Effect of combined enzyme and ultrasound treatment on the structure and gel properties of soy protein isolate: A comparative study of alkaline protease and pepsin

This study aimed to research the effects of enzyme and ultrasound combined modification on the structure and gel properties of soy protein isolate (SPI). Different modified protein samples were obtained by hydrolyzing soy isolate with alkaline protease and pepsin at 0%, 0.1%, 0.5%, and 1.0% of hydrolysis by ultrasound treatment (400 W, 20 min). The results showed that the SPI particle size, which modified by combined enzyme and ultrasound treatment (co-modified), was significantly lower (P < 0.05) and the particle size was more evenly distributed compared with those of the single modified samples. The best improvement in solubility and turbidity was observed for the co-modified SPI. The Fourier transform–infrared spectroscopy showed the most significant changes in the secondary structure content of the co-modified SPI. The protein structure was unfolded, and the contents of free sulfhydryl and H0 were improved. Additionally, the co-modified SPI gel had a uniform and compact gel network and higher water-holding capacity (WHC) and gel strength. After frozen storage, the combined enzyme and ultrasound treatment showed better frozen storage stability than single treatment: the WHC of the gel decreased slightly, the gel strength increased slowly, and the gel network was less damaged. In conclusion, combined enzyme and ultrasound treatment could be effective in improving the SPI structure and gel properties.