Corn Starch Edible Films: Extrusion & Casting Characterization

Physicochemical and Microstructural
Characterization of Corn Starch Edible Films
Obtained by a Combination of Extrusion
Technology and Casting Technique
Perla Rosa Fitch-Vargas, Ernesto Aguilar-Palazuelos, José de Jesús Zazueta-Morales, Misael Odı́n Vega-Garcı́a,
Jesús Enrique Valdez-Morales, Fernando Martı́nez-Bustos, and Noelia Jacobo-Valenzuela
E: Food Engineering &
Materials Science
Abstract: Starch edible films (EFs) have been widely studied due to their potential in food preservation; however, their
application is limited because of their poor mechanical and barrier properties. Because of that, the aim of this work was to
use the extrusion technology (Ex T) as a pretreatment of casting technique to change the starch structure in order to obtain
EFs with improved physicochemical properties. To this, corn starch and a mixture of plasticizers (sorbitol and glycerol,
in different ratios) were processed in a twin screw extruder to generate the starch modification and subsequently casting
technique was used for EFs formation. The best conditions of the Ex T and plasticizers concentration were obtained using
response surface methodology. All the response variables evaluated, were affected significatively by the Plasticizers Ratio
(Sorbitol:Glycerol) (PR (S:G)) and Extrusion Temperature (ET), while the Screw Speed (SS) did not show significant
effect on any of these variables. The optimization study showed that the appropriate conditions to obtain EFs with the best
mechanical and barrier properties were ET = 89 °C, SS = 66 rpm and PR (S:G) = 79.7:20.3. Once the best conditions
were obtained, the optimal treatment was characterized according to its microstructural properties (X-ray diffraction,
Scanning Electron Microscopy and Atomic Force Microscopy) to determine the damage caused in the starch during Ex T
and casting technique. In conclusion, with the combination of Ex T and casting technique were obtained EFs with greater
breaking strength and deformation, as well as lower water vapor permeability than those reported in the literature.
Keywords: casting technique, extrusion technology, microstructural properties, physicochemical properties, starch edible
films
Starch is becoming an environmentally friendly alternative to produce films due to its low cost,
high biodegradability, and thermoplastic properties. However, native starches have many disadvantages, limiting their
broader applications and industrial use. For this reason, the use of Ex T as a pretreatment combined with the conventional
method (casting technique) to elaborate EFs, could be an alternative to change the structure of starch in order to generate
films with improved mechanical and barrier properties.
Practical Application:
Introduction
Edible films (EFs) are thin layers of edible materials applied
on food products that play an important role in their preservation, distribution and marketing (Fakhouri and others 2015).
These materials act as barriers producing modified atmospheres,
reducing moisture exchange, controlling microbial growth, and
carrying functional ingredients (antioxidants, antimicrobials)
(Treviño-Garza and others 2015). The most common polymers
used in formulation of EFs are proteins (gelatin, wheat gluten
and zein), lipids (waxes), and polysaccharides (starch, chitosan
and, cellulose) which are used alone or combined (Vanin and
others 2005; Chiumarelli and Hubinger 2014; Dai and others
MS 20160061 Submitted 1/11/2016, Accepted 7/12/2016. Authors Fitch-Vargas,
Aguilar-Palazuelos, Zazueta-Morales, Vega-Garcı́a, Valdez-Morales, and JacoboValenzuela are with Facultad de Ciencias Quı́mico-Biológicas, Univ. Autonóma de
Sinaloa, Cd. Universitaria, Av. de las Américas y Josefa Ortiz S/N, 80010 Culiacán, Sinaloa, Mexico. Author Martı́nez-Bustos is with Centro de Investigación y
de Estudios Avanzados, Libramiento Norponiente, Fracc. Real de Juriquilla 76230
Querétaro, Querétaro, Mexico. Direct inquiries to author Aguilar-Palazuelos (E-mail:
[email protected]).
