Academic Journal of Polymer Science - Juniper Publishers
The purpose of this study was to evaluate the effect
of adding various concentrations of castor oil (2% and 4% v/v) on the
properties of edible films based on carboxymethyl cellulose (CMC).
Moisture content, water solubility, tensile strength, elongation at
break, elastic modulus, water vapor permeability, optical and thermal
properties and antioxidant activity of the films were examined. The
results demonstrated that the presence of castor oil led to a decrease
in moisture content, water solubility and mechanical strength of the
films. The film containing 4% of oil showed the highest water vapor
permeability. The optical properties measurement represented that all of
the samples were transparent. By the addition of castor oil, the
antioxidant activity of the films improved largely. Thermal properties
of the samples had also been investigated and it was determined that the
effect of castor oil on the melting point was negligible. Finally, the
performance of the coatings for protecting fresh apples was studied with
measuring the pH value of apples and performing some field test.
Every year a large portion of foods, especially
fruits and vegetables, from the time they are harvested until they reach
to consumers, go through different kinds of qualitative and
quantitative deterioration and loss [1]. Various factors are involved in
food deterioration such as microorganisms, mechanical damages, time and
light. Therefore, if we fail to protect the food, it can be damaged
within hours or days [2]. Currently, there are numerous technologies
used for the purpose of fruits and food preservation. Some include
controlled atmosphere storage and modified atmosphere packaging,
innovative osmotic dehydration technologies, electro-osmotic dewatering ,
thermal pasteurization, and gaseous ozone and ozonated water treatments
[3,4]. One way to prevent or delay the food deterioration is the use of
edible coatings. During the past decade, edible coating has been widely
used for this purpose [5-9]. Some advantages of edible coatings include
extending shelf-life of fresh fruits, reducing amount of waste produced
from packaging processes, improving appearance and fulfilling
environmental safety, as well as enhancing nutritional properties due to
containing lots of beneficial biopolymers and biological compounds
including polysaccharides, proteins, and lipids. Moreover, they decrease
water loss and retard ripening of fruits due to barrier mechanism, and
are also able to enhance antioxidant properties of fruits and vegetables
[10-13].
Any material used to cover foods in order to enhance
the shelf life, which may be eaten with the food, is an edible coating
[2]. Various studies have been conducted on the properties and
applications of edible films and coatings [14-18]. The initial studies
on edible coatings can be traced back to 1967, where Hardenburg studied
the application of wax used by Chinese on citrus fruits [19]. Since 1986
there were various studies on edible coating including wax emulsions,
oils, cellulose, chitin and chitosan, and their impact on diverse
properties of fruits and food like shelf-life [20-26]. However, more
research is needed because there are no edible films that can be used
for all purposes. As other technologies, edible coatings can also bring
some challenges like undesirable tastes caused by presence of wax
materials, hydrophilic nature of edible coatings which makes them
moisture-sensitive and limits their moisture-barrier performance, poor
adhesion and coverage, and insufficient mechanical properties of
biopolymers comparing to the synthetic ones [27,28]. In general, an
ideal edible film must have characteristics such as being nontoxic,
allergenic or indigestible materials, protecting foods from mechanical
damages, good adhesion, maintaining the appearance of the product and
having an easy and economical production [2]. Proteins, carbohydrates
and lipids are the main film forming agents in edible coatings [29].
Generally, lipids will reduce water transmission in edible coatings,
polysaccharides are good for controlling the transmission of gases and
protein-based films have proper mechanical strength [29]. These
materials can be used either individually or as a blend (composite
films) in edible coatings [29].
