Viscous behaviour of soy milk colloidal dispersion depending on its flocculation

Abstract The aim of this study is to clarify the aggregation mechanisms of colloidal dispersion systems using soy milk as a model. This study investigated the aggregation mechanisms of colloidal dispersion systems using soy milk, focusing on the effects of magnesium chloride addition and pH variation. Upon Mg2+ addition, the absolute value of the zeta potential sharply decreased beyond approximately 9.90 mM, accompanied by a rapid increase in particle size, indicating aggregation through charge neutralization and ionic bridging. While, the zeta potential showed slightly change during pH variation, but particle size markedly increased around pH 5.7, suggesting non-electrostatic aggregation dominated by depletion flocculation rather than isoelectric precipitation. Parameters; hc and Kc derived from the extended Krieger–Dougherty equation: ηc = ηc0 (1-Kc・hc・C)-2.5/Kc were analysed to clarify aggregation mechanisms. Under Mg2+ addition, Kc values differed among soy milk samples at identical hc, implying charge-related aggregation effects. Conversely, under pH variation, hc and Kc plotted on a single linear trend, confirming depletion flocculation independent of surface charge. The Mg2+-induced aggregation occurred via electrostatic interactions involving charge neutralization, while pH-induced aggregation was governed by non-electrostatic depletion forces. These findings revealed aggregation mechanisms underlying soy milk coagulation, enhancing understanding of protein–ion interactions in colloidal systems.