Content uploaded by Aytunga Arık Kibar
Author content
All content in this area was uploaded by Aytunga Arık Kibar on May 21, 2018
Content may be subject to copyright.
Thermal, mechanical and water adsorption properties of corn
starch–carboxymethylcellulose/methylcellulose biodegradable films
E. Aytunga Arık Kibar
a,
⇑
, Ferhunde Us
b,1
a
TÜB_
ITAK, Marmara Research Center, Food Institute, PO Box 21, 41470 Gebze, Kocaeli, Turkey
b
Hacettepe University, Faculty of Engineering, Department of Food Engineering, 06800 Beytepe, Ankara, Turkey
article info
Article history:
Received 24 January 2012
Received in revised form 23 July 2012
Accepted 29 July 2012
Available online 4 August 2012
Keywords:
Biodegradable film
Starch
Methylcellulose
Carboxymethylcellulose
Glycerol
Polyethylene glycol
abstract
The objective of this study was to investigate the effect of the addition of methylcellulose and carboxy-
methylcellulose on the thermal, mechanical and water adsorption properties of starch-based films plas-
ticized with glycerol or polyethylene glycol (PEG). Mechanical tests showed that as the methylcellulose
and carboxymethylcellulose proportion increased, starch films became more resistant to break, resulting
in higher TS values. Besides there has been a positive effect on the elasticity of starch films realized by a
considerable increase in E% values. Depending on the plasticizer type, either single or dual glass transi-
tions were seen in DSC thermograms. One glass transition temperature was observed for films plasticized
with glycerol, on the contrary, dual glass transitions were detected for PEG plasticized films. This behav-
ior was attributed to the phase separation of the PEG. In addition, the presence of an endothermic peak in
the thermograms of PEG plasticized films was taken as another indicator of the phase separation. As a
result, it was suggested that PEG was not as compatible as glycerol with the composite polysaccharide
matrix and plasticizer type was the main factor that shaped the thermal profiles of the film samples.
Water adsorption isotherm data showed that samples displayed nonlinear sorption profile which is typ-
ical for hydrophilic films. In all films tested, equilibrium moisture contents, increased almost linearly up
to a a
w
of 0.65–0.85, beyond where a sharp increase was noted. Adsorption data was adequately fitted by
BET and GAB models. Eventually, it can be concluded that film forming properties of starch can be
improved by incorporation of methylcellulose and carboxymethylcellulose to the polymer matrix.
Ó2012 Elsevier Ltd. All rights reserved.
1. Introduction
The current global consumption of plastics is more than
200 million tones; with an annual grow of approximately 5%,
which represents the largest field of application for crude oil. It
emphasizes how dependent the plastic industry is on oil and con-
sequently how the increasing of crude oil and natural gas price can
have an economical influence on the plastic market. Therefore it
has been becoming increasingly important to utilize alternative
raw materials. Until now petrochemical-based plastics have been
increasingly used as packaging materials because of their large
availability at relatively low cost, good mechanical performance,
good barrier to oxygen, carbon dioxide, water vapor and aroma
compounds, heat sealability, and so on (Siracusa et al., 2008). But
the improper disposition of the enormous volume of petroleum-
derived plastics in the environment has led to pollution and raised
much interest in the biodegradable and renewable resources (Ma
et al., 2008b). In addition, there has been a considerable interest
in biodegradable films made from starch (Lawton, 1996). Several
studies have been performed to analyze the properties of starch-
based biodegradable films (Bertuzzi et al., 2007; Chang et al.,
2010; Mali et al., 2005; Parra et al., 2004; Romero-Bastida et al.,
2004; Talja et al., 2007; Zhang and Han, 2006a,b, 2008). Starch
films generally have good barrier properties to oxygen, carbon
dioxide and lipids, however they have limitations in mechanical
and water vapor permeability properties (Kester and Fennema,
1986). Three common ways have been used in order to overcome
these limitations: genetic modification; such as production of high
amylose starch (Ryu et al., 2002), chemical modification (Parra
et al., 2004) and blending with appropriate materials. Chemical
or genetic modifications are useful methods to get new substances
with well-defined properties but they are often time consuming
and not seldom costly. On the other side blending is a well-known,
efficient way to prepare new materials with improved properties
(Vasile et al., 2004). Agar (Wu et al., 2009), chitosan (Bourtoom
and Chinnan, 2008; Xu et al., 2005), cellulose fibers (Muller et al.,
2009) cellulose crystallites (Ma et al., 2008b), pullulan (Kristo
and Biliaderis, 2007), nanoclay (Almasi et al., 2010), nano-SiO
2
(Tang et al., 2009) have been added to enhance film forming
0260-8774/$ - see front matter Ó2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jfoodeng.2012.07.034
⇑
Corresponding author. Tel.: +90 262 677 32 26; fax: +90 262 641 23 09.
E-mail addresses: aytunga.kibar@tubitak.gov.tr (E.A. Arık Kibar), ferosh@hacet-
tepe.edu.tr (F. Us).
1
Tel.: +90 312 297 71 05; fax: +90 312 299 21 23.
Journal of Food Engineering 114 (2013) 123–131
Contents lists available at SciVerse ScienceDirect
Journal of Food Engineering
journal homepage: www.elsevier.com/locate/jfoodeng
properties of starch. These studies have demonstrated that
mechanical and permeability properties of starch films could be
improved in some cases.
In the present article we have reinforced the starch film by mix-
ing with methylcellulose and carboxymethylcellulose. Those cellu-
lose ethers have no harmful effects on human health, and are used
as highly effective additive to improve the product and processing
properties in various fields of application, from foodstuffs, cosmet-
ics and pharmaceuticals to products for the paper and textile
industries (Feller and Wilt, 1990). Methylcellulose has been widely
used to prepare edible films and documented in several publica-
tions (Debeaufort and Voilley, 1997; Donhowe and Fennema,
1993a,b; Turhan and Sahbaz, 2004). Carboxymethylcellulose is an
anionic linear polysaccharide derived from cellulose. It is an impor-
tant industrial polymer with a wide range of applications (Biswal
and Singh, 2004).
Plasticizers are added to polymers to reduce brittleness, since
they increase the free volume between polymer chains, decreasing
intermolecular forces and thus increasing flexibility and extensibil-
ity of polymers (Romero-Bastida et al., 2005). Many researchers
studied the effects of various polyols on starch-based films (Yang
and Paulson, 2000; Zhang and Han, 2006a,b, 2008). The most pre-
ferred polyols were glycerol, sorbitol, and PEG (Mali et al., 2002;
Mchugh et al., 1993). In the presented study glycerol and PEG have
been used as plasticizers.