E2224
Journal of Food Science r Vol. 81, Nr. 9, 2016
2015). Starch is the most important polysaccharide used in the
formulation of biodegradable films and EFs, being, probably, the
most promising renewable naturally biodegradable polymer since
it is a versatile, cheap, and, abundant biopolymer (Chiumarelli and
Hubinger 2014; Li and others 2015). Among starches, corn starch
has been widely used on the formulation of EFs due to its availability and relative low price (Garcia and others 2000; Maran
and others 2013; Fakhouri and others 2015). However, to overcome several disadvantages of starch films, such as their strong
hydrophilic character and poor mechanical and barrier properties
compared to synthetic polymers, further modification is usually
necessary (Dai and others 2015). A modified and soluble or pregelatinized starch also has been used to obtain starch films (Jiménez
and others 2012). An alternative technology to produce modified
starches, is extrusion. Twin-screw extruder, particulary, could be
used as a tool to produce modified starches in a continuous process having a consistent product with better quality. The extruder
is an excellent mixing device, which it is particularly suitable for
highly viscous fluids as gelatinized starch (Woggum and others
2014). Prior studies reported that, under the high-shear and hightemperature conditions that exist during such process, starches
R
C 2016 Institute of Food Technologists
doi: 10.1111/1750-3841.13416
Further reproduction without permission is prohibited
Characterization of corn starch EFs . . .
Materials and Methods
Film preparation
To obtain the EFs, corn starch (Ingredion, Jalisco, Mexico),
R
glycerol (JT Baker
, Pa., U.S.A.) and sorbitol (Cedrosa, Edo.
Mexico, Mexico) were used. The corn starch presented a 25.58%
of amylose and 74.72% of amylopectin.
The film preparation was carried out in 2 stages. At the first
stage, thermoplastic starch was obtained using the extrusion technology. The mixture was prepared by mixing 80% corn starch with
20% plasticizers (sorbitol and glycerol, in different ratios according to Table 1). A twin screw extruder (Shandong Light M&E,
Model LT32L, China) with a L/D of 18.5, screw compression
ratio of 2:1, and a circular die with 19 mm of length, 42 mm of
diameter and an output of 5 mm was used. The feed rate was kept
constant at 22.82 g/min, while the moisture content was 20 ±
1%. Barrel temperatures in the feeding and transition zones were
kept constant at 70 and 90 °C, respectively; the output die temperature (ET) varied according to the experimental design. The
3 heating zones were independently electrically heated, and aircooled. The thermoplastic starch was received in water in a ratio
1:4 to avoid its retogradation. The product obtained was named
extruded formulation.
Casting technique was used to produce EFs at the second stage.
Briefly, 300 mL of the extruded formulation were collected and
heated for 10 minutes on a plate (Fisher Scientific, Mass., U.S.A.)
at 80 °C. Subsequently, 25 mL of the gelled formulation were
poured spilled out into acrylic molds, and then were placed in an
oven at 60 °C for 2 h. The thickness of the films was measured using a digital micrometer (Digital Insize, Model 3109–25A, Spain),
obtaining values of 50 ± 5 μm. Finally, films were conditioned
in a desiccator with a saturated solution of Mg(NO3 )2 •6H2 O
R
) to maintain the relative humidity at 53%.
(JT Baker
Table 1–Experimental design of the extrusion process for 3
factors.
Independent Variables
Codified
Assay
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Decodified
X1
X2
X3
ET
(°C)
–1
1
–1
1
–1
1
–1
1
–1.6818
1.6818
0
0
0
0
0
0
0
0
0
0
–1
–1
1
1
–1
–1
1
1
0
0
–1.6818
1.6818
0
0
0
0
0
0
0
0
–1
–1
–1
–1
1
1
1
1
0
0
0
0
–1.6818
1.6818
0
0
0
0
0
0
89.05
100.95
89.05
100.95
89.05
100.95
89.05
100.95
85
105
95
95
95
95
95
95
95
95
95
95
SS
(rpm)
PR
(S:G)
66
66
91
91
66
66
91
91
79
79
57
100.21
79
79
79
79
79
79
79
79
20.27:79.73
20.27:79.73
20.27:79.73
20.27:79.73
79.73:20.27
79.73:20.27
79.73:20.27
79.73:20.27
50:50
50:50
50:50
50:50
0:100
100:0
50:50
50:50
50:50
50:50
50:50
50:50
X1 = ET = extrusion temperature, X2 = SS = screw speed.
X3 = PR (S:G) = plasticizers ratio (sorbitol:glycerol).
Physicochemical Characterization
Mechanical properties
Breaking Strength (BS) and Deformation (D) of EFs were evaluated using the methodology described by Gontard and others
(1993), using a universal texture analyzer (INSTRON, Model
3342, Mass., U.S.A.). Changes of maximum force just before the
break, measured in Newtons (N) and the cutting distance from the
contact with the sample until the break, measured in millimeters
(mm) were evaluated. Twenty samples of EFs from each treatment
were used for measuring mechanical properties.
