Modeling the effect of temperature and relative humidity on ...

<strong>Modeling</strong> <strong>the</strong> <strong>effect</strong> <strong>of</strong> <strong>temperature</strong> <strong>and</strong> <strong>relative</strong> <strong>humidity</strong> on physicochemical
properties <strong>of</strong> honey
Laleh Mehryar a , Mohsen Esmaiili a , Ali Hassanzadeh b
a
Department <strong>of</strong> Food Science <strong>and</strong> Technology, University <strong>of</strong> Urmia, Urmia, Iran
(laleh.mehryar@gmail.com) & (m.esmaiili@urmia.ac.ir)
b
Department <strong>of</strong> Chemistry, University <strong>of</strong> Urmia, Urmia, Iran (a.hassanzadeh@urmia.ac.ir)
ABSTRACT
Several physico-chemical, <strong>the</strong>rmal <strong>and</strong> rheological characteristics <strong>of</strong> six Iranian honey samples from
various floral sources were investigated. The studied parameters were: moisture content, water activity (a w ),
water insoluble solids, diastase activity, pH, free acidity, total sugar content, reducing sugar, fructose to
glucose ratio, sucrose content, ash, fat, total nitrogen content, HMF, density, specific weight, color with
hunter parameters (L, a <strong>and</strong> b), electrical conductivity, glass transition <strong>temperature</strong> (T g ), viscosity, stickiness
<strong>and</strong> equilibrium moisture content. In order to have a precise evaluation <strong>of</strong> honey samples, some <strong>of</strong> its
physical properties such as density <strong>and</strong> viscosity were investigated at three <strong>temperature</strong> levels. To study <strong>the</strong>
<strong>temperature</strong> <strong>effect</strong> on viscosity Arrhenius, VTF <strong>and</strong> power law models were considered. The evaluations <strong>of</strong>
<strong>the</strong> models were done by R 2 , P%, χ 2 <strong>and</strong> RMSE indices. There were some relationships with high coefficient
<strong>of</strong> determination between physical <strong>and</strong> chemical properties. At <strong>the</strong> studied shear rate (0.04-0.63 s -1 ), all <strong>of</strong> <strong>the</strong>
samples showed non-Newtonian (pseudoplastic <strong>and</strong> dilatant) behavior. Among three applied models, samples
1, 2 <strong>and</strong> 3 showed a well accordance with Arrhenius <strong>and</strong> samples 4, 5 <strong>and</strong> 6 with VTF models. Equilibrium
moisture content (EMC) <strong>of</strong> <strong>the</strong> samples was increased by increasing water activity but <strong>the</strong> inversion <strong>effect</strong>
has appeared at a constant water activity (>0.5) among different <strong>temperature</strong>s <strong>of</strong> <strong>the</strong> samples.
Keywords: Honey; physicochemical properties; glass transition; stickiness
1. INTRODUCTION
Among most <strong>of</strong> <strong>the</strong> natural foods, honey still has its own valuable place for long centuries. This unique
nutritional <strong>and</strong> prophylactic-medicinal substance is a sweet, viscous <strong>and</strong> aromatic fluid elaborated by <strong>the</strong>
honeybee from <strong>the</strong> nectar <strong>of</strong> plants or honeydew. Considering <strong>the</strong> various climatic conditions present in Iran
expecting many different kinds <strong>of</strong> honeys from sensory <strong>and</strong> physicochemical aspects is not amazing. In order
to give an authoritative statistic <strong>of</strong> <strong>the</strong> country’s production, <strong>the</strong> 11th rank was attributed to it by <strong>the</strong> Food <strong>and</strong>
Agricultural Organization (FAO) in <strong>the</strong> year 2008 <strong>and</strong> surely it has improved since <strong>the</strong>n [1]. According to
ISNA by producing over 14 thous<strong>and</strong> tons <strong>of</strong> honey, West Azerbaijan province (located in North West <strong>of</strong>
Iran) has <strong>the</strong> 1st rank in <strong>the</strong> total production <strong>of</strong> <strong>the</strong> country [2]. Physical <strong>and</strong> rheological properties <strong>of</strong> honey
are very useful in its processing, h<strong>and</strong>ling <strong>and</strong> storage [3]. Most <strong>of</strong> <strong>the</strong> physicochemical properties <strong>of</strong> honey
such as viscosity, hydroscopicity, <strong>and</strong> granulation are due to its composition that is sugars <strong>and</strong> moisture
content [4]. The composition <strong>of</strong> honey depends highly on <strong>the</strong> type <strong>of</strong> flowers utilized by <strong>the</strong> bee as well as
climatic conditions [5].