Carboxymethyl cellulose (CMC) is a conventional
polysaccharide, which is one of the common derivatives of cellulose. CMC
is an anionic polysaccharide, which unlike cellulose is soluble in
water [30]. CMC has wide variety of uses in different industries such as
foods and coatings [31] and therefore is an excellent choice for edible
coatings because it has no toxic or allergic effects. It is one of the
most desirable polysaccharide polymers due to its excellent properties
including perfect film forming ability, availability, low price and high
viscosity, which can be used to produce both edible and degradable
films and coatings [31]. In most cases, a plasticizer must be added to
the film forming solution in order to reduce the brittleness of CMC
films. However, using plasticizers will affect the mechanical and
permeability properties of the films [32]. The most common food grade
plasticizers are some polyols including glycerol, mannitol, sorbitol,
and sucrose [33]. Glycerol is a clear, colorless and odorless liquid,
which is soluble in water due to its hydrogen bonds. This component
reduces the film's fragility by being located between the CMC chains
(enhancing chain mobility) and also by absorbing water [34]. The
beneficial properties of some lipids, including their good compatibility
with other film constituents and good barrier properties against water
vapor and other gases, make them an ideal choice for edible coatings and
films [35,36]. Lipid compounds that are commonly used to make films and
edible coatings include edible oils, fatty acids and waxes [37]. The
efficiency of lipid used in edible films and coatings depends on the
nature of the lipid, particularly its structure, hydrophobicity and its
interaction with other components in the system [2]. Castor oil is a
viscous and non-volatile liquid with a pale yellow color. High amount of
both resinoleic acid (RA) and double bonds results in oxidative
stability of the castor oil as well as long shelf life [37]. The
antioxidant activity of castor oil also makes it an excellent choice for
use in edible coatings in order to prevent food degradation.
In this study, the effect of castor oil on
mechanical, optical, physical and thermal properties of CMC-based edible
films along with their antioxidant activity have been investigated. The
results showed potential use for castor oil in edible coatings.
Carboxymethyl Cellulose (CMC) with purity of 99.6%
and viscosity of 2787 cps ; Castor oil with density of 0.959 g/mL [38]
and Polysorbate 80 (Tween 80) were purchased from Pasargad Novin
chemical Co. Glycerol (USP grade) was purchased from PALMAC. Analytical
grade of Ascorbic acid was obtained from Merck (Darmstadt, Germany). CMC
is a white, granule-shaped powder used for increasing the viscosity.
Castor oil consists of various fatty acids mostly containing ricinoleic
acid (12-hydroxyoctadecenoic acid) [39]. Tween 80 which is a highly
viscous liquid, is used as a surfactant [40,41].
Film forming solution was prepared by adding 1 g of
CMC to 100 ml of distilled water (1% w/v). The system was under
continuous agitation by magnet stirrer and the temperature was set on
75℃. After 40 minutes, a clear solution was achieved. Then 1 ml of
Glycerol (as plasticizer) was added to the film forming solution and
stirring went on for another 15 minutes. At this point, the film forming
solution was ready in order to prepare the control films (films
containing no castor oil). To do so, 25 g of the solution was poured in
the middle of plastic circular plates (with 10 cm diameter and 1 cm
height) and dried at room temperature in about 48 hours. Other film
samples were prepared as described below:
a. CO-2: 0.2 ml of Tween 80
(proportional to castor oil) was added to the film forming solution
under continuous stirring. After 15 minutes, 2 ml of castor oil was also
added to the solution and the stirring continued for another 30
minutes.
b. CO-4: 0.4 ml of Tween 80 was
added to the film forming solution under continuous stirring. After 15
minutes, 4 ml of castor oil was also added to the solution and the
stirring continued for another 30 minutes.
c. AA1: 1 g of ascorbic acid was added to the film forming solution and the system was under stirring for 10 minutes.
d. AA2: 0.4 ml of Tween 80 was added
to the film forming solution under continuous stirring. After 15
minutes, 4 ml of castor oil was also added to the solution and the
stirring continued for another 30 minutes. Following that, 1 g of
ascorbic acid was added to the solution and stirring continued for 10
minutes.
The procedure of preparing film from all of the
mentioned solutions was similar to the control films. Also, to do some
field tests and measuring pH the sample films were applied on apple by
silicon brush. All the dried films were peeled of the plates and kept in
a desiccator at 25℃ and 50% relative humidity (RH) for 72 hours until
further evaluations.
A hand-held micrometer was used to measure the film
thickness. For each film, the thickness was measured in several areas
(at least at five locations) and the final thickness was reported as the
average value.
