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Research Article
Received: 12 June 2016 Revised: 28 December 2016 Accepted article published: 19 January 2017 Published online in Wiley Online Library:
(wileyonlinelibrary.com) DOI 10.1002/jsfa.8220
Mechanical and barrier properties of maize
starch–gelatin composite films: effects of
amylose content
Kun Wang,aWenhang Wang,a* Ran Ye,bJingdong Xiao,aYaowei Liu,a
Junsheng Ding,aShaojing Zhangaand Anjun Liua*
Abstract
BACKGROUND: In order to obtain new reinforcing bio-fillers to improve the physicochemical properties of gelatin-based films,
three types of maize starch, waxy maize starch (Ap), normal starch (Ns) and high-amylose starch (Al), were incorporated into
gelatin film and the resulting film properties were investigated, focusing on the impact of amylose content.
RESULTS: The thickness, opacity and roughness of gelatin film increased depending on the amylosecontent along with the starch
concentration. The effects of the three starches on the mechanical properties of gelatin film were governed by amylose content,
starch concentration as well as environmental relative humidity (RH). At 75% RH, the presence of Al and Ns in the gelatin matrix
increased the film strength but decreased its elongation, while Ap exhibited an inverse effect. Starch addition decreased the
oxygen permeability of the film, with the lowest value at 20% Al and Ns. All starches, notably at 30% content, led to a decrease
in the water vapor permeability of the film at 90% RH, especially Ns starch. Furthermore, the starches improved the thermal
stability of the film to some extent. Fourier transform infrared spectra indicated that some weak intermolecular interactions
such as hydrogen bonding occurred between gelatin and starch. Moreover, a high degree of B-type crystallinity of starch was
characterized in Gel-Al film by X-ray diffraction.
CONCLUSION: Tailoring the properties of gelatin film by the incorporation of different types of maize starch provides the
potential to extend its applications in edible food packaging.
© 2017 Society of Chemical Industry
Keywords: corn starch; gelatin; edible film; amylose; crystallinity
INTRODUCTION
Gelatin, obtained from the thermal or chemical degradation of
collagen, is a partial triple-helix network including zones of inter-
molecular microcrystalline junctions.1It has been widely used as a
gelatinizer, stabilizer, thickener, emulsifier and film-forming agent
in food, pharmaceutical and cosmetic industries owing to its rel-
atively low cost and excellent functional properties.2,3Therefore
gelatin provides great potential in the development of biodegrad-
able and/or edible packaging.4Generally, gelatin film is prepared
via a sol–gel transition of gelatin followed by a drying process.
Upon heating, gelatin is dissolved into random chains in water
and then reassembles into a partial collagen-like triple-helix struc-
ture upon cooling, which is called renatured or structural gelatin.
This structural gelatin is linked by the interaction among chains,1
which contributes to the formation of films. However, compared
with petrol-derived films, gelatin-based films are more fragile and
susceptible to breakage, especially at low humidity, attributed to
the powerful cohesive energy density of this biopolymer.1Many
studies have been conducted to enhance gelatin film perfor-
mance using a variety of approaches, including chemical5and
enzymatic6crosslinking and the incorporation of external sub-
stances such as other proteins,7polysaccharides,8nanoscale inor-
ganic substances9and biomaterials.10
Starch, a heterogeneous material containing different concen-
trations of amylose and amylopectin, is a polysaccharide com-
posed of linked anhydroglucose units. Starch has great capabil-
ity to form a colorless and transparent polymer matrix and is
widely used in food-packaging materials for its biodegradabil-
ity and low cost.11,12 The amylose/amylopectin ratio ranges from
15:85 to 35:65 in different kinds of starch, except in waxy starch and
corn starch with ∼5 and 50– 80% amylose respectively.11 Amylose
is a linear polymer with a molecular weight of ∼106Da composed
of 𝛼-1,4-linked glucose units with a large number of branches.13
Amylopectin is a highly branched polymer with glucose units in
𝛼-1,6 linkage with a molecular weight of ∼108Da, which is much
higher than that of amylose.14
∗Correspondence to: W Wang or A Liu, Key Laboratory of Food Nutrition and
Safety, Ministry of Education, College of Food Engineering and Biotechnology,
Tianjin University of Science and Technology,Tianjin 300457, China.
E-mail: wangwenhang@tust.edu.cn (Wang); laj@tust.edu.cn (Liu)
aKey Laboratory of Food Nutrition and Safety, Ministry of Education, College
of Food Engineering and Biotechnology, Tianjin University of Science and
Technology,Tianjin, China
bRoha USA LLC, St Louis,MO, USA
J Sci Food Agric (2017) www.soci.org © 2017 Society of Chemical Industry
www.soci.org K Wang et al.
