (Patent No.

KR101443455).

Lee, Y. R., Kim, J., & Ahn, W. S. (2013). Synthesis of metal-organic frameworks: A mini

review. Korean Journal of Chemical Engineering, 30(9), 1667–1680.

https://doi.org/10.1007/s11814-013-0140-6

Lee, Y. S., Beaudry, R., Kim, J. N., & Harte, B. R. (2006). Development of a 1-

methylcyclopropene (1-MPC) sachet release system. Journal of Food Science, 71(1), 1–6.

https://doi.org/10.1111/j.1365-2621.2006.tb12380.x

Lelah, M. D., Kampa, J. J., & Barenberg, S. . (2008). Antimicrobial body covering articles

(Patent No. EP1542556A2).

Levrand, B., Fieber, W., Lehn, J. M., & Herrmann, A. (2007). Controlled release of volatile

aldehydes and ketones from dynamic mixtures generated by reversible hydrazone

formation. Helvetica Chimica Acta, 90(12), 2281–2314.

https://doi.org/10.1002/hlca.200790237

Li, H., Shi, L., Li, C., Fu, X., Huang, Q., & Zhang, B. (2020). Metal-Organic Framework Based

on α-Cyclodextrin Gives High Ethylene Gas Adsorption Capacity and Storage Stability.

ACS Applied Materials and Interfaces, 12(30), 34095–34104.

207

https://doi.org/10.1021/acsami.0c08594

Li, H., Zhang, B., Li, C., Fu, X., Wang, Z., & Huang, Q. (2019). CO2 inclusion complexes of

Granular V-type crystalline starch: Structure and release kinetics. Food Chemistry, 289,

145–151. https://doi.org/10.1016/j.foodchem.2019.03.037

Li, J. R., Kuppler, R. J., & Zhou, H. C. (2009). Selective gas adsorption and separation in metal-

organic frameworks. Chemical Society Reviews, 38(5), 1477–1504.

https://doi.org/10.1039/b802426j

Li, J. R., Sculley, J., & Zhou, H. C. (2012). Metal-organic frameworks for separations. Chemical

Reviews, 112(2), 869–932. https://doi.org/10.1021/cr200190s

Li, W. J., Laurencin, C. T., Caterson, E. J., Tuan, R. S., & Ko, F. K. (2002). Electrospun

nanofibrous structure: A novel scaffold for tissue engineering. Journal of Biomedical

Materials Research, 60(4), 613–621. https://doi.org/10.1002/jbm.10167

Li, X., Jin, Z., & Wang, J. (2007). Complexation of allyl isothiocyanate by α- and β-cyclodextrin

and its controlled release characteristics. Food Chemistry, 103(2), 461–466.


Li, Y., Dong, Q., Chen, J., & Li, L. (2020). Effects of coaxial electrospun eugenol loaded core-

sheath PVP/shellac fibrous films on postharvest quality and shelf life of strawberries.

Postharvest Biology and Technology, 159, 111028.

https://doi.org/10.1016/j.postharvbio.2019.111028

Li, Z., Huang, J., Ye, L., Lv, Y., Zhou, Z., Shen, Y., He, Y., & Jiang, L. (2020). Encapsulation of

Highly Volatile Fragrances in Y Zeolites for Sustained Release: Experimental and

Theoretical Studies. ACS Omega, 5(49), 31925–31935.

https://doi.org/10.1021/acsomega.0c04822

Lichter, A., Zutahy, Y., Kaplunov, T., & Lurie, S. (2008). Evaluation of table grape storage in

boxes with sulfur dioxide-releasing pads with either an internal plastic liner or external

wrap. HortTechnology, 18(2), 206–214. https://doi.org/10.21273/horttech.18.2.206

Lim, L.-T., Mendes, A. C., & Chronakis, I. S. (2019). Electrospinning and electrospraying

technologies for food applications. In Advances in food and nutrition research (pp. 167–

234). Elsevier.

Lim, L. T. (2011). Active and Intelligent Packaging Materials. In Comprehensive Biotechnology,

Second Edition (pp. 629–644). https://doi.org/10.1016/B978-0-08-088504-9.00308-1

Lim, Loong Tak. (2015). Encapsulation of bioactive compounds using electrospinning and

electrospraying technologies. In Nanotechnology and Functional Foods: Effective Delivery

of Bioactive Ingredients (pp. 297–317). https://doi.org/10.1002/9781118462157.ch18

208

Liu, L., & Guo, Q. X. (2002). The driving forces in the inclusion complexation of cyclodextrins.

Journal of Inclusion Phenomena, 42(1), 1–14. https://doi.org/10.1023/A:1014520830813

Liu, Yanbo, Hao, M., Chen, Z., Liu, L., Liu, Y., Yang, W., & Ramakrishna, S. (2020). A review

on recent advances in application of electrospun nanofiber materials as biosensors. Current

Opinion in Biomedical Engineering, 13, 174–189.

https://doi.org/10.1016/j.cobme.2020.02.001

Liu, Yuyu, Deng, L., Zhang, C., Feng, F., & Zhang, H. (2018). Tunable Physical Properties of

Ethylcellulose/Gelatin Composite Nanofibers by Electrospinning. Journal of Agricultural

and Food Chemistry, 66(8), 1907–1915. https://doi.org/10.1021/acs.jafc.7b06038

Liu, Yuyu, Li, Y., Deng, L., Zou, L., Feng, F., & Zhang, H. (2018). Hydrophobic

Ethylcellulose/Gelatin Nanofibers Containing Zinc Oxide Nanoparticles for Antimicrobial

Packaging. Journal of Agricultural and Food Chemistry, 66(36), 9498–9506.


Loftsson, T., & Duchêne, D. (2007). Cyclodextrins and their pharmaceutical applications.

International Journal of Pharmaceutics, 329(1–2), 1–11.

https://doi.org/10.1016/j.ijpharm.2006.10.044

Lovely, C. F. (1968). Pulverulent chlorine dioxide compositions (Patent No. US3591515).

Lu, J. W., Zhu, Y. L., Guo, Z. X., Hu, P., & Yu, J. (2006). Electrospinning of sodium alginate

with poly(ethylene oxide). Polymer, 47(23), 8026–8031.

https://doi.org/10.1016/j.polymer.2006.09.027

Lukic, I., Vulic, J., & Ivanovic, J. (2020). Antioxidant activity of PLA/PCL films loaded with

thymol and/or carvacrol using scCO2 for active food packaging. Food Packaging and Shelf

Life, 26, 100578. https://doi.org/10.1016/j.fpsl.2020.100578

Ma, S., & Zhou, H. C. (2010). Gas storage in porous metal-organic frameworks for clean energy

applications. Chemical Communications, 46(1), 44–53. https://doi.org/10.1039/b916295j

Mahajan, P. V., Caleb, O. J., Singh, Z., Watkins, C. B., & Geyer, M. (2014). Postharvest

treatments of fresh produce. Philosophical Transactions of the Royal Society A:

Mathematical, Physical and Engineering Sciences, 372, 20130309.

https://doi.org/10.1098/rsta.2013.0309

Mahnaj, T., Ahmed, S. U., & Plakogiannis, F. M. (2013). Characterization of ethyl cellulose

polymer. Pharmaceutical Development and Technology, 18(5), 982–989.

https://doi.org/10.3109/10837450.2011.604781

Maisanaba, S., Llana-Ruiz-Cabello, M., Gutiérrez-Praena, D., Pichardo, S., Puerto, M., Prieto, A.

