Effects of Water Activity and pH on the Preparation and Properties of Soy Protein Sheets

ABSTRACT This study investigated the effects of water activity () and pH on the properties of hot‐pressed soy protein isolate (SPI) sheets. SPI and glycerol (1:1 mass ratio) were equilibrated under various (~0–0.75) prior to fabrication. Transparent sheets were obtained at , whereas they became opaque at . Differential scanning calorimetry (DSC) revealed that higher decreased the endothermic peak temperature and increased endothermic heat, while mechanical characterization showed decreased maximum breaking loads and increased fracture strains. Furthermore, successful sheet fabrication was achieved across a broad pH range (1.9–10.9). Notably, alkaline conditions remarkably enhanced toughness, providing both high stiffness and superior extensibility. This is attributed to the synergistic effect of disulfide bond formation and protein chain unfolding via electrostatic repulsion. These findings demonstrate that SPI sheet properties can be precisely engineered through the control of and pH.