## 3 Results and Discussion
### Chemical Interaction Forces Analysis  
The solubility of heat- and cold-induced gels in five different solvents was determined to obtain information about the types of bonding between protein molecules in the two gel types. The differences in gel solubility between solvents S2 and S1 mainly represent ionic bonds; the differences in solubility between S3 and S2 mainly represent the hydrogen bonds; the differences between S4 and S3 mainly represent the hydrophobic interactions; and the differences between S5 and S4 mainly represent the disulfide bonds [39].  
With the addition of Na\({}^{+}\), the bonding for both types of gels was in the order of disulfide bonds > hydrophobic interactions > ionic bonds > hydrogen bonds with increasing concentration (Figure 5a). Disulfide bonds and hydrophobic interactions increased at low ion concentrations (0.005-0.01 M). The addition of Mg\({}^{2+}\) (Figure 5b) had an effect similar to that of Na\({}^{+}\), but the disulfide bonds decreased with increased concentration (0.01-0.015 M). The addition of Ca\({}^{2+}\) (Figure 5c) significantly increased ionic bonds. These results may be due to the functional groups buried in the protein matrix becoming sufficiently exposed to improve the interactions between the protein molecules at low ionic strength; Ca\({}^{2+}\), Mg\({}^{2+}\), and Na\({}^{+}\) ions could bind negatively charged groups on the protein molecules, which leads to conformational changes that make more hydrophobic groups available at the surface [40]. Thus, the hydrophobic interactions and disulfide interactions were increased at low ion concentrations, while the high ionic strength resulted in salting-out as previously discussed, thus affecting the gelation of the protein [41]. Therefore, hydrophobic interactions and disulfide bonds were reduced at high ion concentrations (0.01-0.015 M). In addition, at the same ion concentration, the effect of Ca\({}^{2+}\) was stronger than those of Mg\({}^{2+}\) and Na\({}^{+}\). This may be due to the Ca\({}^{2+}\) forming more hydrophobic interactions and salt bridges, which lead to increased hydrophobic interactions and disulfide interactions [42]. Furthermore, Ca\({}^{2+}\) may be used as a crosslinking agent in the gel [43]. The hydrophobic interactions and disulfide bonds of gels containing Ca\({}^{2+}\) and Mg\({}^{2+}\), which are both divalent ions, were very different. These results were consistent with the hardness of the gels. Other examples of Ca\({}^{2+}\) and Mg\({}^{2+}\) differences are common in biological systems. However, the mechanism accounting for these differences has not yet been completely clarified. [PERSON] et al. (2015) studied the effect of a pH shift combined with a heat treatment on the SPI gel structure, and the results showed that SPI formed disulfide bonds during heating [44]. [PERSON] et al. (2004) suggested that the formation of the soy protein gel network relies mainly on hydrogen bonds and hydrophobic bonds, and heat treatment can increase these forces [45].  
Figure 5: _Cont._Figure 5: (**a**) Forces in heat- and cold-induced gels formed with Na+ (0.005, 0.01, 0.015M) before spray drying; (**b**) Forces in heat- and cold-induced gels formed with Mg2+ (0.005, 0.01, 0.015M) before spray drying; (**c**) Forces in heat- and cold-induced gels formed with Ca2+ (0.005, 0.01, 0.015M) before spray drying.  
Ions could enhance the interactions within the gel system when the ionic concentrations were from 0.005 to 0.01 M, and the hydrophobic interactions and disulfide bond interactions were the main interactions within the gels [23; 46]. Heat-induced gels had stronger interactions than cold-induced gels. This was consistent with previous research on gel texture and WHC [47]. The interaction between salt ions and SPI was increased at low ion concentrations and this interaction was decreased at high ion concentrations.