Food Chemistry 127 (2011) 118–121
Contents lists available at ScienceDirect
Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem
Effect of rice wax on water vapour permeability and sorption properties of edible
pullulan films
F.F. Shih ⇑, K.W. Daigle, E.T. Champagne
Southern Regional Research Center, USDA, 1100 Robert E. Lee Blvd., New Orleans, LA 70124, USA
a r t i c l e
i n f o
Article history:
Received 7 July 2010
Received in revised form 8 December 2010
Accepted 21 December 2010
Available online 28 December 2010
Keywords:
Pullulan
Rice wax
Water permeability
Moisture sorption
a b s t r a c t
Edible films were prepared using various ratios of pullulan and rice wax. Freestanding composite films
were obtained with up to 46.4% rice wax. Water vapour barrier properties of the pullulan film were
improved with increased addition of rice wax. Moisture sorption isotherms were also studied to examine
the impact of rice wax on the water sorption characteristics of the film. The Brunauer–Emmet–Teller
(BET) and Guggenheim–Anderson–de Boer (GAB) sorption models were tested to fit the experimental
data. The models gave a good fit up to the water activity (aw) of 0.55 for BET and a full range of aw from
0.12 to 0.95 for GAB (R2 P 0.98). Changes in the sorption parameters, particularly such as the decrease in
monolayer moisture content (Mo), reflect the trend of reduced hydration capacity with increased addition of rice wax, providing useful information on water activity conditions to achieve stability for the
composite films.
Published by Elsevier Ltd.
1. Introduction
Edible films have been studied extensively in recent years, as a
special form of food or as carriers of flavour and nutritional additives. They are also useful as barriers for separation, protection
and preservation purposes. Edible materials with good filmforming properties include starch, cellulose, protein and their
derivatives that are normally water soluble or dispersible. To develop desirable food-use properties, highly hydrophilic starch or
polysaccharide films need modifications to improve their strength
and moisture resistance. For that purpose, the incorporation of
hydrophobic ingredients, such as oil and wax, has normally been
practiced (Chen & Nussinovitch, 2001; Garcia, Martino, & Zaritzky,
2000; Kester & Fennema, 1989; Koelsch & Labuza, 1992;
Shelhammer & Krochta, 1997). Polysaccharide-lipid films were
developed using cornstarch and soybean oil with methylcellulose
as the stabilising agent (Bravin, Persessini, & Sensidoni, 2006).
When these films were used to coat bakery products, such as
crackers, the coating was found to improve water vapour
resistance and to extend shelf life of the dry bakery product.
Pullulan is a microbial polysaccharide and has a primary structure of linear chains of maltotriose subunits in a [1–6] linkages
(Yuen, 1974). These a [1–6] bonds interrupt the regularity of what
would otherwise be an amylose chain, resulting in enhanced solubility and structural flexibility. Consequently, pullulan possesses
distinctive film- and fibre-forming characteristics not found in
⇑ Corresponding author. Tel.: +1 504 286 4354.
E-mail address: [email protected] (F.F. Shih).
0308-8146/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.foodchem.2010.12.096
amylose (Gounga, Xu, Wang, & Yang, 2008). Pure pullulan films
are transparent, water sensitive and mechanically weak. Improved
film strength and water vapour resistance were achieved for edible
films from pullulan and rice protein by the addition of small
amounts of propylene glycol alginate as the cross-linking agent
(Shih, 1996). The impact of beeswax lamination on the water
vapour permeability properties of pullulan-based films has also
been characterised (Kristo, Biliaderis, & Zampraka, 2007).
Rice bran oil (RBO) is a valuable milled rice co-product. However, only about 800 MT of RBO, out of an estimated 7 MMT annual
total, is currently available in the market (Hui, 1996). The main
reason for this lack of availability is the high content of waxy materials (up to 5% of RBO) that cause difficulties in the RBO processing.
