ORIGINAL PAPER

Ultrasound-Assisted Chitosan–Surfactant NanostructureAssemblies: Towards Maintaining Postharvest Qualityof Tomatoes

Maysoun A. Mustafa & Asgar Ali & Sivakumar Manickam &

Yasmeen Siddiqui

Received: 13 June 2013 /Accepted: 12 August 2013 /Published online: 24 August 2013# Springer Science+Business Media New York 2013

Abstract A novel ultrasound-assisted chitosan–surfactantnanostructure assembly was developed to allow better deliv-ery of chitosan particles into intact fruit tissues for extensionof postharvest life. Three solutions of 1 % chitosan–surfactantnanostructure assembly with micelle sizes of 400, 600 and800 nm were prepared and applied as an edible coating ontomatoes. The fruits were stored at 15±2 °C and 70–80 %relative humidity for 20 days. The indicators of tomato ripen-ing, which included loss of firmness, decline of titratableacidity, decline of chlorophyll content and increase in solublesolid content, were delayed in the treated fruits in comparisonto the control (p <0.05). A delay of approximately 5 days inripening was observed by evaluating the colour evolutionfrom green to red. The treatment significantly (p <0.05) en-hanced the phenolic content while maintaining a lower levelof respiration in comparison to the control, throughout most ofthe storage duration. However, weight loss was higher in thetreated fruits in comparison to the control. No significantdifference in the resultant effects was observed by usingchitosan–surfactant nanostructure assemblies of different mi-celle sizes (p >0.05).

Keywords Chitosan . Edible coatings . Nanostructure .

Micelle . Respiration . Ripening

Introduction

A sustainable agricultural industry has become a central focusin an era challenged with food security. Coupled with the urgeto reduce disease incidence and negative health effects onhumans that have resulted from the use of synthetic chemicalsin agriculture, an increasing interest in the use of natural biode-gradable polymers in agricultural activities has been witnessed(Ziani et al. 2009). This has been particularly apparent in thepostharvest management of fruits and vegetables where exten-sive research has gone into the use of edible coatings derivedfrom agricultural goods and food waste products, which typi-cally incorporate polysaccharides, proteins and lipids (Comaet al. 2002).

Edible coatings form a protective barrier and create anartificial environment where selective transfer of gases occursresulting in reduced moisture loss and metabolism rates (Souzaet al. 2009). A highly researched and valued polysaccharide ischitosan, which is the N-deacetylated derivative of chitin,obtained from the outer shells of crustaceans, such as crab(Ziani et al. 2009). Chitin is the secondmost abundant naturallyoccurring polymer after cellulose (Shahidi et al. 1999).Chitosan is a nontoxic, biocompatible and biodegradable chem-ical that has been used to make semi-permeable films andcoatings for perishable food products owing to its fungicidalproperties and ability to elicit plant defence responses (Lin andZhao 2007; Romanazzi et al. 2007; Maqbool et al. 2010; Aliet al. 2011 and 2012).

To date, only a few studies have been conducted on chito-san edible coatings or films, where different techniques havebeen utilized to improve the stability and quality of the coat-ings (Ziani et al. 2009). Nanotechnology has played a crucial

M. A. Mustafa :A. Ali (*)Centre of Excellence for Postharvest Biotechnology (CEPB), Schoolof Biosciences, The University of Nottingham Malaysia Campus,Jalan Broga, 43500 Semenyih, Selangor D.E., Malaysiae-mail: Asgar.Ali@nottingham.edu.my

S. ManickamDepartment of Chemical and Environmental Engineering, Faculty ofEngineering, The University of Nottingham Malaysia Campus,Jalan Broga, 43500 Semenyih, Selangor D.E., Malaysia

Y. SiddiquiInstitute of Tropical Agriculture (ITA), Universiti Putra Malaysia,Serdang 43400, Selangor, Darul Ehsan, Malaysia

Food Bioprocess Technol (2014) 7:2102–2111DOI 10.1007/s11947-013-1173-x

role in food packaging and has been successfully applied inimproving the quality of packaged food (Traver 2006). Apossible key to future advances in edible coatings for fruitsand vegetables could be through the use of nanostructuredcoatings made of chitosan. Chitosan nanoparticles have beenwidely used as drug carriers due to their anti-tumour and anti-microbial properties and unique biological activities attributedto their small size and quantum size effect (Qi et al. 2004,2005, 2007). It has been found that nanoparticles have ahigher affinity towards biological membranes carrying a neg-ative charge (Qi et al. 2005). Such site-specific targeting maybe beneficial to elicit plant defence responses. Meanwhile, thegreatly reduced size of chitosan nanoparticles could decreasewater vapour and gas permeability, while maintaining theother desirable properties of chitosan coatings.

