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 the addition of Mg2+, the absolute value of the zeta potential sharply decreased beyond approximately 9.90 mM, accompanied by a rapid increase in particle size, which indicated aggregation through charge neutralisation and ionic bridging. While the zeta potential showed a slight change during pH variation, the particle size markedly increased around pH 5.7, suggesting that non-electrostatic aggregation is 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 the addition of Mg2+, Kc values differed among soy milk samples at an identical hc, implying charge-related aggregation effects. Conversely, under pH variation, hc and Kc were plotted on a single linear trend, confirming depletion flocculation independently of surface charge. The Mg2+-induced aggregation occurred through electrostatic interactions involving charge neutralisation, while pH-induced aggregation was governed by non-electrostatic depletion forces. These findings reveal the aggregation mechanisms underlying soy milk coagulation, enhancing the understanding of protein–ion interactions in colloidal systems.