Physicochemical properties of gelatin gels from walleye ... - YIC-IR

Physicochemical properties of gelatin gels from walleye pollock (Theragra
chalcogramma) skin cross-linked by gallic acid and rutin
Mingyan Yan a,b , Bafang Li a, *, Xue Zhao a , Jibing Yi a
a College of Food Science and Technology, Ocean University of China, No. 5, Yushan Road, Qingdao, Shandong Province 266003, PR China
b Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, No. 17, Chunhui Road, Yantai, Shandong Province 264003, PR China
article info
Article history:
Received 24 March 2010
Accepted 24 August 2010
Keywords:
Physicochemical properties
Gelatin gels
Walleye pollock
Cross-link
Gallic acid
Rutin
1. Introduction
abstract
Gelatin is denatured collagen with low antigenicity and being
relatively inexpensive. It has been widely applied as fundamental
materials for microspheres, sealants, tissue adhesives and carriers
for drug delivery systems (Saito et al., 2004). Traditional sources of
gelatin are mainly pig skin and cowhide and their bones. However,
the outbreak of mad cow disease (BSE) in the 1980s accelerated the
search for a mammalian gelatin alternative. Another motivation for
finding an alternative to mammalian gelatin is that Muslims, Jews
and Hindus do not accept gelatin produced from porcine sources.
Researches show that an alternative to mammalian gelatin is fish
gelatin (Haug, Daget, & Smidsrød, 2004a). Fish gelatins from cold
water species show low gelling and melting temperature and have
low gel modulus (Leuenberger, 1991). Gelatins from warm water
fish species have physical properties more similar to those of
mammalian gelatins (Sarabia, Gómez-Guillén, & Montero, 2000).
* Corresponding author. Tel.: þ86 532 82031936.
E-mail address: mingyan012003@163.com (B. Li).
0268-005X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodhyd.2010.08.019
Food Hydrocolloids 25 (2011) 907e914
Contents lists available at ScienceDirect
Food Hydrocolloids
journal homepage: www.elsevier.com/locate/foodhyd
Gelatin gels were cross-linked by gallic acid and rutin. The gel strength, viscoelastic properties, thermal
stability, swelling property, ultrastructure, X-ray diffraction patterns and FTIR spectra were determined
to evaluate the physicochemical properties of the modified gels. The gel strength increased with
increasing gallic acid concentration up to 20 mg/g dry gelatin, and then decreased at further elevated
gallic acid concentration, while it continuously increased with increasing levels of rutin. Either crosslinking
agent could enhance the elastic modulus (G 0 ) and the viscous modulus (G 00 ) of hydrogels, but the
gelling and melting points didn’t show a notable improvement. Rutin boosted the thermal stability of
xerogels, but decreased the equilibrium swelling ratio significantly, while as for gallic acid, there were no
obvious effects on the thermal stability and equilibrium swelling ratio of xerogels. Scanning electron
microscopy (SEM) was applied to observe the ultrastructure changes of the modified xerogels suggesting
that gelatin xerogel at rutin concentration of 8 mg/g dry gelatin showed the highest cross-linking density.
X-ray diffraction revealed that both gallic acid and rutin could enter the spacing of polypeptide chains of
gelatin to reinforce the intermolecular interaction. And FTIR spectra verified that gallic acid and rutin
molecules mainly interacted with skeletal CeNeC group and carboxyl group of gelatin molecules in the
formation of gels. The results suggested that rutin was a better cross-linking agent for gelatin, and gels
treated with rutin could be found with different physicochemical properties.
Ó 2010 Elsevier Ltd. All rights reserved.
However, quantitatively, gelatin from cold water fish species is still
preferred due to the greater availability of by-products (e.g. skin
and bone) (Haug, Daget, & Smidsrød, 2004b). Therefore, fish gelatin
modification is being developed gradually. Chemical and physical
treatments can be applied to modify the gelatin network through
cross-linking of the gelatin chains to improve gel properties (Cao,
Fu, & He, 2007). Chemical cross-linking agents have been used to
cross-link gelatin, including glutaraldehyde, genipin, carbodiimides,
calcium salts and transglutaminase (Chiou et al., 2006).