Nevertheless, food-use RBO has been known to have health benefits (Wilson, Idreis, Taylor, & Nicholosi, 2002; Xu, Hua, & Godber,
2001). Recently, the waxy component that causes processing problems has been found to be a rich source of high molecular weight
fatty alcohols, known as policosanols, which are useful ingredients
for the treatment of hypercholesterolaemia (Gouni-Berthold &
Berthold, 2002; Mas, 2002). As a result, extensive studies have
been reported on the production, purification and characterisation
of RBO, as well as rice bran wax and its policosanol components
(Cravotto, Binello, Merizzi, & Avogadro, 2004; Gunawan, Vali, &
Ju, 2006; Vali, Yu, Kaimal, & Chern, 2005). Food grade rice bran
wax is now commercially available, and is becoming a popular
ingredient in foods for health-conscious consumers. Therefore,
the development of edible films with rice wax is desirable. These
films could be useful as carriers of flavour and nutritional additives
or as water barrier coatings for foods such as bakery products.
F.F. Shih et al. / Food Chemistry 127 (2011) 118–121
119
Moisture sorption isotherms are useful thermodynamic tools
for determining interactions of water and food substances, assessing food processing operations, and establishing parameters for
optimum storage and stability conditions (Apostolopoulos &
Gilbert, 1990; Diosady, Rizvi, Cai, & Jagdeo, 1996; Samapundo
et al., 2007). Moisture sorption studies have often been conducted
on edible films. For instance, effects of surfactants on water sorption properties of films made from hydroxypropyl methylcellulose
were characterised, and the data, fitted into GAB and BET sorption
models, indicated that for a specific hydrocolloid/surfactant ratio,
equilibrium moisture content of the films decreased as the hydrophilic/lipophilic balance of the surfactant mixture increased
(Villalobos, Hernandez-Munnoz, & Chiralt, 2006). Similarly, when
data of whey protein films were fitted using the GAB model, it
was found that solubility and equilibrium moisture contents of
the films were influenced by plasticiser and lipid incorporation
(Kim & Ustunol, 2001).
In this study, we prepared edible films using various ratios of
pullulan and rice wax. The films could be used in foods as water
vapour barriers and carriers of health-enhancing additives. For
these purposes, the films were examined and characterised for
their physicochemical and functional properties. Water sorption
properties of the films were also investigated by studying their
water vapour isotherms and the BET and GAB sorption models
were tested to fit the experimental data.
20 cm2 Plexiglas plate using a 1-mm thin-layer casting rod. The
films thus prepared were pullulan-based films with the incorporation of (A) 0% rice wax as control, (B) 15.5% rice wax, (C) 30.9% rice
wax and (D) 46.4% rice wax. After cooling to room temperature and
air drying overnight, the films were peeled off easily from the
Plexiglas plates and stored at 65% RH before analysis.
2. Materials and methods
where Dm = weight gain of the cell in g with calcium chloride,
A = film area in m2 and Dt = time (days)
2.1. Materials
Water vapour permeability ðWVPÞ ¼ WVTR h=Dp
Wax isolated from rice bran oil, Rice Bran Flakes Wax, was
obtained from Liberty Natural Products, Inc. (Portland, OR), pullulan from Hayashibara Co. (Tokyo, Japan), and arabic gum from Frutarom Meer Co. (North Bergen, NJ). Salts to prepare saturated
solutions for the determination of sorption isotherms, including
sodium nitrite, potassium chloride, magnesium chloride, potassium carbonate, lithium chloride and potassium acetate were supplied by Sigma Chemical (St. Louis, MO).
where h = film thickness (mm) and Dp = gradient of partial vapour
pressure across the film in kPa.