I., Jos, A., & Cameán, A. M. (2017). New advances in active packaging incorporated with

essential oils or their main components for food preservation. Food Reviews International,

209

33(5), 447–515. https://doi.org/10.1080/87559129.2016.1175010

Malanca, F. E., Fraire, J. C., & Argüello, G. A. (2009). Kinetics and reaction mechanism in the

oxidation of ethyl formate in the presence of NO2: Atmospheric implications. Journal of

Photochemistry and Photobiology A: Chemistry, 204(1), 75–81.

https://doi.org/10.1016/j.jphotochem.2009.02.015

Mao, L., Roos, Y. H., Biliaderis, C. G., & Miao, S. (2017). Food emulsions as delivery systems

for flavor compounds: A review. Critical Reviews in Food Science and Nutrition, 57(15),

3173–3187. https://doi.org/10.1080/10408398.2015.1098586

Marcuzzo, E., Debeaufort, F., Sensidoni, A., Tat, L., Beney, L., Hambleton, A., Peressini, D., &

Voilley, A. (2012). Release behavior and stability of encapsulated D-limonene from

emulsion-based edible films. Journal of Agricultural and Food Chemistry, 60(49), 12177–

12185. https://doi.org/10.1021/jf303327n

Marques, H. M. C. (2010). A review on cyclodextrin encapsulation of essential oils and volatiles.

Flavour and Fragrance Journal, 25(5), 313–326. https://doi.org/10.1002/ffj.2019

Maruthupandy, M., & Seo, J. (2019). Allyl isothiocyanate encapsulated halloysite covered with

polyacrylate as a potential antibacterial agent against food spoilage bacteria. Materials

Science and Engineering C, 105, 110016. https://doi.org/10.1016/j.msec.2019.110016

Mastromatteo, M., Mastromatteo, M., Conte, A., & Del Nobile, M. A. (2010). Advances in

controlled release devices for food packaging applications. Trends in Food Science and

Technology, 21(12), 591–598. https://doi.org/10.1016/j.tifs.2010.07.010

Matencio, A., Navarro-Orcajada, S., García-Carmona, F., & López-Nicolás, J. M. (2020).

Applications of cyclodextrins in food science. A review. Trends in Food Science &

Technology.

Mathkar, S., Kumar, S., Bystol, A., Olawoore, K., Min, D., Markovich, R., & Rustum, A. (2009).

The use of differential scanning calorimetry for the purity verification of pharmaceutical

reference standards. Journal of Pharmaceutical and Biomedical Analysis, 49(3), 627–631.

https://doi.org/10.1016/j.jpba.2008.12.030

McAirlaid’s. (n.d.). McAirlaid’s CO2 Pad -for fish and poultry. Retrieved January 20, 2021,

from

https://www.mcairlaids.net/_Resources/Persistent/283d580d70642bbc75e5b9aff3e6bfb6f30

0bcca/CO2_fish_poultry_eng.pdf

McKinlay, A. C., Xiao, B., Wragg, D. S., Wheatley, P. S., Megson, I. L., & Morris, R. E. (2008).

Exceptional behavior over the whole adsorption-storage-delivery cycle for NO in porous

metal organic frameworks. Journal of the American Chemical Society, 130(31), 10440–

10444. https://doi.org/10.1021/ja801997r

210

Melendez-Rodriguez, B., Figueroa-Lopez, K. J., Bernardos, A., Martínez-Máñez, R., Cabedo, L.,

Torres-Giner, S., & Lagaron, J. M. (2019). Electrospun antimicrobial films of poly(3-

hydroxybutyrate-co-3-hydroxyvalerate) containing eugenol essential oil encapsulated in

mesoporous silica nanoparticles. Nanomaterials, 92(227).

https://doi.org/10.3390/nano9020227

Mihindukulasuriya, S. D. F., & Lim, L. T. (2014). Nanotechnology development in food

packaging: A review. Trends in Food Science and Technology, 40(2), 149–167.


Mishra, M. (2015). Overview of Encapsulation and Controlled Release. In Handbook of

Encapsulation and Controlled Release (pp. 27–44). CRC press.

https://doi.org/10.1201/b19038-3

Mitsubishi-Chemical. (n.d.). Antibactical agents. Retrieved January 20, 2021, from

https://www.m-

chemical.co.jp/en/products/departments/group/mfc/product/1201446_7739.html

Mohamad, N., Mazlan, M. M., Tawakkal, I. S. M. A., Talib, R. A., Kian, L. K., Fouad, H., &

Jawaid, M. (2020). Development of active agents filled polylactic acid films for food

packaging application. International Journal of Biological Macromolecules, 163, 1451–

1457. https://doi.org/10.1016/j.ijbiomac.2020.07.209

Monedero, F. M., Hambleton, A., Talens, P., Debeaufort, F., Chiralt, A., & Voilley, A. (2010).

Study of the retention and release of n-hexanal incorporated into soy protein isolate-lipid

composite films. Journal of Food Engineering, 100(1), 133–138.

https://doi.org/10.1016/j.jfoodeng.2010.03.037

Monro, H. A. U. (1971). Manual of Fumigation for Insect Control. Anzeiger Für

Schädlingskunde Und Pflanzenschutz, 44(11), 176–176.

https://doi.org/10.1007/BF02026755

Monteagudo-Olivan, R., Cocero, M. J., Coronas, J., & Rodríguez-Rojo, S. (2019). Supercritical

CO 2 encapsulation of bioactive molecules in carboxylate based MOFs. Journal of CO2

Utilization, 30, 38–47. https://doi.org/10.1016/j.jcou.2018.12.022

Moreira, J. B., Lim, L. T., Zavareze, E. da R., Dias, A. R. G., Costa, J. A. V., & Morais, M. G.

de. (2019). Antioxidant ultrafine fibers developed with microalga compounds using a free

surface electrospinning. Food Hydrocolloids, 93, 131–136.


Morinaga, H., Morikawa, H., Sudo, A., & Endo, T. (2010). A new water-soluble branched

poly(ethylene imine) derivative having hydrolyzable imidazolidine moieties and its

application to long-lasting release of aldehyde. Journal of Polymer Science, Part A:

Polymer Chemistry, 48(20), 4529–4536. https://doi.org/10.1002/pola.24244

211

Mourtzinos, I., Kalogeropoulos, N., Papadakis, S. E., Konstantinou, K., & Karathanos, V. T.

(2008). Encapsulation of nutraceutical monoterpenes in β-cyclodextrin and modified starch.

Journal of Food Science, 73(1), 89–94. https://doi.org/10.1111/j.1750-3841.2007.00609.x

Mourtzinos, Ioannis, & Biliaderis, C. G. (2017). Principles and applications of encapsulation

technologies to food materials. In Thermal and Nonthermal Encapsulation Methods.

https://doi.org/10.1201/9781315267883

Mousavi Khaneghah, A., Hashemi, S. M. B., & Limbo, S. (2018). Antimicrobial agents and

packaging systems in antimicrobial active food packaging: An overview of approaches and

interactions. Food and Bioproducts Processing, 111, 1–19.

https://doi.org/10.1016/j.fbp.2018.05.001

Mu, H., Gao, H., Chen, H., Fang, X., & Han, Q. (2017). A novel controlled release ethanol

emitter: preparation and effect on some postharvest quality parameters of Chinese bayberry

during storage. Journal of the Science of Food and Agriculture, 97(14), 4929–4936.

https://doi.org/10.1002/jsfa.8369

Muppalla, S. R., Kanatt, S. R., Chawla, S. P., & Sharma, A. (2014). Carboxymethyl cellulose-

polyvinyl alcohol films with clove oil for active packaging of ground chicken meat. Food

Packaging and Shelf Life, 2(2), 51–58. https://doi.org/10.1016/j.fpsl.2014.07.002

Mura, P. (2014). Analytical techniques for characterization of cyclodextrin complexes in

aqueous solution: A review. Journal of Pharmaceutical and Biomedical Analysis, 101, 238–

250. https://doi.org/10.1016/j.jpba.2014.02.022

Nayik, G. A., & Muzaffar, K. (2014). Developments in packaging of fresh fruits-shelf life

perspective: A review. American Journal of Food Science and Nutrition Research, 1(5),

34–39.