There are some studies about the carboxymethylcellulose and/
or methylcellulose starch composite films in the literature.
Peressini et al. (2003) have investigated the rheological properties
of starch–methylcellulose blends and exhibited the compatibility
of two polysaccharides in the film forming dispersionsIn their sub-
sequent work, starch–methylcellulose–lipid film has been devel-
oped and the influence of deposition process of film-forming
dispersion on the shelf-life of dry bakery food has been examined
(Bravin et al., 2006). Ma et al. (2008a) have studied the thermo-
plastic starch/cellulose derivatives as potential biodegradable
packaging materials. They have proposed that the introduction of
carboxymethylcellulose and methylcellulose increased the glass
transition temperature and improved the tensile stress and elonga-
tion at break, as well as the barrier property against water vapor. In
a recent study of Tongdeesoontorn et al. (2011) mechanical prop-
erties of CMC reinforced cassava starch films have been investi-
gated. They have reported that addition of CMC to the cassava
starch films has increased tensile strength and reduced elongation
at break (Tongdeesoontorn et al., 2011). However, there is negligi-
ble data available about the physicochemical properties of corn
starch–carboxymethylcellulose and corn starch–methylcellulose
based films. Thus the objective of this study is to determine the ef-
fect of blending level and plasticizer type on the physicochemical
properties of carboxymethylcellulose and methylcellulose–corn
starch composite films and investigate the potential usage as bio-
degradable packaging material.
2. Materials and methods
2.1. Materials
Normal corn starch (Unmodified regular corn starch containing
approximately 73% amylopectin and 27% amylose) and methylcel-
lulose (Molecular weight of 41,000 and degree of substitution of
1.5–1.9) were purchased from Sigma Chemical CO. (St. Luis, Mis-
souri, USA). Carboxymethylcellulose, with a molecular weight of
90,000 and degree of substitution of 0.7 was purchased from Acros
Organics (Geel, Belgium). Analytical grade glycerol (GLI; Merck;
Darmstadt, Germany) and polyethylene glycol 400 (PEG; Merck;
Hohenbrunn, Germany) were used as plasticizer.
2.2. Film preparation and casting
Film-forming solutions were prepared with different blending
levels of carboxymethylcellulose/corn starch (CMC/CS) and meth-
ylcellulose/corn starch (MC/CS) to study the roles of these compo-
nents on the physical properties of the composite films. The blends
of CMC/CS and MC/CS (0:100; 20:80; 40:60; 60:40; 80:20; 100:0)
and the plasticizer content (50% w/w on dry basis) were estab-
lished according to the preliminary tests. In each formulation, the
weight of dry matter was maintained at a constant value of 1.5 g
per 100 mL water. Film-forming dispersions were obtained by
the dispersion and solubilization of CMC and MC in 50 mL of water
at room temperature and at 95 °C, respectively. CS was gelatinized
in 50 mL of water at 95 °C for 45 min in the presence of the plasti-
cizer. When the CS/plasticizer solution temperature was around
50 °C, solution was added to the CMC or MC solution. Then the
mixture was homogenized using an Ultra Turrax T25 (Ika Labor-
technick, Staufen, Germany) for 2 min at 13,000 rpm, followed by
2 min at 11,000 rpm. In order to remove air bubbles, the solutions
were placed in an ultrasonic water bath (Elma LC 30 H, Singen, Ger-
many) for 30 min and finally, solutions were allowed to stabilize at
room temperature overnight. Films were cast by pouring 30 mL of
solution onto the 85 mm internal diameter Petri dishes and dried
in a climatic room with controlled conditions (25 °C and 45% RH)
for at least 3 days. Thickness of films was determined using a dig-
ital micrometer (Mitutoyo, Manufacturing Co. Ltd., Japan,
0.001 mm accuracy). Reported thickness values were the mean va-
lue of five measurements for each film sample.
2.3. Differential scanning calorimetry (DSC) analysis
DSC experiments were carried out using TA Q20 model DSC
apparatus (TA Instruments, USA). The calorimeter was calibrated
with indium (melting point = 156.6 °C,
D
H= 28.5 J/g). The DSC runs
were operated under nitrogen gas atmosphere (30 mL/min) and an
empty pan was used as the reference. The film samples, approxi-
mately 3 mg, were hermetically sealed in aluminum pans after
equilibration over P
2
O
5
for 10 days. The pans were heated from
90 °C to 100 °C at the scanning rate of 10 °C/min. The DSC ther-
mograms were evaluated to characterize the onset, peak and end
temperatures and the enthalpy changes of the phase transitions.
The glass transition temperature was determined by taking the
first derivative of the thermograms. Glass transition was analyzed
for the onset, mid, end points and the midpoint temperatures were
reported as glass transition temperatures of the samples.
2.4. Mechanical properties
A TA Plus Texture Analyzer (Lloyd Instruments, West Sussex,
England) was used to determine the tensile strength and percent-
age of elongation at break. Film specimens were tested as sug-
gested by ASTM D683M (ASTM, 1993). All film strips were
equilibrated for ten days to 52 ± 2% RH in a cabinet using saturated
magnesium nitrate solution at room temperature (25 ± 1 °C). At
least 10 replications of each test sample were run. Tensile strength
(MPa) was calculated by dividing maximum load by cross-sec-
tional area of the film. Per cent elongation at break was expressed
as percentage of change of the original length of a specimen be-
tween grips at break.
2.5. Moisture adsorption isotherm
Moisture adsorption isotherm of films was determined at 25 °C
for a
w
varying from 0.11 to 0.92 using saturated salt solutions
(Merck; Darmstadt, Germany) in desiccators [LiCl, a
w
0.11; CH
3-
COOK, a
w
0.22; MgCl
2
,a
w
0.33; K
2
CO
3
,a
w
0.43; Mg(NO
3
)
2
,a
w
124 E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131
0.52; NaNO
2
,a
w
0.64; NaCl, a
w
0.75; KCl, a
w
0.84; KNO
3
,a
w
0.92].
Film samples were dried over P
2
O
5
at 25 °C for 10 days prior to
adsorption analysis. Samples were checked at certain time inter-
vals to ensure saturation. Equilibrium was judged to have attained
when the difference between two consecutive sample weightings
was less than 1 mg/g dry solid (40 days). Moisture content was
determined in the equilibrated samples as the difference in weight
before and after drying in an oven at 130 °C for 1 h (AACC, 1995).
