## 4 Conclusions  
The effect of the addition of salt ions before spray drying to improve heat- and cold-induced gel properties of soy protein isolate was investigated. The results showed that appropriate concentrations of Na\({}^{+}\), Mg\({}^{2+}\), and Ca\({}^{2+}\) (0.005-0.01 M) significantly increased the hardness, springiness, cohesiveness, chewiness, gumminess, resilience, and WHC of the gels. Presumably, this effect arises predominantly due to the increased exposure at the surface of functional groups buried in the protein matrix, which leads to improvement of the interactions between the protein molecules. Disulfide and hydrophobic interactions were the major forces in maintaining the gel network structure. The hardness, springiness, gumminess, and chewiness of the heat-induced gels were greater than those of the cold-induced gels. However, the cohesiveness and resilience were smaller than those of the cold-induced gels. Moreover, Ca\({}^{2+}\) addition had a greater effect compared with Mg\({}^{2+}\) and Na\({}^{+}\). This effect is probably due to Ca\({}^{2+}\) forming more hydrophobic interactions, possibly through increased salt bridge formation. Therefore, Ca\({}^{2+}\) (0.005 M), Mg\({}^{2+}\) (0.005 M), and Na\({}^{+}\) (0.01 M) can be used to improve the hardness, springiness, and structural uniformity of heat- and cold-induced gel products. This study is useful for the industrial production of gels. Adding salt ions before spray drying could easily improve the gel properties of SPI. In addition, other ions such as anions can also be considered. Overall, adding salt ions before spray drying could offer great potential for the development of SPI with enhanced functionality suitable for comminuted meat products.  
Figure 6: Scanning electron microscopy of heat- and cold-induced gels with 1000\(\times\) magnification: (**A**) Cold-induced control gels; (**B**) Cold-induced gels with Na\({}^{+}\) (0.01 M); (**C**) Cold-induced gels with Mg\({}^{2+}\) (0.005 M); (**D**) Cold-induced gels with Ca\({}^{2+}\) (0.005 M); (**a**) Heat-induced control gels; (**b**) Heat-induced gels with Na\({}^{+}\) (0.01 M); (**c**) Heat-induced gels with Mg\({}^{2+}\) (0.005 M); (**d**) Heat-induced gels with Ca\({}^{2+}\) (0.005 M).  
Author Contributions:[PERSON], [PERSON], [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON], [PERSON]-N.Z., and [PERSON] analyzed the data; [PERSON], [PERSON], [PERSON]-[PERSON] contributed reagents/ materials/analysis tools; [PERSON] wrote the paper. Funding: This research was funded by the 13 th Five-Year Plan (research grant numbers: 2016 YFD0401402, 2016 YFD0400402, and 2016 YED0400702), the National Natural Science Foundation of China (research grant numbers: 31430067, 31601475, 31571876, and 31671807), Heilongjiang Province Applied Technology Research and Development Technology Major Project (GA17B002), Heilongjiang Province General Undergraduate Higher Education Youth Innovative Talents Training Program (NYPXST-2018163); Heilongjiang Provincial Science Foundation Project (C2018024), China Postdoctoral Science Foundation Funded Project (Project No.: 2018M641798).  
The authors would like to acknowledge the support of the above sponsors of this study.  
The authors declare no conflict of interest.