Composition, thermal and rheological behaviour of selected Greek honeys

Journal of Food Engineering 64 (2004) 9–21

www.elsevier.com/locate/jfoodeng

Composition, thermal and rheological behaviourof selected Greek honeys

Athina Lazaridou a, Costas G. Biliaderis a,*, Nicolaos Bacandritsos b,Anna Gloria Sabatini c

a Laboratory of Food Chemistry and Biochemistry, Food Science and Technology Department, School of Agriculture,

Aristotle University of Thessaloniki, Thessaloniki, Greece 541 24b Institute of Veterinary Research of Athens, N.AG.RE.F., 25 Neapoleos Street, Agia Paraskevi, 153 01 Athens, Greece

c Instituto Nazionale di Apicoltura, Via di Saliceto 80, I-41128 Bologna, Italy

Received 6 May 2003; accepted 13 September 2003

Abstract

Several chemical and physicochemical properties (sugar composition, water content, water activity, colour, viscosity, thermal

properties) were determined for 33 Greek honeys from different botanical and geographical origin. The water content and water

activity values varied within 13.0–18.9 g/100 g and 0.528–0.663, respectively. Steady shear and dynamic rheological tests revealed

Newtonian behaviour for all samples examined over the temperature range of 20–60 �C. The steady shear viscosity (g) and loss

modulus (G00) were inversely related to the water content of honey. The temperature dependence of viscosity followed both the

Arrhenius and the Williams–Landel–Ferry models; for the latter model the viscosity data of different samples fitted very well into a

common master curve. The glass transition temperature (Tg) of honeys, as determined by differential scanning calorimetry, varied

between )34 and )47 �C depending on their composition. The plasticizing action of water on honey solids was evident for native

samples as well as among diluted and concentrated honeys; Tg decreased with increasing water content. Despite a broad variation in

sugar composition among the samples, the Tg values vs. water content fitted reasonably well to the Gordon–Taylor empirical

equation.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Honey; Moisture content; Water activity; Colour; Rheology; Arrhenius model; Williams–Landel–Ferry model; Glass transition

1. Introduction

Honey, the viscous and aromatic product prepared

by bees, mainly from the nectar of flowers or honeydew,is a concentrated solution of various sugars. Honey

contains fructose and glucose (60–85%) as the predomi-

nant monosaccharides, maltose and sucrose as the most

important disaccharides, melezitose as the main trisac-

charide and other low molecular weight oligosaccha-

rides (Doner, 1977; Doner & Hicks, 1982). The

composition of honey (sugars and moisture content) is

responsible for many of the physicochemical propertiesof honey, such as viscosity, hydroscopicity, and granu-

lation. Most honeys are supersaturated solutions of

glucose, which have a tendency to crystallize spontane-

*Corresponding author. Tel.: +2310-471467/+30-310-998-785; fax:

+2310-471257/+30-310-471-457.

E-mail address: [email protected] (C.G. Biliaderis).

0260-8774/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2003.09.007

ously at room temperature in the form of glucose

monohydrate. Crystallization of honey, commonly

called granulation, is an undesirable process in liquid

honey because it affects the textural properties, makingit less appealing to the consumer. Moreover, in many

cases, crystallization of honey results in increased

moisture of the liquid phase which can allow naturally

occurring yeast cells to multiply causing fermentation of

the product (Doner, 1977). Water content as well as

water activity are the major factors that influence the

keeping quality or storability of honey.

Sensory and physicochemical properties are veryimportant parameters in determining the quality and

acceptability of honey and many studies have been

devoted to explore such determinants of product qua-

lity (Al-Khalifa & Al-Arify, 1999; Anupama, Bhat, &

Sapna, 2003; Bath & Singh, 1999; Popek, 2003; Singh &

Bath, 1997). The composition and properties of honey

vary with the floral and honeydew sources utilized by

mail to: [email protected]

Nomenclature

a�, b� chromatic components (red, yellow)

aw water activity

C1, C2 coefficients of WLF equation

Ea activation energy (kJmol�1)

G0 storage modulus (Pa)G00 loss modulus (Pa)

G–T Gordon–Taylor equation

k constant

L� lightness component

p probability level

r correlation coefficient

R universal gas constant (8.314 Jmol�1 K�1)

T temperature (�C, K)

Tg glass transition temperature

Tg1 glass transition of dry sample

Tg2 glass transition of glassy water

w2 weight fraction of waterWLF Williams–Landel–Ferry equation_c shear rate (s�1)

g viscosity (Pa s)

g� complex viscosity (Pa s)

gTg sample viscosity at Tgr shear stress (Pa)

x angular frequency (rad s�1)

10 A. Lazaridou et al. / Journal of Food Engineering 64 (2004) 9–21

honey-bees, as well as by regional and climatic condi-

tions. Some physicochemical parameters have already

been studied for their use in the identification of the

botanical and geographical origin of honey (Gomez

Barez et al., 2000; Popek, 2002; Terrab, Diez, & Here-

dia, 2002).

The rheological behaviour of honey has been investi-

gated for shelf-life, proper handling, packing and pro-cessing issues (White, 1978). The rheological properties

of honey, like many other physical properties, depend on

many factors, including composition and temperature.

The Arrhenius model is widely used for temperature

dependence of a property but models such Williams–

Landel–Ferry (WLF) that include the glass transition

temperature (Tg) as a parameter, have proved equally

useful for the viscosity–temperature relationship of foodsystems (Ollett & Parker, 1990; Soesanto & Williams,

1981; Williams, Landel, & Ferry, 1955). Depending on

concentration, and heating and cooling rates, aqueous

carbohydrate solutions exhibit several thermal events,

the most important being the Tg. At the glass transition

temperature, an amorphous material changes from the

rubbery to the glassy state upon cooling, leading to the

formation of a hard solid. As the stability of foods ismainly dependent on the water content and because Tg isalso highly sensitive to this parameter, the glass transi-

tion concept has been proposed as a useful tool for un-

derstanding the mechanisms of deteriorative processes in

food products and for controlling their shelf-life (Slade &

Levine, 1991). Indeed, the glass transition temperature is

often considered as a reference temperature; below Tg,the food is expected to be stable and above this tem-perature, the difference (T � Tg) between Tg and the

storage temperature T is assumed to control the rate of

physical, chemical and biological changes (Roos, 1995).