Nedovic, V., Kalusevic, A., Manojlovic, V., Levic, S., & Bugarski, B. (2011). An overview of

encapsulation technologies for food applications. Procedia Food Science, 1, 1806–1815.

https://doi.org/10.1016/j.profoo.2011.09.265

Neoh, L., Yamauchi, K., Yoshii, H., & Furuta, T. (2007). Kinetics of molecular encapsulation of

1-methylcyclopropene into α-cyclodextrin. Journal of Agricultural and Food Chemistry,

55(26), 11020–11026. https://doi.org/10.1021/jf072357t

Neoh, T. L., Ariyanto, H. D., Menéndez Galvan, P., & Yoshii, H. (2017). Controlled release of

1-methylcyclopropene from its functionalised electrospun fibres under constant and linearly

ramped humidity. Food Additives and Contaminants - Part A Chemistry, Analysis, Control,

Exposure and Risk Assessment, 34(10), 1690–1702.

https://doi.org/10.1080/19440049.2017.1325520

Neoh, T. L., Koecher, K., Reineccius, G., Furuta, T., & Yoshii, H. (2010). Dissociation

characteristic of the inclusion complex of cyclomaltohexaose (α-cyclodextrin) with 1-

212

methylcyclopropene in response to stepwise rising relative humidity. Carbohydrate

Research, 345(14), 2085–2089. https://doi.org/10.1016/j.carres.2010.07.006

Neoh, T. L., Yoshii, H., & Furuta, T. (2006). Encapsulation and release characteristics of carbon

dioxide in α-cyclodextrin. Journal of Inclusion Phenomena and Macrocyclic Chemistry,

56(1–2), 125–133. https://doi.org/10.1007/s10847-006-9073-6

Neon, T. L., Yamauchi, K., Yoshii, H., & Furuta, T. (2008). Kinetic study of thermally

stimulated dissociation of inclusion complex of 1-methylcyclopropene with α-cyclodextrin

by thermal analysis. Journal of Physical Chemistry B, 112(49), 15914–15920.

https://doi.org/10.1021/jp806233c

Nerin, C., Silva, F., Manso, S., & Becerril, R. (2016). The Downside of Antimicrobial

Packaging: Migration of Packaging Elements into Food. In Antimicrobial Food Packaging

(pp. 81–93). Academic Press. https://doi.org/10.1016/B978-0-12-800723-5.00006-1

Nicholls, P. (1975). Formate as an inhibitor of cytochrome c oxidase. Biochemical and

Biophysical Research Communications, 67(2), 610–616. https://doi.org/10.1016/0006-

291X(75)90856-6

Nie, H., He, A., Wu, W., Zheng, J., Xu, S., Li, J., & Han, C. C. (2009). Effect of poly(ethylene

oxide) with different molecular weights on the electrospinnability of sodium alginate.

Polymer, 50(20), 4926–4934. https://doi.org/10.1016/j.polymer.2009.07.043

NOVIPAX. (n.d.). Slows Fresh-Cut Fruit’s Respiration Rate. Retrieved January 20, 2021, from

http://www.novipax.com/products/active-absorbents/ultrazapxtendapak-produce/

Nutan, M. T. H., & Reddy, I. K. (2010). General principles of suspensions. In Pharmaceutical

Suspensions: From Formulation Development to Manufacturing (pp. 39–65).

https://doi.org/10.1007/978-1-4419-1087-5_2

Oh, S., & Liu, Y. B. (2020). Effectiveness of nitrogen dioxide fumigation for microbial control

on stored almonds. Journal of Food Protection, 83(4), 599–604.

https://doi.org/10.4315/0362-028X.JFP-19-281

Oliveira, J. E., Moraes, E. A., Marconcini, J. M., Mattoso, L. H. C., Glenn, G. M., & Medeiros,

E. S. (2013). Properties of poly(lactic acid) and poly(ethylene oxide) solvent polymer

mixtures and nanofibers made by solution blow spinning. Journal of Applied Polymer

Science, 129(6), 3672–3681. https://doi.org/10.1002/app.39061

Ortiz, C. M., Mauri, A. N., & Vicente, A. R. (2013). Use of soy protein based 1-

methylcyclopropene-releasing pads to extend the shelf life of tomato (Solanum

lycopersicum L.) fruit. Innovative Food Science and Emerging Technologies, 20, 281–287.

https://doi.org/10.1016/j.ifset.2013.07.004

Ozdemir, M., & Floros, J. D. (2004). Active food packaging technologies. Critical Reviews in

213

Food Science and Nutrition, 44(3), 185–193. https://doi.org/10.1080/10408690490441578

Pace International. (n.d.). FYSIUM® A ripening management technology for pome fruits.

Retrieved January 20, 2021, from https://www.paceint.com/product/fysium/

Pajnik, J., Lukić, I., Dikić, J., Asanin, J., Gordic, M., Misic, D., Zizović, I., & Korzeniowska, M.

(2020). Application of supercritical solvent impregnation for production of zeolite modified

starch-chitosan polymers with antibacterial properties. Molecules, 25(20), 4717.

https://doi.org/10.3390/molecules25204717

Pan, S., Saha, R., Mandal, S., Mondal, S., Gupta, A., Fernández-Herrera, M. A., Merino, G., &

Chattaraj, P. K. (2016). Selectivity in Gas Adsorption by Molecular Cucurbit[6]uril. Journal

of Physical Chemistry C, 120(26), 13911–13921. https://doi.org/10.1021/acs.jpcc.6b02545

Park, M. G., Park, C. G., Yang, J. O., Kim, G. H., Ren, Y., Lee, B. H., & Cha, D. H. (2020).

Ethyl Formate as a Methyl Bromide Alternative for Phytosanitary Disinfestation of

Imported Banana in Korea with Logistical Considerations. Journal of Economic

Entomology, 113(4), 1711–1717. https://doi.org/10.1093/jee/toaa088

Pavela, R. (2015). Essential oils for the development of eco-friendly mosquito larvicides: A

review. Industrial Crops and Products, 76, 174–187.

https://doi.org/10.1016/j.indcrop.2015.06.050

Pavia, D., Lampman, G., Kriz, G., Vyvyan, J. (2013). Introduction to Spectroscopy. In Journal

of Magnetic Resonance, Series A. Nelson Education.

https://doi.org/10.1006/jmra.1996.0145

Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to

spectroscopy. Cengage Learning.

Persano, L., Camposeo, A., Tekmen, C., & Pisignano, D. (2013). Industrial upscaling of

electrospinning and applications of polymer nanofibers: A review. Macromolecular

Materials and Engineering, 298(5), 504–520. https://doi.org/10.1002/mame.201200290

Personna, Y. R., Slater, L., Ntarlagiannis, D., Werkema, D., & Szabo, Z. (2013). Electrical

signatures of ethanol-liquid mixtures: Implications for monitoring biofuels migration in the

subsurface. Journal of Contaminant Hydrology, 144(199–107).

https://doi.org/10.1016/j.jconhyd.2012.10.011

Petersen, K., Væggemose Nielsen, P., Bertelsen, G., Lawther, M., Olsen, M. B., Nilsson, N. H.,

& Mortensen, G. (1999). Potential of biobased materials for food packaging. Trends in

Food Science and Technology, 10(2), 52–68. https://doi.org/10.1016/S0924-

2244(99)00019-9

Petersen, L. C. (1977). The effect of inhibitors on the oxygen kinetics of cytochrome c oxidase.

BBA - Bioenergetics, 460(2), 299–307. https://doi.org/10.1016/0005-2728(77)90216-X

214

Piran, P., Kafil, H. S., Ghanbarzadeh, S., Safdari, R., & Hamishehkar, H. (2017). Formulation of

menthol-loaded nanostructured lipid carriers to enhance its antimicrobial activity for food

preservation. Advanced Pharmaceutical Bulletin, 7(2), 261.

https://doi.org/10.15171/apb.2017.031

Pisano, R., Bazzano, M., Capozzi, L. C., Ferri, A., & Sangermano, M. (2015). Controlled release

of ethylene via polymeric films for food packaging. AIP Conference Proceedings, 1695(1),

020009. https://doi.org/10.1063/1.4937287

Ponce Cevallos, P. A., Buera, M. P., & Elizalde, B. E. (2010). Encapsulation of cinnamon and

thyme essential oils components (cinnamaldehyde and thymol) in β-cyclodextrin: Effect of

interactions with water on complex stability. Journal of Food Engineering, 99(1), 70–75.