Many studies have been concentrated on high-amylose starches
because of their good film-forming properties and other multiple
applications. Amylose is reported to be an excellent film-forming
material, with linear chains and a network stabilized by hydro-
gen bonds.15,16 In contrast, the polymeric chains of amylopectin,
because of their branching and shorter length, can be highly tan-
gled together, leading to the formation of inter-chain hydrogen
bonds.17,18 The differences in mechanical properties and barrier
capacity of starch films can be illustrated by the retrogradation
phenomenon, in which starch molecules recrystallize during gela-
tion and aging.19 It was reported that amylose retrogradation
involved a crystallization process and the retrograded amylose was
considered to be the crystal nuclei, functioning as a site for crystal
growth.20 A higher concentration of amylose caused faster starch
retrogradation, generating B-type crystallites and gel hardness.21
It was demonstrated that films prepared from high-amylose corn
starch displayed better oxygen barrier properties, lower water
vapor permeability, lower retrogradation temperature and more
stable mechanical properties at high relative humidity compared
with films obtained from normal starch owing to the former’s
strong gelation properties and helical linear polymer structure.16
Meanwhile, starch could improve self-filming and related prop-
erties in other biopolymers, including gelatin,22 carboxymethyl
cellulose23 andamaranthprotein.
24
The objective of this study was to investigate the effects of dif-
ferent types of corn starch (i.e. with different amylose/amylopectin
ratios) on the physicochemical properties of maize starch– gelatin
composite films, including mechanical characteristics, moisture
and oxygen barriers as well as thermal stability. Moreover, the
intermolecular interactions and crystallinity of gelatin and starch in
the films were analyzed by Fourier transforminfrared spectroscopy
and X-ray diffraction. In addition, the impact of starch concentra-
tion on the composite performance was also studied in order to
assess the enhanced properties of the films.
MATERIALS AND METHODS
Chemicals
Bovine-hide gelatin of type A (Bloom 220) was purchased from
Aladdin Industrial Corporation (Shanghai, China). High-amylose
starch from corn (Al) was donated by National Starch Co. (Shang-
hai, China). Amylopectin from waxy corn (Ap) was purchased from
Tokyo Chemical Industry company (Tokyo, Japan). Normal corn
starch (Ns) was purchased from Aladdin Industrial Corporation.
The amylose content of the three starches is 72, 0 and 27% (w/w)
respectively based on the manufacturers’ manuals. All commercial
chemicals were of analytical grade and used without further
purification.
Film preparation
Gelatin powder was added to 10 mL of distilled water and stirred
with a magnetic stirrer at 40 ∘C for 2 h to form a 12.5% (w/v) gelatin
solution. Given the very broad endotherm obtained in the tem-
perature range 65– 115 ∘C for high-amylose corn starch,25 Al sus-
pensions at different concentrations were gelatinized at 125 ∘C
for 80 min in an autoclave to form a homogeneous dispersion. Ns
andApwereheatedat95∘C for 30 min in order to completely
melt starch granules according to the published methodology.16
Aliquots (10 mL) of 12.5% (w/v) gelatin solution were mixed with
30% (w/w, gelatin basis) glycerol, then 15 mL aliquots of starch
solutions were added to reach a series of final starch concentra-
tions of 0, 10, 20, 30, 40 and 50% (w/w, gelatin basis) respectively.
After mixing thoroughly, the mixtures were instantly casted onto
flat plates (11 cm ×11 cm) and oven dried at 30 ∘Cfor5h.The
formed films were wiped off and kept in a desiccator with saturated
Mg(NO3)2solution at 25 ∘C and 52% relative humidity (RH) for 48 h.
Film thickness
Film thickness was assessed using a hand-held micrometer (Mitu-
toyo No. 293-766, Tokyo, Japan) to 0.001 mm accuracy. Measure-
ments were conducted at ten different locations on the film.
Optical properties
The transparency of films was evaluated by measuring their light
absorption at a wavelength of 600 nm using a UV– visible spec-
trophotometer according to the previously published method.26
Samples were cut into strips (1 cm ×4cm)andplacedinaglass
cuvette for detection. The air value was used as reference. Film
transparency (T) was calculated as
T=A600∕M
where A600 is the absorbance at 600 nm and Mis the film thickness
(mm). The equation indicates that a higher value of Trefers to a
lower degree of transparency.
Film color was determined by measuring L* (lightness or bright-
ness) and chromaticity parameters a* (redness/greenness) and b*
(yellowness/blueness) using a CM-700d Konica Minolta colorime-
ter (Osaka, Japan). Total color difference (ΔE*) was calculated as
ΔE∗=[(ΔL∗)2+(Δa∗)2+(Δb∗)2]1∕2
where ΔL*, Δa*andΔb* are the differences in corresponding color
parameters between the sample and standard white paper.
Scanning electron microscopy
Film morphology was investigated with a scanning electron micro-
scope (SEM) (SU1510, Hitachi, Tokyo, Japan) as described in the
literature.27 Before scanning the film surface, samples were sputter
coated with a layer of gold in a vacuum and fixed to the stage with
double-sided adhesive tape. For observing the film cross-section,
films were first fractured by liquid nitrogen and then treated as
for surface scanning. Every sample was delivered to the cold stage
of the SEM chamber and observed at an acceleration voltage
of 5 kV.