Recently, ultrasound has been well-exploited to generatemicelle structures (Tang et al. 2012a, b; Tang and Sivakumar2012) and a similar idea has been used to form the chitosan–surfactant nanostructure assembly. Souza et al. (2013) havealso utilised ultrasonication for the development of biofilmsbased on the biodegradable polymer, chitosan. The applica-tion of chitosan–surfactant nanostructure assembly as an edi-ble coating is a novel strategy that has been demonstrated to bemore effective than conventional chitosan in the control ofColletotrichum spp. owing to the reduced size (Zahid et al.2012). It was also hypothesized that as a nanostructure, amuch lower quantity of chitosan in comparison to conven-tional coatings will be required to create a coating due toimproved absorption. Thus, this has encouraged research intothe functionality of chitosan nanostructures as a coating toextend the shelf life of fruits.

Although application of chitosan–surfactant nanostructurecoating is pertinent to a variety of fruits and vegetables, thisinvestigation uses tomato as a model fruit. Tomato ripening is ahighly coordinated and genetically programmed physiologicalprocess that involves several physical, biochemical and physi-ological changes which ultimately render the fruit more palat-able (Javanmardi and Kubota 2006; Valero and Serrano 2010).Postharvest handling conditions largely influence key qualityattributes of tomato fruits, such as firmness. The decline inmarketability of tomatoes during storage occurs concomitantlywith a decline in nutritional value of the fruit. Thus, it isimportant to assess the changes in quality parameters of toma-toes during ripening and retard the undesirable changes asmuchas possible.

To the best of our knowledge, chitosan-based nanoparticleshave not been previously studied as a coating for the extensionof the shelf life of tomatoes. Furthermore, our aim was to uselocally prepared chitosan to promote the potential of the localindustry. Therefore, the objectives of this study are to generatea chitosan–surfactant nanostructure coating and to evaluatethe effect of chitosan–surfactant nanostructure on physico-chemical properties, antioxidant properties and respiration

rates of tomatoes during storage at 15±2 °C and 70–80 %relative humidity.

Materials and Methods

Preparation of Chitosan–Surfactant Nanostructure Assembly

Locally prepared crab shell chitosan (MW 350 kDa, 95 %deacetylated) was purchased from the Chitin-ChitosanResearch Centre of University Kebangsaan Malaysia. Basedon the preliminary observations, sonication was found to affectthe micelle size of chitosan–surfactant nanostructure (resultsnot shown) and 1 % (w /v) chitosan solution was selected asthe optimum concentration for the coating. To prepare 1 %chitosan–surfactant nanostructure assembly of the desirablesizes, 6.25 % (w /v ) chitosan solution was mixed with theemulsifiers Brij 56 and Span 20 (ratio of 1:1 w /w) and distilledwater in a ratio of 1:4:20 (v /v). It was then homogenized in anultrasonic water bath (Bandelin Sonorex, Germany) with a highintensity processor (130 W) operating at 20 kHz with specifiedamplitude of 40 for 5 to 25 min at 25 °C, to achieve the micellesizes of 400, 600 and 800 nm which were confirmed bydynamic light scattering technique using Zetasizer Nano ZS(Malvern, UK), utilizing an argon laser (λ =633 nm) at ascattering angle of 173° (Zahid et al. 2012). The chitosan–surfactant nanostructure was then centrifuged to remove excesssurfactant (Eppendorf 5810R centrifuge, Germany) at 6797×gfor 10 min, and the supernatant was subsequently used afterconfirming the micelle size (Teo et al. 2010).