The present study was undertaken to determine cer-

tain physicochemical properties of selected Greek hon-

eys, and explore some relationships between them.

2. Materials and methods

2.1. Samples

Thirty-three honey samples were provided by bee-

keepers with guaranteed botanic origin. These honeys

were divided into four main groups according to the

variety type: honeydew from pine, honeydew from fir,mixed floral type, and floral from orange blossom.

Samples were sourced from different geographical areas

of Greece; pine honeydew from Thasos (6 samples),

Halkidiki (6 samples) and Evia (2 samples), fir honey-

dew from Helmos (2 samples) and Vytina (8 samples),

mixed floral from Livadia (3 samples), and orange

blossom floral from Argos (4 samples) and Sparti (2

samples). The botanical/geographical identification wasbased on their colour, aroma, taste and location of the

hives.

2.2. Moisture content and water activity, sugar composi-

tion

Refractive indices of honey samples were measured

using a refractometer at 20 �C and corresponding

moisture content (%) was calculated using the relation-

ship between refractive index and water content

(AOAC, 1990).Water activity (aw) of samples was measured at 20 �C

using an Aqualab 3TE water activity meter (Decagon

Devices Inc., Pullman, WA, USA). The determination

of aw values was performed twice; before and after the

heating of the samples at 50 �C for 1 h. This heat

treatment was carried out to dissolve crystals or nuclei,

which might be present in honey and can influence the

water activity of the system.The sugar composition was determined by a gas

chromatography (GC) method as described by Sabatini,

Marcazzan, Colombo, Carpana, and Serra (2001).

A. Lazaridou et al. / Journal of Food Engineering 64 (2004) 9–21 11

2.3. Colour

Colour was determined by a Metertech UV/VIS

SP8001 spectrophotometer (Metertech Inc., Taipei,

Taiwan) and a Minolta Dimage 5 digital camera. Sam-

ples were heated at 50 �C for 1 h before measurement to

ensure melting of any possibly formed crystals. Colour

was determined as absorbance at 420 nm after dilution

of honey with distilled water at a ratio 1:5 (Bath &Singh, 1999). Before measurement, mixtures of honey

and water were heated for better mixing and filtered for

removal of any coarse particles (plant residues, pollen),

which may also influence the colour. Images of samples

were taken by the digital camera with a proper lighting

system as described by Papadakis, Abdul-Malek,

Kamdem, and Yam (2000). L�, a� and b� colour para-

meters (CIE, 1976) were obtained using the Photoshopsoftware (v6.0, Adobe Systems Inc., San Jose, CA). L� is

the luminance or lightness component, which ranges

from 0 to 100, and a� and b� are the two chromatic

components, which range from )120 to 120 (a� from

green to red and b� from blue to yellow) (Adobe

Photoshop 5.0 User Guide for Machintosh and

Windows, 1998). The software uses a scale, ranging

from 0 to 255, to characterize Lightness, as well as thevalues of a and b. To convert these parameters to L�, a�,and b� the following formulas were used:

L� ¼ ðLightness=250Þ � 100 ð1Þ

a� ¼ ð240a=255Þ � 120 ð2Þ

b� ¼ ð240b=255Þ � 120 ð3Þ

2.4. Rheology

Rheological properties of honey were studied by arotational Physica MCR 300 rheometer (Physica Mess-

technic GmbH, Stuttgart, Germany) using a concentric

cylinder (diameter of cup and bob, 28.92 and 26.66 mm,

respectively) geometry; temperature was regulated by a

Paar Physica circulating bath and a controlled peltier

system (TEZ 150P/MCR) with an accuracy of ±0.1 �C.The data of the rheological measurements were analyzed

with the supporting rheometer software US200 V2.21.All honey samples were heated at 50 �C for 1 h before

rheological measurements to melt any crystals present

and to remove the air bubbles, factors that can influence

the viscosity of honey. Two types of measurements were

performed: (a) flow behaviour by measuring steady

shear viscosity (g) and shear stress (r) over a range of

shear rates ( _c) of 0.1–500 s�1 at 20, 30, 40, 50 and 60 �C;and (b) oscillatory measurements to obtain the storageand loss moduli (G0, G00) and complex viscosity (g�) at astrain level of 0.1% and a range of angular frequencies of

3–300 rad s�1 at 20 �C.

Temperature effects on steady shear viscosity wereanalysed using the Arrhenius relationship:

g ¼ g0 eðEa=RT Þ ð4Þ

where g is the viscosity at temperature T , g0 is a pre-

exponential factor, Ea is the activation energy for flow, Ris the perfect gas constant and T is the absolute tem-

perature (K).

The temperature dependence of honey viscosity wasalso described using the WLF model (Williams et al.,

1955):

logggTg

!¼ �C1ðTg � T Þ

C2 þ ðTg � T Þ ð5Þ

where Tg is the glass transition temperature, g is the

viscosity at temperature T , gTg is the viscosity of sample

at Tg and C1 and C2 are the WLF constants. Experi-

mental data were fitted to the model using the TableCurve 2D software (v4.0, SPSS Inc., Chicago, IL).

2.5. Differential scanning calorimetry

The glass transition temperature (Tg) of honeys was

determined by differential scanning calorimetry (DSC)

using a PL DSC-Gold calorimeter (Polymer Labs. Ltd,

Epsom, UK). Temperature calibration was made with

cyclohexane, dodecane and octane, whereas heat flow

calibration was made by reference to the known meltingenthalpy of indium and gallium (purity 99.99%) from

Goodfellows Metals (Biliaderis, Lazaridou, & Arvani-

toyannis, 1999).