Pongprasert, N., & Srilaong, V. (2014). A novel technique using 1-MCP microbubbles for

delaying postharvest ripening of banana fruit. Postharvest Biology and Technology, 95, 42–

45. https://doi.org/10.1016/j.postharvbio.2014.04.003

Porat, R., Lichter, A., Terry, L. A., Harker, R., & Buzby, J. (2018). Postharvest Biology and

Technology Postharvest losses of fruit and vegetables during retail and in consumers ’

homes : Quantifications , causes , and means of prevention. 139, 135–149.


PPS Packaging Company. (n.d.). SO2 Pads. Retrieved January 20, 2021, from

http://www.ppspackaging.com/so2-pads/

Preslar, A. T., & Mouat, A. R. (2018). Compositions for controlled release of active ingredients

and methods of making same (Patent No. US20190037839A1).

Prodana, M., Albu, M. G., Kaya, D. A., Negru, A., Bojin, D., & Enachescu, M. (2015). New

Method for Encapsulation of Oregano Essential Oil into Carbon Nanotubes. The 39th ARA

Proceedings. https://doi.org/10.14510/39ara2015.3918

Ragaert, P., Devlieghere, F., & Debevere, J. (2007). Role of microbiological and physiological

spoilage mechanisms during storage of minimally processed vegetables. Postharvest

Biology and Technology, 44(3), 185–194. https://doi.org/10.1016/j.postharvbio.2007.01.001

Ramakrishna, S., Fujihara, K., Teo, W. E., Lim, T. C., & Ma, Z. (2005). An introduction to

electrospinning and nanofibers. In An Introduction to Electrospinning and Nanofibers.

World Scientific. https://doi.org/10.1142/5894

Ramamoorthy, M., & Rajiv, S. (2014). L-carvone-loaded nanofibrous membrane as a fragrance

delivery system: Fabrication, characterization and in vitro study. Flavour and Fragrance

Journal, 29(6), 334–339. https://doi.org/10.1002/ffj.3209

Ramos, M., Beltrán, A., Peltzer, M., Valente, A. J. M., & Garrigós, M. del C. (2014). Release

215

and antioxidant activity of carvacrol and thymol from polypropylene active packaging

films. LWT - Food Science and Technology, 58(2), 470–477.


Ramos, M., Jiménez, A., Peltzer, M., & Garrigós, M. C. (2012). Characterization and

antimicrobial activity studies of polypropylene films with carvacrol and thymol for active

packaging. Journal of Food Engineering, 109(3), 513–519.


Ranjan, S., Chandrasekaran, R., Paliyath, G., Lim, L. T., & Subramanian, J. (2020). Effect of

hexanal loaded electrospun fiber in fruit packaging to enhance the post harvest quality of

peach. Food Packaging and Shelf Life, 23, 100447.

https://doi.org/10.1016/j.fpsl.2019.100447

Raouche, S., Mauricio-Iglesias, M., Peyron, S., Guillard, V., & Gontard, N. (2011). Combined

effect of high pressure treatment and anti-microbial bio-sourced materials on

microorganisms’ growth in model food during storage. Innovative Food Science and

Emerging Technologies, 12(4), 425–434. https://doi.org/10.1016/j.ifset.2011.06.012

Ravindra, R., Krovvidi, K. R., Khan, A. A., & Kameswara Rao, A. (1997). FTIR, diffusivity,

selectivity, and aging studies of interactions of hydrazine, water, and hydrazine hydrate with

the ethylcellulose membrane. Macromolecules, 30(11), 3288–3292.

https://doi.org/10.1021/ma961779t

Ray, S. S., Chen, S. S., Li, C. W., Nguyen, N. C., & Nguyen, H. T. (2016). A comprehensive

review: Electrospinning technique for fabrication and surface modification of membranes

for water treatment application. RSC Advances, 6(88), 85495–85514.

https://doi.org/10.1039/c6ra14952a

Ray, Sohini, Raychaudhuri, U., & Chakraborty, R. (2016). An overview of encapsulation of

active compounds used in food products by drying technology. Food Bioscience, 13, 76–83.

https://doi.org/10.1016/j.fbio.2015.12.009

Ray, Soumi, Jin, T., Fan, X., Liu, L., & Yam, K. L. (2013). Development of Chlorine Dioxide

Releasing Film and Its Application in Decontaminating Fresh Produce. Journal of Food

Science, 78(2), 276–284. https://doi.org/10.1111/1750-3841.12010

Ren, Y L, & Desmarchelier, J. M. (2002). Natural occurrence of carbonyl sulfide and ethyl

formate in grains. Proceedings of the International Conference on Controlled Atmosphere

and Fumigation in Stored Products, 639–649.

Ren, Yong Lin, & Mahon, D. (2006). Fumigation trials on the application of ethyl formate to

wheat, split faba beans and sorghum in small metal bins. Journal of Stored Products

Research, 42(3), 277–289. https://doi.org/10.1016/j.jspr.2005.04.002

Reneker, D. H., Yarin, A. L., Zussman, E., & Xu, H. (2007). Electrospinning of Nanofibers from

216

Polymer Solutions and Melts. Advances in Applied Mechanics, 41, 341–346.

https://doi.org/10.1016/S0065-2156(07)41002-X

Requena, R., Vargas, M., & Chiralt, A. (2017). Release kinetics of carvacrol and eugenol from

poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) films for food packaging applications.

European Polymer Journal, 92, 185–193. https://doi.org/10.1016/j.eurpolymj.2017.05.008

Restuccia, D., Spizzirri, U. G., Parisi, O. I., Cirillo, G., Curcio, M., Iemma, F., Puoci, F., Vinci,

G., & Picci, N. (2010). New EU regulation aspects and global market of active and

intelligent packaging for food industry applications. Food Control, 21(11), 1425–1435.


Ribeiro-Santos, R., Andrade, M., Melo, N. R. de, & Sanches-Silva, A. (2017). Use of essential

oils in active food packaging: Recent advances and future trends. Trends in Food Science

and Technology, 61, 132–140. https://doi.org/10.1016/j.tifs.2016.11.021

Rilo, E., Vila, J., García-Garabal, S., Varela, L. M., & Cabeza, O. (2013). Electrical conductivity

of seven binary systems containing 1-ethyl-3-methyl imidazolium alkyl sulfate ionic liquids

with water or ethanol at four temperatures. Journal of Physical Chemistry B, 117(5), 1411–

1418. https://doi.org/10.1021/jp309891j

Robles, J. L., & Bochet, C. G. (2005). Photochemical release of aldehydes from α-acetoxy

nitroveratryl ethers. Organic Letters, 7(16), 3545–3547. https://doi.org/10.1021/ol051280w

Ruiz, M. A., Sua, A., & Tian, F. (2018). Covalent attachment of metal-organic framework thin

films on surfaces. In Encyclopedia of Interfacial Chemistry: Surface Science and

Electrochemistry (pp. 646–671). https://doi.org/10.1016/B978-0-12-409547-2.14124-1

Ryan, R., & Bishop, S. (2003). Vapormate TM: Non-flammable ethyl formate/liquid carbon

dioxide fumigant mixture. Proceedings of the Australian Postharvest Technical Conference

on Stored Grain in Australia, 25–27.

Saifullah, M., Shishir, M. R. I., Ferdowsi, R., Tanver Rahman, M. R., & Van Vuong, Q. (2019).

Micro and nano encapsulation, retention and controlled release of flavor and aroma

compounds: A critical review. Trends in Food Science and Technology, 86, 230–251.