Mechanical properties
The stress– strain properties including tensile strength (TS) and
elongation at break (EAB) of film samples were assessed using
a texture analyzer (Stable Micro Systems Ltd, Godalming, UK) as
described in the literature.28 The test was performed at 25 ∘Cand
two RH levels (50 and 75%). Film samples were cut into rectangular
test pieces with a width of 20 mm and a length of 70 mm, clamped
and deformed under tensile load with a 100 N load unit at a
crosshead speed of 3 mm s−1untiltheybroke.TS(MPa)andEAB
(%) were calculated as
TS =Tmax∕A
EAB =[(L1−L0)∕L0]×100
where Tmax is the maximum load needed to pull the composite film
apart (N), Ais the cross-sectional area of the composite film (m2), L0
is the initial length of the film (mm) and L1is its length at breakage
(mm).
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Water vapor permeability
Water vapor permeability (WVP) of the starch–gelatin films was
determined as described in the literature.6The films were cut to
a diameter of 6 cm and put on cups containing anhydrous calcium
sulfate,withasealofmoltenparaffinovertherimofthecups.
The cups were then placed in desiccators of 52, 75 and 90% RH
respectively at 25 ∘C. The cup weight was measured indirectly at
96 h. WVP (g Pa−1s−1m−1) of the films was calculated as
WVP =(Δm×d)∕(A×t×Δp)
where Δmis the increase in cup weight (g), dis the film thickness
(m), tis the measuring time (s), Ais the measurement area (m2)and
Δpis the pressure difference between the outside and the inside
of the cup (Pa).
Oxygen permeability
The gelatin films were cut to a diameter of 5 cm and put on
cups containing 5 g of peanut oil, with a seal of molten paraffin
over the rim of the cups. The cups were then kept in an oven
at 50 ∘C for 7 days. The peroxide value (POV) of each extract
was used to estimate the oxygen permeability of the film by the
published method.29 Inbrief,2gofpeanutoiland30mLofsolvent
(chloroform/acetic acid, 2:3 v/v) was dissolved in an iodine bottle.
Then the solution was added with 1 mL of saturated potassium
iodide and kept in darkness for 3 min. After stabilization, the
solution was added with 100 mL of distilled water and 1 mL of
1% (w/v) starch solution and titrated with a standard solution of
Na2S2O3until colorless. POV (meq kg−1) was calculated as
POV =C×(V1−V2)×0.1269 ×78.8 ×100∕M
where Cis the concentration of the standard solution of Na2S2O3,
V1is the titration amount of the sample, V2is the titration amount
of the blank, Mis the quality of peanut oil (0.1269) and 78.8 is a
conversion factor.
Differential scanning calorimetry (DSC)
Peak melting temperature (Tm)andenthalpy(ΔH) of the films were
determined by DSC (DSC-60 plus, Shimadzu, Kyoto, Japan). Film
samples of ∼3 mg were weighed in aluminum pans and analyzed
between 25 and 200 ∘C using a heating rate of 10 ∘Cmin
−1and
atmospheric air as material reference.14
Thermogravimetric analysis (TGA)
TGA of the starch– gelatin samples was performed using a ther-
mogravimetric/differential thermogravimetric (TG/DTG) appara-
tus (Model 2010, TAI nstruments,New Castle, DE, USA) as described
in the literature.4Film samples of ∼2 mg weresubjec ted to temper-
atures ranging from 25 to 600 ∘Cataheatingrateof10∘Cmin
−1.
Nitrogenwasusedaspurgegasataflowrateof50mLmin
−1. Films
were conditioned in a desiccator (25 ∘C, 0% RH) for 2 weeks prior
to detection.
X-ray diffraction (XRD)
The crystallinity index of samples was measured as described
previously4using a Shimadzu X-ray diffractometer. Film samples
stored at 52% RH and 25 ∘C were cut into 3.5 cm×3.3 cm pieces
and fixed in one of the circular clamps of the instrument. The
conditions were as follows: 30 mA and 40 kV; from 5 to 35∘at a
speed of 1∘min−1.
Fourier transform infrared spectroscopy (FTIR)
According to the published method,6FTIR spectra of films were
recorded using a Nicolet Avatar 370 FTIR spectrometer with a
DTGS KBr detector (Thermo, Shanghai, China). Each sample was
subjected to 32 scans at 1.9 cm−1resolution in the wavenumber
range 4000–475 cm−1.
RESULTS AND DISCUSSION
Film morphology
Figure 1 presents the morphology of the surface and cross-section
of gelatin films with different types of corn starch at 30% (w/w)
concentration. All films except Gel-Al film exhibited a homoge-
neous and compact structure, indicating that the starch gran-
ules were sufficiently gelatinized and completely compatible with
gelatin. Furthermore, from the SEM images, it was observed that a
more uneven surface occurred in composite films with increasing
concentration of amylose in the starch. This might be attributable
to remnants of amylose granules after gelatinization and/or fast
aggregation and retrogradation of amylose during the cooling
process, resulting in a rough surface.30 Compared with gelatin film,
there were no significant differences in the surface of Gel-Ap film,
suggesting good compatibility between amylopectin and gelatin
and no significant crystal aggregation in the drying process, in
agreement with the calm peaks reflected in the XRD patterns
(Fig.7). Moreover, the cross-section of gelatin films in the pres-
ence of different types of starch displayed a heterogeneously com-
pact and stiff microstructure, with an amylose content-dependent
behavior.