Plant Material

Mature green tomatoes were purchased from MalaysianAgriFood Corporation (MAFC), washed with 0.05 % (v /v )sodium hypochlorite and dried at ambient temperature for30 min with the aid of a standing fan (Ali et al. 2010).

Fruits were separated into five treatments, each with fourreplicates. After drying, fruits were coated with chitosan–sur-factant nanostructure with the micelle sizes of 400, 600 and800 nm by dipping for 1 min in the respective solutions. Twocontrols were set up; a water control where fruits were dipped indistilled water only, and emulsifier control where fruits weredipped in the emulsifier mixture for 1 min. The data wererecorded every 4 days after the storage of fruits at 15±2 °Cand 70–80 % relative humidity for 20 days. The temperatureand relative humidity were monitored using a Hobo data logger(Hobo U14 LCD Logger, Bourne, MA).

Weight Loss, Firmness and Colour

Weight loss, firmness and colour were assessed as indicatorsof the shelf life of the fruit. The same fruits were weighted

Food Bioprocess Technol (2014) 7:2102–2111 2103

using a digital balance (ANDGF-6100, Japan) on day 0 and atthe end of each storage period. For each treatment there werefour replicates, and weight loss was expressed as percentageloss of initial weight.

Firmness was determined as the maximum force requiredto puncture the fruit, using an Instron texture analyser (Instron2519–104, Norwood, MA) with an 8.0-mm plunger tip. Themaximum force (N) required to penetrate the fruit wasrecorded as the mean of readings taken at four positions onthe equator of fruit that were randomly selected.

Colour was determined using a HunterLab MiniScan XEPlus, which was first standardized using a black tile and awhite tile (X =79.0, Y=83.9, Z =87.9). Peel colour of the fruits(n =10) was determined at two specific points along the equa-tor of the fruit, and recorded as the mean ± SE of parametersL* , a* and b* .

Soluble Solids Content and Titratable Acidity

Soluble solid content was determined with a digital refractom-eter (Atago, Japan) calibrated against sucrose and expressed inBrix. Samples (10 g) were homogenized with 40 mL ofdistilled water and filtered through cheesecloth.

Titratable acidity was measured using the AOAC method942.15 (AOAC 2002) by titrating tomato juice, extracted asdescribed for soluble solids content, against 0.1 N NaOH using0.1 % (v /v ) phenolphthalein as an indicator. The obtainedresults were expressed as the percentage of citric acid in tomato(percent titratable acidity (TA), which was determined by themethod described by Ranganna (1979).

Chlorophyll Content

Chlorophyll content was determined spectrophotometrically(Mejia-Torres et al. 2009), where 1 g of tomato puree was addedto an Eppendorf tube containing 10 mL of 80 % (v/v) acetone,then centrifuged (Eppendorf 5810R centrifuge, Germany) at3,331×g for 15 min at 20 °C. The clear supernatant was usedto measure the absorbance at 647 nm (A647) and at 664 nm(A664) with a BioChrom LibraS12 Spectrophotometer (England)and 80 % (v/v) acetone as a blank. The chlorophyll content wasthen determined using the following equation (Mejia-Torres et al.2009):

Total chlorophyll microgrammes per grammeð Þ¼ 17:76A647 þ 7:34A664ð Þ � Volume=Sample weight

Total Phenolic Content

The Folin Ciocalteu spectrophotometric method, establishedby Singleton and Rossi (1965), was used to determine the total

phenolics content, which was expressed in milligrammes ofgallic acid equivalent to 100 g of fresh weight of fruit sample.

Respiration Rate

The respiration rate was measured using a PerkinElmer Clarus500 Gas Chromatograph (GC). The fruits were incubated for1 h at room temperature in a 1 L of gas tight container to allowadequate time for respiration. A sample from the collected gasin the headspace was then injected into the GC using a 1-mLgas tight syringe injection (SGE Analytical Science SyringePerfection, Australia). The GC was equipped with a stainlesssteel column (Porapak R 80/100) and a thermal conductivitydetector (TCD) to analyse the CO2 content of the injected gas.The carrier gas for CO2 was helium, and the temperatures ofthe GC were set at 60 °C for the oven, 80 °C for the injectorand 200 °C for the TCD.

