3.4.3. S-S Stretching

The formation and interchange reactions of disulfide are important for non-reversible heat gelation [9]. Moreover, spectral features in the 500–550 cm−<sup>1</sup> region were correlated with the structural parameters of the disulfide bands (Figure 5c, d). Stretching vibrations located at 510 cm−1, 516–530 cm−1, and 535–545 cm−<sup>1</sup> have been assigned to gauche– gauche–gauche, gauche–gauche–trans, and trans–gauche–trans, where the –C–S–S–C– torsion angle of 90◦ was represented as gauche form and that of 180◦ was represented as trans form [45]. Due to its low potential energy, the gauche–gauche–gauche form was presumably the most stable among the others [46]. Quantitative analysis of FT-IR spectroscopy showed stable bands located at 510 cm−<sup>1</sup> in CG and SCG and disappeared with the increment of starch addition. The weak bands vibrations of CG significantly increased at 516–530 cm−1. It meant that not only disulfide bond conformation changed from gauche–gauche–gauche to gauche–gauche–trans but also new bonds emerged. The variation was consistent with the decrease in total sulfhydryl groups, indicating that more disulfide bonds formed in CG. Therefore, the conformational shift occurred by a combination with excessive starch, which was conducive to the breaking and reformation of disulfide bonds during the direct heating process. The vibrations rose near 518 cm−<sup>1</sup> and 525 cm−<sup>1</sup> in SCG, accompanied by a decrease in bonds near 510 cm−1. Thus, the setting effect resulted in micro-changes in the conformation of the disulfide bonds in SCG. In brief, the results showed that the matrix after the setting process had a specific compressive capacity, causing fewer vibrations (S–S) changes.