The color of each film was monitored using a
calorimeter in the CIE Lab system. The films were placed on a standard
white background (L*= 89.7584, a* = -0.5083 and b* = -2.0585) and their
“Lab” values were determined. The color difference (ΔE) between the
films and the standard background, whiteness index (WI) and yellowness
index (YI) were calculated using the following equations:
Where “L” value represents the lightness of the
sample (varies from 0 (black) to 100 (white)), “a” represents the
redness-greenness balance of the sample (from negative (green) to
positive (red) values) and “b” indicates the yellowness-blueness balance
of the film (between negative (blue) and positive (yellow) values)
[37].
The moisture content of each film was determined
based on weight loss. In this method, the film was first weighed and
then placed inside the oven at 110°C. The weight of the film was
measured every hour until it reached a constant value. Finally, the
amount of moisture in the film was obtained from the difference between
the initial and the final weight. The percentage of water in the film
was calculated by the following equation:
Where Wi is the weight of the film before being
placed in the oven and Wf is the weight of the film after that. The test
was repeated three times for each sample to secure a more accurate
result.
The solubility of the film in water was evaluated
based on the method described by Gontard et al. with some modifications
[42]. The initial weight of the film was measured after drying the
sample at 110 °C. Then the dry samples were immersed in 100 ml water for
5 hours at 25°C under continuous stirring. After filtration, to
determine the final dry weight, the portion of the film, which was
undissolved in water, was placed in an oven at 110°C to reach a constant
weight. The water solubility percentage of the film was calculated by
the following equation:
Where Wi is the initial dry weight and Wf is the final dry weight of the films (after immersing in water).
In order to obtain a consistent result, water
solubility of each film was evaluated three times and the average value
was reported.
The water vapor permeability of each film was
determined based on the instructions provided by ASTM E96 [43]. Cups
with an average diameter of 3.5 cm and a depth of 1 cm were used to
determine WVP. The films were cut in circle shapes with slightly larger
diameter than of the cup. After placing 3 g of water (RH = 100%) in each
cup, they were covered with sample films. Each cup was placed in a
desiccator containing silica gel (RH = 0%) and the temperature was
maintained at 25°C. The cups were weighed every 24 hours (With a
precision of 0.0001 g). The weight change of the cup was plotted as a
function of time and the slope of this curve (weight change versus time)
was calculated by a linear regression. Water vapor transmission rate
(WVTR) was obtained by dividing this slope by the transmission area (m2). Finally, the WVP was evaluated from the following equation:
Where P is the saturated vapor pressure of water at test temperature (25℃), R1 is the RH inside the desiccator, R2 is the RH inside the cup and X is the film thickness (in meter).
The antioxidant activity of the film samples was
investigated through DPPH radical scavenging activity based on
instruction of Brand-Williams et al. [44] with some modification. This
test is based on the ability of the samples to donate hydrogen or
electron and is evaluated by measuring the amount of color reduction of
2,2-diphenyl-1-picrylhydrazyl (DPPH) solution in methanol (a solution
with purple hue). DPPH radical is absorbed at 517 nm but the amount of
absorbance decreases when it is exposed to an antioxidant. To evaluate
this property, 25 mg of the sample film was added to 5 ml of distilled
water and the mixture was stirred continuously by a magnet Stirrer. Then
0.1 ml of the resulted solution was added to 3.9 ml of DPPH solution
(0.1 mM methanol solution) and the sample was incubated in darkness for
one hour at room temperature. The sample’s absorbance was then measured
at 517 nm and the scavenging activity was calculated by the following
equation:
Where A is the absorbance at 517 nm.
Tensile strength, elongation at break and elastic
modules of the films were measured by the universal testing machine. The
test was conducted based on the ASTM D882-91 standard [45]. For this
purpose, the films were cut into rectangle shapes (about 2 cm x 10 cm)
and placed between the grips. The initial space between the grips and
the crosshead speed were set at 50 mm and 10 mm/min, respectively (with 1
kg load cell). Mechanical properties were evaluated for three
replications of each sample.