Water activity was evaluated at 25 °C by means of an AquaLab Ser-
ies CX2 model instrument (Decagon Devices, Inc., Washington,
USA). Equilibrium moisture content (X
e
) was expressed as grams
per 100 g of dry solid.
2.6. Sorption models
A number of sorption isotherm models that have been reported
in the literature. In the present study BET (Labuza, 1968) and GAB
(Zhang and Han, 2008) models were used for fitting the sorption
data.
The BET model : X
X
m
¼Ca
w
1a
w
ðÞ1a
w
þCa
w
ðÞ½
ð1Þ
The GAB model : X
X
m
¼Cka
w
1ka
w
ðÞ1ka
w
þCka
w
ðÞ½
ð2Þ
In Eqs (1) and (2),X
m
; the monolayer moisture content, C; a con-
stant related to thermal effects and k; the GAB constant related to
the properties of multilayer water molecules with respect to bulk
liquid. The sorption data was analyzed according to the models
and the corresponding constants were determined. The goodness
of fit of each model was computed in terms of coefficient of regres-
sion, R
2
and root mean square error per cent (%RMS) values, as
%RMS ¼100 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
n
i¼1
X
oi
X
pi
ðÞ
X
oi
2
n
v
u
u
u
tð3Þ
where X
oi
is the observed equilibrium moisture content, X
pi
is the
predicted equilibrium moisture content and n is the number of
observations. The isotherm equation with a %RMS value of less than
or equal to 10 was considered to be a good fit (Yanniotis et al.,
1990).
2.7. Statistical analysis
Experimental data was subjected to one-way analysis of vari-
ance (ANOVA), using SPSS version 11.5 (SPSS Inc., USA). Treatment
means were tested separately for least significant difference (LSD)
test. Nonlinear curve fitting was performed by using Origin 7.0
software (Northampton, USA).
3. Results and discussion
3.1. Thermal properties and glass transition temperature
In order to improve the processability of polymer films, incor-
poration of a plasticizer is required. Plasticizers reduce intermolec-
ular forces and increase the mobility of polymer chains. In this
way, plasticizers decrease the glass transition temperature of these
materials and improve their flexibility (Mali et al., 2005). Glass
transition temperatures of plasticizers used in this study were de-
tected as 78.6 ± 2.1 °C and 65.0 ± 1.3 °C for glycerol and PEG,
respectively. Moreover, a melting endotherm of PEG was detected
at 6.50 ± 0.05 °C which had a melting enthalpy of 107.1 ± 2.2 J/g
(Fig 1). Similar results have been reported in the literature. Averous
et al. (2000) and Buera et al. (1999) have measured the glass tran-
sition temperature of glycerol by DSC technique and reported as
78 °C and 77 °C, respectively. The glass transition temperature
of PEG has been reported as 67 °C(Feldstein, 2001) and 70 °C
(Feldstein et al., 2003). Feldstein et al. (2003) have reported also
a fusion endotherm of PEG at 6 °C with a melting enthalpy of
118 J/g (Feldstein et al., 2003).
In our study, depending on the plasticizer type, DSC thermo-
grams of biodegradable films have showed either single or dual
glass transitions. One glass transition has been observed for films
plasticized with glycerol which had a midpoint temperature be-
tween 76.5 °C and 38.1 °C(Fig 2). The presence of one single
glass transition temperature for multiple polymer blends may be
attributed to their similar plasticization behavior in the presence
of plasticizers (Arvanitoyannis and Biliaderis, 1999). As previously
reported by Bizot et al. (1997) polysaccharides (starch, pullulan,
dextran, phytoglycogen, fructoslated amylose and amylopectin),
widely differing in their molecular structure, branching and con-
formation of glycosidic linkages, exhibited parallel trends in the
glass transition temperature-moisture content plots, indicative of
similar plasticization responses. It is unlikely, therefore, to expect
multiple glass transitions of composite polysaccharide matrices
because of their similar plasticization behavior in the presence of
plasticizers. As a result, the observed single glass transition for
all glycerol plasticized films was attributed to the whole polymer
matrix. Similarly Arvanitoyannis and Biliaderis (1999) have re-
ported only one glass transition temperature for methylcellu-
lose–soluble starch–glycerol blends.
On the contrary of glycerol plasticized films, two glass transi-
tions have been detected for films plasticized with PEG. The
‘‘upper’’ and ‘‘lower’’ glass transitions have been observed at tem-
peratures between (46 °C) to (55 °C) and (80 °C) to (67 °C),
respectively (Fig. 3). Dual glass transitions in DSC thermograms
are typical of a phase separated system (Feldstein et al., 2003;
Forssell et al., 1997). As suggested by the earlier observations,
Fig. 1. DSC thermograms of (a) glycerol and (b) polyethylene glycol. (The glass transition temperatures were pointed out on the insert view.)
E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131 125
composite films have showed only one glass transition in the pres-
ence of glycerol. Therefore, dual glass transitions in PEG plasticized
films seemed to be due to the phase separation of PEG. This phe-
nomenon has been previously reported for poly(N-vinyl pyrroli-
done)-PEG blends, starch–glycerol–water mixtures and amylose–
amylopectin films by Feldstein et al. (2003), Forssell et al. (1997)
and Myllarinen et al. (2002), respectively. In these studies, the
phase separation of the plasticizer has been reported and the upper
transition has been attributed to a polymer-rich phase, whereas
the lower transition has been due to the existence of
Fig. 2. DSC thermograms of glycerol plasticized (a) methylcellulose–corn starch and (b) carboxymethylcellulose–corn starch-based films. (The first derivatives of heat flow
curves were given on the right side of DSC curves.) (Glass transition temperatures (T
g
) were pointed out on the first derivatives of heat flow curves.) (Cellulose ether: corn starch
blending ratios were given next to each curve.)
Fig. 3. DSC thermograms of PEG plasticized (a) methylcellulose–corn starch and (b) carboxymethylcellulose–corn starch-based films. (The first derivatives of heat flow curves
were given on the right side of DSC curves.) (Glass transition temperatures (T
g
) were pointed out on the first derivatives of heat flow curves.) (Cellulose ether: corn starch blending
ratios were given next to each curve.)
126 E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131
plasticizer-rich microdomains. Moreover, we have found an endo-
thermic peak between 6.6 °C to 4.6 °C in DSC thermograms of
films plasticized with PEG (Fig. 3). The presence of a fusion endo-
therm of PEG in the film structure could also be considered as an-
other evidence for partial phase separation of PEG.