Statistical Analysis

The experiment was conducted using four replicates (twofruits per replicate) of each treatment in a completelyrandomised design. Analysis of variance was used to measurethe treatment effect using SAS® 9.1, and means were separat-ed using least significant difference test at (p <0.05).

Results and Discussion

Preparation of Chitosan–Surfactant Nanostructure Assembly

As sonication time increased, the micelle size of the chitosan–surfactant nanostructure assembly decreased (Table 1). Thedesired sizes for the nanostructure assembly were obtained for1 % chitosan solution, which was consequently selected as theconcentration for the coating. Moreover, in vivo screeningrevealed that the nanostructure assembly coating caused anundesirable speckling on the fruit surface. This may have beenan effect of sedimentation of excess emulsifier on the fruitsurface. To overcome this negative effect, the nanostructure

Table 1 Average diameter of the chitosan–surfactant nanostructure assem-blies obtained by dynamic light scattering at a scattering angle of 173°

Sonication time (min)

10 min 20 min 30 min

0.5 % chitosan 917±48 680±49 635±7

1.0 % chitosan 836±17 620±20 452±10

1.5 % chitosan 632±9 565±20 438±8

Results are the Z-average size (nanometre) expressed as the mean (n= 3)± SEM

2104 Food Bioprocess Technol (2014) 7:2102–2111

assembly was centrifuged after a stable structure was formed,to reduce the concentration of the surfactant, and the superna-tant was immediately used for the coating.

The instability of the chitosan–surfactant nanostructureassembly can be attributed to the fact that such nanostructureassemblies are thermodynamically unfavourable due to grav-itational separation, flocculation, coalescence and Ostwaldripening (McClements and Rao 2011; Porras et al. 2004).Most of these phenomena can be minimised by reducing themicelle size and with the addition of surfactant beforeultrasonication (Sharma et al. 2010). The addition of excesssurfactant minimizes the incidence of gravitational separation,flocculation and coalescence. On the other hand, Ostwaldripening, which results from the surface energy differencesof the micelles generating attractive forces that lead to theaggregation of the micelles, can be overcome by prolongingthe sonication period. However, the sonication period shouldnot exceed a certain threshold as intense sonication results inthe degradation of the chitosan molecule (Taghizadeh andAbdollahi 2011).

Weight Loss, Firmness and Colour

Cumulative percentage weight loss increased linearly during thestorage period for all the fruits, but it was significantly higher forthe coated fruits in comparison to the controls (Fig. 1). Thelowest percentage weight loss was observed for the emulsifiercoating, and there was no significant difference (p>0.05) be-tween the percentage weight loss for the coated fruits.

Application of coating usually increases water retention infruits extending the shelf life (Chien et al. 2007); however, thiswas not the case with the chitosan–surfactant nanostructurecoating which resulted in excessive water loss and shrivelling

of the fruits. This implies that the water barrier properties ofchitosan were impaired, resulting in increased water permeabil-ity and hence excessive water loss (Lin and Zhao 2007). Watervapour permeability is largely dependent on the hydrophilic/hydrophobic properties of the edible coating as well as the poresize, as it was observed by Souza et al. (2009) when usinghigher concentrations of chitosan which resulted in a filmnetwork with larger pore size. The adhesion properties of thechitosan–surfactant nanostructure coating on the whole fruitsurface might have been inherently poor, which would resultin a coating that was absorbed by the surface rather thanforming a smooth layer. A nanostructure assembly offers novelproperties to the chitosan edible coating, some of which dependon the properties of the emulsifier, more specifically the hydro-philic–lypophilic balance (HLB). Surfactants, based on theirspecific HLB value, are usually added to solutions to modifytheir wettability and adhesion properties as coatings (Lin andZhao 2007). Furthermore, studies have demonstrated that sur-factants reduce the polymer chain interactions (Navarro-Tarazaga et al. 2008). Thus, it is possible that using surfactantsthat do not offer the appropriate HLB value resulted in ananostructure assembly with poor adhesion properties.

