The role of pH decrease in tofu-like precipitate formation using magnesium chloride

Uncertainty surrounds the role of pH in tofu-like precipitate (TLP) formation using MgCl₂. In this study, to clarify the role of MgCl₂-induced pH decrease in TLP formation, the protein concentration in the supernatant, weight of wet precipitate, and the solubility in 2 M urea at constant pH (6.5 or 6.2) were measured using a buffer and comparing the buffering effect under variable pH conditions to the effect observed without buffer. Analysis of protein concentration in the supernatant showed no precipitation of soymilk proteins under stable pH conditions of 6.5. Wet precipitate formation varied substantially during TLP formation at three different pHs; the weight slightly increased with the increase in MgCl₂ concentration up to 20 mM at a constant pH of 6.5. However, wet precipitation was substantially enhanced by MgCl₂ (≥10 mM) at a constant pH of 6.2 but rapidly increased by MgCl₂ (≥7 mM) under variable pH conditions. The protein concentrations and wet precipitate weight at 20 mM MgCl₂ under constant pH conditions showed contrasting patterns. However, the weight of dry precipitate was almost the same under both constant pH (6.5 and 6.2) conditions. Urea solubility under constant and variable pH conditions differed; no urea-insoluble precipitates were produced under constant pH conditions. These results suggest that pH decrease is essential for bringing proteins closer together for MgCl₂-mediated TLP formation. This study will aid in understanding the mechanisms underlying TLP formation and support further discovery of new coagulants for the effective production of new tofu-like and tofu-based foods.