The Arrhenius, VTF <strong>and</strong> Power law models are useful for <strong>temperature</strong> dependence <strong>of</strong> viscosity [3, 6].
The honey viscosity depends on <strong>the</strong> honey water content, <strong>temperature</strong> <strong>and</strong> chemical constitution [7, 8]. Most
honey varieties show Newtonian behavior [3-4, 8-10] while <strong>the</strong>re are some which show non-Newtonian
behavior [7-8, 11-12].
Materials with amorphous or partially amorphous structure undergo a transition from a glassy solid state
to a rubbery viscous state at a material-specific <strong>temperature</strong> range, which is called <strong>the</strong> glass transition
<strong>temperature</strong> (T g ). Measuring <strong>the</strong> Glass Transition Temperature <strong>of</strong> honeys by <strong>the</strong> use <strong>of</strong> DSC was done by
most scientists in literature [3-4, 6, 13-16].
Moisture sorption/desorption iso<strong>the</strong>rms are useful in food dehydration, storage <strong>and</strong> packaging. Because <strong>of</strong>
<strong>the</strong> complex food composition, experimental measurements are necessary for prediction <strong>of</strong> <strong>the</strong> iso<strong>the</strong>rms.

Although <strong>the</strong>re have exists more studies about moisture iso<strong>the</strong>rms <strong>of</strong> lots <strong>of</strong> foodstuffs, but still in honey case
<strong>the</strong>re have been done little or no work [17].
Stickiness is a phenomenon that reflects <strong>the</strong> tendency <strong>of</strong> some materials to agglomerate <strong>and</strong>/or adhere to
contact surfaces. Its control is critical for proper operations in Food Industries. So far little attention has been
given to <strong>the</strong> cohesion <strong>and</strong> adhesion phenomena that contribute to stickiness.
In order to know <strong>the</strong> characteristics <strong>and</strong> properties <strong>of</strong> honey, to maintain or improve its quality, <strong>the</strong> present
study has been undertaken to determine <strong>the</strong> most important chemical, physical <strong>and</strong> rheological properties <strong>of</strong>
Iranian honey <strong>and</strong> <strong>the</strong>refore to study <strong>the</strong> relationships between <strong>the</strong> <strong>temperature</strong> <strong>and</strong> its physicochemical <strong>and</strong>
calorimetric properties.
2. MATERIALS & METHODS
2.1 Samples
Six varieties <strong>of</strong> West Azerbaijan regions’ honeys were provided by <strong>the</strong> Agricultural Organization with
guaranteed botanic <strong>and</strong> natural origin. Preparation <strong>of</strong> <strong>the</strong> samples was done according to <strong>the</strong> related AOAC
method [18].