Water vapor permeability (WVP)
The WVP of EFs was determined using the gravimetric method
according to McHugh and others (1993). Films were placed on the
R
top of glass containers with 15 g of calcium chloride (JT Baker
)
and were secured with parafilm. After that, the containers were
placed into a desiccator (Dry Keeper, Sanplatec Corp., Osaka,
Japan) with a saturated solution of sodium chloride to generate
75% of relative humidity. The weight gain of calcium chloride
was recorded every 12 h during 4 d by quintuplicate; these data
were used to generate a graph of weight gain compared with time.
WVP was determined according to Eq. 1:
WVP =
Mp × E
A × t × p
(1)
where Mp is the moisture absorbed mass (g), E is the film thickness
(m), A is the exposed film area (m2 ), t is the time (s), and p =
partial pressure difference through the film (Pa).
Water solubility (S)
The S was determined according to the methodology reported
by Gontard and others (1992). Five measurements per treatment
Vol. 81, Nr. 9, 2016 r Journal of Food Science E2225
E: Food Engineering &
Materials Science
can be exposed to a variety of changes at the intra and intermolecular level (Sagar and Merrill 1995), which can improve the
properties of the obtained products, such as, EFs. However, the
processing of starch is much more complex and difficult to control
than for many other polymers. The processing of starch involves
its transformation (granular disruption, crystalline melting, and so
on) from a native granular state to a molten state (Xie and others 2012). Under specific extrusion conditions and with the help
of a plasticizer, such as glycerol or sorbitol, thermoplastic starch
can be obtained after disruption and plasticization of native starch,
which is modified by the temperature and mechanical damage of
the processing. Thermoplastic starch is considered one of the most
attractive materials for short-life products due to its low cost and
because it is a biodegradable material obtained from renewable
sources (Cyras and others 2008). Because of these features, this
polymeric material can be used in EFs formulation.
Studies related with the processes to obtain films suggests that,
probably, the casting technique is the traditional procedure applied
in research areas (Flores and others 2010). The casting technique
begins with a solution that contains the polymer former and also
a heating with excess water is applied. The solution is spilled out
into plates and after that it is dried under specific conditions of
temperature and relative humidity (Zamudio-Flores and others
2007), however when native starch is used, the EFs formation
is difficult. For this reason the aim of this work was to use the
extrusion technology as a pretreatment of the casting technique
to produce a change in the starch structure in order to obtain EFs
with improved mechanical and barrier properties.
Characterization of corn starch EFs . . .
were done. The results were expressed as percentage of disinte- Table 2–Analysis of variance for the responses of BS, D, WVP,
and S of edible films of corn starch with a mixture of sorbitol–
grated material, as it is shown in the Eq. 2:
glycerol.
wi − wf
× 100
(2) Response R2 Adjusted CV (%) F Value P of F (Model) Lack of Fit
%S =
wi
BS
where %S is the water solubility percentage, wi is the initial weight D
WVP
of sample, and wf is the final weight of sample.
S
E: Food Engineering &
Materials Science
Experimental design
Three numerical factors were evaluated, Extrusion Temperature
(ET, 85 to 105 °C), Screw Speed (SS, 57 to 100.21 rpm) and
Plasticizers Ratio (Sorbitol:Glycerol) (PR (S:G), 0:100), employing a central composite rotable design with α = 1.6817. The 3
independent variable levels used were selected based on preliminary experiments and technical limitations of the study. All assays
were performed randomly (Table I). A second order polynomial
was used to predict the experimental behavior (Eq. 3):
yi = b 0 + b 1 X1 + b 2 X2 + b 3 X3 + b 12 X1 X2 + b 13 X1 X3
0.69
0.82
0.80
0.80
12.34
5.61
16.25
4.05
21.40
18.18
20.06
79.34
<0.0001
<0.0001
<0.0001
<0.0001
0.003
0.135
0.868
0.110
CV, coefficient of variation; BS, breaking strenght; D, deformation; WVP, water vapor
permeability; S, water solubility.
Scanning electron microscopy (SEM)
SEM analysis was performed according to the procedure described by Rodrı́guez-Castellanos and others (2015). A Scanning
R
Electron Microscope (Philips
, Model XL30 ESEM, Eindhoven,
Holland) was used, employing a secondary electron detector with
15 KV of acceleration. The microphotographs were obtained
using ESEM XL-30 software.