For studying the effect of moisture on Tg, 14 honey

samples were chosen covering a moisture content range

of 13.0–18.9%. For increasing the above range the

sample with the lowest moisture content (13%) was

either diluted with water or concentrated under vacuum(at 50 �C) to the following levels of moisture content:

26.9%, 23.4%, 20.0%, 16.5%, 11.9%, 11.0% and 10.2%.

The native honey samples as well as the diluted and

concentrated samples were hermetically sealed into

stainless steel pans (�40–45 mg) and analysed by calo-

rimetry under continuous flow of dry N2 gas (20

mlmin�1) to avoid condensation of moisture. First, the

pans were heated from +20 to +50 �C at a heating rateof 10 �Cmin�1 and kept at +50 �C for 3–5 min to ensure

the melting of any crystals and reach at thermodynamic

equilibrium. The samples were then quenched–cooled

with liquid N2 to )100 �C and reheated to +50 �C at the

same heating rate (10 �Cmin�1). The Tg was determined

in the latter heating scans as the onset temperature of

the step-like decrease in the heat flow.

Data analysis to fit experimental values of Tg to theempirical Gordon–Taylor (G–T) model (Gordon &

Taylor, 1952) was performed using the Table Curve 2D

software:


Tg ¼w1Tg1 þ kw2Tg2

w1 þ kw2

ð6Þ

where Tg1 is the glass transition temperature of the

sample at zero moisture content, w1 is the weight frac-

tion of dry solids, Tg2 is the glass transition temperature

for glassy water, w2 is the weight fraction of water and kis a constant. The constructed G–T plots were based onthe best data fitting to the equation (i.e. optimization for

both parameters, k and Tg1), where a Tg of )138 �C was

used for water (Sugisaki, Suga, & Seki, 1968).

3. Results and discussion

3.1. Water content, water activity, sugar composition,

colour

The results of analysis of some physicochemical pa-

rameters namely, moisture, water activity (aw), and

colour (absorbance at 420 nm and L�, a�, and b� colourparameters) for the Greek honeys are summarized in

Table 1. The refractive index varied from 1.4892 to

1.5043 and the corresponding moisture content ranged

between 13.0% and 18.9%; these values are within the

range found by other researchers and indicate a proper

degree of maturity for these honey samples. In general,

the moisture content in different varieties of honey may

be as low as 13% (White, 1978) and as high as 29%(Junzheng & Changying, 1998). For example, moisture

contents have been found in the range of 14.0–16.9% for

Saudi honeys (Al-Khalifa & Al-Arify, 1999), 13.8–17.8%

for Spanish honeys (Gomez Barez et al., 2000), 15.4–

18.1% for Polish honeys (Popek, 2003), 16.8–20.3% for

Moroccan honeys (Terrab et al., 2002), and 18.7–21.8%

for Indian honeys (Singh & Bath, 1997). The difference

in moisture content was significant between all Greekhoneys; however, Greek regulations require <21%

moisture for safety from fermentation.

Honey is an intermediate moisture food with a water

activity of about 0.6 and is therefore shelf stable for a

reasonable period of time. The low aw (high osmotic

environment) does not support microbial growth, pre-

venting fermentation of honey by osmophilic yeast. The

aw values of Greek honeys obtained after heating thesamples at 50 �C varied within the range 0.528–0.663

(Table 1), whereas for most of the samples the corre-

sponding values before heating were found higher. It is

well known that crystal formation in sugars results to

water release, thus increasing water availability.

Table 2 shows the composition of sugars identified in

the honey samples. The monosaccharides fructose (22.1–

41.3%) and glucose (13.5–36.3%) were the main sugars,with fructose being always the most abundant. The

honeys with lower percentages of fructose and glucose

were those with a non-floral origin (i.e. honeydew hon-

eys). Among the disaccharides, maltose was the mostabundant one, ranging between 1.9% and 6.7%. The

relatively low levels of sucrose for most samples indicate

that the selected honeys were at an advanced stage of

ripening. Several trisaccharides were also identified and

quantified, namely raffinose, erlose, melezitose, panose,

isomaltotriose and maltotriose. It was of interest to note

that melezitose was present in relatively high amounts

(9.1–14.4%) for most of the honeydew-fir samples.The colour of honey is related to the content of

phenolics, HMF, pollen and minerals (Perez-Arquille,

Conchello, Arino, Juan, & Herresa, 1994). The absor-

bance at 420 nm varied between 0.113 and 0.915 (Table

1) and is in agreement with the finding of other authors

(Bath & Singh, 1999; Singh & Bath, 1997). It is known

that orange blossom honeys are honeys with very light

colour, which concurs with the lowest values in the ab-sorbance range for the samples shown in Table 1. The

colour parameters L�, a� and b� measured using the

digital camera were within the range of 35.79–59.56,

()5.06)–27.27 and 16.91–42.92, respectively. These val-

ues are in close agreement with those found by others

researchers using chromatometers (Anupama et al.,

2003; Popek, 2002, 2003). It is worthy to note, that or-

ange blossom honeys were found to have high values forlightness (L�), and low values for red (a�) and yellow (b�)components, showing similar responses to the results

from absorbance measurements at 420 nm.

3.2. Rheological behaviour

Fig. 1 illustrates the steady shear flow curves (Fig. 1a)

and a typical mechanical spectrum (Fig. 1b) for a Greek

honey sample. All honey samples behaved as Newtonian

fluids at all temperatures of measurement (Fig. 2).