Plasticizing activity of polyols has been related to various fac-
tors including plasticizer molecular size and 3-dimensional com-
patibility between the plasticizers and polymers (Zhang and Han,
2006a). When the differences between the phase separation
behaviors of glycerol and PEG were taken into consideration, it
could be concluded that glycerol was a more compatible plasticizer
than PEG with the polysaccharide matrix. This result could be due
to PEG’s bigger molecular size that could reduce its plasticizing
efficiency. Compared to the PEG, glycerol’s smaller size facilitates
its penetration into the polymer matrix. Zhang and Hang (2006a)
have obtained the same conclusion in their study where the plas-
ticization effect of various polyols in starch films has been investi-
gated. They have suggested that sorbitol is a larger molecule when
compared to glycerol, as a result it has a limited accessibility to the
high-density junction zones of the polymer matrix.
On the other hand, better plasticizing efficiency of glycerol
should be evaluated in detail. Because in our study polymer matrix
contains three types of polysaccharides and their individual com-
patibility with the plasticizer might be different. The glass transi-
tion temperatures of composite film samples have been
evaluated in order to compare this property (Table 1). Plasticizers
decrease the glass transition temperature, as a result; observing
lower glass transition temperature at the same plasticizer content,
indicates a better compatibility of the polymer matrix with the
plasticizer. When the glass transition temperatures of glycerol
plasticized films have been considered, the lowest glass transition
temperature has been detected as 76.5 °C for the methylcellulose
Table 1
Thermal properties of methylcellulose–corn starch and carboxymethylcellulose–corn starch films.
Blending
ratio
T
g
(°C)
i
Blending
ratio
T
g,1
(°C)
i
T
g,2
(°C)
i
T
p
(°C)
ii
D
H(J/g)
ii
Methylcellulose:corn starch
Plasticizer:glycerol
100:0 76.5 ± 0.2
g
Methylcellulose:corn starch
Plasticizer:PEG
100:0 67.0 ± 0.2
a
46.3 ± 0.5
c
6.6 ± 1.1
e
0.78 ± 0.03
f
80:20 74.6 ± 0.3
f
80:20 69.2 ± 0.2
b
46.7 ± 0.5
c
5.0 ± 0.8
de
4.29 ± 0.50
e
60:40 73.5 ± 0.6
f
60:40 71.1 ± 1.0
b
47.8 ± 0.1
b
3.1 ± 0.9
d
6.61 ± 0.63
d
40:60 67.2 ± 0.3
e
40:60 74.7 ± 0.4
c
48.7 ± 0.1
b
3.0 ± 0.8
d
7.68 ± 1.20
cd
20:80 63.8 ± 0.2
d
20:80
nd
nd
nd
nd
0:100 62.5 ± 0.5
d
0:100
nd
nd
nd
nd
Carboxymethylcellulose:corn
starch
Plasticizer:glycerol
100:0 38.4 ± 0.9
a
Carboxymethylcellulose:corn
starch
Plasticizer:PEG
100:0 80.2 ± 0.2
d
55.4 ± 1.1
a
4.6 ± 0.4
a
21.98 ± 0.66
a
80:20 38.1 ± 0.7
a
80:20 79.9 ± 0.2
d
48.5 ± 0.6
b
2.8 ± 0.2
ab
11.50 ± 1.69
b
60:40 47.6 ± 0.2
b
60:40 78.9 ± 0.8
d
49.0 ± 2.1
b
2.3 ± 0.7
bc
14.90 ± 2.93
b
40:60 57.2 ± 1.2
c
40:60 75.4 ± 0.4
c
50.1 ± 2.2
b
2.1 ± 0.3
bc
14.53 ± 3.01
b
20:80 63.7 ± 0.1
d
20:80 75.9 ± 1.2
c
54.0 ± 1.3
a
0.6 ± 0.3
c
13.41 ± 2.68
bc
0:100 62.5 ± 0.5
d
0:100
nd
nd
nd
nd
nd, not determined;
All values shown are means ± standard deviations.
Data with the same letter (a–f) within a column are not statistically different at a (p< 0.05) level.
i
T
g
: Glass transition temperature, T
g,1
and T
g,2
: ‘‘upper’’ and ‘‘lower’’ glass transition temperatures, respectively,
ii
T
p
;
D
H: peak temperature and enthalpy of melting endotherm, respectively.
Fig. 4. Mechanical properties of methylcellulose–corn starch and carboxymethylcellulose–corn starch composite films. (For each property mean values in the same graph with
different letters are not statistically different at a (p < 0.05) level.)
E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131 127
film and the highest one at 38.1 °C for the carboxymethylcellu-
lose film. In addition, the glass transitions have increased from
74.6 °Cto63.8 °C as the methylcellulose proportion has de-
creased for the composite film samples. On the contrary, the glass
transition temperatures decreased from 38.1 °Cto63.7 °Cas
the carboxymethylcellulose proportion has decreased. This result
indicated that glycerol had its best plasticizing effect on methylcel-
lulose and it decreased in the sequence of methylcellulose, starch
and carboxymethylcellulose, respectively.
The thermal properties of fusion endotherm detected in PEG
plasticized samples have been given in Table 1. If the melting en-
thalpy of this endotherm has been considered as corresponding to
the relative amount of phase separated PEG, it could be said that
the compatibility of PEG increased as the proportion of methylcel-
lulose increased, while that of carboxymethylcellulose decreased in
the formulation. That was also obvious when the peak tempera-
tures of the melting endotherm have been examined. The peak tem-
peratures increased in the similar trend with the melting enthalpies
(Table 1). The proximity of the peak temperature to that of pure PEG
(Fig. 1) could be considered as an indirect evidence of the interac-
tion between the polymeric matrix and the PEG molecules. There-
fore it may be suggested that as the interaction declines, the peak
temperature of fusion endotherm diverges from that of pure PEG.
Eventually it could be said that type of the plasticizer is the
main factor that affects the thermal profiles. In this context, it
may be concluded that glycerol is highly efficient plasticizer that
is compatible with all of the polymers considered in this study.
On the contrary, PEG has been compatible only with the methylcel-
lulose portion.
3.2. Tensile properties
Mechanical properties of films have been characterized by the
tensile strength (TS) and elongation% (E%) values and high values
are generally required, which are the indicators of the film’s
strength and flexibility. Mechanically, starch, methylcellulose and
carboxymethylcellulose films have behaved differently, which
can be seen easily in Fig 4. Methylcellulose and carboxymethylcel-
lulose films have showed higher TS and E% values than starch film.
In this case, it is expected that mechanical properties of starch film
can be improved by incorporation of methylcellulose and carboxy-
methylcellulose into the film formulation.