2.2. Preparation of composite films
Freestanding pullulan films with various degrees of rice wax
substitution were formulated. To prepare the films, a 20% pullulan
solution was prepared by dissolving 20 g pullulan in 80 g deionised
water. The required amounts of ingredients for each film, as listed
in Table 1, were then introduced into a beaker (50 mL), on a hot
plate, in the order of water, arabic gum, pullulan solution and rice
wax. The mixture was stirred with continuous heating at 65 °C for
a total of 10 min. The beaker with the heated mixture was removed
from the heat source and the mixture was immediately homogenised for 2 min with a T 25 homogeniser (S25N – 10G probe)
(IKA, Wilmington, NC). The homogenisation effectively transformed the mixture into a homogenous emulsion. The resulting
warm slurry emulsion was easily cast into a film on a 20 Table 1
Formulation of pullulan films with various degrees of rice wax (g of components).
Ingredients
A (0% rice
wax)
B (15.5% rice
wax)
C (30.9% rice
wax)
D (46.4% rice
wax)
20% Pullulan Soln
(Pullulan)
Rice wax
Water
Gum arabic
Total mixture
(Total solid)
22.50
(4.50)
0
7.15
0.35
30.00
(4.85)
18.75
(3.75)
0.75
10.15
0.35
30.00
(4.85)
15.00
(3.00)
1.5
13.15
0.35
30.00
(4.85)
11.25
(2.25)
2.25
16.15
0.35
30.00
(4.85)
2.3. Water vapour permeability
Films were preconditioned at 65% relative humidity and at
25 °C for 3 days. Water vapour permeability (g/m2 day kPa) of the
film was determined by the ASTM method (ASTM E96-80, 1996–
98). It was measured using Carson type test dishes (Thwing-Albert
Instrument Co., Philadelphia, PA). Anhydrous calcium chloride was
placed in the bottom of the test dish and molten beeswax was used
to seal the 7 cm diameter film samples to the dish. Moisture permeation through the 0.0021-m2 opening was calculated by weighing the dishes to the nearest 0.0001 g on an O’Haus Galaxy 160
analytical balance (Pine Brook, NJ). Film thickness was measured
to the nearest 0.0001 in (0.0025 mm) using an Ames dial micrometre (B.C. Ames Co., Waltham, MA). The mean of five measurements
for each film was used.
The water vapour permeability parameters are calculated using
the following equations (ASTM E96-80):
Water vapour transmission rate ðWVTRÞ ¼ Dm=A Dt
2.4. Moisture adsorption isotherm
Moisture sorption studies were conducted, based on the AACC
method (1995), by equilibrating film samples (1 g cut in 1 cm2
pieces in triplicate) at 25 °C in desiccators with saturated salt solutions [aw = 0.11, LiCl2; 0.22, CH3COOK; 0.33, MgCl2; 0.39, NaI; 0.43,
K2CO3; 0.53, Mg(NO3)2; 0.65, NaNO2; 0.84, KCl]. The sample was
weighed twice a week till reaching equilibrium (deviation <
1 mg/g in 3 weeks). Moisture content of the sample was determined as the weight gain at equilibrium. Dry weight of the sample
was obtained by freeze-drying. Water activity of the sample was
evaluated by using an Aqualab CS-2 water activity metre (Decagon
Devices, Pullman, WA).
The Brunauer–Emmet–Teller (BET) and Guggenheim–Anderson–
de Boer (GAB) models have been commonly used to describe
the sorption behaviour of foods. They were used to calculate parameters describing the sorption isotherms. The equations are as
follows:
BET (Timmermann, Chirife, & Iglesias, 2001)
aw =½ð1 aw ÞM ¼ 1ðMoCÞ þ ½ðC 1Þ=ðMoCÞaw
where M = dry basis moisture content, aw = water activity,
Mo = monolayer moisture content, C = surface heat constant
C ¼ expðQs=RTÞ
where Qs = excess heat capacity at the monolayer (cal/mol) and
R = the gas constant (1.986 cal/mol K), T = temperature (K).
GAB (Lewicki, 1997)
aw =½ð1 kaw ÞM ¼ 1=ðMoCkÞ þ ½ðC 1Þ=ðMoCÞaw
The GAB equation has a similar form to BET, but has an extra constant k, where k = constant (0.7–1.0) and C = constant (not equal
to the C of BET).