Apparent viscosity (g) and complex viscosity (g�) wereconstant, regardless of the shear rate and angular fre-quency, respectively (Fig. 1). Moreover, the G00 was

dependent on frequency and greater than G0 at all fre-

quencies (Fig. 1b). In most of the published works,

honey was reported to have a Newtonian behaviour

(Abu-Jdayil, Al-Majeed Ghzawi, Al-Malah, & Zaitoun,

2002; Bhandari, D’Arcy, & Chow, 1999; da Costa &

Pereira, 2002).

Values of various rheological parameters obtainedfrom steady shear and dynamic measurements for all

samples are summarized in Table 3. These values ob-

tained from measurements at 20 �C varied within the

wide range of 9.9–200.0 (Pa s) for apparent viscosity (g),0.15–19.10 (Pa) for storage modulus (G0), 64–1682 (Pa)

for loss modulus (G00), and 7.7–164.4 (Pa s) for complex

viscosity. The differences among samples could be at-

tributed to natural variations in composition (individualsugars and water content), as they belong to different

plant species-specific varieties and collected from dif-

ferent geographical locations in Greece. It is clear that

Table 1

Some physicochemical parameters in 33 Greek honeys

Sample No. Botanic/geographical origin Moisture content,

Xw (g/100 g)

Water activity

(20 �C) aftermelting at 50 �C

Colour

Absorbance at

420 nm

L� a� b�

1 Honeydew (pine)/Thasos 18.9 0.610 0.387 45.48 12.13 40.15






7 Honeydew (pine)/Halkidiki 15.0 0.580 0.703 45.38 9.48 38.48






13 Honeydew (pine)/Evia 16.3 0.570 0.593 40.14 19.36 39.96

14 Honeydew (pine)/Evia 14.8 0.663 0.525 44.04 16.58 41.05

15 Honeydew (fir)/Helmos 13.4 0.562 0.557 41.63 19.36 41.53

16 Honeydew (fir)/Helmos 13.0 0.565 0.476 41.63 17.35 40.11

17 Honeydew (fir)/Vytina 13.9 0.570 0.428 39.79 18.49 40.59








25 Floral/Livadia 13.8 0.528 0.915 42.97 27.27 39.50

26 Floral/Livadia 14.1 0.528 0.831 35.79 23.49 35.59

27 Floral/Livadia 15.1 0.550 0.682 39.30 21.30 40.84

28 Floral (Orange blossom)/Argos 15.1 0.542 0.183 47.34 1.70 22.72

29 Floral (Orange blossom)/Argos 15.8 0.540 0.161 45.75 0.89 19.85

30 Floral (Orange blossom)/Argos 16.2 0.584 0.127 59.56 )5.06 16.91

31 Floral (Orange blossom)/Argos 17.9 0.577 0.113 45.76 )2.79 18.93

32 Floral (Orange blossom)/Sparti 15.6 0.548 0.134 51.62 7.29 28.93

33 Floral (Orange blossom)/Sparti 15.6 0.546 0.115 47.18 2.96 23.31

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13

Table 2

Distribution of the levels of various sugars (%) among the selected honey samples

Sample

No.

Fructose

(g/100 g)

Glucose

(g/100 g)

Sucrose

(g/100 g)

Trehalose

(g/100 g)

Maltose

(g/100 g)

Isomal-

tose

(g/100 g)

Raffinose

(g/100 g)

Erlose

(g/100 g)

Melezi-

tose

(g/100 g)

Panose

(g/100 g)

Isomalto-triose

(g/100 g)

Malto-triose

(g/100 g)

Malto-tetraose

(g/100 g)

