Abstract: This study determined the subunit composition and structure, free amino group content, free sulfhydryl content, surface hydrophobicity index and gel strength of soybean protein isolate (SPI). Furthermore, the effect of heat treatment on physicochemical properties, structural characteristics and gel property of SPI was explored. The results showed that with the increase in heat treatment temperature, the subunit dissociation of SPI was aggravated; the β-sheet content decreased significantly, whereas the random coil content increased significantly. The free amino group content, free sulfhydryl content, and surface hydrophobicity tended to increase with elevated temperature from 70 to 95 ℃ when the heat treatment lasted 10 min. In addition, the gel strength of heat induced gel decreased after an initial increase, and its water loss continuously dropped. In the case of heat treatment at 90 ℃, the free amino group content gradually increased, and the free sulfhydryl content and surface hydrophobicity initially increased and thereafter declined with increasing the treatment time from 5 to 60 min; the gel strength and the water loss of heat induced gel increased and declined, respectively. The gel strength and the water loss gradually increased and decreased respectively with the increase in temperature and holding time. To sum up, the optimal conditions to obtain SPI gel should be heat treatment at 90 ℃ for 15 min, and then maintenance at 90 ℃ for 30 min.

Key words: soybean protein isolate, protein subunits, secondary structure, surface properties, gel properties