Film thickness
As shown in Table1, the thick ness of starch– gelatin films gradually
increased with increasing amylose content in the starch, compared
with pure gelatin film with the lowest thickness of 73 ±2μm. At the
same starch concentration, Gel-Al film had the highest thickness,
followed by Gel-Ns and Gel-Ap films. Film thickness is highly asso-
ciated with the alignment, sorting and compacting of molecules
during the drying process of the film and is intensively affected by
the components of the film-forming solution.31 The differences in
thickness of the starch–gelatin films in the present study may be
due to the compatibility between gelatin and starch driven by the
binding of gelatin chains with starch molecules, leading to the low-
est thickness of Gel-Ap film. In addition, the formation crystallinity
of amylose in the early stage of the film-forming process might
explain the highest thickness of Gel-Al film. Moreover, with increas-
ing amount of the same type of starch, the thickness of the film
also increased. Similarly, an increase in thickness was detected in
composite films containing fish gelatin and rice flour, attributed to
changes in the density of the materials and the structure, allowing
for a high concentration of water in the film structure.32
Mechanical properties
As shown in Table 1 and Fig. 2, the effects of amylose content on
mechanical properties of the gelatin films varied with the humidity
conditions. At 75% RH, increasing amylose content led to a sig-
nificant increase in TS in Gel-Al film, suggesting a positive effect
of incorporating Al into gelatin film. Lower concentrations of Ns
(≤20%) and Ap (≤30%) initially caused a decrease in TS compared
with gelatin film, while higher concentrations (≥40%) gradually
increased the TS value. Moreover, a decrease in EAB was observed
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www.soci.org K Wang et al.
AB
Figure 1. SEM images of (A) surface (1000×) and (B) cross-section (800×) of gelatin film (Gel, a) and gelatin films with 30% (w/w) waxy maize starch (Gel-Ap,
b), normal starch (Gel-Ns, c) and high-amylose starch (Gel-Al, d).
Table 1. Thickness, TS and EAB of gelatin film (Gel) and gelatin films with different concentrations (% w/w) of high-amylose starch (Gel-Al), normal
starch (Gel-Ns) and waxy maize starch (Gel-Ap) at 50 and 75% RH
50% RH 75% RH
Sample
Thickness
(μm) TS (MPa) EAB (%) TS (MPa) EAB (%)
Gel 73 ±2a 51.95 ±1.69f 5.93 ±0.06g 12.61 ±0.23cde 65.93 ±1.50g
Gel-Al 10% 89 ±1c 45.19 ±2.74bcd 5.16 ±0.09cd 13.15 ±0.66de 49.28 ±3.97e
20% 99 ±1d 44.91 ±2.35bcd 6.33 ±0.07de 14.49 ±0.81ef 40.03 ±3.09d
30% 103 ±2e 42.36 ±0.69bc 5.16 ±0.12cd 16.96 ±0.82g 32.11 ±3.18d
40% 114 ±2g 40.42 ±2.93b 5.27 ±0.11d 22.05 ±1.84h 21.66 ±3.06bc
50% 130 ±3h 34.17 ±2.84a 4.89 ±0.10c 29.09 ±1.63i 10.16 ±4.17a
Gel-Ns 10% 80 ±2b 49.59 ±5.67def 4.48 ±0.77b 9.68 ±1.05b 56.55 ±3.68f
20% 90 ±1c 46.09 ±1.09bcd 5.74 ±0.13fg 12.42 ±1.31cde 46.48 ±1.74e
30% 91 ±1c 45.32 ±4.19bcd 4.14 ±0.08a 13.49 ±0.55de 35.91 ±2.43d
40% 109 ±2f 41.44 ±1.53bc 5.13 ±0.08cd 15.95 ±1.95fg 26.63 ±1.36c
50% 103 ±2de 34.38 ±4.29a 4.85 ±0.10c 21.33 ±1.70h 20.08 ±2.08b
Gel-Ap 10% 76 ±2a 53.10 ±2.14ef 5.60 ±0.16efg 5.61 ±1.22a 95.55 ±6.70j
20% 81 ±2b 49.41 ±3.22def 4.13 ±0.14a 10.63 ±0.56bc 88.81 ±1.37i
30% 83 ±1b 48.36 ±1.53de 4.10 ±0.14a 11.81 ±0.67bcd 73.94 ±1.95h
40% 89 ±1c 46.35 ±3.89cd 5.77 ±0.09g 12.71 ±0.60cde 67.80 ±1.03g
50% 103 ±2de 44.30 ±2.75bcd 5.43 ±0.09def 13.82 ±2.12f 57.56 ±4.52f
Values are mean ±standard deviation. Means with different letters within the same column are significantly different (P<0.05).