1 29.9 26.3 0.7 <0.1 4.0 1.4 0.6 3.1 0.1 0.2 0.1 0.2 7.7

2 29.0 25.6 0.8 0.1 3.9 1.4 0.6 3.3 0.1 0.2 0.1 0.2 7.9

3 30.5 26.4 0.8 <0.1 4.2 1.4 0.6 3.3 0.1 0.2 0.1 0.2 8.0

4 30.5 24.5 0.2 0.1 6.7 3.0 0.5 0.6 0.1 0.5 0.2 0.3 0.7

5 30.5 23.6 0.1 <0.1 6.1 3.2 0.2 0.6 0.1 0.5 0.2 0.3 0.7

6 30.6 23.7 0.1 <0.1 6.2 2.9 0.2 0.6 0.1 0.5 0.2 0.3 0.6

7 22.5 18.4 0.1 0.1 4.4 2.4 0.3 1.0 0.1 0.4 0.1 0.3 0.4

8 23.0 19.6 0.1 <0.1 4.4 2.4 0.6 1.1 0.1 0.4 0.1 0.3 0.4

9 22.2 19.0 0.1 <0.1 4.1 2.0 0.6 1.0 0.1 0.2 0.1 0.1 0.2

10 23.9 21.1 0.1 0.1 4.6 2.1 0.5 0.8 0.1 0.3 0.1 0.2 0.4

11 24.1 21.1 0.1 0.1 4.7 2.2 0.5 0.8 0.1 0.4 0.2 0.3 0.6

12 24.1 21.2 0.1 0.1 4.6 2.0 0.5 0.8 0.1 0.3 0.1 0.3 0.4

13 32.3 28.7 0.6 0.6 4.3 1.2 0.5 1.4 2.0 0.2 <0.1 0.2 0.3

14 29.7 25.5 1.4 0.4 4.2 1.0 0.6 2.8 1.9 0.2 <0.1 0.2 0.2

15 27.3 18.6 0.1 1.7 3.5 1.1 0.8 0.6 9.1 0.6 0.1 0.3 0.3

16 24.1 16.0 0.1 2.9 2.7 0.8 0.9 0.8 10.5 0.4 <0.1 0.3 0.3

17 25.7 16.7 0.1 2.1 3.1 1.1 0.6 0.4 11.0 1.0 0.1 0.4 0.3

18 28.1 19.0 0.2 1.6 3.6 1.0 0.5 0.5 9.8 0.7 <0.1 0.3 0.2

19 32.5 24.5 0.2 0.8 5.4 1.2 1.0 2.1 1.3 0.1 <0.1 0.5 0.5

20 31.1 25.0 0.2 0.8 5.3 0.9 1.0 2.1 1.4 0.1 <0.1 0.4 0.3

21 22.1 13.6 0.1 3.1 1.9 0.8 0.9 0.5 14.4 1.0 <0.1 0.4 0.4

22 22.2 13.5 0.1 3.0 1.9 0.7 0.9 0.5 14.2 0.9 <0.1 0.3 0.3

23 29.0 19.7 0.1 1.5 3.6 0.9 0.3 0.6 10.5 0.6 <0.1 0.2 0.2

24 22.1 14.2 0.2 2.9 2.2 1.0 0.7 0.4 13.0 1.1 0.1 0.4 0.3

25 34.8 29.2 0.2 0.6 4.7 0.9 0.4 0.8 2.5 0.2 <0.1 0.1 0.1

26 34.1 29.0 0.1 0.6 4.6 1.1 0.3 0.8 2.4 0.2 <0.1 0.2 0.2

27 34.9 30.0 0.1 0.7 4.9 1.3 0.4 0.4 2.2 0.2 <0.1 0.3 0.3

28 39.1 32.9 2.7 0.1 4.4 0.4 <0.1 0.9 0.3 <0.1 <0.01 <0.1 <0.01

29 38.1 32.8 2.7 0.1 4.3 0.4 0.5 0.8 0.3 <0.1 <0.01 <0.1 <0.01

30 39.1 36.3 0.6 0.1 3.0 0.4 0.5 0.6 0.2 <0.1 <0.01 <0.1 <0.01

31 39.9 34.7 0.6 0.1 3.3 0.5 0.5 0.6 0.2 <0.1 <0.01 0.1 <0.01

32 39.9 35.4 1.6 0.1 3.6 0.6 0.5 0.8 <0.1 0.1 <0.1 0.1 <0.01

33 41.3 32.7 1.7 <0.1 3.6 0.4 0.5 0.7 <0.1 <0.1 <0.01 <0.1 <0.01

14

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(a) (b)

Fig. 1. Steady shear viscosity profiles of three Greek honey samples (a) and a representative mechanical spectrum of a Greek honey (b) at 20�C.

Fig. 2. Temperature effect on viscosity of a representative Greek honey

and its Arrhenius plot of viscosity (at 10 s�1) vs. temperature (inset).


the viscosity of honey decreases with water content. As

shown in Fig. 3a there was a significant relationship

between moisture content and apparent viscosity, as

described by an exponential function: g ¼ 90071

e�0:4997Xw (r2 ¼ 0:80, p < 0:05). Such moisture content

dependence of viscosity has been also noticed by otherauthors (Anupama et al., 2003; Junzheng & Changying,

1998; Sopade et al., 2002; Zaitoun, Ghzawi, Al-Malah,

& Abu-Jdayil, 2001). Moreover, a relationship between

moisture content and the loss modulus, estimated by

dynamic rheological measurements was observed (Fig.

3b); the experimental data fitted into an exponential

equation: G00 ¼ 2 � 106 e�0:5547Xw (r2 ¼ 0:72, p < 0:05). Itis worthy here to note that the g values were very closewith the respective g� values for each individual sample

(Table 3). It appears therefore that the viscosity data

follow the Cox–Merz rule (Cox & Merz, 1958), as ex-

pected for Newtonian fluids. According to this rule the

complex viscosity from small-deformation oscillation

measurements and the steady shear viscosity from ro-

tational measurements superimpose closely at equiva-

lent numerical values of angular frequency (x, rad s�1)

and shear rate ( _c, s�1) for non-interacting moleculardispersions.

The temperature effects on honey viscosity are shown

in Fig. 2; as expected, the viscosity was reduced with

increasing temperature. The temperature dependence of

g is adequately described using the Arrhenius relation-

ship (Fig. 2, inset); the correlation coefficients (r2) for

each sample are given in Table 3 and were all greater

than 0.91. The observation that the temperature de-pendence of viscosity follows the Arrhenius relationship

has been also reported by other researchers for honey

(Al-Malah, Abu-Jdayil, Zaitoun, & Ghzawi, 2001; Bath

& Singh, 1999; Bhandari et al., 1999; Sopade et al.,

2002) as well as for other sugar-rich liquid foods such

as fruit juice concentrates and sugar syrups (Khalil,

Ramakrishna, Nanjundaswamy, & Patwardhan, 1989;

Manohar, Ramakrishna, & Udayasankar, 1991). Theactivation energies (Ea) for flow estimated from the

slope of the linear relationship of logðgÞ vs. ð1=T Þ are

presented in Table 3 for all samples and varied between

69.1 and 93.75 kJmol�1. Activation energy reflects the

sensitivity of viscosity to temperature changes; higher Ea

means that the viscosity is relatively more sensitive to

a temperature change. There was an inverse linear

relationship between Ea and moisture content (r2 ¼ 0:61,p < 0:01). Although the Arrhenius formalism seems

to adequately describe the temperature dependence

of honey viscosity, it gives relatively high values for

the activation energy, which are more typical of

chemical reactions. Similar observations concerning the

Table 3

Rheological parameters of 33 Greek honeys

Sample No. Viscosity (Pa s)

(20 �C)Ea (kJmol�1) G0 (Pa) (x ¼ 10 rad s�1)

(20 �C)G00 (Pa) (x ¼ 10 rad s�1)

(20 �C)Complex viscosity

(Pa s) (20 �C)