The firmness of treated fruits decreased less rapidly (p<0.05)as compared to both the controls (Fig. 2). For the control fruits,the firmness experienced a sharp drop to below 25 N by day 5.Meanwhile, the treated fruits retained their firmness at day 5,dropping to below 25 N by day 10. As the storage periodprogressed, firmness continued to decline at a slower rate.However, by day 10 the control fruits had reached a value below20Nwhich rendered the fruits moderately soft and susceptible todamage. This level of firmness was observed in the coated fruitson day 15. By day 15, the firmness values for the coated fruitsand the control were not significantly different (p>0.05). There

Storage Time (Days)0 5 10 15 20 25

Wei

ght L

oss

(%)

0

5

10

15

20

ControlEmulsifier800 nm600 nm400 nm

Fig. 1 Effect of coating onweight loss of tomato fruit duringstorage (15 °C, 70–80 % RH).Values are the means ± SE.Relative humidity (RH)

Food Bioprocess Technol (2014) 7:2102–2111 2105

was no significant difference (p>0.05) in the decrease in firm-ness between the coated fruits.

Firmness retention in the treated fruits is attributed to thereduced activity of cell wall degrading enzymes. Pectinesteraseand polygalacturonase are the key enzymes responsible for thedepolymerisation of pectin substances found within the cellwall, which result in fruit softening (Zapata et al. 2008). It hasbeen postulated that the modified atmosphere with the reducedO2 and increased CO2 content inhibits ethylene productionwhich in turn reduces the activity of these cell wall degradingenzymes (Zapata et al. 2008). The reduced enzyme activityallows for longer firmness retention, in comparison to thecontrols. This is a key aspect in the extension of the fruit shelflife, owing to the fact that excessive fruit softening may limitshelf life and marketability of the fruit as well as increase thesusceptibility of the fruit to pests and pathogens (Valero andSerrano 2010).

Initially, the a* value was negative for all the fruits (Fig. 3),indicating their initial green colour. By day 5, the a* value waspositive for both controls, while a positive value for the threetreatments was recorded on day 10. This positive value indi-cates that the fruits are at ripening stage 4. The a* value of fruitscoated with 400 nm micelle size was significantly (p <0.05)less than the a* value of fruits coated with 600 and 800 nmmicelle size on day 10. Although all of the treatments ripened tomoderate quality (stage 6) by day 15, the control fruits alwaysmaintained a significantly higher a* value (p <0.05) through-out the storage period.

The results showed a 5-day delay in the colour change of thecoated fruits from green to red, in comparison to the controls, andthis suggests the effect of coating on delaying ripening withoutactually impeding the biochemical and physiological changesassociated with it. Colour is an important developmental factor

since natural fruit colours are an integral part of human diet anddefine the aesthetic value and marketability of the fruit.Moreover, colour is not easily retained during storage due tovarious factors such as light, temperature, oxidation and endog-enous enzymes (Valero and Serrano 2010). Consequently, theability of coatings to retain colour is a highly desirable attributeand one of the primary indicators of longer shelf life.

Soluble Solids Content and Titratable Acidity

With storage, soluble solids content (SSC) increased more inthe control treatments than in the treated fruits (p <0.05)(Fig. 4). The increase in SSC for the coated fruits was veryslow during the initial 15 days of storage. There was nosignificant difference (p >0.05) between the SSC of the threecoatings; however, the difference between the SSC of both thecontrols and the coated fruits was significant, reaching almost2 % by the final day of storage.

Titratable acidity decreased for all the treatments during thestorage period (Fig. 5) with a greater decrease in both thecontrols (from 0.42 % to 0.19 %) in comparison to the coatedfruits (p <0.05). The 400-nm coated fruits maintained thehighest TA during storage (0.27 % at day 20). During most ofthe storage period, there was no significant difference (p >0.05)between the 600- and 800-nm coated fruits. During the first15 days of storage, the difference between the emulsifier controland fruits coated with 600 and 800 nm micelle sizes was notsignificant (p >0.05).