(3) Atomic force microscopy (AFM)
A contact atomic force microscope (Bruker/Veeco/Digital Inwhere: yi is the generic response; b1 . . . 12 is the regression coef- struments Dimension 3100 Nanoscope IV, Germany) was used
ficients; X1 is the extrusion temperature, X2 is the screw speed, to carry out atomic force acoustic microscopy measurements.
and X3 is the plasticizers ratio (sorbitol:glycerol). The numerical Diamond-coated silicon AFM probe (BudgetSensors, Model
method was applied as optimization technique where the main ContDLC) with nominal length of 450 mm, first resonance frecriteria for determining the optimal treatment was to identify quency of 13 KHz and spring constant of 0.2 N/m was used.
the processing conditions (ET and SS) and PR (S:G) that could
provide the highest BS and D values and lowest WVP and S values. Results and Discussion
+ b 23 X2 X3 + b 11 X1 2 + b 22 X2 2 + b 33 X3 2
Data analysis
The data were analyzed using the surface response methodology
R
Software Version 6 (Stat-Ease, Inc., Minn.,
with Design Expert U.S.A.). The significance of the models was tested using variance
analysis (F test).
Microstructural Characterization
Physicochemical characterization
Breaking strength. BS showed a significant model of regression with values of R2 = 0.69, coefficient of variation CV =
12.34, and P of F < 0.0001 (Table 2). ET and PR (S:G) in their
linear terms (P < 0.0001 and P = 0.03, respectively) were the
factors that had the most significant effect on the BS. The Eq. 4
shows the mathematical model used for this response:
Once the optimal treatment was obtained, it was characterB S = +7.16 − 1.47 ET + 0.54 P R (S : G)
(4)
ized according to its microstructural properties (X-ray diffraction,
Scanning Electron Microscopy and Atomic Force Microscopy) to
In Figure 1(A) is showed that when the PR (S:G) was from
determine the damage caused in the mixture of starch and plasti- 0:100 to 100:0, the film strength increased slightly. Also, when the
cizers and EFs during extrusion process and casting technique.
ET increased from 85 to 105 °C the BS decreased considerably.
Therefore, with a combination of low temperatures of extrusion
X-ray diffraction
and high ratio of sorbitol, it is possible to get resistant EFs. The
Samples were milled to achieve a size of less than 420 μm and average of BS value was 10.39 ± 2.73 N and this result was
a small piece of EFs were packed into a glass sample holder with a better than the one reported by Zhong and Li (2014) for kudzu
depth of 0.5 mm and mounted on an X-ray diffractometer (Rigaku starch EFs (7.82 ± 0.19 N) with 20% of glycerol and 53% relative
Model Last D/Max-2100, Rigaku Denki Co. Ltd., Japan). Three humidity, where they only employed the casting technique for the
measurements per treatment were done. The diffractograms were film formation.
Liu and others (2005) thought that high temperatures during
obtained with a sweep angle of Bragg of 4 to 60° over a scale
of 2θ with intervals of 0.02, operating at 30 KV and 16 mA, the starch processing in an extruder generate a dissociation of the
with CuKα radiation and a wavelength λ = 1.5406 Å. Relative polymer structure, which could avoid the formation of resistant
crystallinity was calculated using Herman’s method, as described matrices. For this reason it is considered that increasing the ET
by Gomez and others (1989). The percentage of relative crys- may have generated more hydrolyzed starches, which were less able
tallinity of the starch was measured by separating the crystalline to form a polymer network with greater BS. Garcia and others
and amorphous areas in the X-ray diffractograms. The percentage (2000) developed corn starch films with glycerol or sorbitol and
of relative crystallinity was calculated as crystalline area/total area × found that sorbitol has a better ability to interact with the polymer
100 (Aguilar-Palazuelos and others 2006). Statistical analysis of data chains of starch. This was attributed to its molecular structure simwas performed through analysis of variance (ANOVA) using Stat- ilar to glucose, thus the plasticized films with sorbitol had higher
graphics plus 6.0 (Manugistics Corp., Rockville, MD., U.S.A.) resistance to rupture. Al-Hassan and Norziah (2012) developed
and the means were compared using Fisher’s least significant EFs with sago starch and fish gelatin with glycerol or sorbitol,
showing that those who have sorbitol had higher tensile strength.
difference (LSD) test (P < 0.05).
E2226 Journal of Food Science r Vol. 81, Nr. 9, 2016
Meanwhile, Krogars and others (2003) consider that different size
of the polyols (glycerol and sorbitol) may induce more interactions
between the constituents, resulting in a more stable film. These
results agree with those obtained on this work, since increasing
slightly the sorbitol content in the plasticizers mixture, tougher
films were generated. In the other hand, the glycerol played an
important role related with the improving interactions and flexibility, this because when it was not present at least 5%, the films
were fragile.