Sajid, M. (2016). Toxicity of nanoscale metal organic frameworks: a perspective. Environmental

Science and Pollution Research, 23(15), 1405–14807. https://doi.org/10.1007/s11356-016-

7053-y

Sanches-Silva, A., Costa, D., Albuquerque, T. G., Buonocore, G. G., Ramos, F., Castilho, M. C.,

Machado, A. V., & Costa, H. S. (2014). Trends in the use of natural antioxidants in active

food packaging: a review. Food Additives and Contaminants - Part A Chemistry, Analysis,

Control, Exposure and Risk Assessment, 31(3), 374–395.

https://doi.org/10.1080/19440049.2013.879215

217

Sannino, A., Demitri, C., & Madaghiele, M. (2009). Biodegradable cellulose-based hydrogels:

Design and applications. Materials, 2(2), 353–373. https://doi.org/10.3390/ma2020353

Santiago, L. G., & Castro, G. R. (2016). Novel technologies for the encapsulation of bioactive

food compounds. Current Opinion in Food Science, 7, 78–85.

https://doi.org/10.1016/j.cofs.2016.01.006

Santos, E. H., Kamimura, J. A., Hill, L. E., & Gomes, C. L. (2015). Characterization of carvacrol

beta-cyclodextrin inclusion complexes as delivery systems for antibacterial and antioxidant

applications. LWT - Food Science and Technology, 60(1), 583–592.


Sarker, M. I., Fan, X., & Liu, L. S. (2015). Boron derivatives: As a source of 1-MCP with

gradual release. Scientia Horticulturae, 188, 36–43.

https://doi.org/10.1016/j.scienta.2015.03.017

Sarker, M. I., Liu, L. S., Shahrin, T., Fan, X., & Tomasula, P. (2020). Synthetic Platform for

Controlled Delivery of 1-MCP: An Effective Approach to the Protection of Crops and Fresh

Produce. In ACS Symposium Series (pp. 109–127). https://doi.org/10.1021/bk-2020-

1347.ch006

Sartori, R., Sepulveda, L., Quina, F., Lissi, E., & Abuin, E. (1990). Binding of Electrolytes to

Poly(ethylene oxide) in Aqueous Solutions. Macromolecules, 23(17), 3878–3881.

https://doi.org/10.1021/ma00219a002

Satish Kuchi, V., & Sai Ratna Sharavani, C. (2019). Fruit Physiology and Postharvest

Management of Strawberry. In Strawberry - Pre- and Post-Harvest Management

Techniques for Higher Fruit Quality. IntechOpen. https://doi.org/10.5772/intechopen.84205

Scaffaro, R., Maio, A., Gulino, E. F., Morreale, M., & Mantia, F. P. La. (2020). The effects of

nanoclay on the mechanical properties, carvacrol release and degradation of a pla/pbat

blend. Materials, 13(4), 983. https://doi.org/10.3390/ma13040983

Seglie, L., Martina, K., Devecchi, M., Roggero, C., Trotta, F., & Scariot, V. (2011). The effects

of 1-MCP in cyclodextrin-based nanosponges to improve the vase life of Dianthus

caryophyllus cut flowers. Postharvest Biology and Technology, 59(2), 200–205.


Seydim, A. C., & Sarikus, G. (2006). Antimicrobial activity of whey protein based edible films

incorporated with oregano, rosemary and garlic essential oils. Food Research International,

39(5), 639–644. https://doi.org/10.1016/j.foodres.2006.01.013

Shahidi Noghabi, M., & Molaveisi, M. (2020). Microencapsulation optimization of cinnamon

essential oil in the matrices of gum Arabic, maltodextrin, and inulin by spray-drying using

mixture design. Journal of Food Process Engineering, 43(2), e13341.

https://doi.org/10.1111/jfpe.13341

218

Shao, P., Yan, Z., Chen, H., & Xiao, J. (2018). Electrospun poly(vinyl alcohol)/permutite fibrous

film loaded with cinnamaldehyde for active food packaging. Journal of Applied Polymer

Science, 135(16), 4617. https://doi.org/10.1002/app.46117

Sharanyakanth, P. S., & Radhakrishnan, M. (2020). Synthesis of metal-organic frameworks

(MOFs) and its application in food packaging: A critical review. In Trends in Food Science

and Technology. https://doi.org/10.1016/j.tifs.2020.08.004

Sherje, A. P., Dravyakar, B. R., Kadam, D., & Jadhav, M. (2017). Cyclodextrin-based

nanosponges: A critical review. Carbohydrate Polymers, 173(37–49).


Shewan, H. M., & Stokes, J. R. (2013). Review of techniques to manufacture micro-hydrogel

particles for the food industry and their applications. Journal of Food Engineering, 119(4),

781–792. https://doi.org/10.1016/j.jfoodeng.2013.06.046

Shi, C., Jash, A., & Lim, L. T. (2021). Activated release of hexanal and salicylaldehyde from

imidazolidine precursors encapsulated in electrospun ethylcellulose-poly(ethylene oxide)

fibers. SN Applied Sciences, 3(3), 1–13.

Shi, L., Fu, X., Huang, Q., & Zhang, B. (2017). Single helix in V-type starch carrier determines

the encapsulation capacity of ethylene. Carbohydrate Polymers, 174, 798–803.


Shi, L., Fu, X., Tan, C. P., Huang, Q., & Zhang, B. (2017). Encapsulation of Ethylene Gas into

Granular Cold-Water-Soluble Starch: Structure and Release Kinetics. Journal of

Agricultural and Food Chemistry, 65(10), 2189–2197.


Shi, L., Wang, W., Fu, X., Yuan, Y., Zhang, B., & Huang, Q. (2019). Encapsulation and

controlled release characteristics of ethylene gas in cucurbit[: N] urils. Polymer Chemistry,

10(44), 6021–6030. https://doi.org/10.1039/c9py01303b

Shi, L., Zhong, L., Zhang, B., Fu, X., & Huang, Q. (2020). Encapsulation and release

characteristics of ethylene gas from V6- and V7-type crystalline starches. International

Journal of Biological Macromolecules, 156, 10–17.

https://doi.org/10.1016/j.ijbiomac.2020.03.240

Shin, J., Kathuria, A., & Lee, Y. S. (2019). Effect of hydrophilic and hydrophobic cyclodextrins

on the release of encapsulated allyl isothiocyanate (AITC) and their potential application for

plastic film extrusion. Journal of Applied Polymer Science, 136(42), 48137.

https://doi.org/10.1002/app.48137

Silvestre-Albero, A., Silvestre-Albero, J., Sepúlveda-Escribano, A., & Rodríguez-Reinoso, F.

(2009). Ethanol removal using activated carbon: Effect of porous structure and surface

chemistry. Microporous and Mesoporous Materials, 120(1–2), 62–68.

219

https://doi.org/10.1016/j.micromeso.2008.10.012

Simpson, T., Bikoba, V., Al, E., & Mitcham, E. J. (2004). Effects of ethyl formate on fruit

quality and target pest mortality for harvested strawberries. Postharvest Biology and

Technology, 34(3), 313–319.

Simpson, T, Bikoba, V., Tipping, C., & Mitcham, E. J. (2007). Ethyl formate as a postharvest

fumigant for selected pests of table grapes. Journal of Economic Entomology, 100(4),

1084–1090. https://doi.org/10.1603/0022-0493(2007)100[1084:EFAAPF]2.0.CO;2

Simpson, Tiffanie, Bikoba, V., & Mitcham, E. J. (2004). Effects of ethyl formate on fruit quality

and target pest mortality for harvested strawberries. Postharvest Biology and Technology,

34(3), 313–319. https://doi.org/10.1016/j.postharvbio.2004.05.015

Sircar, S., Golden, T. C., & Rao, M. B. (1996). Activated carbon for gas separation and storage.