In <strong>the</strong> following part <strong>the</strong> methods, applied instruments <strong>and</strong> <strong>the</strong> related equations <strong>of</strong> <strong>the</strong> parameters are
summarized:
2.2 Moisture content: (AOAC, Method 969.38)-(NAR-3T, Atago, Japan Abbe Refractometer) [18]
2.3 Water activity (a w ) (25.18 o C): Ms1-a w , Novasina, Switzerl<strong>and</strong>, water activity meter
2.4 Water insoluble solids: (International Honey Commission <strong>and</strong> ISIRI, No.92) [19, 20]
2.5 Diastase content: (AOAC, Method 958.09) [18]
2.6 pH <strong>and</strong> free acidity: (AOAC, Method 962.19)- (INOLABPH720, WTW, Germany, pH meter) [18]
2.7 Sugar contents: (International Honey Commission <strong>and</strong> ISIRI, No.92) [19, 20]
2.8 Ash content: (AOAC, Method 920.181)- (FMS4, Fan Azma Gostar, Iran) [18]
2.9 Total nitrogen content: (AOAC, Method 962.18)- (V40, Bakhshi, Iran) [18]
2.10 Fat content: (Soxhlet method [21])-( Bakhshi, Iran)
2.11 HMF content: (International Honey Commission <strong>and</strong> ISIRI, No.92) [19, 20]
2.12 Density: (Mass to volume ratio [22])-(Calibrated density bottle with <strong>the</strong>rmometer – 25ml, ISO-Lab,
Germany).
Specific gravity <strong>of</strong> <strong>the</strong> samples was calculated by dividing <strong>the</strong> honey density value to <strong>the</strong> density <strong>of</strong>
distilled water at a st<strong>and</strong>ard <strong>temperature</strong> [23]). The honey density was <strong>the</strong>n calculated according to <strong>the</strong>
following equation (derived from <strong>the</strong> pearson's square <strong>and</strong> mass balance basis):
ρm
× ρ
w
ρ
H
= (1)
2 ρ
w
− ρm
Where ρ H is <strong>the</strong> density <strong>of</strong> honey (g.cm -3 ), ρ m is <strong>the</strong> density <strong>of</strong> <strong>the</strong> diluted mixture (g.cm -3 ) <strong>and</strong> ρ w is <strong>the</strong>
density <strong>of</strong> <strong>the</strong> distilled water (g.cm -3 ).
The experimental density data were correlated with <strong>temperature</strong> (T, o C) <strong>and</strong> dry matter (DM, g/100g),
using non-linear regression method <strong>and</strong> by <strong>the</strong> Levenberg-Marquardt estimation method, supplied in <strong>the</strong>
STATISTICA s<strong>of</strong>tware release 7.
2.13 Color: (Minolta, CR-410, Japan)
2.14 Electrical conductivity: (International Honey Commission)-(AZ-86P3, ACCURACY THE ZENIT,
Taiwan) [19]
2.15 Glass transition <strong>temperature</strong> (T g ): (Thermal scan range: -100 to +140 o C, Scan rate: 5 o Cmin -1 )
(METTLER.Star s<strong>of</strong>tware 9.10, USA)
2.16 Dynamic viscosity: (BROOKFIELD, DV-II+Pro No. M/03-165-b0707, viscometer)
2 2
2ω
R . R
(R
c s
s < r

2.16.1 VTF model:
µ
⎡ B
= A.exp⎢
⎢⎣
( T −
T g
⎤
⎥
) ⎥⎦
Where µ is <strong>the</strong> viscosity (Pa.s) at <strong>temperature</strong> T ( o K), A (Pa.s) <strong>and</strong> B are constants <strong>and</strong> T g is glass
transition <strong>temperature</strong> ( o K).
B
2.16.2 Power law model: µ = A ( T − )
(5)
T g
Where µ is <strong>the</strong> viscosity (Pa.s) at <strong>temperature</strong> T ( o K), A <strong>and</strong> B are constants <strong>and</strong> T g is glass transition
<strong>temperature</strong> ( o K).
−
2.16.3 Arrhenius model:
Ea
µ = A.exp(
)
(6)
R.
T
Where µ is <strong>the</strong> viscosity (Pa.s) at <strong>temperature</strong> T ( o K), A is a constant (Pa.s), E a <strong>the</strong> activation energy
(kJ.mol -1 ) <strong>and</strong> R <strong>the</strong> gas constant (0.008314472 kJ/mol K).