TS and E% values of carboxymethylcellulose–starch blend films
have been determined between 3.6–24.1 MPa and 2.5–136.1%,
respectively. TS values of composite films have increased as the
carboxymethylcellulose level has increased. These results are con-
sistent with corn starch films (Ghanbarzadeh et al., 2010), cassava
starch films (Tongdeesoontorn et al., 2011) and pea starch films
(Ma et al., 2008a) in which TS has improved as the concentration
of added carboxymethylcellulose has been increased. Furthermore,
Tongdeesoontorn et al. (2011) reported that the increase in the TS
of cassava starch–carboxymethylcellulose films could be attrib-
uted to the formation of intermolecular interaction between the
hydroxyl group of starch and carboxyl group of carboxymethylcel-
lulose. The flexibility of composite films has been also affected by
the blending level. As it is shown in the Fig. 4 there has been a syn-
ergistic effect between carboxymethylcellulose and starch on the
elasticity by a considerable increase in E% values for glycerol plas-
ticized films. Indeed the highest E% values among all samples in
this study have been measured for glycerol plasticized carboxy-
methylcellulose–starch blend films. This synergistic effect could
be attributed to the carboxymethylcellulose–starch interaction
which has been also reported in the literature. Aguirre-Cruz et al.
(2005) notified that carboxymethylcellulose has increased the vis-
cosity of corn starch, and this was mainly due to the three-dimen-
sional network formed by the carboxymethylcellulose–starch
association. Lee et al. (2002) have also reported the interaction of
carboxymethylcellulose and potato starch and they have proposed
a mechanism in order to explain this interaction: carboxymethyl-
cellulose associates with swollen starch or leached amylose chains.
However, this synergistic effect on E% has not been observed for
PEG plasticized carboxymethylcellulose–starch blend films,
Fig. 5. Moisture adsorption isotherms of methylcellulose–corn starch and carboxymethylcellulose–corn starch films at 25 °C. (Corn starch: methylcellulose and corn starch:
carboxymethylcellulose blending ratios were given in data labels.)
128 E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131
besides significantly lower E% values have been measured. Perhaps
this could be related to the phase separation behavior of PEG,
which has induced the crystallization and reduced the amount of
plasticizing portion of PEG. As a result this might decrease the car-
boxymethylcellulose–starch association in polymer network and
thus allowed changes in elongation properties.
Similarly for the methylcellulose–starch blend films, the TS and
E% values increased as the methylcellulose level increased. TS of
methylcellulose–starch blend films have been determined to be
2.5–28.4 MPa and E% measured between and 8.8–109.7%. It is obvi-
ous that incorporation of methylcellulose has improved both the
mechanical strength and flexibility of starch films (Fig. 4). When
the mechanical test results were inspected with respect to the
plasticizer type, an interesting trend has been detected. In the for-
mulations where methylcellulose proportion exceeded 60%, the
films plasticized by PEG have been stronger than those plasticized
by glycerol. On the contrary, when the starch portion dominated
the formulation, better tensile properties have been measured for
the glycerol plasticized formulations than that of PEG plasticized
films. It could be associated with the difference in the compatibil-
ity of the plasticizers; that is, glycerol was miscible with both
starch and methylcellulose while PEG was compatible only with
the methylcellulose fraction.
If the tensile test results obtained in this study have been com-
pared to the synthetic polymers, they had comparable TS values
with low and high density polyethylene, which have been reported
between 10–20 MPa and 16–41 MPa, respectively (Cuq et al., 1995)
and also E% values were better than cellophane and cellulose ace-
tate which have been reported between 15–25% and 15–70%,
respectively (Briston, 1986; Cuq et al., 1995). Eventually it might
be concluded that incorporation of methylcellulose and carboxy-
methylcellulose into the starch films could be a potential solution
to the classical problem encountered with this kind of films and
thus widen the application of starch films in food packaging.
3.3. Moisture adsorption isotherm
Water acts as a good plasticizer in most hydrophilic films and
water adsorption of hydrophilic films depends on the environmen-
tal relative humidity (van Soest et al., 1995). The moisture adsorp-
tion isotherm data of films have been displayed in Fig. 5. In general,
the moisture adsorption isotherms of films displayed sigmoid
shaped curvatures. In all films tested equilibrium moisture con-
tents, X
e
, (g/100 g dry solid) has increased almost linearly up to a
a
w
of 0.65–0.85, beyond a sharp increase has been noted. That type
of nonlinear sorption profile is typical for hydrophilic films (de la
Cruz et al., 2001; Turhan and Sahbaz, 2004). The sorption levels
have been determined within the range of high amylose corn
starch (Bader and Goritz, 1994; Bertuzzi et al., 2007) and cellulosic
films (de la Cruz et al., 2001; Turhan and Sahbaz, 2004).
Carboxymethylcellulose and methylcellulose are hydrophilic
polymers; as expected, incorporation of carboxymethylcellulose
and methylcellulose have not decreased the moisture adsorption
of starch films, in fact slightly increased the adsorption capacity
(Fig. 5). This could be due to the etheric groups on the cellulose
ethers. The repeating side groups on the polymer chains could have
led to the higher moisture adsorption capacity by increasing the
intermolecular distance between the polymer chains, and hence
facilitates the penetration of water into the polymer matrix.
In starch films, plasticizers are generally more hygroscopic than
starch. Thus, the difference in the water adsorption capacity of
starch films is mostly dependent on the type of the plasticizers
when the starch content remains constant (Zhang and Han,
2006a). In line with this context, when a comparison has been car-
ried out with respect to the plasticizer type, moisture contents
have been higher in films containing glycerol at constant starch/
cellulose ether blending ratios (Fig. 5). This could be related to
the better plasticizing efficiency of glycerol than PEG as shown in
the DSC results previously. Glycerol had a better influence on
decreasing the attractive forces between the polymer chains, in-
creased the free volume and segmental motions, hence water mol-
ecules entered more easily and higher moisture contents resulted.
Similar results were reported in the literature. Zhang and Han
(2006a, 2008) have noted that glycerol-plasticized starch films
contained significantly higher level of moisture than the other
polyols plasticized film and it has been attributed to the high
polarity of glycerol. They have suggested that glycerol is acting like
a ‘‘water holding agent’’ and therefore entrapped large amount of
Table 2
Monolayer moisture contents (X
m
), coefficient of regression, (R
2
) and root mean square% (RMS%) values of GAB and BET models for moisture adsorption isotherms of
methylcellulose–corn starch and carboxymethylcellulose–corn starch composite films.