Carbon, 34(1), 1–12. https://doi.org/10.1016/0008-6223(95)00128-X

Siriwardane, R. V., Shen, M. S., Fisher, E. P., & Poston, J. A. (2001). Adsorption of CO2 on

molecular sieves and activated carbon. Energy and Fuels, 15(2), 279–284.

https://doi.org/10.1021/ef000241s

Sivertsvik, M. (2003). Active packaging in practice: Fish. In Novel Food Packaging Techniques

(pp. 384–400). https://doi.org/10.1016/B978-1-85573-675-7.50022-5

Smit, R., Jooste, M. M., Addison, M. F., & Johnson, S. A. (2020). Ethyl formate fumigation: Its

effect on stone and pome fruit quality, and grain chinch bug (Macchiademus diplopterus)

mortality. Scientia Horticulturae, 261, 108845.

https://doi.org/10.1016/j.scienta.2019.108845

Smith, J. P., Ooraikul, B., Koersen, W. J., van de Voort, F. R., Jackson, E. D., & Lawrence, R.

A. (1987). Shelf life extension of a bakery product using ethanol vapor. Food Microbiology,

4(4), 329–337. https://doi.org/10.1016/S0740-0020(87)80007-2

Son, W. K., Youk, J. H., Lee, T. S., & Park, W. H. (2004). The effects of solution properties and

polyelectrolyte on electrospinning of ultrafine poly(ethylene oxide) fibers. Polymer, 45(9),

2959–2966. https://doi.org/10.1016/j.polymer.2004.03.006

Song, C., & Scharf, M. E. (2008). Formic acid: A neurologically active, hydrolyzed metabolite

of insecticidal formate esters. Pesticide Biochemistry and Physiology, 92(2), 77–82.

https://doi.org/10.1016/j.pestbp.2008.06.005

Speronello, B. K., Thangaraj, A., & Yang, X. (1997). Method, composition and system for the

controlled release of chlorine dioxide gas (Patent No. US6077495).

Starkenmann, C., Troccaz, M., & Howell, K. (2008). The role of cysteine and cysteine-S

conjugates as odour precursors in the flavour and fragrance industry. Flavour and

220

Fragrance Journal, 23(6), 369–381. https://doi.org/10.1002/ffj.1907

Stewart, J. K., & Mon, T. . (1984). Commercial-scale vacuum fumigation with ethyl formate for

postharvest control of the green peach aphid (Homoptera: Aphididae) on film wrapped

leattuce. Journal of Economic Entomology, 77(3), 569–573.

Sun, X., Baldwin, E., & Bai, J. (2019). Applications of gaseous chlorine dioxide on postharvest

handling and storage of fruits and vegetables – A review. Food Control, 95, 18–26.


Sun, X., Baldwin, E., Plotto, A., Narciso, J., Ference, C., Ritenour, M., Harrison, K., Gangemi,

J., & Bai, J. (2017). Controlled-release of chlorine dioxide in a perforated packaging system

to extend the storage life and improve the safety of grape tomatoes. Journal of Visualized

Experiments, 122. https://doi.org/10.3791/55400

Szejtli, J. (1989). Downstream processing using cyclodextrins. Trends in Biotechnology, 7(7),

170–174. https://doi.org/10.1016/0167-7799(89)90094-2

Tackenberg, M. W., Marmann, A., Thommes, M., Schuchmann, H. P., & Kleinebudde, P.

(2014). Orange terpenes, carvacrol and α-tocopherol encapsulated in maltodextrin and

sucrose matrices via batch mixing. Journal of Food Engineering, 135, 44–52.


Talón, E., Vargas, M., Chiralt, A., & González-Martínez, C. (2019). Antioxidant starch-based

films with encapsulated eugenol. Application to sunflower oil preservation. LWT, 113,

108290. https://doi.org/10.1016/j.lwt.2019.108290

Tampau, A., González-Martinez, C., & Chiralt, A. (2017). Carvacrol encapsulation in starch or

PCL based matrices by electrospinning. Journal of Food Engineering, 214, 245–256.


Tanabe, K. K., & Cohen, S. M. (2011). Postsynthetic modification of metal–organic

frameworks—a progress report. Chemical Society Reviews, 38(5), 1315–1329.

https://doi.org/10.1039/c0cs00031k

Tang, Y., Zhou, Y., Lan, X., Huang, D., Luo, T., Ji, J., Mafang, Z., Miao, X., Wang, H., &

Wang, W. (2019). Electrospun Gelatin Nanofibers Encapsulated with Peppermint and

Chamomile Essential Oils as Potential Edible Packaging. Journal of Agricultural and Food

Chemistry, 67(8), 2227–2234. https://doi.org/10.1021/acs.jafc.8b06226

Tao, F., Hill, L. E., Peng, Y., & Gomes, C. L. (2014). Synthesis and characterization of β-

cyclodextrin inclusion complexes of thymol and thyme oil for antimicrobial delivery

applications. LWT - Food Science and Technology, 59(1), 247–255.


Tapia-Hernández, J. A., Torres-Chávez, P. I., Ramírez-Wong, B., Rascón-Chu, A., Plascencia-

221

Jatomea, M., Barreras-Urbina, C. G., Rangel-Vázquez, N. A., & Rodríguez-Félix, F. (2015).

Micro- and Nanoparticles by Electrospray: Advances and Applications in Foods. Journal of

Agricultural and Food Chemistry, 63(19), 4699–4707.


TESSARA. (n.d.). What is Uvasys Green? Retrieved January 20, 2021, from

https://www.tessara.co.za/products/uvasys/uvasys-green/

Theron, S. A., Zussman, E., & Yarin, A. L. (2004). Experimental investigation of the governing

parameters in the electrospinning of polymer solutions. Polymer, 45(6), 2017–2030.

https://doi.org/10.1016/j.polymer.2004.01.024

Toffano, L., Fialho, M. B., & Pascholati, S. F. (2017). Potential of fumigation of orange fruits

with volatile organic compounds produced by Saccharomyces cerevisiae to control citrus

black spot disease at postharvest. Biological Control, 108, 77–82.

https://doi.org/10.1016/j.biocontrol.2017.02.009

Trachsel, A., Chapuis, C., & Herrmann, A. (2013). Slow release of fragrance aldehydes and

ketones in functional perfumery from dynamic mixtures generated with N-

heteroarylmethyl-substituted secondary diamines. Flavour and Fragrance Journal, 28(5),

280–293. https://doi.org/10.1002/ffj.3170

Trotta, F., Cavalli, R., Martina, K., Biasizzo, M., Vitillo, J., Bordiga, S., Vavia, P., & Ansari, K.

(2011). Cyclodextrin nanosponges as effective gas carriers. Journal of Inclusion

Phenomena and Macrocyclic Chemistry, 71(1–2), 189–194. https://doi.org/10.1007/s10847-

011-9926-5

Tucker, G., Yin, X., Zhang, A., Wang, M., Zhu, Q., Liu, X., Xie, X., Chen, K., & Grierson, D.

(2017). Ethylene and fruit softening. Food Quality and Safety, 1(4), 253–267.

https://doi.org/10.1093/fqsafe/fyx024

Ulloa, P. A., Guarda, A., Valenzuela, X., Rubilar, J. F., & Galotto, M. J. (2017). Modeling the

release of antimicrobial agents (thymol and carvacrol) from two different encapsulation

materials. Food Science and Biotechnology, 26(6), 1763–1772.

https://doi.org/10.1007/s10068-017-0226-8

UniQuest. (n.d.). RIPESTUFFTM a tactical tool for accelerated fruit ripening. Retrieved

January 20, 2021, from https://uniquest.com.au/available-technologies/?view=list

Upadhye, S. B., & Rajabi-Siahboomi, A. R. (2013). Properties and Applications of Polyethylene

Oxide and Ethylcellulose for Tamper Resistance and Controlled Drug Delivery. In Melt

Extrusion (pp. 145–158). Springer. https://doi.org/10.1007/978-1-4614-8432-5_6

Utama, I. M. S., Wills, R. B. H., Ben-Yehoshua, S., & Kuek, C. (2002). In vitro efficacy of plant

volatiles for inhibiting the growth of fruit and vegetable decay microorganisms. Journal of

Agricultural and Food Chemistry, 50(22), 6371–6377. https://doi.org/10.1021/jf020484d

222

VdP International. (n.d.). SuperFresh CO2 pad. Retrieved January 20, 2021, from

http://www.vdpinternational.nl/vartdal-superfresh

Vega-Lugo, A. C., & Lim, L. T. (2008). Electrospinning of soy protein isolate nanofibers.

