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Binding of phytate to soybean protein during the heat treatment of soymilk and its effect on protein aggregation

Wang, Ruican, Liu, Jingyuan, Guo, Shuntang
Food hydrocolloids 2018 v.84 pp. 368-378
amino acids, beta-conglycinin, calcium, dialysis, glycinin, heat treatment, hydrocolloids, magnesium, phytic acid, polypeptides, solubility, soy protein, soymilk, temperature, urea
To investigate the interactions between phytates and soybean proteins during heat treatment, as well as their effects on the heat-induced protein aggregation and the formation of soymilk protein particles, the amount of free phytate, calcium, and magnesium in both unheated and heated soymilk were measured by equilibrium dialysis. It was found that about one-third of the free phytate became attached to proteins and was transferred to the particulate protein fraction after heat treatment. By heating purified soy protein solutions, it was shown that the soy glycinin had considerably higher phytate-binding capacity than β-conglycinin and that the binding reaction only occurred when the glycinin was denatured by being either heated to a temperature higher than 75 °C or treated with urea and SDS. It was assumed that when soymilk was heated to high temperatures, there was a critical “opening period” when the compact structure of glycinin unfolded and dissociated into the basic and acidic polypeptides, with plenty of the inner basic amino acids exposed to the surface, which attracted phytate and other denatured negatively charged protein fractions. The competitive binding of phytate with the basic polypeptides inhibited the binding of α/α’ subunits and acidic polypeptides to some extent, resulting in a smaller ratio of particulate proteins. Phytate enhanced the solubility and inhibited the thermal aggregation of glycinin by increasing the negative charges on the protein surface.