Physical treatments, such as UV- and g-irradiation (Chambi &
Grosso, 2006) and high pressure (Montero, Fernández-Díaz, &
Gómez-Guillén, 2002), have been applied.
Plant polyphenols are an important species of compounds, and
mainly occur in rigid tissues, such as the hulls of cereal grains, cell
walls of fruits (e.g. grapes, apples), coffee beans, tea leaves, and
tubers (e.g. potatoes) (Balange & Benjakul, 2009). They have had
extensive applications in the field of pharmaceutical, food processing
and tanning industries. Their ability to form complexes
with proteins or related biopolymers has been correlated with
some protection of the plants from predators such as animals,
insects and microbes (Madhan, Subramanian, Rao, Nair, &

908
Ramasami, 2005). In this research, gallic acid and rutin were used as
the cross-linking agents to modify the gelatin gels. Gallic acid
(2,3,4-trihydroxybenzoic acid) is one of natural phenolic
compounds with lower molecular weight, widely available in the
plant kingdom and showing pharmacological properties, e.g. strong
antimutagenic, anticarcinogenic, and antioxidant activities (Wang,
Wang, & Yang, 2007). Rutin (quercetin-3-O-beta-rutinoside) is the
glycosylated form of quercetin (3 0 ,4 0 -dihydroxyflavonol) with the
molecular weight of 610.51, and one of the primary flavonoids in
a number of plants.
Walleye pollock is one of the commercially important fish
species in China, mainly in Shandong Province. More than 30
thousand tons of fish skins were generated through the processing
of pollock for food service per year. The skin is dumped without
utilization except for use in food feed, causing environmental
pollution and resources waste. However, 70% of the pollock skin dry
matter is collagen (Yan et al., 2008; Zhang, Luo, Zhang, Song, &
Jiang, 2003). For making effective use of the dumped skin as
a gelatin resource, it is necessary to study the characteristics of the
pollock skin gelatin. The main objective of our research was to
examine the gel behavior of gelatin from walleye pollock skin and
to evaluate to what extent their properties could be modified by
cross-linking agentsdgallic acid and rutin.
2. Materials and methods
2.1. Materials
Walleye pollock (Theragra chalcogramma) were caught from
Bering Sea, a northern extension of the Pacific Ocean, by commercial
fishing boat, stored at 18 C immediately after gutting, and
transported to the dock in Qingdao in China. After arrival at a local
fish processing factory, frozen fish were thawed using running
water, and skins were removed and descaled manually. These skins
were transported to the laboratory and stored at 20 C until used.
All other reagents used were of analytical grade.
2.2. Gelatin extraction
All procedures were performed as previously described (Sarabia
et al., 2000) with a slight modification. Cleaned walleye pollock
skins were soaked with 0.05 M sodium hydroxide (1:6 w/v) at 4 C
for 30 min (repeated three times). Samples were drained and
rinsed with tap water after each time. Subsequently, skins were
swollen with 0.05 M sulphuric acid (1:6 w/v) at room temperature
for 3 h, rinsed with distilled water and then extracted with distilled
water overnight at 45 C. The extract was centrifuged at 4000 g for
30 min. The supernatant was collected and air-dried in a convection
oven at 40e42 C until the final moisture content was less than 15%,
and then lyophilized.
2.3. Gel sample preparation
The lyophilized gelatin was dissolved in 50 C water bath with
mechanical stirring until completely dissolved followed by incorporation
of cross-linking agents previously dissolved in 0.2 M
sodium hydroxide at the final concentration of 10, 20, 30, 40 mg/g
dry gelatin of gallic acid, and 2, 4, 6, 8 mg/g dry gelatin of rutin,
respectively. The solutions with the final gelatin concentration of
6.67% were adjusted to pH 5e6 and left for about 30 min at 50 C
for cross-linking reactions to occur. Subsequently, they were cooled
to the room temperature and kept at 4 C for about 24 h to form
hydrogels, and then lyophilized to get the xerogels. In the next
study, hydrogels were used to determine the gel strength, and
xerogels were applied in differential scanning calorimetry (DSC),
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914
swelling test, scanning electron microscopy (SEM), X-ray diffraction
analysis and Fourier transform infrared spectroscopy (FTIR).