in the composite films, except in those containing lower concen-
trations of Ap and Ns. This phenomenon might suggest that Ap
and Ns acted as plasticizing agents owing to their lower amylose
content, increasing the mobility of the polymer matrix and sub-
sequently increasing film elongation.33 The fact that higher amy-
lose content gave rise to higher TS and lower EAB of the films
revealed that starch crystallization enabled the stiffness of films
and a higher modulus, but less extendibility. The addition of Al
to gelatin film to improve its strength has been reported previ-
ously, potentially arising from the fact that linear amylose forms an
ordered structure in gelatin by intermolecular hydrogen bonds.34
Moreover, the high TS of Gel-Al film may be attributed to recrys-
tallization of amylose after gelatinization, leading to a decrease in
water sorption of Al, compared with gelatin film incorporating Ap
or Ns.35
In addition, at 50% RH, the presence of starch failed to show
a positive influence on the strength of the composite films
from the TS values (Table 1). All starch–gelatin films had an
undesired decrease in TS compared with gelatin film, and with
increasing addition of starch the TS of films decreased, which is
in contrast to amaranth protein films reinforced with maize starch
nanocrystals.24 It may be that under lower RH a reduced content
of moisture as a plasticizer caused a loss of cohesion forces in
the film matrix, resulting in severe phase separation of starch and
gelatin and consequently increasing film discontinuity, contribut-
ing to weak resistance of the film to fracture and reflecting in a
decrease in TS. In addition, owing to its stiff nature, the incorpo-
ration of starch granules, especially at high concentrations, also
interfered with the original gelatin network, causing a decrease
in TS. Generally, protein films such as gelatin film have poor flex-
ibility and high Young’s modulus under lower RH because of the
strong adhesive energy of the polymer,36 driving the high TS of
gelatin film under lower RH in the present study. Furthermore,
no observable tendency was obtained in the EBA values of the
films, because the low values detected under lower RH failed to
differentiate them.
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Figure 2. Stress –strain curves of gelatin film (Gel) and gelatin films with 30% (w/w) high-amylose starch (Gel-Al), normal starch (Gel-Ns) and waxy maize
starch (Gel-Ap) at (A) 75 and (B) 50% RH.
Water vapor permeability of films
WVP of the starch–gelatin composite films is shown in Table 2.
In general, under low RH (52%), Al led to an undesired increase,
originating from the aggregation of Al and the high thickness of
Gel-Al film. Moreover, it was reported that the WVP of films is
directly proportional to the amylose content,37 since the presence
of Al resulted in a high WVP of gelatin film under low RH. The
increase in RH improved the WVP of gelatin films containing Al.
When the experiments were placed in a desiccator of 90% RH,
Al decreased the WVP of gelatin films, which can be observed
in Table 2. These phenomena are due to the fact that under
high-humidity conditions the increased concentration of moisture
as a plasticizer enhanced the adhesion forces between gelatin and
amylose in the film matrix. Under all RH conditions, the decrease
in WVP of Gel-Ns and Gel-Ap films may be due to the formation
of a compact structure between starch and gelatin, inhibiting the
permeation of water into the matrix. Furthermore, with increasing
concentration, the three types of starch had a similar tendency
for the WVP value: first a decrease and then a gradual increase.
Theoretically, moreserpentine p aths in the layersof starch – gelatin
films for water molecules lengthen the distance for the delivery of
vaporized water molecules through the films. However, when the
starch amount is beyond a certain value, starch particles tend to
aggregate together, resulting in a reduction in the path for water
to pass through and an increase in WVP.24 In the present study,
the WVP values of all composite films were lower than those of
gelatin–rice flour films reported by Ahmad et al.,32 attributed to
differences in starch origin.
Oxygen permeability of films
As shown in Fig. 3, the POVs of starch–gelatin films were lower
than that of gelatin film, suggesting that the addition of corn starch
offered resistance to the transfer of gases. Generally, at the same
concentration of starch, the high-amylose starch causes a decrease
in POV of the composite film. This can be attributed to the bar-
rier properties of amylose, which has higher crystallinity compared
with amylopectin,17 coherently with the X-ray pattern detected in
this study (see Fig. 7). A similar result was obtained that unplas-
ticized amylose films exhibited lower oxygen permeability than
amylopectin films.35 The POVs of Gel-Ns and Gel-Ap films were also
lower than that of gelatin film owing to the formation of a com-
pact structure between starch and gelatin, hindering the perme-
ation of oxygen. When the concentration of starch was above 20%,
the POV presented a starch concentration-dependent behavior,
because the proper concentration of starch particles could effec-
tively interact with gelatin by the hydrogen bonds between starch
and gelatin and retard oxygen permeation, leading to a decrease in
POV. However, as the starch content increased further, starch parti-
cles in the gelatin films tended to aggregate tightly and generated
straight paths for oxygen molecules to pass through, causing an
increase in POV.