Deformation. The statistical analysis of the D data for EFs
showed a significant regression model (R2 = 0.82, CV = 5.61, P
of F < 0.0001) and did not show lack-of-fit (P > 0.99) (Table 2).
Linear (P < 0.0001) and quadratic terms (P = 0.0002) of the
PR (S:G) were the factors that had the greatest effect on the EFs
deformation. The prediction model coefficients obtained for D
variable is presented in the Eq. 5:
Figure 1(B) shows the behavior of D related with ET and PR
(S:G). It was observed that when the PR (S:G) was increased from
50:50 to 100:0, the deformation degree was greater. Therefore, a
high sorbitol ratio in the plasticizers mixture in a range of extrusion
temperatures of 85 to 105 °C could produce films with good
deformation. The best value of D in this work was 15.01 ± 1.23
mm; Zhong and Li (2014) obtained a value of 4.11 ± 0.02 mm
in kudzu starch EFs, while Farahnaky and others (2013) registered
1.35 ± 0.03 mm for wheat starch EFs with 20% of glycerol and
64% relative humidity obtained by casting technique.
Deformation is expressed as the ratio between the sample
lenght to the breaking point and its original length, this property
is strongly influenced by the concentration of plasticizer used
(Guilbert and Gontard 2005). Polyols such sorbitol and glycerol
have been reported as good plasticizers due to their ability to
reduce hydrogen bonds and increase intermolecular space (Vanin
and others 2005). Piermaria and others (2011), Farahnaky and
others (2013), and Chiumarelli and Hubinger (2014) agree that
D = +10.25 − 0.18 ET − 0.25 SS + 1.27 P R (S : G)
higher glycerol content in EFs increases elongation percentage
+ 0.77 P R (S : G)2 + 0.40 ET ∗ SS
(5) due to it improves the starch–plasticizer interactions, producing a
molecular displacement.
Figure 1–Effects of extrusion temperature and plasticizers ratio (sorbitol:glycerol) on: (A) breaking strength, (B) deformation, (C) water vapor permeability (WVP) and (D) water solubility of edible films of corn starch with a mixture of sorbitol–glycerol.
Vol. 81, Nr. 9, 2016 r Journal of Food Science E2227
E: Food Engineering &
Materials Science
Characterization of corn starch EFs . . .
Characterization of corn starch EFs . . .
E: Food Engineering &
Materials Science
In this study, it was observed that when sorbitol portion was
higher, the deformation increased. It is known that sorbitol has
greater ability to interact with other molecules due to the number
of hydroxyl groups present on its structure (Vanin and others
2005). For this reason, sorbitol may have contributed to the
formation of a greater number of links with the different components (starch and glycerol) of the polymeric matrix, generating
flexibility, or high deformation of the EFs. In the other side,
when the glycerol portion was higher, deformation was reduced,
generating gummy EFs. However, the glycerol plays an important
role in the EFs formation, this because it may disrupts the strong
interactions between the network formed by starch-sorbitol,
allowing thus, greater mobility between molecules. Nevertheless,
the amount of this plasticizer should not be exceeded as this may
prevent the formation of a strong and deformed network.
Water vapor permeability. The data analysis showed that
linear (P < 0.0001) and quadratic term (P = 0.0003) of PR
(S:G) and the quadratic term (P = 0.001) of ET were the factors that had significant effect on the experimental model at P
of F < 0.0001, R2 = 0.80 and CV = 16.25 (Table 2). The
Eq. 6 shows the coefficients obtained for the WVP experimental
model:
WV P = + 5.875 x 10−11 + 1.27 x 10−12 ET
− 1.381 x 10−11 P R (S : G)
− 8.161 x 10−12 ET 2
− 9.234 x 10−12 P R (S : G)2
(6)
The behavior related to ET and PR (S:G) for WVP is shown in
Figure 1(C). It can be observed that when the PR (S:G) increased
from 50:50 to 100:0, the WVP decreased, while as when the ET
increased there was an increase in permeability. The lowest value
Figure 2–X-ray diffractograms of formulation without extrusion
(FWE), optimized extruded formulation (Ex F) and optimized edible
film (OEF ).
Figure 3–Relative crystallinity of formulation
without extrusion (FWE), optimized extruded
formulation (Ex F) and optimized edible film (OEF ).
