Abstract: The effects of different heat treatment conditions (60–121 ℃ for 5, 10 and 30 min) on the flexibility of soy protein isolate (SPI) as characterized by its sensitivity to trypsin and the relationship between flexibility and structure were investigated. The results suggested that heating at temperature below 80 ℃ had no significant impact on the flexibility of SPI, while the flexibility of SPI increased with increase in temperature (above 80 ℃) and heating time. Flexibility was very significantly correlated with turbidity, free sulfhydryl group content and surface hydrophobicity under heating conditions at 60–100 ℃, with correlation coefficients of 0.956, 0.954 and 0.954, respectively. Turbidity and surface hydrophobicity declined with increase in flexibility, while the content of free sulfhydryl group increased when heating temperature was higher than 100 ℃. The content of free sulfhydryl group reached the maximum level at 5 min of heating at 121 ℃, and then decreased. Ultraviolet absorption spectroscopy and endogenous tryptophan fluorescence spectroscopy suggested that the structure of SPI was more stretched with increase in SPI flexibility under heating conditions at 80–100 ℃.

Key words: soy protein isolate, heat treatment, flexibility, structure, correlation analysis