2.4. Gel strength
Gel strength was determined on a 6.67% hydrogel (w/v) modified
by cross-linking agents. The gelatin solutions were cooled in
a refrigerator at 4 C for 24 h to get the cylindrical hydrogels,
50 mm in diameter and 45 mm in height. Gel strength, expressed in
break force, was examined on the TMS-PRO food texture analyzer
(Food Technology Co., USA) with a load cell of 5 kN, cross-head
speed 1 mm/s, and equipped with a 1.27-cm diameter flat-faced
cylindrical plunger. The maximum force (in grams) was recorded as
the plunger compressing the hydrogel. Break force was calculated
as follows,
F 9:81
s ¼ N=mm2
p r2 where s, F and r are the break force, the maximum force and the
radius of cylindrical plunger, respectively.
2.5. Viscoelastic properties
Dynamic studies were performed on a Physica MCR 101
rheometer rotary viscometer (Anton Paar Physica, Graz, Austria)
using a coneeplate geometry (cone angle 4 , gap ¼ 1.0 mm).
Temperature ramps were implemented from 0 to 50 C and back 50
to 0 C, and were performed at a scan rate of 0.5 C/min, frequency
1 Hz, and applied strain of 2%. The elastic modulus (G 0 ), viscous
modulus (G 00 ) and the relation between the two, i.e. the phase angle
( ), were represented as a function of temperature. The error in the
reproducibility of the parameters considered in different determinations
of a single sample was 6% or less.
2.6. Differential scanning calorimetry
Differential scanning calorimetry (DSC) was performed on
a Netzsch DSC 200PC (Netzsch, Bavaria, Germany) fitted with an air
cooling compressor and a liquid nitrogen cooler at ambient
temperature (Yan et al., 2008). The temperature was calibrated
effectively using indium as standard. Sample was weighed (5 mg)
accurately and sealed in aluminium pans (BO 6.239.2-64.502). At
least triplicate samples were heated from 30 to 120 C at a scanning
rate of 2 K/min, with an empty sealed pan as a reference.
2.7. Swelling test
The swelling test was performed according to the method of
Nam et al. (Nam, Kimura, & Kishida, 2007). Each sample was tested
by cutting the xerogel into small pieces and putting them into
distilled water at 4 C. The xerogels were gently shaken for 24 h and
removed for weighing every hour. The swelling ratio (SR) was
determined using the following equation,
SR ¼ Wt W 0
W 0
100%
where Wt is the hydrated weight of xerogel at definite time and W0
is the weight of xerogel. When Wt tended to be constant, the SR of
xerogel was defined as the equilibrium swelling ratio (ESR).
2.8. Scanning electron microscopy (SEM)
Ultrastructures of xerogels were determined using scanning
electron microscope (JSM-840, Tokyo, Japan). The xerogels were

Fig. 1. Gel strengths of hydrogels from walleye pollock skin with and without crosslinking
agents (gallic acid or rutin).
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914 909
fixed on stubs using copper-conducting adhesive tap, and sputtercoated
with gold. The coated samples were observed under SEM at
an acceleration voltage of 20 kV.
2.9. X-ray diffraction studies
X-ray diffraction patterns of xerogels cross-linked by gallic acid
and rutin were obtained using X-ray diffractometer (Rigaku D/MAX
2500, Tokyo, Japan) in the angular range of 5e50 (2q) with Nickelfiltered
Cu Ka radiation (l ¼ 0.154 nm) at a voltage of 40 kV and
current of 40 mA.
2.10. Fourier transform infrared spectroscopy (FTIR)
FTIR spectra were obtained from discs containing xerogels
treated with gallic acid or rutin and potassium bromide (KBr)
ground together under drying condition. The spectra were recorded
using infrared spectrophotometer (Nicolet 200SXV) from 4000 to
500 cm 1 at a data acquisition rate of 2 cm 1 per point at room
temperature. The resulting spectra were analyzed using Omnic 6.0
Fig. 2. Elastic modulus (G 0 ), viscous modulus (G 00 ) and phase angle of hydrogels from walleye pollock skin with and without cross-linking agents (gallic acid or rutin).