120
F.F. Shih et al. / Food Chemistry 127 (2011) 118–121
Goodness of fit is evaluated using the mean relative deviation
modulus (%P), (Lomauro, Bakshi, & Labuza, 1985) defined by the
equation:
%P ¼ 100=n
X
jM i M ei j=M i
where Mi is the moisture content at observation i, and Mei is the predicted moisture content at that observation and n is the number of
observations.
The parameters of both models were calculated using the nonlinear regression program Water Analysis v97.4, developed by
Professor T.P. Labuza (Webb Tech Pty Ltd., Australia).
3. Results and discussion
3.1. Film preparation
Wax films are normally difficult to prepare. The wax needs to be
melted with heat and the molten wax or wax–water emulsion cast
into films while still hot. The formation of wax-in-water emulsion
is enhanced by the addition of arabic gum, which serves as an
emulsifier (Kim & Ustunol, 2001). Nevertheless, the resulting freestanding films are often rigid and brittle after cooling to the room
temperature. We melted the rice wax at about 65 °C and heating
was continued when water and pullulan were added into the mixture. The hot slurry was homogenised immediately and cast into
films with no further heating. The pullulan–rice wax emulsion
films thus prepared, with up to 30.9% wax, were homogeneous
and peeled off easily from the fibreglass plate. Pure pullulan film
(0% wax) was transparent but became increasingly opaque and rigid with increased incorporation of the rice wax. At 46.4% wax, the
slurry was viscous and difficult to cast into films, and the resulting
films were brittle and not totally homogeneous.
wax resulted in emulsion films more opaque in appearance with
significantly lower WVP. Films with 30.9% wax remained homogeneous and flexible, and they showed superior water vapour resistance with WVP at about (6 g mm)/(m2 day kPa) and a thickness
of 0.15 ± 0.04 mm.
3.3. Sorption isotherms
Fig. 2 shows GAB moisture sorption isotherm curves for the
pullulan films with various degrees of rice wax substitution. In
general, the curves display the familiar sigmoidal shape with a
trend that the equilibrium moisture content (EMC) increases
slowly with increasing environmental water activities up to about
0.7 and then a steep rise thereafter. At low moisture contents,
water is adsorbed at the strongest binding sites of the surface of
the solid. As moisture increases, the material swells, opening up
new sites for water to bind, resulting in the upswing of the profile.
Furthermore, other than the curve for the control (pullulan film
with no rice wax), the sorption isotherm has lower EMC with increased rice wax at a given water activity, reflecting the impact
of decreasing hydrophilic pullulan in the composition on the water
adsorption capacity of the composite films.
Table 2 summarises the parameters as calculated using the BET
and GAB equations. BET is for water activities up to about 0.5 and
GAB is for the entire range of water activities. The monolayer values of both BET and GAB decreased with increased substitution of
the rice wax in the film, reflecting the more hydrophilic nature of
pullulan compared to rice wax. The BET values of Mo and C were
calculated based on a simplified monolayer concept as a special
case of the GAB model. However, despite the theoretical
A
3.2. Water vapour permeability
Moisture (g water/ g solids)
0.30
Wax films, such as those from beeswax, canelilla and carnauba,
have been reported to display low water vapour permeability
(Donhowe & Fennema, 1993; Kristo et al., 2007). Rice wax is similar to carnauba wax in composition and expected to form effective
water resistant films comparable to those of the carnauba wax. In
the current experiments, composite films of pullulan and up to
46.4% rice wax were prepared. Fig. 1 shows the effect of rice wax
on the water vapour permeability (WVP) of the films. At 0% wax
incorporation, the pure pullulan film was highly hydrophilic and,
as expected, showed the highest WVP. With the addition of
15.5% wax, the effect on WVP was minimal and the film remained
equally highly permeable to water vapour. Further increases in
B
C
D
0.20
0.10
0.00
0
0. 2
0. 4
0. 6
0. 8
1
WVP
(g x mm)/(m 2 x day x kPa)
aw
10.00
0.82
Fig. 2. Experimental and calculated GAB water isotherm of pullulan firms with
various ratios of rice wax. Film A (0% rice wax), Film B (15.5% rice wax), Film C
(30.9% rice wax), Film D (46.4% rice wax).
