Abstract: The effect of pH-shifting (pH 7.0, 2.0, and 11.0) combined with heat treatment (90 and 120 ℃) on the flexibility of soy protein isolate (SPI) and the relationships between flexibility and structure or emulsifying properties were investigated. The results suggest that the flexibility of SPI was increased with heating temperature under different pH-shifting treatment conditions. pH-shifting treatment from 2.0 or 11.0 to 7.0 contributed to the increased flexibility of SPI during heating as compared with that at pH 7.0. The heat treatment increased the flexibility more significantly at pH 11.0. At pH 7.0, the content of free sulfhydryl group was increased with heating temperature, indicating that the increase in the flexibility was related to the cleavage of the disulfide bond of SPI. Under pH-shifting treatment from 11.0, the dissociation of the SPI molecule occurred during the heating process, thereby increasing the flexibility. Under the experimental conditions, the flexibility showed a significantly positive correlation with the emulsifying activity and emulsion stability, with correlation coefficients of 0.945 and 0.936, respectively. UV spectroscopy and endogenous tryptophan fluorescence spectroscopy suggested that the SPI structure became more unfolded as the flexibility increased.

Key words: pH-shifting, heat treatment, soy protein isolate, flexibility, emulsifying properties