910
software (Thermo-Nicolet, Madison, Wisconsin). The spectra
obtained were used to determine possible interactions of functional
groups in gelatin molecules and gallic acid or rutin.
2.11. Statistical analysis
Statistical data were analyzed using Microsoft Excel 2000 and
Origin 7.5. Student’s t-test was applied to compare the averages of
properties with a level of 95% confidence interval.
3. Results and discussion
3.1. Gel strength
Gel strength is one of the important properties of gels, and the
specific application of a gel is determined by the range of gel
strength values (Cho, Gu, & Kim, 2005). As shown in Fig. 1, we
plotted gel strength of the cross-linked hydrogels as a function of
cross-linker concentration. The results showed that gel strength of
hydrogels cross-linked by gallic acid increased and then decreased
as gallic acid concentration increased from 0 to 40 mg/g dry gelatin,
and the highest value was found to be at 20 mg/g dry gelatin of
gallic acid. The introduction of gallic acid at a level of 20 mg/g dry
gelatin resulted in increase in the gel strength by 32.5% (p < 0.05)
compared with that of untreated hydrogel. As for hydrogels treated
with rutin, gel strength continuously increased as rutin concentration
increased from 0 to 8 mg/g dry gelatin, and the maximum
value appeared at 8 mg/g dry gelatin, which was also the highest
one in all studied hydrogels. With the addition of 8 mg/g dry gelatin
of rutin, the gel strength increased by 76.3% (p < 0.01) compared
with that of untreated hydrogel. However, when rutin was further
increased to 9 mg/g dry gelatin, there was granule precipitate in
hydrogels, which could also be found in previous studies (Naczk,
Grant, Zadernowski, & Barre, 2006). Silber, Davitt, Khairutdinov,
and Hurst (1998) postulated that protein could be precipitated by
polyphenols when the number of polyphenol molecules interacting
with a protein molecule reached a critical value. The gel strength of
modified hydrogel decreased evidently after inclusion of 30 mg/g
dry gelatin of gallic acid, the reason of which might be the formation
of a sufficient coating of protein surfaces by polyphenol
molecules (Charlton et al., 2002). The further decrease in strength
of modified hydrogels cross-linked with gallic acid at 40 mg/g dry
gelatin, and the appearance of granule precipitate in hydrogel
modified with 9 mg/g dry gelatin of rutin, may be due to the
polyphenol binding to the protein surface and cross-linking of
different protein molecules with polyphenols (Charlton et al., 2002;
Papadopoulou & Frazier, 2004). It could be found that as a crosslinking
agent, the concentration of rutin needed in the cross-linked
hydrogel was much lower than that of gallic acid, which might
mainly be related to the molecular weight and molecular structure
of rutin and gallic acid.
3.2. Viscoelastic properties
Viscoelastic properties refer to the changes of viscoelastic
modulus in solehydrogel conversion. The viscoelastic properties of
hydrogels, after the addition of cross-linking agent, are shown in
Fig. 2. The modulus of elasticity (G 0 ), modulus of viscosity (G 00 ) and
the phase angle (d) all displayed sigmoidal curves during both
heating (from 0 to 50 C) and subsequent cooling (from 50 to 0 C)
ramp, similar to other reports (Cho et al., 2005; Fernández-Díaz,
Montero, & Gómez-Guillén, 2001; Gómez-Guillén et al., 2002).
Either ingredient could improve the G 0 and G 00 of hydrogel, the most
effective being rutin at 8 mg/g dry gelatin. In general, the increase
of G 0 is due to the increasing number of chemical junctions
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914
responsible for the formation of the amide bonds (Saito et al.,
2007). That is to say, the hydrogels modified with gallic acid and
rutin had greater cross-linking density than the untreated hydrogel,
which could be observed in the following studies. However, as
deduced from the evolution of the phase angle, gallic acid and rutin
had no significant effect on the hydrogelesol transition temperature.