Sample Plasticizer Model (a
w
) Parameter Blending ratio
100:0 80:20 60:40 40:60 20:80 0:100
Methylcellulose:corn starch Glycerol BET (0.1–0.4) X
m
15.7 21.4 21.4 17.5 17.6 18.9
R
2
0.977 0.984 0.979 0.968 0.980 0.976
RMS% 4.05 2.66 2.58 4.26 3.20 3.43
GAB (0.1–0.9) X
m
16.3 20.0 20.8 17.8 18.8 22.0
R
2
0.995 0.990 0.991 0.987 0.986 0.990
RMS% 5.07 6.45 6.28 8.54 8.88 7.52
Polyethylene glycol BET (0.1–0.4) X
m
12.3 12.6 11.5 10.4 – –
R
2
0.979 0.951 0.967 0.961 – –
RMS% 3.60 4.79 4.05 5.04 – –
GAB (0.1–0.9) X
m
13.1 13.1 13.4 13.4 – –
R
2
0.996 0.997 0.995 0.988 – –
RMS% 5.96 5.33 7.31 11.54 – –
Carboxymethylcellulose:corn starch Glycerol BET (0.1–0.4) X
m
15.7 19.8 19.1 17.3 21.0 21.1
R
2
0.977 0.956 0.951 0.962 0.928 0.984
RMS% 4.05 4.39 4.34 3.43 6.07 2.97
GAB (0.1–0.9) X
m
16.3 19.0 18.7 17.4 20.2 19.6
R
2
0.995 0.990 0.996 0.999 0.996 0.999
RMS% 5.07 5.44 4.11 2.56 5.09 2.15
Polyethylene glycol BET (0.1–0.4) X
m
12.6 9.3 9.4 10.7 9.9 –
R
2
0.983 0.966 0.989 0.917 0.936 –
RMS% 3.10 6.97 3.68 12.81 10.22 –
GAB (0.1–0.9) X
m
14.6 13.8 13.0 15.8 15.1 –
R
2
0.997 0.993 0.993 0.990 0.992 –
RMS% 6.60 12.12 9.70 13.25 12.84 –
E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131 129
water molecules inside the starch polymer network (Zhang and
Han, 2008). Also, Talja et al. (2007) have reported that glycerol in-
creased the moisture adsorption of starch films, and this was re-
lated to the lower molecular weight of the glycerol.
Table 2 shows the estimated parameters and goodness of fit of
BET and GAB models to experimental data of films between the a
w
ranges of 0.1–0.4 and 0.1–0.9, respectively. Among the sorption
isotherm models that have found in the literature, the GAB model
has received the most attention in practical applications. It is re-
garded as reliable in modeling sorption data for many food mate-
rials for almost the entire sorption isotherm (Biliaderis et al.,
1999). The BET model could also be used to provide the estimates
of the monolayer value. However, the BET model does not take into
account the effect of water on structural change of the films. When
dissolution or swelling of the films occur, the BET model is not use-
ful in providing insight into the sorption process. Therefore, the
BET model is usually restricted to a narrow a
w
range where the
change in film structure occurs hardly (Mathlouthi, 2001; Zhang
and Han, 2008).
The monolayer water content values of glycerol plasticized
films have been determined to be 15.7–21.4 g/100 g dry solid
and 16.3–22.1 g/100 g dry solid as predicted by BET and GAB equa-
tions, respectively. Similarly monolayer moisture values of PEG
plasticized films ranged from 10.4 to 13.4 g/100 g dry solid and
from 9.3 to 15.8 g/100 g dry solid as predicted by BET and GAB
models, respectively. These values have been comparable with
monolayer water contents reported for high amylose corn starch
films (Bertuzzi et al., 2007), potato starch films (Talja et al.,
2007), pea starch films (Zhang and Han, 2008) and methyl and
ethyl cellulose based films (de la Cruz et al., 2001). The monolayer
moisture contents of glycerol plasticized films have been higher
than that of PEG plasticized films (Table 2). The value of the mono-
layer moisture content is of particular interest, since it indicates
the amount of water that is strongly adsorbed to specific sites at
the surface. In other words monolayer value can be used to express
the number of active sites available to the water adsorption
(Inchuen et al., 2009). Therefore, it could be proposed that incorpo-
ration of glycerol into the structure led to an increase in the
number of available sorption sites. This result is not surprising
when the molecular weight of glycerol has been taken into consid-
eration. Due to its smaller weight, at constant plasticizer content,
glycerol had larger number of hydroxyl groups compared to the
PEG. In the literature many authors reported that glycerol in-
creased the monolayer moisture content of films compared to
other plasticizers (Cho and Rhee, 2002; Mali et al., 2005; Martelli
et al., 2006). It could be concluded that the presence of
cellulose ethers in the starch film has increased hygroscopic
characteristics.
4. Conclusions
In this study carboxymethylcellulose–corn starch and methyl-
cellulose–corn starch biodegradable blend films have been pre-
pared and characterized and the following conclusions have been
derived;
(i) In the DSC thermograms the glycerol plasticized blend films
have showed one glass transition while, PEG plasticized
films, two step transitions, which suggested two glass tran-
sitions of a phase separated system. When this behavior has
been taken into consideration, it has been suggested that
PEG was not as compatible as glycerol with the composite
polysaccharide matrix.
(ii) Composite films have been more elastic and resistant to
break when compared to starch-based film. Thus, addition
of methylcellulose and carboxymethylcellulose to starch-
based films could be a potential solution to the classical
problem encountered with this kind of films.
(iii) The moisture adsorption isotherms have showed the mois-
ture adsorption capacity of the films have increased in the
presence of glycerol and also it has increased as the methyl-
cellulose and carboxymethylcellulose level in the formula-
tion has been increased. Mathematical fitting of adsorption
data to BET and GAB models have given the monolayer val-
ues and an opportunity to assess the amount of available
sorption sites on the composite polymer matrix.
(iv) Eventually, it can be concluded that film forming properties
of starch can be improved by incorporation of methylcellu-
lose and carboxymethylcellulose to the polymer matrix.
Acknowledgments
The authors thank to TÜB_
ITAK (Project number: TBAG-
107T899) and Hacettepe University Research Centre Office (Project
number: 010 T02 604 001) for providing funds in the form of a re-
search project. The authors also wish to thank to Prof. Dr. Piotr P.
Lewicki (recently passed away) in the Faculty of Food Sciences,
Warsaw University of Life Sciences for his contribution to the intel-
lectual content.