The thermal properties of the films were investigated
by DSC test, which is described below. Approximately 5 mg of the film
was placed in an aluminum pan. The reference was also an empty aluminum
pan. Thermal behavior of different films were measured at a thermal
scanning rate of 5℃/min in three steps. First heating from 30℃ to 100°C,
then cooling from 100℃ to 30°C and last heating again from 30℃ to
250°C. The melting point (Tm) of the films was determined in this
process.
To evaluate the pH of apples, 10 g of each sample
(with its coating) was added to 100 ml of distilled water and then
stirred for 30 minutes. After filtering the mixture, its pH was measured
and reported by a digital pH-Meter. In order to compare the results,
the same process was performed on control apples (apples with no
coating). The pH measurement was started immediately after applying
coating on the apples and was repeated every 4 days. During the test
process, apples were kept at room temperature.
To evaluate the performance of the coating, three apples were picked from a store and processed as described below:
a. The first apple was kept unwashed.
b. The second one was washed.
c. The last one was washed and then the CO-4 film forming solution was applied on it with a silicon brush.
All three samples were then kept at a dark place at room temperature for six months.
All the Data were analyzed using MATLAB and
Statistics Toolbox Release R2018b, The MathWorks, Inc., Natick,
Massachusetts, United States. The results were reported as mean ±
standard deviation.
Thickness is an important parameter that affects some
properties of the films such as transparency, permeability and
mechanical strength [46]. The thickness of the films varied from 45 to
95 microns. As shown in Table 1, the thickness of the control film
(without castor oil) was 45 microns and with increasing the oil content
in the samples, their thickness increased until it reached 95 microns
for the film containing 4% castor oil. The reason is that by adding oil
to the film-forming solution, the solid content of the final coating
increases, because castor oil enters the CMC matrix, which results in an
increase in thickness. Same result have been reported by
Shojaee-Aliabadi et al. [47].
Optical properties or appearance is an important
factor of edible coatings and films as they affect the acceptancy of a
product by the consumers. The least expectation from a coating is to
maintain the appearance of the product if cannot improve it. The CIE
Lab, color difference (ΔE), whiteness index (WI) and yellowness index
(YI) values used to describe the optical properties are given in Table
2. The spectral reflectance curves of the samples are also shown in
Figure 1. As illustrated, the control film had the highest area under
the curve and therefore the highest brightness. The L value (brightness)
of the samples decreased slightly with increasing oil content. In
addition, the yellowness index of the specimens increased marginally
with increasing oil content in the film structure, which could be due to
the absorption of light by castor oil at low wavelengths. For the same
reason, the color difference of the samples increased moderately with
the addition of oil to the film (the color difference between the CO-2
and CO-4 samples having different oil content was not significant).
Finally, the whiteness index of the samples decreased slightly with the
addition of oil to the system. The presence of castor oil and increasing
the amount of it reduced the transparency of the film to some
negligible degree. The reason is that by adding oil to the film forming
solution, which is a water-insoluble fraction, the system becomes an
emulsion, resulting in a decrease in the transparency of the film
relative to the oil-free sample [48]. Overall, the addition of castor
oil to CMC films had no major negative effects on their appearance.
Moisture content of films is an important factor in
determining coatings quality. High moisture content can provide an ideal
environment for the growth of microorganisms and cause deterioration
during long storage periods. The MC also helps edible films to melt in
mouth when eaten [2]. The MC of each specimen is reported in Table 1. As
can be seen, samples containing castor oil had a lower moisture content
than the control film, and the MC decreases with increasing oil content
from 2% to 4%. The reason for this decrease can be mainly attributed to
the increased hydrophobicity of the films due to the increased oil
content. In addition to increasing hydrophobicity, the interaction
between castor oil and the hydroxyl groups present in the CMC impedes
water absorption by these groups, thereby reducing the amount of water
in the film. Similar results were found by previous studies [47,49].
