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

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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.