Vertical bars indicate LSD = 1.17 (P ࣘ 0.05).
E2228 Journal of Food Science r Vol. 81, Nr. 9, 2016
obtained for WVP on this work was 1.07×10−11 ± 1.58×10−12
g.m.Pa-1.s-1.m2, where the processing conditions were, PR (S:G)
= 100:0, ET = 95 °C and SS = 79 rpm. Fakhouri and others
(2015) obtained a value of 4.37 ± 0.16×10−7 g.m.Pa-1.s-1.m2
for EFs of corn starch, gelatin and glycerol, while Zhong and
Li (2014) found 3.15 ± 0.45×10−11 g.m.Pa-1.s-1.m2 for kudzu
starch EFs. The results found on this work were better than those
reported by both references.
WVP results can be useful to understand possible mass transfer
mechanisms and solute and polymer interactions in EFs. According to the thermodynamic of irreversible process, water chemical
potential difference is the driving force of the water transfer
through a film. This property is usually affected for parameters such
as temperature, film thickness and plasticizer content (Bertuzzi
and others 2007). Generally, the plasticizers addition increases the
WVP by reducing intermolecular bonds between the polymer
chains (Maran and others 2013). However, on this work was observed that increasing the sorbitol ratio in the plasticizers mixture
reduced the WVP, this could be due to the chemical structure of
this plasticizer. Talja and others (2008) reported that some plasticizers generate low permeability because of hydrogen bonds arising
between the hydroxyl groups of the polyols and starch, which
decreases free water absorption sites. Garcı́a and others (1999)
and Maran and others (2013) developed corn starch films with
sorbitol or glycerol, resulting in lower permeability those which
were formed with sorbitol. Moreover, Jiménez and others (2012)
indicated that the thermal processing of starch involves a serie of
chemical and physical changes such as decomposition, melting,
and crystallization. This could be the reason why high extrusion
temperatures increaments the WVP of the EFs, as they can be generate hydrolyzed starches that have not the ability to form a strong
polymeric network that prevents the water diffusion. Therefore,
it is possible to obtain EFs with low WVP using a high ratio of
sorbitol in the plasticizers mixture and low extrusion temperatures.
Water solubility. This response showed a significant model of
regression (R2 = 0.80, CV = 4.05, and P of F < 0.0001) and did
not show lack-of-fit (Table 2), where the linear term (P < 0.0001)
of PR (S:G) was the factor that showed significant effect. The
Eq. 7 shows the mathematical model for S:
S = +70.54 − 6.88 P R (S : G)
(7)
Figure 1(D) shows that when the PR (S:G) varied from 0:100
to 100:0, the solubility decreased, which is considered favorable
because of the resistant water films are sought. So, the combination of high sorbitol ratio in the plasticizers mixture and low
extrusion temperatures could generate EFs with a low percentage
of solubility. The water solubility ranged from 52.97 to 81.68%.
Solubility in water is an important property of starch based
films. Potential applications may need water insolubility to enhance product integrity and water resistance (Bertuzzi and others
2007). Chiumarelli and Hubinger (2014) reported that solubility
of biodegradable films is influenced by the plasticizer type and
concentration used in their preparation. According to Matta Jr
and others (2011) the plasticizers addition, especially glycerol,
has a great influence on the solubility of starch films due to its
hydrophilic behavior; glycerol interacts with the polymeric matrix
by increasing the space between the chains, which facilitates the
water diffusion, and therefore increases the solubility of the film.
Similar results were obtained in this work, since by increasing glycerol portion; an increase on solubility percentage was observed.
Moreover, when the ratio of sorbitol increased, the solubility
decreased, so it can be assume that sorbitol interacted strongly with
the amylose and amylopectin chains during the extrusion process,
which may have generated some protection in the starch granules
to degradation and therefore could keep some insoluble starches.
Numerical optimization. A numerical optimization was
performed to determine the best extrusion process conditions
and concentration of sorbitol:glycerol with the aim to obtain EFs
with the highest BS and D values and lowest WVP and S values.
The numerical method was used for this procedure and different
criteria for each of the response variables were established. According to the optimization, the best process conditions were: ET =
89 °C, SS = 66 rpm and PR (S:G) = 79.7:20.3. With these optimum conditions were obtained the following predicted values for
each of the corresponding mathematical models: BS = 9.1 N, D
= 13 mm, WVP = 2.6×10−11 g.m.Pa-1.s-1.m2 and S = 63.4%.