0.30
7.50
0.45
5.00
0.25
2.50
Table 2
BET and GAB parameters from isotherms of composite films and pure components of
the films.
Filmsa
0.00
A
B
C
D
(0)
(15.5)
(30.9)
(46.4)
Composite Films (% Rice Wax)
Fig. 1. Water vapour permeability (WVP) of pullulan films with various ratios of
rice wax.
A
B
C
D
BET
GAB
Mo
C
Qs
Mo
C
K
R2
%P
0.055
0.041
0.035
0.029
2.705
10.935
11.737
10.289
590
1417
1459
1381
0.063
0.045
0.034
0.031
0.287
10.457
17.405
11.305
0.857
0.898
0.919
0.887
0.98
0.98
0.99
0.99
7.81
9.72
10.03
6.19
a
Film composition A, 0% rice wax; B, 15.5% rice wax; C, 30.9% rice wax; D, 46.4%
rice wax.
F.F. Shih et al. / Food Chemistry 127 (2011) 118–121
limitations of the BET adsorption analysis, the BET equation was
useful in predicting the monolayer value of coverage and the heat
of adsorption Qs, which are of the most concern to processing and
storage (Labuza, Kaanane, & Chen, 1985). The GAB parameters
were calculated using the full range of aw from 0.11 to 0.95, postulating that the states of water molecules in the second and higher
layers were the same as each other but different from those in the
liquid state. This assumption introduces an additional degree of
freedom by which the GAB model gains its greater versatility.
Generally, the value of monolayer (Mo) indicates the amount of
water that is strongly adsorbed to specific sites of food for optimum stability. The value of C, which may lack physical meaning,
is the result of mathematical compensation among parameters,
as often occurs in curve fitting procedures. Our results show that
the Mo is consistently slightly smaller from the BET calculation
than the one from GAB. Similar findings have been reported in
the literature, for example, that the estimates of the monolayer
moisture capacity (mol/105 g) were 535 (BET) and 638 (GAB) for
collagen and 470 (BET) and 572 (GAB) for gelatin (Villalobos
et al., 2006).
The sorption parameters for GAB as shown in Table 2 enable the
calculation of EMC values within the temperature and relative
humidity ranges involved. The values of coefficient of determination (R2) range from 0.98 to 0.99 and those of the percent deviation
(%P) from 7.8 to 10.7. The higher the values of R2 and the lower the
values of %P, the better will be the performances of the equations
(Lomauro et al., 1985). Our values indicate that the GAB equation
gives a good fit to the experimental data.
The value of K provides a measure of the interaction of the
molecules in the multilayer with the adsorbent, and tends to fall
between the energy value of the molecules in the monolayer and
that of liquid water (K = 1). Lewicki (1997) concluded that when
the K and C values of the GAB model fall within the intervals
0.24 6 K 6 1, 5.67 6 C 6 1, the calculated monolayer values are
within ±15.5% of the true monolayer capacity. In the present study,
the K and C values fell within the aforementioned ranges, with K
values ranging from 0.85 to 0.91, and C (except for Film A) from
10.4 to 17.4.
4. Conclusion
Freestanding edible films can be obtained using a composite of
pullulan and up to 46.4% rice wax. The added rice wax is not only a
health-enhancing ingredient but also a function-modifying agent
for improved water vapour barrier properties of the composite
films. For instance, films with 30.9% rice wax, which showed superior water vapour resistance, could be used such as in the coating
of fruit products, for protection and preservation purposes. Sorption isotherm studies, with data fitted into BET and GAB models,
provide sorption parameters characterising the hydration properties of the films. The information is useful in establishing the range
of water activity conditions for desirable food-use properties of the
films.
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