Aggregation and gelation of soymilk protein after alkaline heat treatment: Effect of coagulants and their addition sequences

This study investigated the effect of different coagulants, including the addition sequence, on the aggregation behavior of soymilk protein and the quality of tofu to improve the gel formation ability and conversion efficiency of soymilk protein after alkaline heat treatment (AHT). The results showed that the coagulation reaction of AHT soymilk was delayed compared with normal soymilk when introducing magnesium chloride (MC). The pH (6.7–6.8) was much higher than that of normal soymilk curd (5.7–5.8) when the coagulation reaction reached the endpoint, and a large number of 7S α′ and α subunits and 11S acidic (A) subunits did not coagulate. However, when introducing lactic acid (LA) or LA combined with MC as a coagulant, the proteins could be almost completely converted into curd, and the recovery rate of the protein increased by about 10%. Inducing coagulation by adding LA and MC simultaneously (MIXTURE) or adding LA first, followed by MC (LA-MC), allowed each protein subunit to aggregate into particles indiscriminately, forming a continuous and uniform network structure via salt bridges, disulfide bonds, or hydrophobic interactions. This structure differed from the "cluster-like" network structure of MC-MC tofu or the "fine chain-like" network structure of LA-LA tofu. Furthermore, the textural properties, such as hardness, springiness, water holding capacity (WHC), and cohesiveness of the tofu, were improved. The results of this study reveal that regulating the aggregation patterns of soymilk protein subunits is an effective way to directionally control the quality of gel products.