Moisture content of a film can also affect its
solubility in water. The higher the moisture content of the film, the
greater the solubility in water. The water solubility of each film
sample is reported in Table 1. As shown in the diagram, the CMC film
containing no castor oil (control sample) is completely soluble in
water. The solubility in water decreased with increasing castor oil
concentration in samples due to the high amount of hydrophobicity in the
film matrix and preventing CMC hydroxyl groups from absorbing water.
The lowest water solubility was observed in CO-4 sample containing 4%
castor oil in its structure.
WVP is an important factor to investigate the
performance of edible films and coatings and can be influenced by
characteristics such as film integrity, hydrophobic ratio and film
thickness [50]. Knowing WVP of the film is very helpful for preventing
the mass transfer from food to the surrounding environment and therefore
extending the shelf life of the product. The values of G/t, which is
the slope of the weight change curve over time of the samples, are shown
in Figure 2. Using these slopes, the WVP values were calculated for the
samples (Table 1). It is observed that with increasing oil content, the
slope of the curve increased, resulting in a higher WVP. The film
containing 4% oil shows the highest amount of WVP. While the common
expectation is that increasing oil content would decrease the
permeability of the samples due to the increased hydrophobicity of the
films, the exact opposite result has been achieved. This could be due to
the fact that the increase in castor oil concentration in CMC films,
besides increasing the hydrophobicity ratio, had a negative effect on
the cohesion of the film matrix. The weakening of the cohesion forces in
the matrix accelerates the transfer phenomenon and ultimately increased
the water vapor permeability. Furthermore, castor oil also has a
softening effect similar to glycerol. This makes the movement of polymer
chains in the film easier, which increases permeability. Similar result
was observed in the work of Dashipour et al. [49].
The ability of an edible coating to prevent foods
from oxidation is an important factor in determining the coating's
performance to extend the shelf life of the product. The higher the
antioxidant activity of the coating, the better its function in
protecting the food. In this study, the antioxidant activity of the
samples was measured by DPPH scavenging activity method, and the results
are shown in Figure 3. As expected, the control film showed no
antioxidant activity, as there is no compound in its structure capable
of radical absorption. The amount of antioxidant activity in
oil-containing films is significantly higher than that of control
sample. By increasing the oil concentration from 2% to 4%, the
scavenging activity of the film also increased and reached 46.3%, which
is desirable. The antioxidant activity of films containing just castor
oil was lower than that of ascorbic acid containing samples. The highest
scavenging activity was for AA2 sample, which had both castor oil and
ascorbic acid (95.4%). The radical scavenging activity of the
oil-containing specimens results from the unsaturated double bands
present in castor oil structure. These double bands become saturated by
absorbing free radicals; therefore, prevent the product from oxidizing.
Characteristics of edible films such as tensile
strength (TS), elastic modulus (EM) and elongation at break (EB) are
very important in coatings and packaging and can be helpful to predict
coatings performance [51]. Table 1 shows the TS, EM and EB values of
samples. As illustrated, the tensile strength of the control sample was
10.14 MPa, which decreased significantly with increasing oil content and
reached 3.43 MPa in CO-4 film. Similarly, the elastic modulus of the
films decreased with increasing castor oil from 76.91 MPa to 27.75 MPa.
On the other hand, with increasing castor oil concentration, the
flexibility of the films improved and therefore their elongation at
break increased from 24.81% for control sample to 49.08% for CO-4
sample. This can be due to the fact that the addition of castor oil
created a heterogeneous film, which results in decreasing mechanical
strength and increasing flexibility [52]. In addition, as mentioned
earlier, castor oil has some level of softening property. Therefore, its
molecules can be placed between CMC chains and facilitate their
movements and as a result increased the films’ flexibility, desirably.