2.17 Stickiness (24.5 o C): (TA.XT Plus, Stable Micro Systems, UK, Texture Analyzer)
2.18 EMC: (Static-gravimetric) [17]-( (MgCl 2, Mg(NO 3 ) 2 <strong>and</strong> NaCl) [24]
2.19 Statistical analysis: (St<strong>and</strong>ard statistical packages (STATISTICA release 7 <strong>and</strong> Excel 2003)-( P
value, Chi square <strong>and</strong> RMSE statistical indices) [25]
3. RESULTS & DISCUSSION
The results <strong>of</strong> most important physicochemical parameters <strong>of</strong> <strong>the</strong> investigated honeys are listed in Table1.
(4)
3.1 Moisture content <strong>and</strong> water activity
The obtained relationships between water activity (y) <strong>and</strong> moisture content (x) <strong>of</strong> <strong>the</strong> samples <strong>and</strong> <strong>the</strong>ir
linear regression is: y = ( 0.113234) + (0.027032)
× x , r=0.901 <strong>and</strong> between water activity (y) <strong>and</strong> <strong>the</strong> content
<strong>of</strong> reducing sugars (x) (Fructose <strong>and</strong> Glucose) is: y=(0.906883)+((-0.00535)*(x)), r =-0.899.
3.2 Total sugars, reducing sugars, fructose to glucose ratio <strong>and</strong> sucrose content
The values obtained for total sugar <strong>and</strong> reducing sugar contents, fructose to glucose ratio <strong>and</strong> sucrose
content are at <strong>the</strong> range <strong>of</strong>: 73.97-81.82, 68.21-78.49, 0.85-1.23 <strong>and</strong> 1.73-5.74 respectively. Higher Sucrose
content can be attributed to over feeding <strong>of</strong> honeybees with sucrose syrup, adulteration or earlier harvesting
<strong>of</strong> honey in which sucrose hasn’t been yet converted to Glucose or Fructose [26-28].
3.3 Ash, total nitrogen, fat, Diastase <strong>and</strong> Hydroxymethylfurfural (HMF) content
There has been seen a straight linear relationship between Free acidity <strong>and</strong> ash content <strong>of</strong> <strong>the</strong> samples
with a high coefficient <strong>of</strong> determination (R 2 = 0.81). Ano<strong>the</strong>r linear relationship has been obtained between
pH <strong>and</strong> ash content <strong>of</strong> <strong>the</strong> samples with <strong>the</strong> coefficient <strong>of</strong> determination <strong>of</strong> R 2 = 0.94. Such a relationship has
been reported in o<strong>the</strong>r scientific studies too [29]. Protein levels depend on flowers’ type <strong>and</strong> so are variable
[28]. Low levels <strong>of</strong> proteins indicate that honey is genuine [21]. A low level <strong>of</strong> fat in honey is an indication
<strong>of</strong> its virginity [21].
3.4 Density, specific gravity, color <strong>and</strong> electrical conductivity
Values <strong>of</strong> <strong>the</strong> regression analysis <strong>of</strong> density are illustrated in <strong>the</strong> following equation:
ρ = 96.9363 + 0.0309T - 0.00002T 2 - 0.00037(T× DM) – 2.2665(DM) + 0.0134(DM) 2 (R 2 = 0.945)
The measured color parameters L, a <strong>and</strong> b by using a hunter lab were within <strong>the</strong> range <strong>of</strong> 18.95-21.42, (-
0.003)-0.46 <strong>and</strong> 0.47-1.62 respectively. The L value is designated by <strong>the</strong> Hunter Colorimeter to measure <strong>the</strong>
degree <strong>of</strong> whiteness <strong>and</strong> blackness in <strong>the</strong> product. The a index relates to <strong>the</strong> degree <strong>of</strong> greenness or redness <strong>of</strong>
a sample <strong>and</strong> <strong>the</strong> b index shows <strong>the</strong> degree <strong>of</strong> blueness or yellowness <strong>of</strong> <strong>the</strong> sample.