The hydrogels, whether the cross-linking agent was introduced
or not, showed the gelling temperature at 4e6 C and the
melting temperature at 11e13 C. The reason is not clear, however,
it may be hypothesized that this might be related to the forces of
interaction between polypeptide chains of gelatin.
3.3. Thermal stability
It is important to carry out studies on the thermal properties and
stability of gels containing gallic acid and rutin for their application
in food and pharmaceutical industry as the gels may be subjected to
heat processes during their preparation, processing or consumption
(Mathew & Abraham, 2008). Changes in thermal stability are
good indicator of proteinephenol interactions. Shrinkage temperature
Ts is usually used to describe the thermal stability of gels, and
is defined as the temperature corresponding to the rupture of the
inter-chain bonds resulting in the fusion of the oriented peptide
chains (Flory & Garrett, 1958). Differential scanning calorimetry
thermograms of gallic acid and rutin incorporated xerogels are
shown in Fig. 3. The xerogels with and without cross-linking agents
all exhibited a single peak, and the peak value corresponded to the
shrinkage temperature. So the Ts was 89 C for untreated xerogel,
90 C for gallic acid-modified xerogel and 100 C for rutin-modified
xerogel. The Ts of xerogels treated with 6 and 8 mg/g dry gelatin of
rutin were both 11 C higher than that of the untreated xerogel
implying that rutin was able to further impart thermal stability to
xerogel, but Ts of xerogels modified with 20 and 30 mg/g dry gelatin
of gallic acid showed no distinct variation compared with that of
untreated xerogel. A rise in Ts reflects an increase in the average
number of cross-linking junctions per molecule, which suggests
that xerogels treated with rutin had better cross-linking network
than gallic acid-modified xerogels and the untreated xerogel.
Fig. 3. Thermal transition curve of xerogels from walleye pollock skin with and
without cross-linking agents (gallic acid or rutin), as shown by DSC.

Fig. 4. Swelling kinetics of xerogels from walleye pollock skin with and without crosslinking
agents (gallic acid or rutin).
3.4. Swelling property
When a xerogel is soaked in water, the water penetrates into the
xerogel in the form of a front, which shifts from surface to the core.
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914 911
Fig. 4 shows the swelling kinetics of xerogels cross-linked by gallic
acid or rutin. Whether the cross-linking agent was introduced or
not, the swelling ratio of xerogel increased significantly within the
first 7e8 h, and then leveled off to the maximum, which was
defined as the equilibrium swelling ratio (ESR). The ESR of the
untreated xerogel, and the xerogels treated with 20 and 30 mg/g
dry gelatin of gallic acid were 2126.95%, 1944.26% and 1789.17%,
respectively, all higher than the maximum water content of
1399.25% of hydrogel prior to lyophilisation, while as for the
xerogels treated with 6 and 8 mg/g dry gelatin of rutin, the ESR
were 1240.71% and 1160.38%, respectively, both lower than the
value of 1399.25% of hydrogel before freeze-drying. It could be
found that gallic acid and rutin both decreased the equilibrium
swelling ratio of xerogel. The ESR of xerogels cross-linked with 20
and 30 mg/g dry gelatin of gallic acid decreased by 8.59% and
15.89%, respectively, compared with that of the untreated xerogel.
Rutin caused a maximum reduction in the equilibrium swelling
ratio of xerogel. When it was added at concentrations of 6 and
8 mg/g dry gelatin, the ESR decreased by 41.67% and 45.44%
(p < 0.01), respectively. The result indicated that the xerogel
modified with rutin had the least ability to entrap water. It has been
proposed that increase in degree of cross-linking in a gel results in
decrease in the extent of its association with water (Cao et al., 2007;
Charulatha & Rajaram, 2003; Saito et al., 2004; Strauss & Gibson,
2004). Therefore, it may be concluded that rutin was a better
Fig. 5. SEM images of xerogels from walleye pollock skin with and without cross-linking agents (gallic acid or rutin). (aee) correspond to untreated xerogel, xerogels with 20 and
30 mg/g dry gelatin of gallic acid, and xerogels with 6 and 8 mg/g dry gelatin of rutin, respectively.