1 9.9 72.69 (r2 ¼ 0:97) 3.01 99 9.0

2 10.5 73.62 (r2 ¼ 0:97) 0.40 80 7.7

3 10.7 74.09 (r2 ¼ 0:97) 0.33 111 10.4

4 61.1 89.09 (r2 ¼ 0:97) 4.46 665 62.2

5 55.5 88.13 (r2 ¼ 0:97) 2.88 526 51.4

6 63.6 88.74 (r2 ¼ 0:97) 4.87 708 66.2

7 42.8 82.79 (r2 ¼ 0:97) 3.02 476 43.1

8 44.4 83.79 (r2 ¼ 0:97) 2.39 467 42.2

9 46.8 84.15 (r2 ¼ 0:97) 3.91 469 42.4

10 39.0 80.88 (r2 ¼ 0:96) 2.87 424 38.4

11 37.6 80.55 (r2 ¼ 0:96) 2.22 418 37.8

12 39.8 82.29 (r2 ¼ 0:97) 2.79 429 38.8

13 38.5 79.39 (r2 ¼ 0:95) 2.10 928 34.1

14 38.4 80.36 (r2 ¼ 0:96) 6.51 265 35.9

15 86.3 86.20 (r2 ¼ 0:96) 7.83 980 88.6

16 200.0 93.75 (r2 ¼ 0:96) 10.70 1682 164.4

17 73.2 84.00 (r2 ¼ 0:96) 3.40 772 69.7

18 80.3 85.23 (r2 ¼ 0:96) 4.91 988 89.3

19 67.4 87.33 (r2 ¼ 0:97) 3.50 708 64.0

20 59.0 84.21 (r2 ¼ 0:96) 2.98 624 56.4

21 154.0 89.77 (r2 ¼ 0:96) 19.10 1177 115.0

22 160.0 89.56 (r2 ¼ 0:96) 10.20 1701 167.0

23 67.6 89.65 (r2 ¼ 0:98) 5.58 683 67.5

24 118.0 93.33 (r2 ¼ 0:98) 4.30 1074 105.0

25 79.8 90.22 (r2 ¼ 0:97) 5.64 839 75.8

26 77.9 89.74 (r2 ¼ 0:97) 4.40 803 78.9

27 73.8 88.59 (r2 ¼ 0:97) 5.90 773 69.8

28 28.8 78.87 (r2 ¼ 0:96) 3.75 376 34.0

29 26.2 79.61 (r2 ¼ 0:96) 0.94 108 20.5

30 12.6 83.06 (r2 ¼ 0:99) 0.33 106 10.4

31 13.2 75.40 (r2 ¼ 0:98) 0.15 100 9.9

32 28.6 89.75 (r2 ¼ 0:99) 0.31 208 20.4

33 26.8 90.17 (r2 ¼ 0:99) 2.35 64 26.8


magnitude of Ea derived from viscosity data of three

Jordanian honey samples (Ea 95.6–97.7 kJmol�1, tem-

perature range 20–50 �C) have been made in a previous

study by Al-Malah et al. (2001).

3.3. DSC thermal behaviour

The effect of moisture content (m.c.) on the DSC

thermal traces of four representative native honey

samples is demonstrated in Fig. 4a. The glass transition

temperature shifted to lower temperatures with increaseof moisture content due to the plasticization effect of

water. Water is a very effective plasticizer for hydro-

philic components, such as low molecular weight car-

bohydrates (Levine & Slade, 1988); this effect has been

related to the ability of water molecules to weaken hy-

drogen bonds, dipole–dipole, and intra- and inter mole-

cular interactions (Matveev, Grinberg, & Tolstoguzov,

2000). The glass transition temperatures of the honeysamples are summarized in Table 4, varying between

)34.6 and )47.2 �C for a moisture content range of

13.0–18.9 g/100 g. These values are similar to those

found by other researchers; i.e. a range from )40 to )46�C for honey samples with 15.8–18.0% moisture content

(Sopade, Bhandari, Halley, D’Arcy, & Caffin, 2001;

Sopade et al., 2002), from )37.5 to )42.5 �C for samples

with m.c. about 17.5% (Cordella et al., 2002), and from)42.5 to )50.7 �C for samples with undetermined m.c.

(Kantor, Pitsi, & Thoen, 1999) have been reported. It is

generally accepted that differences in the composition of

carbohydrate solutions could contribute to the variation

in the Tg; i.e. the glass transition temperature is a func-

tion of both moisture content and the type of solute

(Slade & Levine, 1991). Recently, the Tg value responsesto the modification of chemical composition of honeyhas been used, concomitantly with other thermal events

detected by DSC, to develop a new method for adulte-

ration detection in this product (Cordella et al., 2002);

these authors have found that adulterations of honey by

industrial sugar syrups can be detected calorimetrically

up to a minimum of 5–10% addition.

The plasticizing action of water is also obvious at

comparative DSC traces of diluted and concentratedhoney samples using a native sample (16), as illustrated

(a)

(b)

Fig. 3. Effect of moisture content on viscosity (g) (a) and loss modulus

(G00) (b) of Greek honeys.

(a)

(b)

Fig. 4. DSC thermal scans for four honey samples with different

moisture content (a) and for sample 16, and diluted and concentrated

sample 16 (b); arrows indicate the position of the onset temperature

considered as glass transition.


in Fig. 4b. With increasing moisture content of honey

from 10.2 to 26.9 g/100 g the Tg decreased significantly

from )25.0 to )69.5 �C. Similar trends in Tg for dilutedhoneys samples have been reported by Kantor et al.

(1999). These authors have also noticed onto the DSCscans water crystallization from honey/water mixtures

with less than approximately 85% honey content. Their

DSC traces showed the presence of a typical endother-

mic peak in the range )20 to 0 �C linked to the free

water of the sample; with decreasing honey content this

peak moved towards 0 �C and the enthalpy of melting

increased. In the present study, the sample with the

highest moisture content (26.9%) corresponds to 84%honey content. For all the examined samples there was

no evidence of water or sugar crystallization (Fig. 4).