During fruit ripening, translocated assimilates are inten-sively metabolised to form fructose and glucose. This resultsin the accumulation of these sugars and an increase in SSC(Pila et al. 2010; Valero and Serrano 2010). The decline in TAis characteristic of ripening in climacteric fruits and is

Storage Time (Days)0 5 10 15 20 25

Firm

ness

(N

)

10

15

20

25

30

35

40

45

ControlEmulsifier800 nm600 nm400 nm

Fig. 2 Effect of coating onfirmness of tomato fruit duringstorage (15 °C, 70–80 % RH).Values are the means ± SE.Relative humidity (RH)

2106 Food Bioprocess Technol (2014) 7:2102–2111

attributed to the metabolism of organic acids during ripening(Valero and Serrano 2010). The rate of increase in SSC anddecline in TA depend on the metabolic activity occurringwithin the fruit. Thus, the coated fruits, which are assumedto be metabolising at a slower rate due to the reduced O2

content of the modified atmosphere, accumulate sugars andmetabolise organic acids at a slower rate, which indicatesdelayed ripening (Valero and Serrano 2010).

Chlorophyll Content

Chlorophyll content decreased rapidly from the beginning ofthe experiment to almost undetectable levels at day 20 (Fig. 6).The rate of decline was significantly (p <0.05) lower for the

treated fruits in comparison to the controls, with a time differ-ence of almost 5 days observed for the values of total chloro-phyll content for the control and treated fruits. There was nosignificant difference between the 400-, 600- and 800-nm coat-ed fruits (p >0.05). The decrease in chlorophyll content was inparallel with the other parameters observed during ripening,more specifically the change in colour from green to red.Chlorophyll is degraded as the fruit ripens with the concom-itant synthesis of the antioxidant lycopene which accountsfor the characteristic red colour of ripened tomatoes (Mejia-Torres et al. 2009). The slower rate of chlorophyll degrada-tion in the coated fruits is another indication of slowerripening occurring in these treatments, in comparison to thecontrol.

Storage Time (Days)0 5 10 15 20 25

Sol

uble

Sol

ids

Con

tent

(%

)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Control Emulsifier800 nm600 nm400 nm

Fig. 4 Effect of coating onsoluble solids content of tomatofruit during storage (15 °C, 70-80 % RH). Values are themeans ± SE. Relative humidity(RH)

Storage Time (Days)0 5 10 15 20

a* v

alue

-15

-10

-5

0

5

10

15

20

25

ControlEmulsifier800 nm600 nm400 nm

Fig. 3 Effect of coating on a*value of tomato fruit duringstorage (15 °C, 70–80 % RH).Values are the means ± SE.Relative humidity (RH)

Food Bioprocess Technol (2014) 7:2102–2111 2107

Total Phenolic Content

Total phenolic content decreased during the first 5 days ofstorage (Fig. 7). There was a more rapid decline in the controlsthan in the treated fruits (p <0.05). After day 5, the totalphenolics content increased for all the treatments, peaking atday 15 before a further decline. During the total storage period,the total phenolics content was significantly (p <0.05) higherfor the coated fruits in comparison to the controls, with the600 nm coated fruits maintaining the highest phenolics contentduring most of the storage period.

According to literature, the total phenolic content of tomatoesdecreases as ripening progresses (Valero and Serrano 2010). Thisphenomenonwas observed for all the treatments during the initial5 days, with a slower rate of decline observed for the coated fruitssuggesting the role of coating in delaying the progression ofripening associated parameters. Interestingly, the total phenolicscontent peaked at day 15, which is very different from what has

been reported in the literature on tomato ripening. This mightindicate an increase in the phenolic activity as a result of height-ened plant defence system due to the better delivery of chitosanparticles into the cells of the intact tissue (Souza et al. 2009). In astudy by Badawy and Rabea (2009), increased total phenoliccontent was observed in the chitosan-treated tomatoes, and thiswas related to enhanced defence mechanism.

Respiration Rate

In the control treatments, no respiration peak was observed(Fig. 8). A climacteric respiration pattern was clear for thetreated fruits, peaking at day 10, with the highest peak observedfor the 400-nm coated fruits. Fruit in the control treatmentsgenerally maintained a significantly (p <0.05) higher rate ofrespiration in comparison to the coated fruits. With the coatedfruits, the respiration for the 400 nm treatment was significantly(p <0.05) higher than the other treatments during most of the

Storage Time (Days)0 5 10 15 20 25

Titr

atab

le A

cidi

ty (

%)

0.15

0.20

0.25

0.30

0.35

0.40

0.45

ControlEmulsifier800 nm600 nm400 nm

Fig. 5 Effect of coating ontitratable acidity of tomato fruitduring storage (15 °C, 70–80 %RH). Values are the means ± SE.Relative humidity (RH)