The physicochemical characterization of EFs elaborated under
optimum conditions gave the following average values and standard deviations: BS = 10.39 ± 2.73 N, D = 12.41 ± 1.34 mm,
WVP = 3.05×10−11 ± 3.5×10−12 g.m.Pa-1.s-1.m2 and S =
65.98 ± 4.15%. The experimental values did not showed a significant difference(P < 0.05) regarding to the predicted values with
the mathematical models. Therefore, the model used experimentally showed a good fit to find the best conditions of extrusion
process and plasticizers ratio in the manufacture of EFs with good
mechanical and barrier properties.
Microstructural analysis
X-ray diffraction. Figure 2 shows the patterns of X-ray
diffraction as effect of the extrusion process and casting technique.
The formulation without extrusion (FWE), whose predominant
component is corn starch, presented a diffraction pattern type A,
which is characteristic of cereals, with 2 main peaks at values 2θ of
ࣈ 18.5° and ࣈ 23.2°. These results are similar to those obtained
by Guimarães and others (2010) who reported values 2θ of ࣈ
17.7° and ࣈ 23.3° for corn starch. Talking about the optimized
extruded formulation (EX F), it showed a diffraction pattern of
rays type A, having a decrease in the peaks intensity, with values of 2θ of ࣈ 18.7° and ࣈ 20.1°. Therefore, it can be inferred
that extrusion process may have caused partial fragmentation and
gelatinization of the starch granules due to the ET and SS are contributing to the formation of amorphous regions. Colonna and
others (1989) reported that during severe processing conditions,
the original structure of the starch is modified. On the other hand,
comparing the diffraction patterns of the FWE and EX F (Figure 2),
it was found that the position of the peaks was similar, however
the intensity was lower, as it is shown in the relative cristallinity
results (Figure 3). This result could mean that the starch gelatinization during extrusion process and the plasticizers addition could
reduce the crystallinity level. The plasticizers addition can block
the reordering of the starch molecules preventing crystal growth
by forming strong hydrogen bonds with the hydroxyl groups of
the starch chains (Farahnaky and others 2013). Meanwhile, the
optimized EFs (OEF ) showed a V-type pattern with a main peak
at 2θ of ࣈ 17°. The helical crystal structure of V-type pattern
could indicate the formation of amylose–glycerol complexes during processing (Zhong and Li 2014). The low crystallinity in the
OEF may be due to the gelatinization occurred during the extrusion process and casting technique, as well as by the presence of
plasticizers. Farahnaky and others (2013) observed that increasing
the content of glycerol in wheat starch EFs, decreased the percentage of crystallinity. Similarly, Talja and others (2008) prepared
potato starch films with a mixture of xylitol and sorbitol, resulting
Vol. 81, Nr. 9, 2016 r Journal of Food Science E2229
E: Food Engineering &
Materials Science
Characterization of corn starch EFs . . .
Characterization of corn starch EFs . . .
E: Food Engineering &
Materials Science
in films with a lower degree of crystallinity because of the addition
of plasticizers.
Extrusion processing destroys the crystal structure of starch,
either partially or completely, depending on the amyloseamylopectin ratio and process conditions as moisture, shear
strength, temperature, and so on (Singh and others 1998). Figure 3
shows the values of relative crystallinity of the materials studied. So,
it can be observed that the crystallinity of FWE was significantly
(P < 0.05) greater than that of EX F, showing that extrusion process
and plasticizers contributed to partial fragmentation of the starch
granules, diminishing some crystalline regions. Figure 3 also shows
that the EFs registered the lowest value in crystallinity, which can
result from casting technique where starches gelatinization finish,
and thus considerably reduce the crystalline regions. The addition
of plasticizers may also have contributed to the decrease.
Scanning electron microscopy. Figure 4(A) shows the microstructure of the FWE. The corn starch granules had spherical and polygonal shapes and diameters ranging between 5 and
25 μm, which is agree with the results reported by Pérez and
Bertoft (2010). The image shows that the main component
of the film forming mixture, corn starch, had not change in
its native composition when it was combined with a mixture
of plasticizers; however it is observed that the starch granules
are agglomerated, which could be attributed to the plasticizers
presence.
The SEM was used to observe the damage caused by the extrusion process on EX F components (Figure 4B). To this, it was
noted that the starch granules presented an irregular and semiopen shape, which may be a consequence of the processing effect.