References
AACC, 1995. Approved Methods of the AACC (American Association of Cereal
Chemists), ninth ed. St. Paul, Minnesota.
Aguirre-Cruz, A., Mendez-Montealvo, G., Solorza-Feria, J., Bello-Perez, L.A., 2005.
Effect of carboxymethylcellulose and xanthan gum on the thermal, functional
and rheological properties of dried nixtamalised maize masa. Carbohydrate
Polymers 62 (3), 222–231.
Almasi, H., Ghanbarzadeh, B., Entezami, A., 2010. Physicochemical properties of
starch–CMC–nanoclay biodegradable films. Biological Macromolecules 46, 1–5.
Arvanitoyannis, I., Biliaderis, C.G., 1999. Physical properties of polyol-plasticized
edible blends made of methyl cellulose and soluble starch. Carbohydrate
Polymers 38 (1), 47–58.
ASTM, 1993. Annual Book of ASTM Standards, Standard Test Methods for Tensile
Properties of Plastics D638M, Philadelphia, pp. 59–67.
Averous, L., Moro, L., Dole, P., Fringant, C., 2000. Properties of thermoplastic blends:
Starch–polycaprolactone. Polymer 41 (11), 4157–4167.
Bader, H.G., Goritz, D., 1994. Investigations on high amylose corn starch films. 2.
Water-vapor sorption. Starch–Starke 46 (7), 249–252.
Bertuzzi, M.A., Vidaurre, E.F.C., Armada, M., Gottifredi, J.C., 2007. Water vapor
permeability of edible starch based films. Journal of Food Engineering 80 (3),
972–978.
Biliaderis, C.G., Lazaridou, A., Arvanitoyannis, I., 1999. Glass transition and physical
properties of polyol-plasticised pullulan–starch blends at low moisture.
Carbohydrate Polymers 40 (1), 29–47.
Biswal, D., Singh, R., 2004. Characterisation of carboxymethyl cellulose and
polyacrylamide graft copolymer. Carbohydrate Polymers 57, 379–387.
Bizot, H., LeBail, P., Leroux, B., Davy, J., Roger, P., Buleon, A., 1997. Calorimetric
evaluation of the glass transition in hydrated, linear and branched
polyanhydroglucose compounds. Carbohydrate Polymers 32 (1), 33–50.
Bourtoom, T., Chinnan, M.S., 2008. Preparation and properties of rice starch–
chitosan blend biodegradable film. LWT – Food Science and Technology 41 (9),
1633–1641.
Bravin, B., Peressini, D., Sensidoni, A., 2006. Development and application of
polysaccharide–lipid edible coating to extend shelf-life of dry bakery products.
Journal of Food Engineering 76 (3), 280–290.
Briston, J., 1986. Films, Plastic. John Wiley and Sons, New York.
Buera, M.P., Rossi, S., Moreno, S., Chirife, J., 1999. DSC confirmation that vitrification
is not necessary for stabilization of the restriction enzyme EcoRI dried with
saccharides. Biotechnology Progress 15 (3), 577–579.
Chang, P.R., Jian, R., Zheng, P., Yu, J., Ma, X., 2010. Preparation and properties of
glycerol plasticized-starch (GPS)/cellulose nanoparticle (CN) composites.
Carbohydrate Polymers 79 (2), 301–305.
Cho, S.Y., Rhee, C., 2002. Sorption characteristics of soy protein films and their
relation to mechanical properties. LWT – Food Science and Technology 35 (2),
151–157.
Cuq, B., Gontard, N., Guilbert, S., 1995. Edible films and coatings as active layers. In:
Rooney, M.L. (Ed.), Active Food Packaging. Chapman & Hall, UK, pp. 111–142.
de la Cruz, G.V., Torres, J.A., Martin-Polo, M.O., 2001. Temperature effect on the
moisture sorption isotherms for methylcellulose and ethylcellulose films.
Journal of Food Engineering 48 (1), 91–94.
130 E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131
Debeaufort, F., Voilley, A., 1997. Methylcellulose-based edible films and coatings. 1.
Mechanical and thermal properties as a function of plasticizer content. Journal
of Agricultural and Food Chemistry 45 (3), 685–689.
Donhowe, I.G., Fennema, O., 1993a. The effects of plasticizers on crystallinity,
permeability, and mechanical-properties of methylcellulose films. Journal of
Food Processing and Preservation 17 (4), 247–257.
Donhowe, I.G., Fennema, O., 1993b. The effects of solution composition and drying
temperature on crystallinity, permeability and mechanical-properties of
methylcellulose films. Journal of Food Processing and Preservation 17 (4),
231–246.
Feldstein, M.M., 2001. Peculiarities of glass transition temperature relation to the
composition of poly(N-vinyl pyrrolidone) blends with short chain poly(ethylene
glycol). Polymer 42 (18), 7719–7726.
Feldstein, M.M., Roos, A., Chevallier, C., Creton, C., Dormidontova, E.E., 2003.
Relation of glass transition temperature to the hydrogen bonding degree and
energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing
plasticizers: 3. Analysis of two glass transition temperatures featured for PVP
solutions in liquid poly(ethylene glycol). Polymer 44 (6), 1819–1834.
Feller, R.L., Wilt, M., 1990. Evaluation of Cellulose Ethers for Conservation. Getty
Conservation Institute, Marina del Rey, CA, USA.
Forssell, P.M., Mikkila, J.M., Moates, G.K., Parker, R., 1997. Phase and glass transition
behaviour of concentrated barley starch–glycerol–water mixtures, a model for
thermoplastic starch. Carbohydrate Polymers 34 (4), 275–282.
Ghanbarzadeh, B., Almasi, H., Entezami, A., 2010. Physical properties of edible
modified starch/carboxymethyl cellulose films. Innovative Food Science and
Emerging Technologies 11, 697–702.
Inchuen, S., Narkrugsa, W., Pornchaloempong, P., 2009. Moisture sorption of Thai
red curry powder. Maejo International Journal of Science and Technology 3 (3),
486–497.
Kester, J., Fennema, O., 1986. Edible films and coatings: A review. Food Technology
40, 47–59.
Kristo, E., Biliaderis, C.G., 2007. Physical properties of starch nanocrystal-reinforced
pullulan films. Carbohydrate Polymers 68 (1), 146–158.
Labuza, T.P., 1968. Sorption phenomena in foods. Food Technology 22 (3), 15.
Lawton, J.W., 1996. Effect of starch type on the properties of starch containing films.
Carbohydrate Polymers 29 (3), 203–208.