912
cross-linking agent for the pollock skin gelatin, which is similar to
the result obtained from the thermal stability of xerogels.
3.5. Ultrastructure observation
The ultrastructures of xerogels with and without gallic acid and
rutin were analyzed by scanning electron microscopy (SEM). Fig. 5
shows SEM images of the morphology of xerogels. The untreated
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914
xerogel and the xerogels treated with 20 mg/g dry gelatin of gallic
acid and 6 and 8 mg/g dry gelatin of rutin formed network structures,
which were similar to those of bovine bone gelatin films
modified with ferulic acid and tannic acid (Cao et al., 2007).
However, there were notable differences in the xerogels studied.
The networks of xerogels cross-linked by gallic acid and rutin were
superior to that of untreated xerogel, and rutin caused the greatest
cross-linking network, which may be because rutin-modified
Fig. 6. X-ray diffraction diagrams of gallic acid, rutin and xerogels from walleye pollock skin with and without cross-linking agents (gallic acid or rutin).

xerogel had a great number of binding sites (Balange & Benjakul
2009). This might be the reason for the higher viscoelastic
modulus and thermal stability and the lower swelling ratio of rutin
cross-linked gel. However, it could also be observed that the xerogel
treated with 30 mg/g dry gelatin of gallic acid didn’t show the
network structure, which might be due to overabundance of
phenolic compounds covering gelatin surface.
3.6. X-ray diffraction
In general, X-ray diffraction diagrams of collagen show three
peaks, the first sharp peak (Peak C) indicating the distance between
the molecular chains, a second broad peak (Peak A1) due to diffuse
scattering and a third peak (Peak A2) corresponding to the unit
height, typical of the triple helical structure (Giraud-Guille,
Besseau, Chopin, Durand, & Herbage, 2000). But gelatin only
displays the first and second order peaks. X-ray diffraction
diagrams of free gallic acid, rutin and various xerogels are shown in
Fig. 6. It could be found that there was no free gallic acid or rutin in
xerogels. When the peaks for the xerogels with and without crosslinking
agents were compared, no significant variations were seen
in the position of the first peak (7 ) and the second peak (20 ).
Bragg equation (2d sin q ¼ l) is the result of experiments into
the diffraction of X-rays or neutron diffraction off crystal surfaces at
certain angles, where l is the X-ray wavelength (0.154 nm), d is the
spacing between the planes in the atomic lattice, and q is the Bragg
diffraction angle (Wang, Chen, He, Li, & Zhang, 2009). The d values
of diffraction peaks in X-ray diffraction diagrams of xerogels are
shown in Table 1. When gallic acid or rutin was introduced, the
d value (corresponding to Peak C) of xerogel decreased, revealing
that gallic acid and rutin could enter the spacing of polypeptide
chains of gelatin to reinforce the intermolecular interaction.
However, the d value (corresponding to Peak C) of xerogel modified
by 6 mg/g dry gelatin of rutin was higher than that of xerogels
treated with gallic acid, may be mainly because of the larger
molecular size of rutin, from which, it was guessed that gallic acidmodified
xerogel should have higher cross-linking density than
rutin-modified xerogel. But SEM revealed that xerogels treated
with 6 mg/g dry gelatin of rutin showed better cross-linking
network than gallic acid-modified xerogels. The reason might be
that there were more binding sites in rutin-modified xerogel. When
the xerogel included 8 mg/g dry gelatin of rutin, the d value (corresponding
to Peak C) showed the lowest one in all studied xerogels,
mainly related to the higher cross-link of polypeptide chains.
X-ray diffraction further verified that rutin was a better crosslinking
agent for pollock skin gelatin.
3.7. Fourier transform infrared spectroscopy (FTIR)
FTIR spectroscopy was used to reveal the changes on the
molecular level caused by interaction between gelatin molecules
Table 1
The d values of diffraction peaks in X-ray diffraction diagrams of xerogels from
walleye pollock skin with and without ingredients (gallic acid or rutin).
