*3.3. Changes in Chemical Interactions of Starch–Surimi Gels with Non-Setting or Setting Effect* 3.3.1. Non-Covalent Bonds

Non-covalent bonds play essential roles in supporting the three-dimensional structure and enhancing gel strength [32]. All of the non-covalent bonds (including the non-specific associations, ionic bonds, hydrogen bonds, and hydrophobic interactions) differed between the two types of heating samples (Table 2). The non-specific associations had significant increases in both CG and SCG (*p* < 0.05) with the increase in starch content, which resulted from a weak link with low molecular proteins in the gel [33]. Li et al. [28] discovered that the non-specific associations of myofibrillar protein were affected by starch with a larger particle size. Hence, the increase in non-specific associations might be attributed to the expansion of potato starch. The ionic bonds generated by electrostatic interactions between peptides also increased [34]. Hydrogen bonds could enhance the rigidity of the gel, but it might be easily destroyed under high temperature. However, the reduction in hydrogen bonds, in turn, allowed the hydration of exposed peptide backbones and, thus, was important in stabilizing bound water [2]. Overall, these weak forces uniformly increased with the increment of starch content, containing non-specific associations, ionic bonds, and hydrogen bonds (*p* < 0.05).

**Table 2.** Non-covalent bonds and total sulfhydryl groups of starch–surimi mixtures subjected with different heating processes.


Uppercase letters indicate significant difference (*p* < 0.05) between different heating processes, lowercase letters indicate the difference between gels with different starch content (*p* < 0.05), and the values are expressed as mean ± SD.

Hydrophobic interactions predominate in the gel matrix compared with other noncovalent bonds [17], which are produced by the unfolding action of the protein (above 60 ◦C) and the exposure of the hydrophobic core. With an increase in starch content, hydrophobic interactions declined in CG (Table 2). The unfolding action of the surimi protein structure during heating contributed to the formation of hydrophobic interactions. It could be presumed that the hydrophilic groups absorbed water, resulting in starch swelling as the temperature rose. Subjected to setting treatment, the hydrophobic interactions tended to increase with relatively minor fluctuations. Compared with CG, the effect of starch addition on hydrophobic interactions in SCG weakened. This was related to the elastic SCG containing disulfide bonds, which had a high extrusion resistance to starch [35].

#### 3.3.2. Total Sulfhydryl Groups

Sulfhydryl groups buried in protein are exposed during heating process and subsequently generate disulfide cross-linking [23]. The disulfide bonds display a type of rheological behavior known as rubber elasticity, and they are critical to maintaining network stability [34]. The concentration of total sulfhydryl groups decreased ceaselessly in CG with increased starch content, as shown in Table 2. It was ascribed to internal changes of protein aggregates in which more peptide chains were unfolded and sulfhydryl groups were exposed. Subsequently, the cross-linking of -SH occurred followed by the formation of more disulfide bonds [36]. Nevertheless, faced with stress from starch, SCG showed minor changes in total sulfhydryl groups, contributing to a more stable structure formed by low-temperature preincubation [37].