Storage Time (Days)0 5 10 15 20 25

Tot

al C

hlor

ophy

ll C

onte

nt (µg

/g)

0

2

4

6

8

10

12

14

16

ControlEmulsifier800 nm 600 nm400 nm

Fig. 6 Effect of coating onchlorophyll content of tomatofruit during storage (15 °C, 70–80%RH). Values are the means ±SE. Relative humidity (RH)

2108 Food Bioprocess Technol (2014) 7:2102–2111

storage period. There was no significant difference (p >0.05)between the respiration of the 400 nm coated fruit and thecontrols on the final day of storage; however, respiration wassignificantly (p <0.05) lower in the other treatments.

Edible coatings create a modified atmosphere around thefruit through a natural interplay between the gas permeability ofthe packaging and the rate of fruit respiration (Fonesca et al.2002). The relatively low permeability of chitosan-based coat-ings results in an atmosphere of decreased O2 and increasedCO2 (Lin and Zhao 2007). This consequently results in reducedrespiration of the product, which was observed in the treatedfruits in comparison to the controls. Meanwhile, the respirationof the fruits coated with chitosan–surfactant nanostructure as-sembly of 400 nm micelle size was considerably higher than

other coatings, which might be an indication of an increase inthe gas permeability of the chitosan coating as the size of thenanostructure assembly decreases. With regard to the climac-teric respiratory peak, it is usually observed for tomatoes be-tween days 2 and 4 (Zapata et al. 2008). Thus, it is likely that thepeak for the controls was not observed due to the fact that CO2

evolution was not documented on a daily basis. Moreover, theclimacteric peak for the coated fruits (observed at day 10) wasrelatively delayed, which might be another indication of the 5-day delay of ripening as noted by the colour change. Thisclimacteric peak for the coated fruits coincides with the declinein firmness and colour evolution to red, the key indicators ofripening assessed in this experiment. These ripening indicatorswere observed for the controls by the 5th day of storage.

Storage Time (Days)0 5 10 15 20 25

Tot

al P

heno

lic C

onte

nt (

mg

(GA

E)/

100g

sam

ple

FW

)

20

30

40

50

60

70

ControlEmulsifier800 nm600 nm 400 nm

Fig. 7 Effect of coating on totalphenolic content of tomato fruitduring storage (15 °C, 70–80 %RH). Values are the means ± SE.Respiration rate (millilitre CO2/kilogramme/hour); relativehumidity (RH)

Storage Time (Days)0 5 10 15 20 25

10

12

14

16

18

20

22

24

26

28

Control

Emulsifier800 nm600 nm

400 nm

Res

pira

tion

rate

(m

l CO

2 / k

g / h

r)

Fig. 8 Effect of coating on CO2

evolution of tomato fruit duringstorage (15 °C, 70–80 % RH).Values are the means ± SE.Relative humidity (RH)

Food Bioprocess Technol (2014) 7:2102–2111 2109

Conclusion

The marketability features of fresh fruit result from an inter-action of appearance, texture, flavour, nutritional quality andsafety. Deterioration of these features can rapidly occur duringpostharvest storage, resulting in a short shelf life and subse-quent postharvest losses. The control fruits ripened after 5 daysof storage at 15 °C and were considered fully ripened by day10, as assessed by firmness and colour change. Application ofthe chitosan–surfactant nanostructure coating resulted in a 5-day delay in ripening as compared to the control and based onthese two key quality attributes. Changes in the various phys-ical and biochemical quality factors were retarded in compar-ison to the controls, with the exception of weight loss whichwas significantly higher for the coated fruits. The desirablequality attributes that were maintained by the coating includefirmness retention, higher total phenolic content as well asslower decline in titratable acidity and chlorophyll content andslower increase in soluble solids content. Respiration rate wasgenerally lower in the coated fruits, with the exception of day10 where the climacteric peak was observed. Further studiesare required to elucidate the surface characteristics of thecoating.

Acknowledgments The authors express their gratitude to the Ministryof Agriculture (MOA) Malaysia for the financial support under projectgrant 05-02-12-SF1003.

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