This could be attributed to the effectiveness of quench-

cooling with liquid nitrogen in preventing freezing of

water during Tg measurement and the absence of �free-water’ from the native honey samples. The minor tran-

sition at 0 �C present in all DSC curves of Fig. 4 was not

related to crystallization of free water from the sample

but is an artifact due to moisture condensation from theatmosphere; i.e. the magnitude of this transition was

found independent of the moisture content of the sam-

ple and it represented ice melting of water in amounts

less than 0.22% of the amount of water present in any-

one of the honey samples examined. With reference

to the concentrated sugar solutions, such as honeys, a

rapid reduction in temperature also prevents solute nu-

cleation as a result of the lack of sufficient mobility ofthe sugar molecules to assemble into a crystal lattice.

The Tg–water content relationship presented in Fig. 5

reveal more clearly the sensitivity of honey to water-

plasticization. Despite the large variation in sugar

composition among the samples (Table 2), the experi-

mental data for all honeys fitted successfully well

(r2 ¼ 0:99) to the empirical G–T model (Eq. (6)). Esti-

mated values of the G–T parameters were 3.14 for the kand 288.0 K (or 15 �C) for the Tg1, i.e. the glass transi-

tion temperature for dry honey solids. Consequently, the

Fig. 5. Relationship of glass transition and moisture content for honey;

the solid line gives the G–T plot of the experimental data from DSC

measurements.

Table

4

Glass

transitiontemperaturesofGreek

honeysandestimatedparametersoftheWLFmodel

Sample

No.

Moisture

con-

tent(g/100g)

T g,K

(�C)experim

ental

data

from

DSC

WLF(�u

niversal’constants)

WLF(T

gfrom

DSC

experim

entaldata)

T g,K

(�C)(predicted)

logðg

T gÞ(Pas)

r2logðg

T gÞ(Pas)

C1

C2(K

)r2

118.9

225.85()47.15)

214.09()58.91)

11.52

0.99

13.10

17.68

30.90

0.99

217.4

227.45()45.55)

213.40()59.60)

11.57

0.99

14.77

18.98

24.81

0.99

413.9

235.77()37.23)

226.73()46.27)

11.57

0.99

15.26

19.38

25.07

0.99

515.4

234.31()38.69)

226.04()46.96)

11.57

0.99

15.73

19.89

24.75

0.99

915.4

233.15()39.85)

222.72()50.28)

11.69

0.99

16.50

20.41

22.44

0.99

13

16.3

232.96()40.04)

219.65()53.35)

11.75

0.98

21.72

25.18

14.89

0.98

16

13.0

237.23()35.77)

230.53()42.47)

11.81

0.99

21.09

24.56

17.11

0.99

17

13.9

238.40()34.60)

223.17()49.83)

11.85

0.99

20.00

23.17

15.12

0.99

19

14.6

234.31()38.69)

225.65()47.39)

11.68

0.99

17.75

21.60

20.92

0.99

21

14.1

236.36()36.64)

227.68()45.32)

11.89

0.99

20.79

24.17

16.91

0.99

26

14.1

235.77()37.23)

227.42()45.58)

11.62

0.99

17.64

21.52

20.92

0.99

30

16.2

229.20()43.80)

209.88()63.12)

11.80

0.95

21.59

25.01

13.95

0.94

31

17.9

229.06()43.94)

214.92()58.08)

11.60

0.99

12.85

17.20

29.71

0.99

33

15.6

228.90()44.09)

219.96()53.04)

11.61

0.99

18.66

22.68

20.20

0.99


G–T model could be an important tool for the predic-

tion of Tg. Another usefulness of the above modeling is

the determination of Tg1, a temperature that it could not

be measured experimentally, as it is impossible to re-

move the strongly bound water from honey.

Being a molecular phenomenon, the glass transitionhas been often related to many food properties that

comprise mechanical/structural relaxation processes. The

WLF equation (Eq. (5)) includes the glass transition

temperature as a parameter and has been suggested to be

applicable in predicting the temperature dependence of

viscosity, a primary property that describes mechanical

relaxation of mobile components. This equation has been

claimed as a more appropriated model than the Arrhe-nius relationship in the rubbery domain; i.e. between Tgand about Tg þ 100 �C (Roos, 1995; Slade & Levine,

1991). The WLF equation specifies a much stronger

temperature dependence of viscosity compared to that

predicted by the Arrhenius formalism. In this context, the

Tg is considered as a reference temperature: below Tg,the viscosity is very high, whereas above this temperature,

the difference between storage and processing tempera-tures (T � Tg) is assumed to control the rate of viscosity

changes in the product. Generally, the viscosity in the

glassy state varies between 107 and 1016 Pa s as has been

estimated and/or extrapolated for a range of materials by

various procedures (Maltini & Manzocco, 1998; Ollett &

Parker, 1990; Peleg, 1992; Slade&Levine, 1993; Soesanto

& Williams, 1981; Williams et al., 1955).

3.4. WLF modeling of viscosity

The viscosity data of 14 honey samples were fitted to

the WLF model to test its applicability at temperatures

Fig. 6. Temperature dependence of viscosity for 14 honey samples

according to the WLF formalism; the solid line shows the WLF

equation using the �universal’ constants.


from 20 to 60 �C (Fig. 6); i.e. between 60 and 125 �Cabove Tg of honeys. In general, as already mentioned,

the WLF equation is not intended for use much below

Tg, (in the glassy state) or in the very low viscosity re-

gime (<10 Pa s) occurring at temperatures typically 100

�C or more above Tg, where Arrhenius kinetics apply

(Slade & Levine, 1991; Soesanto & Williams, 1981; So-

pade et al., 2002). In a study for the application of WLF

relationship in starch hydrolysates, D’Haene and VanLiederkerke (1996) considered the latter limit restrictive

and proved that the principles of the WLF-theory can

be applied down to viscosity levels of 0.1 Pa s. These

viscosity levels are comparable to viscosity values for

Greek honeys measured at high temperatures. Our vis-

cosity data were also extended to higher temperatures

than usual with success. Moreover, these high temper-

atures are usually used in honey processing for ease ofhandling with minimal quality (e.g. hydroxymethyl-

furfural, HMF) degradation and thus it would be useful

the prediction of viscosity changes under these condi-

tions. The WLF equation has been previously found to

adequately describe viscosity data of Jordanian (tem-

perature range 20–50 �C; Al-Malah et al., 2001) and

Australian (temperature range 2–40 �C; Sopade et al.,

2002) honeys.Very good fits (r2 > 0:95) to the WLF model were

obtained for all 14 data sets (Table 4) as showed in Fig.

