Effect of protein concentration on thermal aggregation and Ca2+-induced gelation of soymilk protein

The concentration of protein in soymilk is a key element in the production of soy products. In this study, the thermal aggregation behavior of soymilk proteins and the properties of aggregates formed at different concentrations (33 mg/g-56 mg/g) were analyzed. Results showed that the soymilk proteins formed two kinds of aggregates which demonstrated different properties at particular concentrations. In the low concentration range (<40 mg/g), rod-like aggregates were formed. The content and size of the aggregates increased with growing protein concentration, but the morphology didn't change. Additionally, the surface hydrophobic groups, sulfhydryl groups(-SH) and electrostatic charge of the protein aggregates exposed to the outside also increased, which drastically (20%) reduced the activation energy of the Ca2+-induced gelation and led to the cross-linkage of protein aggregates to form a dense, cluster-like network structure, which facilitated the formation of gels with high levels of hardness and increased water holding capacity (WHC). At protein concentrations greater than 40 mg/g, however, the protein subunits formed aggregates, which also combined with each other along an axial direction through the hydrophobic groups and -SH on the surface, thereby forming linear or even micro-network agglomerates. In this way, the surface hydrophobicity(S0) and -SH content decreased linearly with the rising protein concentration, while the surface charge density increased, the activation energy required for soymilk gelation increased, and the protein aggregates crosslinked to form a coarse network structure with large voids. Hence, the hardness, elasticity, toughness and WHC of the gel decreased with the increase in protein concentration. This study reveals the mechanism by which protein concentration changes the gel-forming ability of soymilk, and have important theoretical reference for enterprises to select the appropriate protein concentration for soymilk preparation, or to optimize the "concentration dilution method" to meet the processing requirements of different types of soy products.