Mechanistic studies on glycinin basic subunit/soybean soluble polysaccharide complexes: Ionic modulation and...

Interactions and complex formation between the incompletely hydrolyzed soybean glycinin basic subunit (B) and soybean soluble polysaccharide (SSPS), which are key components of soy sauce secondary precipitate, were investigated in this study. We systematically analyzed the formation mechanisms of B/SSPS complexes under varying pH conditions and B/SSPS ratios by turbidimetry, zeta potential, particle size analysis, SEM, and structural analysis, and further examined the effects of NaCl concentration as well as different monovalent and divalent ions on their aggregation behavior by turbidimetric titration, fluorescence spectroscopy, and circular dichroism. Results showed that B/SSPS complex formation was driven by electrostatic interactions, and SSPS effectively prevented excessive protein aggregation under acidic conditions through electrostatic repulsion and steric hindrance. Furthermore, monovalent/divalent ions exerted ion-specific effects on B/SSPS complexes aggregation, consistent with the Hofmeister series. Among monovalent ions, NaCl exhibited the most pronounced and ionic strength-dependent effect: at low concentrations (≤0.2 M), it enhanced complex formation by screening electrostatic repulsion, whereas at high concentrations, excessive ionic shielding weakened the interactions and reduced aggregation. Notably, increasing NaCl concentration from 1.0 M to 2.0 M did not significantly reduce turbidity, suggesting that reducing salt content to 1.0 M in soy sauce is feasible without substantially increasing precipitation risk. For divalent ions, the turbidity-promoting effect followed the order: MgCl₂ > CaCl₂ > ZnCl₂. Our findings on the B/SSPS complex mechanism offer key insights for simultaneously achieving salt reduction and controlling secondary precipitate in soy sauce production.