6. All data sets could be fitted to a single master curve

using the WLF model with the fixed �universal’ con-

stants, C1 ¼ 17:44 and C2 ¼ 51:6 K (Williams et al.,

1955). These results concur with the findings of

Al-Malah et al. (2001) who also found that the WLF

equation, using the �universal’ constants, adequatelydescribed the temperature-dependence of viscosity for

three honey samples. The �universal constants’ of theWLF equation are actually average values, which have

been extracted from data on numerous glass-forming

liquids and their use is not successful for all materials.

For each data set of g vs. T (each honey sample con-

sidered individually), the WLF equation with �universal’constants was fitted to yield the corresponding values of

Tg and gTg . Table 3 shows these predicted values for the

glass transition temperature (Tg) ranging from )59.6 to)42.5 �C and for the glass viscosity (gTg ) values which

ranged from 1011:5 to 1011:9 Pa s. Although Tg is a pa-

rameter that is measured by various techniques, a high

viscosity value such as gTg , is not accessible experimen-

tally by any of the existing rheometers and viscometers.

The predicted Tg values differed from the corresponding

DSC values obtained experimentally by 7–19 �C (Table

4). It is well known that the glass transition reflects arange of temperatures rather than a single temperature.

Even for single components (e.g. starch) the glass entails

changes over a temperature range of 10–20 �C (Bili-

aderis, 1998) and thus the Tg value depends on the

technique and experimental conditions used for its de-

termination. Moreover, in a previous study for con-

centrated fructose (69%) solutions differences up to 21

�C between Tgs values obtained by DSC measurementsand Tgs values estimated by the application of WLF on

viscosity data have been reported (Maltini & Manzocco,

1998).

Recently, there have been conflicting reports on how

to handle the constants in the WLF model, whether to

use the �universal’ constants or to allow them to vary for

a good fit to the experimentally data (Peleg, 1992; Roos,

1995). In a second attempt to apply the WLF model onviscosity data of Greek honeys with varying the C1 and

C2 constants and using as reference temperature the Tgvalues experimentally determined from DSC, the cor-

relation coefficients were the same with the previous

analysis using the �universal’ constants (Table 4). The C1

and C2 constants were in the range of 17.20–25.18 and

13.95–30.90 K, respectively (Table 4). Moreover, the

predicted values of glass viscosities varied widely from1012:9 to 1021:7 Pa s and for most of the samples were

extremely high which is probably unrealistic. However,

the latter findings are in agreement with a recent study

by Sopade et al. (2002) for Australian honeys. In their

study, although the temperature dependence of viscosity

was badly predicted using the �universal’ WLF con-

stants, the WLF model was successfully applied if the C1

and C2 were allowed to vary. The estimated values forthe Australian honeys have been reported within the

range 13.7–21.1, 55.9–118.7 �C, and 4 · 107 )4 · 1020Pa s for C1, C2, and gTg , respectively (Sopade et al.,

2002). These researchers claimed that the WLF con-

stants should be allowed to vary in order to use them

as parameters for comparison of the temperature sen-

sitivity among different samples. The WLF model with


different constants than the �universal’ ones has beenalso applied by Kerr and Reid (1994) on viscosity data

of sugar (glucose and sucrose) and maltodextrin solu-

tions. Maltini and Manzocco (1998) have applied the

WLF formalism in both ways (with �universal’ and with

varying constants) on viscosity data for concentrated

solutions of various materials (polymers, fructose, lactic

acid, and glycerol); in their study the estimated values

for glass viscosities have been found about 1014 cP.Soesanto and Williams (1981), using the �universal’constants on viscosity data of highly concentrated (91.9–

97.9%) fructose–sucrose mixtures (1:7), indicated that

gTg may not be invariant; i.e. they showed that gTg variesfrom about 4.5 · 1014 to 1.5 · 1015 mPa s as the mole

fraction of sugar varies from 0.4 to 0.7. Finally, Peleg

(1992) has reported that for a variety of polymers and

amorphous sugars, the magnitude of C1 and C2 mayvary considerably from the �universal’ constants de-

pending on the material studied, the measured property

and the selected reference temperature.

4. Conclusions

All honey samples exhibited Newtonian flow behav-iour and the viscosity in the temperature range of 20–60

�C can be predicted using the Arrhenius relationship; the

calculated activation energies for flow were inversely

related to the moisture content. Steady shear viscosity

and the loss modulus values (G00) at 10 s�1 or rad s�1

decreased exponentially with increasing moisture con-

tent of honey. The glass transition temperature (Tg) ofthe honey samples varied between )34 and )47 �C (overthe entire moisture content range, 13.0–18.9%) and ex-

hibited strong depression with the moisture; the Tg datafitted reasonably well to the Gordon–Taylor empirical

equation. The WLF equation was also suitable to model

the rheological behaviour of honey viscosity above the

glass transition (T � Tg ¼ 60–125 �C) using the universalvalues of the WLF model (C1 ¼ 17:44 and C2 ¼ 51:6 K)

where all samples fitted onto a common master curve.

Acknowledgement

The authors wish to thank Mr. A. Pavlis for his

analytical assistance.

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Composition, thermal and rheological behaviour of selected Greek honeys

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