Journal of Biobased Materials and Bioenergy, 2(3), 223–230.

https://doi.org/10.1166/jbmb.2008.408

Vega-Lugo, A. C., & Lim, L. T. (2009). Controlled release of allyl isothiocyanate using soy

protein and poly(lactic acid) electrospun fibers. Food Research International, 42(8), 933–

940. https://doi.org/10.1016/j.foodres.2009.05.005

Vega-Lugo, A. C., & Lim, L. T. (2012). Effects of poly(ethylene oxide) and pH on the

electrospinning of whey protein isolate. Journal of Polymer Science, Part B: Polymer

Physics, 50(15), 1188–1197. https://doi.org/10.1002/polb.23106

Vemmer, M., & Patel, A. V. (2013). Review of encapsulation methods suitable for microbial

biological control agents. Biological Control, 67(3), 380–389.

https://doi.org/10.1016/j.biocontrol.2013.09.003

Verdugo, M., Lim, L. T., & Rubilar, M. (2014). Electrospun Protein Concentrate Fibers from

Microalgae Residual Biomass. Journal of Polymers and the Environment, 22(3), 373–383.

https://doi.org/10.1007/s10924-014-0678-3

Véronique, C. O. M. A. (2008). Bioactive packaging technologies for extended shelf life of

meat-based products. Meat Science, 78(1–2), 90–103.

https://doi.org/10.1016/j.meatsci.2007.07.035

Vilela, C., Kurek, M., Hayouka, Z., Röcker, B., Yildirim, S., Antunes, M. D. C., Nilsen-

Nygaard, J., Pettersen, M. K., & Freire, C. S. R. (2018). A concise guide to active agents for

active food packaging. Trends in Food Science and Technology, 80(212–222).


Walse, S. S., Krugner, R., & Tebbets, J. S. (2012). Postharvest treatment of strawberries with

methyl bromide to control spotted wing drosophila, Drosophila suzukii. Journal of Asia-

Pacific Entomology, 15(3), 451–456. https://doi.org/10.1016/j.aspen.2012.05.003

Wang, Jinpeng, Qiu, C., Narsimhan, G., & Jin, Z. (2017). Preparation and characterization of

ternary antimicrobial films of β-cyclodextrin/allyl isothiocyanate/polylactic acid for the

enhancement of long-term controlled release. Materials, 10(10), 1210.

https://doi.org/10.3390/ma10101210

Wang, Junya, Huang, L., Zheng, Q., Qiao, Y., & Wang, Q. (2016). Layered double

hydroxides/oxidized carbon nanotube nanocomposites for CO2 capture. Journal of

Industrial and Engineering Chemistry, 36, 255–262.

https://doi.org/10.1016/j.jiec.2016.02.010

223

Wang, L., Sokorai, K., Wu, V. C. H., & Fan, X. (2019). Gaseous chlorine dioxide maintained the

sensory and nutritional quality of grape tomatoes and reduced populations of Salmonella

enterica serovar Typhimurium. Food Control, 96, 299–309.


Wang, X., He, G., Liu, H., Zheng, G., & Sun, D. (2013). Fabrication and morphological control

of electrospun ethyl cellulose nanofibers. 8th Annual IEEE International Conference on

Nano/Micro Engineered and Molecular Systems, IEEE NEMS 2013.

https://doi.org/10.1109/NEMS.2013.6559742

Wang, Z., & Cohen, S. M. (2009). Postsynthetic modification of metal–organic frameworks.

Chemical Society Reviews, 38(5), 1315–1329. https://doi.org/10.1039/b802258p

Watkins, C. (2015). Advances in the Use of 1-MCP. In Advances in Postharvest Fruit and

Vegetable Technology (pp. 117–146). https://doi.org/10.1201/b18489-7

Watkins, C. B. (2006). The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables.

Biotechnology Advances, 24(4), 389–409. https://doi.org/10.1016/j.biotechadv.2006.01.005

Weeks, J. J. (1963). Melting temperature and change of lamellar thickness with time for bulk

polyethylene. Journal of Research of the National Bureau of Standards Section A: Physics

and Chemistry, 67(5), 441. https://doi.org/10.6028/jres.067a.046

Wellinghoff, S. T. (1993). Chlorine dioxide generating polymer packaging films (Patent No.

US5360609).

Wellons, J. D., & Stannett, V. (1966). Permeation, sorption, and diffusion of water in ethyl

cellulose. Journal of Polymer Science Part A-1: Polymer Chemistry, 4(3), 593–602.

https://doi.org/10.1002/pol.1966.150040313

Wen, P., Zhu, D. H., Wu, H., Zong, M. H., Jing, Y. R., & Han, S. Y. (2016). Encapsulation of

cinnamon essential oil in electrospun nanofibrous film for active food packaging. Food

Control, 59, 366–376. https://doi.org/10.1016/j.foodcont.2015.06.005

Wilson, M. D., Stanley, R. A., Eyles, A., & Ross, T. (2017). Innovative processes and

technologies for modified atmosphere packaging of fresh and fresh-cut fruits and

vegetables: A review. Critical Reviews in Food Science and Nutrition, 59(3), 411–422.

https://doi.org/10.1080/10408398.2017.1375892

Womack, G., Vermeer, R., & Kalinoski, H. (2004). Compounds for the controlled release of

active aldehydes (Patent No. US 20050026998 A1).

Wood, J. P., Ryan, S. P., Snyder, E. G., Serre, S. D., Touati, A., & Clayton, M. J. (2010).

Adsorption of chlorine dioxide gas on activated carbons. Journal of the Air and Waste

Management Association, 60(8), 898–906. https://doi.org/10.3155/1047-3289.60.8.898

224

Wood, W. E., Beaverson, N. J., & Kuduk, W. J. (2017). Maturation or ripening inhibitor release

from polymer, fiber, film, sheet or packaging. (Patent No. US9642356B2).

Wu, H., Lu, J., Xiao, D., Yan, Z., Li, S., Li, T., Wan, X., Zhang, Z., Liu, Y., Shen, G., Li, S., &

Luo, Q. (2021). Development and characterization of antimicrobial protein films based on

soybean protein isolate incorporating diatomite/thymol complex. Food Hydrocolloids, 110,

106138. https://doi.org/10.1016/j.foodhyd.2020.106138

Wu, X., Wang, L., Yu, H., & Huang, Y. (2005). Effect of solvent on morphology of

electrospinning ethyl cellulose fibers. Journal of Applied Polymer Science, 97(3), 1292–

1297. https://doi.org/10.1002/app.21818

Wu, Y., Luo, Y., Zhou, B., Mei, L., Wang, Q., & Zhang, B. (2019). Porous metal-organic

framework (MOF) Carrier for incorporation of volatile antimicrobial essential oil. Food

Control, 98, 174–178. https://doi.org/10.1016/j.foodcont.2018.11.011

Xiao, Q., & Lim, L. T. (2018). Pullulan-alginate fibers produced using free surface

electrospinning. International Journal of Biological Macromolecules, 112, 809–817.

https://doi.org/10.1016/j.ijbiomac.2018.02.005

Xin, N., Ren, Y. L., Forrester, R. I., Ming, X., & Mahon, D. (2008). Toxicity of ethyl formate to

adult Sitophilus oryzae (L.), Tribolium castaneum (herbst) and Rhyzopertha dominica (F.).

Journal of Stored Products Research, 44(3), 241–246.

Xing, Y., Yun, J., Li, X., Xu, Q., & Li, W. (2011). The effect of formulation variables on the

encapsulation efficiency and SO2-release behavior of microparticles containing sulphite.

