Effects of Ultrasonication Combined with Enzymatic Treatment on the Structure and Function of Soy Protein Isolate

Natural soy protein isolate (SPI) exhibits suboptimal functional characteristics, including limited solubility, reduced foaming capacity, and diminished emulsifying ability. Conventional singular-modification techniques are unable to enhance multiple functional properties concurrently, thereby posing challenges in fulfilling the varied requirements of food processing. Therefore, this study employed ultrasonic and pepsin enzymatic modification techniques on SPI. By varying ultrasonic frequency (20 kHz, 207 kHz) and sonic energy density (295 W/L, 590 W/L), different modified protein samples were obtained. The effects of single treatment, combined treatment, and varying ultrasonic parameters on their structure and functionality were investigated. The results indicate that compared to single enzymatic hydrolysis, combination-treated SPI exhibited reduced fluorescence intensity and UV absorbance, along with significant decreases in methionine (Met) and free-sulfhydryl (SH) content (p < 0.05). Particle size decreased while distribution became more uniform, and relative molecular weight also diminished. This indicates that combined processing induces more pronounced changes in the protein’s primary to higher-order structures, thereby enhancing functional properties. Specifically, surface hydrophobicity (H0) and emulsification stability (ESI) improved, while emulsifying capacity (EAI) significantly increased (p < 0.05). In summary, ultrasonication combined with enzymatic hydrolysis exhibits synergistic effects, optimizing protein structure and functional characteristics. This approach facilitates the development of functional foods and broadens their application scope.