Effects of combined enzymatic and ultrasonic treatments on the structure and gel properties of soybean protein isolate

This study aimed to investigate the effects of combined treatment of enzyme and ultrasound on the structure of soybean protein isolate (SPI) and the properties of SPI gel cross-linked with glutamine transaminase (TG). The papain-hydrolyzed SPI with hydrolysis degrees of about 0%, 0.1%, 0.5%, and 1.0% was treated with ultrasound (300 W, 20 min) to obtain different modified proteins. The sodium dodecyl sulfate–polyacrylamide gel electrophoresis of SPI after combined treatment showed lower molecular weight, indicating dissociation of some subunits into smaller units. In addition, the protein structure extended, promoting the exposure of free sulfhydryl and hydrophobic groups. Compared with untreated SPI, the SPI gel prepared by the combined treatment formed a uniform and dense gel network, and its gel strength and water-holding capacity (WHC) significantly improved. The gel strength and WHC were higher in the SPI gel treated with a combination of ultrasound and enzyme than that treated with ultrasound or enzyme alone, exhibiting the synergistic effect of enzymatic and ultrasonic treatment. In conclusion, the combined treatment of enzyme and ultrasound might be an effective way of improving the structure and gel properties of SPI.