Sample Peak position
(2q, )
d values (nm)
Peak C Peak A1 Peak C Peak A1
Untreated xerogel 7.22 20.38 1.22 0.44
20 mg gallic acid/g dry gelatin 7.52 20.30 1.17 0.44
30 mg gallic acid/g dry gelatin 7.52 19.82 1.17 0.45
6 mg rutin/g dry gelatin 7.38 19.92 1.20 0.45
8 mg rutin/g dry gelatin 7.62 19.82 1.16 0.45
M. Yan et al. / Food Hydrocolloids 25 (2011) 907e914 913
Table 2
Fourier transform infrared absorption band assignment of xerogels from walleye
pollock skin with and without ingredients (gallic acid or rutin).
Absorption band
assignment
and ingredients. The Fourier transform infrared absorption band of
xerogels treated with gallic acid and rutin are shown in Table 2.
Amide I and amide II of all studied xerogels were observed at
1700e1600 cm 1 and 1560e1500 cm 1 , as reported by Muyonga,
Cole, and Duodu (2004) and Yakimes et al. (2005). However, it
could be found that the most noticeable effect was a decreased
absorption in amide V, attributed to skeletal CeNeC vibration,
suggesting that the interaction of gallic acid or rutin with skeletal
CeNeC group of gelatin molecules most likely caused the conformational
change of xerogels. In addition, some increase in the
absorption at 1418 cm 1 was observed in xerogels after gallic acid
with the concentration of 20 and 30 mg/g dry gelatin and rutin with
6 mg/g dry gelatin were incorporated. Therefore, it was deduced
that gallic acid and rutin might interact with carboxyl group in
gelatin molecules. However, the absorption band of the symmetric
vibration of the carboxyl group of the xerogel cross-linked with
rutin at 8 mg/g dry gelatin was not found in the FTIR spectra. The
reason might be that more rutin molecules interacted with
carboxyl group in gelatin molecules, in addition to the higher
degree of cross-linking in this xerogel. So it was concluded that
gallic acid and rutin molecules mainly interacted with skeletal
CeNeC group and carboxyl group of gelatin molecules in the
formation of gels.
4. Conclusion
Absorption band (cm 1 )
Untreated
xerogel
Gallic acid
(mg/g dry
gelatin)
Rutin
(mg/g dry
gelatin)
20 30 6 8
Amide I 1665 1665 1664 1665 1661
Amide II 1540 1541 1540 1541 1535
Bending vibration of CH2 or CH3 1459 1459 1458 1459 1454
Symmetric vibration of
carboxyl group
1418 1424 1420 1424 e a
Rocking vibration of CH2 1332 1333 1332 1330 1334
Amide V 1243 1238 1205 1239 1237
a
The absorption for the symmetric vibration of the carboxyl group was not
located in the spectra.
In summary, it was possible to improve several physicochemical
properties of gelatin gels from walleye pollock (T. chalcogramma)
skin by addition of gallic acid and rutin. Rutin was found to bring
about maximum gel strength in all studied hydrogels at the
concentration of 8 mg/g dry gelatin, and it could enhance the
thermal stability and decrease the equilibrium swelling ratio of
xerogels significantly. But gallic acid-modified xerogels showed no
marked changes in the thermal stability and equilibrium swelling
ratio. The two phenolics had no obvious effect on the gelling and
melting points of hydrogels. Different ultrastructures were
obtained among xerogels with different phenolics at different
levels, suggesting that rutin-modified xerogels had the higher
cross-linking degree than those treated with gallic acid and the
untreated xerogel. X-ray diffraction and FTIR spectra verified that
gallic acid and rutin molecules mainly interacted with skeletal
CeNeC group and carboxyl group of gelatin molecules to reinforce
the intermolecular interaction. The results above showed that rutin
was a highly effective cross-linking agent for pollock skin gelatin.
Cross-linking was also a very important factor influencing the
physicochemical properties of gels.

914
Acknowledgement
This work was supported by National Natural Science Foundation
of China (No. 30871943) and the High Technology Research
and Development Programme of China (No. 2006AA09Z438).
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