Effects of small molecular compounds in soymilk on the protein coagulation process: Ca2 + as coagulant

•Protein precipitation with Ca2 + could be well fitted mathematically with a Logistic model.•The start and end point of coagulation were positively correlated with soybean phosphorus.•Removal of small molecules resulted in earlier start and higher growth rate of precipitation.•Mechanism of interactions among small molecules, protein and added Ca2 + was established.

In order to clarify the roles played by small molecular compounds in soymilk on its coagulation process, characteristics of protein coagulation for soymilk from 66 soybean varieties were statistically analyzed. The curve, namely the function between protein precipitation amount (mg/mL) and calcium chloride concentration (mM), fitted well with Logistic equation, (y = a/(1 + b * exp(− kx))). The curve included two slowly increasing stages, which indicated the initial and later periods, (x = 0 ~ (ln b − 1.317)/k and x = (ln b + 1.317)/k ∼ ∝) and a sharply increasing stage (x = (ln b − 1.317)/k ~ (ln b + 1.317)/k). Parameter “a” differed among soybean varieties and it was positively correlated with soluble protein content in soymilk, referring to the maximum precipitation amount. Parameters “b” and “k” were correlated with the contents of soluble protein and phosphorus in soymilk, reflecting the odds of interactions between protein and the coagulant. When small molecules were removed or reduced from soymilk using an ultrafiltration module (cut-off 10 kDa), the start and end point of the curve moved forward and aggregation of protein was quicker and more easily happened. It was further verified that small molecules like polyacid ions could lower the odds of direct interactions between protein molecules and Ca2 +, so that the protein became less sensitive to Ca2 +. Therefore, the protein coagulation reaction should occur after the small molecules, mainly phytate and other polyacid ions, interacted with Ca2 + and formed unionizable substances. Then the protein molecules aggregated, grew and form a network.