Complex Formation between Soy Proteins and Potato Starch: Effect of pH, Biopolymer Ratio, and Biopolymer Concentration

Understanding the factors that induce protein–starch complex formation under acidic conditions is important for the production of acidic beverages. This study investigates the formation of complexes between soy proteins (SP) and potato starch (PS). Measurements of turbidity and ζ potential are used to study complex formation as a function of pH (7.0–2.0), SP/PS ratio (1:8–4:1, w/w), biopolymer concentration (Cₚ, 0.1–0.8%, w/v), and PS concentration (CPS, 0.05–0.8%, w/v). For 1:4 SP/PS mixtures, the highest turbidity (OD₆₀₀ = 0.33) is reached at pH 3.2 (defined as the optimal pH [pHₒₚₜ]), where the ζ potential is 0 mV. Thus, biopolymer interactions reach a maximum at the charge balanced point. With increasing SP/PS ratio (1:8–1:4), the pHₒₚₜ increases. At SP/PS ratios > 1:2, soy protein aggregates affect the turbidity. The turbidity increases with increasing biopolymer concentration (0.1%–0.5%), indicating more interactions between SP and PS. Sedimentation measurements prove that the addition of PS reduces SP aggregation at its isoelectric point owing to electrostatic repulsion. Soy protein aggregates are confirmed to be involved in SP/PS complex formation, leading to higher and broader turbidity curves. This study will provide necessary theoretical support for the application of starch in acid protein beverage.