Transparency and color of films
As shown in Table 3, the transparency values of starch–gelatin
films were higher than that of gelatin film, indicating that the
addition of starch to gelatin increased film opacity. Furthermore,
the increased opacity was dependent upon the amylose con-
tent in the film, with Al causing the highest light transmis-
sion barrier at a wavelength of 600 nm. In contrast, the pres-
ence of Ns or Ap led to higher film transparency. This differ-
ence might be due to the presence of crystalline material at
the surface of the film and structural changes of the gelatin
film. It was previously suggested that the opacity was highly
influenced by the crystalline content of a sample and the com-
pact structures of polymer chains which hindered the passage of
light.4
Color parameters of gelatin films added with maize starch at
different concentrations are listed in Table 3. The brightness of
starch– gelatin composite films showed a slight change compared
with that of gelatin film. Gelatin film was the clearest, followed
in order by gelatin films containing Ap, Ns and Al respectively, as
reflected in successively decreasing L* values, presumably associ-
ated with the crystallization degree and whiteness extent of the
starch. The addition of starches led to an increase in both ΔE*and
b* values but a decrease in a* values compared with gelatin film
(P<0.05). This result might be due to the effect of the color of the
starch itself on the film. The key factors governing the color prop-
erties of films depend on the type, nature and concentration of
biopolymer incorporated.32
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www.soci.org K Wang et al.
Table 2. WVP of gelatin film (Gel) and gelatin films with different concentrations (% w/w) of high-amylose starch (Gel-Al), normal starch (Gel-Ns) and
waxy maize starch (Gel-Ap) at 52, 75 and 90% RH
WVP (g Pa−1s−1m−1×10−11)
Sample 52% RH 75% RH 90% RH
Gel 1.52 ±0.12de 4.53 ±0.21hi 11.43 ±0.29i
Gel-Al 10% 1.89 ±0.09g 4.66 ±0.13ij 8.63 ±0.18h
20% 1.84 ±0.06g 4.43 ±0.11ghi 8.22 ±0.21gh
30% 1.61 ±0.06ef 4.27 ±0.12efg 7.87 ±0.23efg
40% 1.68 ±0.08f 4.35 ±0.11fgh 7.95 ±0.16fg
50% 2.02 ±0.07h 4.68 ±0.17j 8.54 ±0.22h
Gel-Ns 10% 1.44 ±0.03cd 3.78 ±0.09abc 7.23 ±0.19cd
20% 1.42 ±0.04bcd 3.66 ±0.17a 6.13±0.23b
30% 1.14 ±0.07a 3.54±0.12a 5.67 ±0.27a
40% 1.20 ±0.06a 3.61 ±0.13a 5.92 ±0.26ab
50% 1.37 ±0.05bc 3.72 ±0.11a 6.05 ±0.31ab
Gel-Ap 10% 1.47 ±0.09cd 4.14 ±0.14def 7.97 ±0.33fg
20% 1.45 ±0.04cd 4.02 ±0.18cde 7.43 ±0.17d
30% 1.34 ±0.06bc 3.76 ±0.14ab 7.01 ±0.22c
40% 1.32 ±0.06b 3.98 ±0.15bcd 7.51 ±0.27de
50% 1.46 ±0.07cd 4.11 ±0.13def 7.65 ±0.20def
Values are mean ±standard deviation. Means with different letters within the same column are significantly different (P<0.05).
Figure 3. POV patterns of gelatin film (Gel) and gelatin films with different
concentrations (% w/w) of high-amylose starch (Gel-Al), normal starch
(Gel-Ns) and waxy maize starch (Gel-Ap) (P<0.05).
Thermal stability of films
As illustrated in Fig. 4, all DSC curves exhibited characteristic
endothermic peaks, with Tmand ΔHof the films in the following
order: Gel-Al >Gel-Ns >Gel-Ap >Gel. The improved thermal sta-
bility could be attributed to the generation of interactions through
inter- and intra-hydrogen bonding of starch and gelatin and con-
sequently the crystalline domains of gelatin–starch matrix com-
plexes, as evidenced from the FTIR and XRD results (see Figs 6
and 7). Owing to higher crystallinity of Al and stronger interac-
tion between gelatin and Al based on hydrogen bonding, it needs
more thermal energy for dissociation of polymer molecules from
the ordered structure of composite films. It was reported that linear
chains tend to orientate chain– chain interactions and yield par-
tial crystallinity, while branched chains have chain-end effects and
Figure 4. DSC curves of gelatin film (Gel) and gelatin films with 30% (w/w)
high-amylose starch (Gel-Al), normal starch (Gel-Ns) and waxy maize starch
(Gel-Ap).
flexible branch points,38 contributing to differences in the thermal
stability of these starches.
Two main stages of weight loss and degradation rate of the films
were apparent in the TGA and DTG curves (Fig. 5). The first-stage
weight loss (Δw=5.449%) of the gelatin film, mainly associated
with the loss of free and bound water adsorbed in the film,32
was higher than that of the starch–gelatin films (Δw=3.8345,
4.885 and 4.991% for gelatin films with Al, Ns and Ap respec-
tively). The lower first-stage weight loss and degradation rate of
the gelatin–starch films suggested lower water desorption from
both film matrices, mostly linked by hydrogen bonds, compared
with the gelatin film, as observed by FTIR spectroscopy (Fig.6).