The linear relationship between electrical conductivity with ash content is characterized by a correlation
coefficient r equal to 0.99. An exponential relationship with <strong>the</strong> coefficient <strong>of</strong> determination <strong>of</strong> R 2 =0.87 has
been obtained between electrical conductivity <strong>and</strong> free acidity <strong>of</strong> <strong>the</strong> honey samples (y = 0.0286e 0.1318x ) (x=
Free acidity <strong>and</strong> y= electrical conductivity). There has been also a linear relationship detected with <strong>the</strong>
coefficient <strong>of</strong> determination <strong>of</strong> R 2 =0.98 between electrical conductivity <strong>and</strong> pH (y= 0.7605x - 2.8575) (x= pH
<strong>and</strong> y= electrical conductivity).

3.5 DSC <strong>the</strong>rmal behavior
The obtained glass transition <strong>temperature</strong>s for <strong>the</strong> investigated honeys are at <strong>the</strong> range <strong>of</strong>:
(-45.24)-(-39.15). The glass transition <strong>temperature</strong>s <strong>of</strong> <strong>the</strong> honey samples show strong dependence on
moisture content (T g =-43.513 Ln(m.c * )+74.716, R 2 =0.85) <strong>and</strong> shifts to lower <strong>temperature</strong>s with increasing <strong>of</strong>
moisture content due to <strong>the</strong> plasticization <strong>effect</strong> <strong>of</strong> water [13, 30].
Physicochemical
properties
Water content
(g/100g)
Water insoluble
solids content
(%)
Total Nitrogen
content
(mg/100g)
Fat content
(g/100g)
Ash content
(g/100g)
Specific
conductivity
(mS.cm -1 )
pH
Table 1. Physicochemical properties <strong>of</strong> honeys
St<strong>and</strong>ard
Samples
values* 1 2 3 4 5 6
Maximum
20
14.44±1.24 14.02±1.30 15.16±1.15 14.13±1.22 13.97±1.12 15.87±1.33
Maximum
0.5 (In
pressed
0.31±0.02 0.20±0.04 0.62±0.02 0.73±0.05 0.52±0.02 0.22±0.01
honey)
- 63.03±0.64 131.33±0.94 29.39±0.22 26.66±0.32 406.51±0.88 52.70±0.55
- 1.16±0.22 0.52±0.05 0.09±0.09 0.14±0.03 0.29±0.05 0.16±0.04
Maximum
0.6%
Maximum
0.8
Minimum
3.5
0.17±0.09 0.16±0.08 0.49±0.09 0.01±0.00 0.11±0.08 0.03±0.01
0.33±0.01 0.32±0.01 0.99±0.01 0.15±0.01 0.24±0.01 0.15±0.01
4.13±0.32 4.21±0.20 5.06±0.28 3.98±0.33 4.01±0.34 4.02±0.25
Free acidity
(meq/kg)
Maximum40 21±6 16±6 25±7 13±6 18±7 12±6
HMF (mg/kg) Maximum40 0.04±0.01 12.1±5.80 3.3±2.21 6.4±0.55 5.2±0.62 17.2±0.65
Diastase content
(DN)
Minimum 3 16.41±0.63 21.21±0.80 9.41±0.53 10.54±0.70 25.20±0.75 13.33±0.62
Water activity
(a w )
- 0.510±0.002 0.500±0.001 0.535±0.003 0.483±0.004 0.485±0.003 0.534±0.005
Specific weight
(20 o C)
- 1.472±0.006 1.478±0.005 1.477±0.007 1.497±0.005 1.489±0.005 1.467±0.005
* Values reported by <strong>the</strong> Iran National St<strong>and</strong>ard (ISIRI No. 92)
Results are expressed as mean values ± st<strong>and</strong>ard deviations.
3.6 Rheological behavior
3.6.1 Viscosity
Values <strong>of</strong> <strong>the</strong> Power law equation parameters for honey samples at 18.7 o C are summarized in Table 2.
Considering this point that <strong>the</strong> <strong>effect</strong> <strong>of</strong> shear rate on rheological behavior is more apparent at low shear rates
is significantly important [4, 11]. At high values <strong>of</strong> shear rate, honey tends to show a Newtonian behavior.