Determination of thermal properties, especially glass
transition temperature, is very effective in evaluating the performance
of edible films and coatings. If the glass transition temperature of
the film is much higher than the ambient temperature, the film would be
very brittle, but it would have low permeability. However, if the Tg
is lower than the ambient temperature, the permeability would be very
high and the film would be very soft and flexible, which will not
provide the proper mechanical protection for the product. As a result,
if the glass transition temperature of the films is slightly above
ambient temperature (close to ambient temperature), it will be in a
desirable range of permeability and mechanical strength [46]. Therefore,
if castor oil could reduce the Tg and Tm of the specimens, it will be
considered a beneficial effect. Thermal diagram of the samples obtained
by performing DSC experiment and is shown in Figure 4. As shown in the
diagrams, in the temperature range of 208℃ to 211°C there are large and
endothermic peaks, which could be attributed to the melting temperature
of the CMC matrix. The Tm of the control sample is 210.99°C, and with
increasing castor oil content to 4% (CO-4), the Tm reached 208.39°C. As
presented, no glass transition temperature was observed in the samples
due to the low sensitivity of the test equipment.
The microbial growth in the food produces
nitrogen-containing compounds that increase the pH of the product [53].
The coating should prevent this increase in pH by restraining the
microbial growth. The better the coating performs in this matter, the
lower the pH increases and the longer the shelf life of the product will
be. The pH values of control and coated apples were measured over 16
days, which are represented in Figure 5. As can be seen, the highest pH
increase was for uncoated apples and the lowest increase was for apples
with CO-4 coating. In general, coatings (especially the ones with castor
oil) were successful in preventing pH increase and their pH was lower
than non-coated apples after 16 days. This result shows that CMC-based
coatings containing castor oil prevented pH value from increasing.
As mentioned earlier, one of the causes of food
deterioration is time. Nowadays, the process of preparing foods until
they are ready for consumers’ use takes a considerable amount of time.
During this time-consuming process, foods could undergo water loss and
deterioration. Therefore, the performance of edible coatings over time
is very important. Figure 6 shows the changes of apples after being
exposed to ambient temperature for 6 months. Although the coated apple
(Figure 6, c2) was also suffered from high deterioration, it had less
water loss and discoloration than the other two apples, which had no
coating. The coated apple also had a much better physical state and was
more firm and less wrinkled than the other two.
The presence of castor oil in the film forming
solution increased the thickness of the resulting CMC films, but due to
its hydrophobic nature, reduced the moisture content and water
solubility of the films. The addition of castor oil in carboxymethyl
cellulose films resulted in higher permeability due to the weakening of
cohesion forces in CMC matrix and softening properties of the oil. The
mechanical strength of the films was also weakened for the same reason.
As the oil content increased, the color difference and yellowness index
of the films increased, and the whiteness index decreased slightly. Due
to the presence of unsaturated double bonds in the castor oil structure,
this oil had the ability of radical absorption and therefore
antioxidant activity. Addition of castor oil had no significant effect
on the thermal properties of the carboxymethyl cellulose films and
reduced the melting temperature of the samples by 3°C. The coatings
prevented the early deterioration of apples and reduced the pH
increasing rate by preventing the growth and activity of the
microorganisms.
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. The resulting equation has the concise form
of
is the local induced
bending moment and is a function of the imposed electric field
E, Y is the Young’s modulus (elastic stiffness) of the strip which
is a function of the hydration H of the ionic polymer metal
nanocomposite and I is the moment of inertia of the strip. Note
that locally M(E) is related to the pressure gradient such that in a
simplified scalar format [15]:
is related to the imposed
electric field E 
Based on this simplified
model the tip bending deflection δmax of an IPMC strip of length lg
can be shown to be almost linearly related to the imposed electric
field. The experimental deformation characteristics of IPMCs are
clearly consistent with the above predictions obtained by the
above linear irreversible thermodynamics formulation which
is also consistent with the above equation in the steady state
conditions. Note also that the value of the Onsager coefficient L
has been estimated to be of the order of 10-8m2/V-s [20-24]. Other
parameters have been experimentally measured to be K~10-
18m2/CP, σ ~1A/mV or S/m. On the other hand one may consider
charge transport modeling of actuation and sensing. The reader is
referred to Bahramzadeh and Shahinpoor [20-24] and Shahinpoor
[22-24] for such Poisson-Nernst-Planck equations. Similar to poly (ethylene-co-methacrylic acid) metal composite, member of
the electroactive polymer can be introduced as biomimetic soft
robotic polymer [28-32].
torsion
of a helix is given by 