As the temperature increased from 120 to 600 ∘C, the tendency
of second-stage weight loss was related to the degradation of
large-size or biopolymer fragments.39 The second-stage weight
loss of the gelatin film (Δw=89.26%) was significantly higher than
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Table 3. Transparency and color properties of gelatin film (Gel) and gelatin films with different concentrations (% w/w) of high-amylose starch
(Gel-Al), normal starch (Gel-Ns) and waxy maize starch (Gel-Ap)
Color parameters
Sample L*a*b*ΔE*Transparency
Gel-Al 10% 89.75 ±0.10de 0.24 ±0.020cd −5.06 ±0.04f 1.51 ±0.04a 0.711 ±0.012de
20% 89.17 ±0.15b 0.01 ±0.002a −3.93 ±0.08i 2.77 ±0.03b 1.263 ±0.051h
30% 89.10 ±0.03b 0.01 ±0.001a −3.68 ±0.02j 2.92 ±0.04c 1.808 ±0.066i
40% 88.73 ±0.42a −0.02 ±0.001a −3.56 ±0.05k 3.26 ±0.03d 2.544 ±0.080j
50% 88.78 ±0.06a −0.08 ±0.004a −3.35 ±0.02l 3.41 ±0.03e 2.828 ±0.060k
Gel-Ns 10% 90.10 ±0.13fg 0.29 ±0.15b −5.27 ±0.17e 1.37 ±0.04f 0.622 ±0.031bc
20% 89.84 ±0.09ef 0.29 ±0.15b −5.01 ±0.03f 1.43 ±0.01f 0.767 ±0.023ef
30% 89.62 ±0.10de 0.24 ±0.015cd −5.02 ±0.01f 1.57 ±0.03g 0.838 ±0.062fg
40% 89.50 ±0.16cd 0.18 ±0.009bc −4.60 ±0.03g 1.98 ±0.02h 0.841 ±0.086fg
50% 89.32 ±0.12bc 0.16 ±0.002bc −4.31 ±0.02h 2.37 ±0.02i 0.854 ±0.019g
Gel-Ap 10% 90.74 ±0.11ij 0.4 ±0.036e −6.80 ±0.02a 0.42 ±0.07j 0.565 ±0.004ab
20% 90.53 ±0.13hi 0.38 ±0.006e −5.87 ±0.03c 0.43 ±0.03j 0.609 ±0.005bc
30% 90.37 ±0.10gh 0.376 ±0.009e −5.82 ±0.02c 0.44 ±0.02j 0.652 ±0.006cd
40% 90.26 ±0.14gh 0.37 ±0.014e −5.53 ±0.12d 0.68 ±0.02j 0.676 ±0.039cd
50% 89.85 ±0.23ef 0.32 ±0.012de −5.33 ±0.02e 1.23 ±0.03k 0.721 ±0.06de
Gel 90.85 ±0.07j 0.37 ±0.014e −6.11 ±0.01b 0.46 ±0.01j 0.523 ±0.017a
Values are mean ±standard deviation. Means with different letters within the same column are significantly different (P<0.05).
Figure 5. (A) TGA and DTG (B) curves of gelatin film (Gel) and gelatin films with 30% (w/w) high-amylose starch (Gel-Al), normal starch (Gel-Ns) and waxy
maize starch (Gel-Ap).
that of the gelatin films incorporating Ap (Δw=81.13%) and Ns
(Δw=69.81%). This was probably due to low decomposition of
the highly interacted starch–gelatin film matrix. The gelatin film
containing Al displayed the lowest weight loss (Δw=61.56%) and
degradation rate, illustrating that the enhanced thermal stabil-
ity was attributable to the crystalline region created by the inter-
twining of amylose side-chains, as observed in the XRD pattern
(see Fig. 7), yielding higher heat resistance of the film.40 Based
on the results of the DTG curves, the addition of starch caused
a slower degradation rate at the first stage (∼120 ∘C), especially
with Al and Ns, indicating a slower loss of water from the film
because of the formation of a tight structure between gelatin
and starch. At the second stage, the first two peaks occurring in
the gelatin film changed into one peak at around 245 ∘Cwith
Ap and Ns addition, which might be due to the disturbance of
embedded starch granules on the original partly crystalline struc-
ture of gelatin based on the triplex. For the Gel-Al film, the dis-
appeared peak occurred again at 267 ∘C, indicating the existence
of B-type crystallinity as expressed in the XRD pattern (see Fig.
7). In addition, the intensity of the main peak (300–350 ∘C) in
the second degradation stage clearly decreased with starch addi-
tion (Ap, 0.7% ∘C−1; Ns, 0.62% ∘C−1; Al, 0.52% ∘C−1) as compared
with the gelatin film (0.9% ∘C−1), which further confirmed the
improvement of the thermal stability of gelatin film by adding
starch.
FTIR analysis of films
Figure 6 depicts the FTIR absorption bands of gelatin films in
the absence and presence of maize starches. The spectrum of
the gelatin film presented major bands at 1630, 1545 and 1238
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Figure 6. FTIR spectra of gelatin film (Gel) and gelatin films with 30% (w/w) high-amylose starch (Gel-Al), normal starch (Gel-Ns) and waxy maize starch
(Gel-Ap).