Advanced Materials Research, 152, 512–515.

https://doi.org/10.4028/www.scientific.net/AMR.152-153.512

Xu, T., Gao, C. C., Yang, Y., Shen, X., Huang, M., Liu, S., & Tang, X. (2018). Retention and

release properties of cinnamon essential oil in antimicrobial films based on chitosan and

gum arabic. Food Hydrocolloids, 84, 84–92. https://doi.org/10.1016/j.foodhyd.2018.06.003

Xu, W., Li, D., Fu, Y., & Wei, H. (2011). Development and application of intelligent release

fresh-keeping composite packaging films. Advanced Materials Research, 174, 480.

https://doi.org/10.4028/www.scientific.net/AMR.174.480

Yang, Y., Wahler, D., & Reymond, J. L. (2003). β-amino alcohol properfumes. Helvetica

Chimica Acta, 86(8), 2928–2936. https://doi.org/10.1002/hlca.200390241

Yao, B., Mandrà, S., Curry, J. O., Shaikhutdinov, S., Freund, H. J., & Schrier, J. (2017). Gas

Separation through Bilayer Silica, the Thinnest Possible Silica Membrane. ACS Applied

Materials and Interfaces, 9(49), 43061–43071. https://doi.org/10.1021/acsami.7b13302

Yao, Z. C., Chen, S. C., Ahmad, Z., Huang, J., Chang, M. W., & Li, J. S. (2017). Essential Oil

Bioactive Fibrous Membranes Prepared via Coaxial Electrospinning. Journal of Food

225

Science, 82(6), 1412–1422. https://doi.org/10.1111/1750-3841.13723

Yoon, J. H., & Huh, M. W. (1994). Encapsulation of simple gases in zeolites. Journal of

Physical Chemistry, 98(12), 3202–3206. https://doi.org/10.1021/j100063a025

Yu, D. G., Li, X. Y., Chian, W., Li, Y., & Wang, X. (2014). Influence of sheath solvents on the

quality of ethyl cellulose nanofibers in a coaxial electrospinning process. Bio-Medical

Materials and Engineering, 24(1), 695–701. https://doi.org/10.3233/BME-130857

Yu, M., Dong, R. H., Yan, X., Yu, G. F., You, M. H., Ning, X., & Long, Y. Z. (2017). Recent

Advances in Needleless Electrospinning of Ultrathin Fibers: From Academia to Industrial

Production. Macromolecular Materials and Engineering, 302(7), 1700002.

https://doi.org/10.1002/mame.201700002

Zaitoon, A., & Lim, L.-T. (2020). Effect of Poly(ethylene oxide) on the Electrospinning

Behavior and Characteristics of Ethyl Cellulose Composite Fibers. Materialia, 10, 100649.

https://doi.org/10.1016/j.mtla.2020.100649

Zaitoon, A., Lim, L. T., & Scott-Dupree, C. (2019). Synthesis and Characterization of Ethyl

Formate Precursor for Activated Release Application. Journal of Agricultural and Food

Chemistry, 67(50), 13914–13921. https://doi.org/10.1021/acs.jafc.9b06335

Zaitoon, A., Lim, L. T., & Scott-Dupree, C. (2021). Activated release of ethyl formate vapor

from its precursor encapsulated in ethyl Cellulose/Poly(Ethylene oxide) electrospun

nonwovens intended for active packaging of fresh produce. Food Hydrocolloids, 112,

106313. https://doi.org/10.1016/j.foodhyd.2020.106313

Zhang, B., Luo, Y., Kanyuck, K., Bauchan, G., Mowery, J., & Zavalij, P. (2016). Development

of Metal-Organic Framework for Gaseous Plant Hormone Encapsulation to Manage

Ripening of Climacteric Produce. Journal of Agricultural and Food Chemistry, 64(25),

5164–5170. https://doi.org/10.1021/acs.jafc.6b02072

Zhang, C., Yuan, X., Wu, L., Han, Y., & Sheng, J. (2005). Study on morphology of electrospun

poly(vinyl alcohol) mats. European Polymer Journal, 41(3), 423–432.

https://doi.org/10.1016/j.eurpolymj.2004.10.027

Zhang, H. (2016). Optimization of encapsulation methods and conditions to maximize 1-MCP

loading in modified beta-cyclodextrins. Rutgers University.

Zhang, L., Huang, C., Xu, Y., Huang, H., Zhao, H., Wang, J., & Wang, S. (2020). Synthesis and

characterization of antibacterial polylactic acid film incorporated with cinnamaldehyde

inclusions for fruit packaging. International Journal of Biological Macromolecules, 164,

4547–4555. https://doi.org/10.1016/j.ijbiomac.2020.09.065

Zhang, Q., Zhen, Z., Jiang, H., Li, X. G., & Liu, J. A. (2011). Encapsulation of the ethylene

inhibitor 1-methylcyclopropene by cucurbit[6]uril. Journal of Agricultural and Food

226

Chemistry, 59(19), 10539–10545. https://doi.org/10.1021/jf2019566

Zhang, Yang, Zhou, Y., Cao, S., Li, S., Jin, S., & Zhang, S. (2015). Preparation, release and

physicochemical characterisation of ethyl butyrate and hexanal inclusion complexes with β

- and γ -cyclodextrin. Journal of Microencapsulation, 32(7), 711–718.

https://doi.org/10.3109/02652048.2015.1073391

Zhang, Yibo, Zhang, Y., Zhu, Z., Jiao, X., Shang, Y., & Wen, Y. (2019). Encapsulation of

thymol in biodegradable nanofiber via coaxial eletrospinning and applications in fruit

preservation. Journal of Agricultural and Food Chemistry, 67(6), 1736–1741.


Zhong, Q., Chen, H., Zhang, Y., Pan, K., & Wang, W. (2015). Delivery systems for food

applications: An overview of preparation methods and encapsulation, release, and

dispersion properties. In Nanotechnology and Functional Foods: Effective Delivery of

Bioactive Ingredients (p. 91). https://doi.org/10.1002/9781118462157.ch6

Zhou, F. L., Gong, R. H., & Porat, I. (2009). Three-jet electrospinning using a flat spinneret.

Journal of Materials Science, 44(20), 5501–5508. https://doi.org/10.1007/s10853-009-

3768-1

Zhou, H. C., Long, J. R., & Yaghi, O. M. (2012). Introduction to metal-organic frameworks.

Chemical Reviews, 112(2), 673–674. https://doi.org/10.1021/cr300014x

Zhou, S., Hu, C., Zhao, G., Jin, T., Sheen, S., Han, L., Liu, L. S., & Yam, K. L. (2018a). Novel

generation systems of gaseous chlorine dioxide for Salmonella inactivation on fresh tomato.

Food Control, 92, 479–487. https://doi.org/10.1016/j.foodcont.2018.05.025

Zhou, S., Hu, C., Zhao, G., Liu, L. S., Sheen, S., & Yam, K. L. (2018b). A novel gaseous

chlorine dioxide generating method utilizing carbon dioxide and moisture respired from

tomato for Salmonella inactivation. Food Control, 89, 54–61.


Zhou, S., Jin, T., Sheen, S., Zhao, G., Liu, L. S., Juneja, V., & Yam, K. (2020). Development of

sodium chlorite and glucono delta-lactone incorporated PLA film for microbial inactivation

on fresh tomato. Food Research International, 132, 109067.

https://doi.org/10.1016/j.foodres.2020.109067

Zuidam, N. J., & Shimoni, E. (2010). Overview of microencapsulates for use in food products or

processes and methods to make them. In Encapsulation Technologies for Active Food

Ingredients and Food Processing (pp. 3–29). Springer, New York, NY.

https://doi.org/10.1007/978-1-4419-1008-0_2

Development of Ethyl Formate Delivery System for Active ...

Documents

Transcript of Development of Ethyl Formate Delivery System for Active ...