Metal ions binding and colloidal destabilization in the model systems: Implication on the magnesium chloride coagulation mechanism in tofu making

MgCl₂-induced soymilk coagulation mechanism in tofu making was explored from perspectives of Mg²⁺ binding and colloidal properties in model systems. Isothermal titration calorimetry of bovine serum albumin (BSA)-small molecule mixtures revealed proteins contributed negligibly to Mg²⁺ binding sites, instead, substantial Mg²⁺ were bound by soymilk-borne small molecules. The results thus suggested the "protein-Mg²⁺-protein bridge" was hardly formed in tofu making. Zeta potentials for both BSA-small molecule system and defatted soymilk changed in a similar pattern relative to the unbound Mg²⁺ concentration, indicating only those small molecules-unbound Mg²⁺ effectively neutralized the electronegative charges on protein colloidal particles. Turbidity and particle size results revealed a critical zeta potential (-11 mV) was required to induce marked Mg²⁺-defatted soymilk coagulation. For heated defatted soymilks, a critical minimum protein concentration (8 mg/mL) was needed to observe the significant Mg²⁺-induced coagulation. This study is expected to deepen our understanding of Mg²⁺ coagulation mechanism in tofu making.