Likewise, it was observed the starch complexes formation with
diameters ranging from 35 to 50 μm, indicating that some starch
granules were intact after processing. This result indicates that the
conditions used (ET and SS) in the extrusion process, may have
generated the partial breaking of the starch granules which allowed
its fragmentation and pregelatinization, maintaining its thermoplastic ability for future application in film formation. These results are similar to those obtained by Aguilar-Palazuelos and others
(2007) during the preparation of corn starch, fibre and glycerol
pellets employing the extrusion process. They observed that fragmentation of the granular structure and partial plasticization of
the material took place; however the pellets were suitable for its
posterior processing by injection moulding. Therefore, it can be
assumed that extrusion technology caused physical modification
of corn starch, possibly favored interactions between starch and
plasticizers, allowing to generate EFs with good mechanical and
barrier properties.
Figure 4–Scanning electron micrographs of: (A)
formulation without extrusion (FWE) and (B)
optimized extruded formulation (Ex F).
E2230 Journal of Food Science r Vol. 81, Nr. 9, 2016
Characterization of corn starch EFs . . .
E: Food Engineering &
Materials Science
Figure 5–(A) Topography of optimized
edible film (OEF ) and (B) its 3-dimensional
image.
Atomic force microscopy. AFM technique is a powerful
tool for the study of films surfaces and has been widely used to
provide quantitative and qualitative information about the compounds that are part of a polymer matrix in a nanometric scale. In
Figure 5(A), the OEF topography is observed, showing a heterogeneous and rough surface. This may be the result of an incomplete
degradation of the starch granules during gelatinization in the casting technique, leaving residues of granules (amylopectin) which
contributed to the formation of a rough surface. Similarly, the
appearance of the starch films surface could be affected by the
conditions used in the formation mechanism, drying time and
thickness (Rindlav-Westling and Gatenholm 2003). Similar results were reported by Mathew and others (2006) who produced
potato starch films, resulting in a film with rough regions due to
the presence of starch granules.
Figure 5(B) shows a map of the topography of the OEF in 3
dimensions, where can be seen some smooth regions and ripples
with sizes ranging from 50 to 300 nm in height. The presence of
smooth areas are related to the content of plasticizer and the presence of extra-granular amylose (Acosta and others 2006). While
the ripples or rough regions depend on the quantity of extragranular amylopectin in the polymer matrix. The EFs roughness
also depends on the changes given into the polymer matrix by
recrystallization of amylose during storage (Mali and others 2002).
Thiré and others (2003) obtained similar results to those found
on this study, reporting rough and smooth surfaces on corn starch
films with glycerol.
adjusted R2 values ࣙ 0.7, CV < 16.17% and P of F (model) <
0.001. The PR (S:G) and ET had significant effect (P < 0.0001)
on all the evaluated responses; meanwhile the SS did not present
significant effect on any response variable. The extrusion process favoured physical modifications and interactions between
the starch matrix and the plasticizers (corroborated by the microstructural analysis). Therefore, the combination of extrusion
technology with the casting technique allowed to obtain EFs of
corn starch with a mixture of plasticizers (sorbitol:glycerol) with
greater BS and D and lower WVP than those reported in the
literature.
Corn starch EFs could be applied on food products to improve
their preservation, distribution, and marketing. These materials
are receiving much attention as an efficient way to protect
fresh and/or minimally processed vegetables from degradation
during storage, this because of they can be used to control the
permeability to water, oxygen, and carbon dioxide, as well as lipid
permeability in a food system. In addition, EFs could improve
the mechanical properties of the food to facilitate handling and
transport and, at the same time could be effective carriers of
many functional ingredients, such as antimicrobial agents to
improve safety and stability of foods, antioxidants to prevent lipid
oxidation, and flavorings and pigments to improve quality of
foods.
Acknowledgments
The authors thank to CONACYT for providing financial support for the development of this work and to CINVESTAV
The mathematical models used to analyze the data from the Querétaro for the support in carrying out the necessary techextrusion study were satisfactory for the evaluated responses, with niques for microstructural analysis.
Conclusion
Vol. 81, Nr. 9, 2016 r Journal of Food Science E2231
Characterization of corn starch EFs . . .
Author Contributions
P.R. Fitch-Vargas designed the study and drafted the manuscript.
J.E. Valdez-Morales and N. Jacobo-Valenzuela collected test data
and interpreted the results. J.J. Zazueta-Morales, M. Vega-Garcı́a
and F. Martı́nez-Bustos provided technical support and revised
the manuscript. E. Aguilar-Palazuelos supervised the study and
prepared the manuscript.
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