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

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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.
Page 1 and 2: Physicochemical properties of gelat
Page 3 and 4: Fig. 1. Gel strengths of hydrogels
Page 5: Fig. 4. Swelling kinetics of xeroge

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

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