There is an inverse significant relationship between moisture content <strong>and</strong> apparent viscosity, which is
described by an exponential function: η = 1084902e −0. 65728Xw (r 2 =0.84, p < 0.05). Honey viscosity also
depends on honey density <strong>and</strong> sucrose content; <strong>the</strong>se linear relationships are defined by <strong>the</strong> correlation
coefficient <strong>of</strong> r which equals to 0.94 for both density <strong>and</strong> sucrose content dependence <strong>of</strong> viscosity.
The results <strong>of</strong> <strong>the</strong> verifications <strong>of</strong> three obtained models for <strong>temperature</strong> dependence <strong>of</strong> viscosity suggests
that Arrhenius model fits well with samples 1,2 <strong>and</strong> 3, <strong>and</strong> for <strong>the</strong> rest <strong>of</strong> <strong>the</strong> samples VTF model matches
good due to low values <strong>of</strong> P, RMSE <strong>and</strong> χ 2 .
* Moisture Content

Table 2. Values <strong>of</strong> <strong>the</strong> Power law equation parameters for honey samples at 18.7 o C
Sample No. Coefficient <strong>of</strong> consistency (Pa.s n ) (18.7 o C) Flow behavior indices (n) r P %
1 58.09 0.902 0.9991 3.55
2 123.79 1.030 0.9999 1.37
3 54.40 0.996 0.9999 1.57
4 101.52 0.966 0.9997 2.64
5 108.81 1.010 0.9999 1.55
6 42.09 1.014 0.9997 2.89
3.6.2 Stickiness
Figure1 shows <strong>the</strong> surface stickiness curvature <strong>of</strong> <strong>the</strong> samples. The highest difference between values <strong>of</strong>
viscosity <strong>and</strong> surface stickiness are for samples No. 2 <strong>and</strong> 5 which own <strong>the</strong> highest values for viscosity. By
considering glass transition <strong>temperature</strong> <strong>of</strong> <strong>the</strong> samples with surface stickiness, a linear relationship with <strong>the</strong>
coefficient <strong>of</strong> determination <strong>of</strong> R 2 = 0.85 between <strong>the</strong>m was apparent.
Figure 1. Force-Time curvature (surface stickiness) <strong>of</strong> honey samples at 24.5 o C
3.7 Equilibrium moisture content (EMC) determination
Results <strong>of</strong> <strong>the</strong> experiments showed that at all <strong>temperature</strong>s EMC <strong>of</strong> honeys increase by increasing water
activity. On <strong>the</strong> contrary at a particular water activity, <strong>temperature</strong> rise does not reduce EMC in all cases but
this principle is to some extent accurate at water activities lower than 0.5. By considering <strong>the</strong> obtained curves
<strong>the</strong> presence <strong>of</strong> a phenomenon as “Inverse <strong>effect</strong>” is apparent. Such an <strong>effect</strong> was observed in o<strong>the</strong>r
researchers investigations too.
4. CONCLUSION
Results <strong>of</strong> <strong>the</strong> physicochemical analysis <strong>of</strong> 6 honey samples from West Azerbaijan province suggests that
flower resources <strong>of</strong> honey are <strong>the</strong> most important factor in its variety. Most <strong>of</strong> <strong>the</strong> investigated
physicochemical indices except some little ones were at <strong>the</strong> range <strong>of</strong> related st<strong>and</strong>ard <strong>and</strong> were similar to <strong>the</strong>
values found by o<strong>the</strong>r researchers. Unlike <strong>the</strong> most reported results from scientific resources, all honey
samples in this study exhibited non-Newtonian flow behavior at <strong>the</strong> related shear rate range. Temperature
dependence <strong>of</strong> viscosity was evaluated with three Arrhenius, VTF <strong>and</strong> Power law models. The first two
models (Arrhenius <strong>and</strong> VTF) have shown good fitting with data. Outcomes <strong>of</strong> this study can be applied in
processing, quality improvement <strong>and</strong> product storage.
ACKNOWLEDGEMENTS
The authors would like to thank <strong>the</strong> University <strong>of</strong> Urmia for its equipped laboratories.

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