Figure 7. XRD patterns of gelatin film (Gel) and gelatin films with 30% (w/w)
high-amylose starch (Gel-Al), normal starch (Gel-Ns) and waxy maize starch
(Gel-Ap).
cm−1, corresponding to amides I– III respectively.41 With the addi-
tion of starches, the peaks in the amide I– III region showed no
obvious shift, indicating no conformational changes of the high
structure of gelatin. The amide A band corresponding to N—H
stretching vibrations occurred at 3284 cm−1for the gelatin film and
was shifted to different wavenumbers for the films with different
types of starch, presumably owing to the hydrogen bonds formed
between starch and gelatin. The absorption bands at ∼1152 and
1079 cm−1observed in the FTIR spectra of the starch –gelatin films
correspond to C—O—C stretching vibrations and a composite
of C—O—C stretching vibrations and C—C skeleton vibrations,
which are typical functional groups of starch.42 The band relat-
ing to the Cbond;N (alkyl) group shifted from 1034 cm−1for the
gelatin film to 1021, 1004 and 1024 cm−1for the Gel-Ap, Gel-Al
and Gel-Ns films respectively, presumably resulting from the dif-
ferences in crystallinity of the starches and their interaction with
gelatin molecules. The absorption at ∼1024 cm−1corresponds
to the typical structure of the non-crystalline region of starch,
which often exists in waxy corn starch and normal starch when
they are completely gelatinized and interacted with gelatin. The
band at 1004 cm−1corresponding to C—OH bending vibrations
arises from the formation of numerous hydrogen bonds between
starch molecules,42 presumably originating from fast aggregation
of amylose during the film-forming process.
X-ray diffraction
Figure 7 shows the XRD spectra of gelatin and gelatin– corn starch
films. For the gelatin film, the XRD pattern had peaks at 2𝜃≈7
and 20∘corresponding to gelatin type A powder. These peaks
demonstrate the reconstitution of the three-dimensional struc-
ture of collagen.1In the literature,43 it is clear that the peak at
2𝜃≈8∘is related to the diameter of the triple helix and its inten-
sity will be related to the triple-helix content of the film. When
maize starches were added, the peaks at 2𝜃≈7and20
∘weak-
ened, indicating that the reconstitution of the triple-helix struc-
ture of gelatin was partially destroyed because of its interaction
with starch. The XRD pattern of Gel-Al showed large characteris-
tic peaks at 2𝜃≈17.2 and 23∘corresponding to the B-type pattern
of high-amylose corn starch.44 The variation in crystallinity con-
centration is linked with the dispersion of incompletely melted
starch during processing and the rearrangement of amylose dou-
ble helices after the gelatinization and drying process.16 For gelatin
films with high-amylose starch, only a fraction of the amylose
reacts with gelatin and generates the crystalline structure owing
to the spontaneous recrystallization of amylose molecules after
gelatinization.11 Crystallization of amylose is considered as a fast
process and occurs mainly at the period of film drying, and the
high water concentration in the medium is responsible for the high
mobility of amylose chains. This conclusion is also supported by
the disappearance of the peak of A-type crystallinity for Ns and
Ap in the gelatin films with added Ns and Ap owing to slow crys-
tallization and more complete interaction between amylopectin
and gelatin than between amylose and gelatin. An increase in
crystallinity with amylose indicates that the amylose helices inter-
acted and formed semi-crystalline units in a random order.20 These
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results are consistent with a previous study reporting that the
crystallinity content of starch films increased with high amylose
content, while films based solely on amylopectin were entirely
amorphous.17
CONCLUSIONS
Three types of maize starch (waxy maize starch, normal starch
and high-amylose starch) were used to reinforce gelatin film in
this study, resulting in a variety of effects on the film properties,
highly associated with amylose content. First, a higher concentra-
tion of amylose in starch (such as high-amylose starch and nor-
mal starch) can improve the mechanical strength of starch– gelatin
composite films under conditions of high humidity (such as 75%
RH). In addition, the addition of these maize starches generally
decreased the POV and WVP of gelatin film under 90% RH. SEM
images of Gel-Al films showed uneven surfaces compared with
gelatin film and other starch– gelatin composite films. The pres-
ence of starches, particularly amylose starch, increased the thermal
stability of gelatin films tested by DSC and TGA. Some intermolec-
ular interactions occurring between gelatin and maize starch
molecules were observed in the FTIR spectra of the films. More-
over, XRD showed that the Gel-Al films exhibited a high degree of
B-type crystallinity. In summary, it is suggested that the amylose
concentration in starch is greatly related to starch crystallinity and
the properties of the composites. In addition, the incorporation of
maize starch provides a feasible way of tailoring the mechanical
and barrier properties of gelatin film and extending its applications
in edible food packaging.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support pro-
vided by the National High Technology Research and Develop-
ment Program of China (No. 2013AA102204) and the Special Fund
for Agro-scientific Research in the Public Interest of China (No.
201303082).
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