Lee, M.H., Baek, M.H., Cha, D.S., Park, H.J., Lim, S.T., 2002. Freeze–thaw stabilization
of sweet potato starch gel by polysaccharide gums. Food Hydrocolloids 16 (4),
345–352.
Ma, X., Chang, P., Yu, J., 2008a. Properties of biodegradable thermoplastic pea
starch/carboxymethyl cellulose and pea starch/microcrystalline cellulose
composites. Carbohydrate Polymers 72, 369–375.
Ma, X., Chang, P.R., Yu, J., 2008b. Properties of biodegradable thermoplastic pea
starch/carboxymethyl cellulose and pea starch/microcrystalline cellulose
composites. Carbohydrate Polymers 72 (3), 369–375.
Mali, S., Grossmann, M.V.E., Garcia, M.A., Martino, M.N., Zaritzky, N.E., 2002.
Microstructural characterization of yam starch films. Carbohydrate Polymers 50
(4), 379–386.
Mali, S., Grossmann, M.V.E., Garcia, M.A., Martino, M.N., Zaritzky, N.E., 2005.
Mechanical and thermal properties of yam starch films. Food Hydrocolloids 19
(1), 157–164.
Martelli, S.M., Moore, G., Paes, S.S., Gandolfo, C., Laurindo, J.B., 2006. Influence of
plasticizers on the water sorption isotherms and water vapor permeability of
chicken feather keratin films. LWT – Food Science and Technology 39 (3), 292–
301.
Mathlouthi, M., 2001. Water content, water activity, water structure and the
stability of foodstuffs. Food Control 12 (7), 409–417.
Mchugh, T.H., Avenabustillos, R., Krochta, J.M., 1993. Hydrophilic edible films –
Modified procedure for water-vapor permeability and explanation of thickness
effects. Journal of Food Science 58 (4), 899–903.
Muller, C.M.O., Laurindo, J.B., Yamashita, F., 2009. Effect of cellulose fibers on the
crystallinity and mechanical properties of starch-based films at different
relative humidity values. Carbohydrate Polymers 77 (2), 293–299.
Myllärinen, P., Buleon, A., Lahtinen, R., Forssell, P., 2002. The crystallinity of amylose
and amylopectin films. Carbohydrate Polymers 48 (1), 41–48.
Parra, D.F., Tadini, C.C., Ponce, P., Lugao, A.B., 2004. Mechanical properties and water
vapor transmission in some blends of cassava starch edible films. Carbohydrate
Polymers 58 (4), 475–481.
Peressini, D., Bravin, B., Lapasin, R., Rizzotti, C., Sensidoni, A., 2003. Starch–
methylcellulose based edible films: Rheological properties of film-forming
dispersions. Journal of Food Engineering 59 (1), 25–32.
Romero-Bastida, C.A., Flores-Huicochea, E., Martin-Polo, M.O., Velazquez, G., Torres,
J.A., 2004. Compositional and moisture content effects on the biodegradability
of zein/ethylcellulose films. Journal of Agricultural and Food Chemistry 52 (8),
2230–2235.
Romero-Bastida, C.A., Bello-Perez, L.A., Garcia, M.A., Martino, M.N., Solorza-Feria, J.,
Zaritzky, N.E., 2005. Physicochemical and microstructural characterization of
films prepared by thermal and cold gelatinization from non-conventional
sources of starches. Carbohydrate Polymers 60 (2), 235–244.
Ryu, S.Y., Rhim, J.W., Roh, H.J., Kim, S.S., 2002. Preparation and physical properties of
zein-coated high-amylose corn starch film. Lebensmittel-Wissenschaft und -
Technologie 35 (8), 680–686.
Siracusa, V., Rocculi, P., Romani, S., Rosa, M.D., 2008. Biodegradable polymers for
food packaging: A review. Trends in Food Science & Technology 19 (12), 634–
643.
Talja, R.A., Helen, H., Roos, Y.H., Jouppila, K., 2007. Effect of various polyols and
polyol contents on physical and mechanical properties of potato starch-based
films. Carbohydrate Polymers 67 (3), 288–295.
Tang, H.L., Xiong, H.G., Tang, S.W., Zou, P., 2009. A starch-based biodegradable film
modified by nano silicon dioxide. Journal of Applied Polymer Science 113 (1),
34–40.
Tongdeesoontorn, W., Mauer, L., Wongruong, S., Sriburi, P., Rachtanapun, P., 2011.
Effect of carboxymethyl cellulose concentration on physical properties of
biodegradable cassava starch-based films. Chemistry Central Journal 5 (1), 6.
Turhan, K.N., Sahbaz, F., 2004. Water vapor permeability, tensile properties and
solubility of methylcellulose-based edible films. Journal of Food Engineering 61
(3), 459–466.
van Soest, J.J.G., Benes, K., De Wit, D., 1995. The influence of acid hydrolysis of
potato starch on the stress-strain properties of thermoplastic starch. Starch–
Starke 47 (11), 429–434.
Vasile, C., Bumbu, G.G., Dumitriu, R.P., Staikos, G., 2004. Comparative study of the
behavior of carboxymethyl cellulose-g-poly(N-isopropylacrylamide)
copolymers and their equivalent physical blends. European Polymer Journal
40 (6), 1209–1215.
Wu, Y., Geng, F., Chang, P.R., Yu, J., Ma, X., 2009. Effect of agar on the microstructure
and performance of potato starch film. Carbohydrate Polymers 76 (2), 299–304.
Xu, Y., Kim, K., Hanna, M., Nag, D., 2005. Chitosan–starch composite film:
Preparation and characterization. Industrial Crops and Products 21, 185–192.
Yang, L., Paulson, A.T., 2000. Effects of lipids on mechanical and moisture barrier
properties of edible gellan film. Food Research International 33 (7), 571–578.
Yanniotis, S., Nikoletopoulos, P., Pappas, G., 1990. Sorption models for dried fruits.
In: Engineering and Food Physical Properties and Process Control. Elsevier
Science Publishers Ltd., Essex, England, pp. 574–582.
Zhang, Y.C., Han, J.H., 2006a. Mechanical and thermal characteristics of pea starch
films plasticized with monosaccharides and polyols. Journal of Food Science 71
(2), E109–E118.
Zhang, Y.C., Han, J.H., 2006b. Plasticization of pea starch films with
monosaccharides and polyols. Journal of Food Science 71 (6), E253–E261.
Zhang, Y., Han, J.H., 2008. Sorption isotherm and plasticization effect of moisture
and plasticizers in pea starch film. Journal of Food Science 73 (7), E313–E324.
E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131 131
