Protein aggregation and Ca2+-induced gelation of soymilk after heat treatment under slightly alkaline conditions

Abstract

Alkaline heat treatment of soymilk is widely used in the production of soy products. This study aimed to clarify the effect of the thermal aggregation state of proteins on the coagulation characteristics of soymilk. Soymilk with different pH levels (6.6–8.0) was preheated, and the gelation processes induced by CaSO4·2H2O and the gel properties were investigated. Results showed that the aggregation and binding pattern of proteins changed after slightly alkaline heat treatment. The ability of 7S and the acidic (A) subunit of 11S to bind to the basic (B) subunit of 11S weakened. These subunits converted into non-particulate proteins, which decreased particulate protein content and particle size. The number of sulfhydryl groups on the protein surface reduced by 45%–62%, and more disulfide bonds formed in the protein subunits during recombination. During gelation, the soymilk after alkaline heat treatment exhibited a slow intermolecular binding in the “pre-aggregation” stage, which led to late gelation onset and a uniform gel network. With the increase in non-gelable protein content in the gel, the water holding capacity (WHC) enhanced and but the hardness decreased. When the pH was further increased (pH ≥ 7.8) to the isoelectric point of the B subunit, the B subunit excessively aggregated and formed large and dense aggregates, which increased gel hardness and produced a rough and uneven network.