Further evaluation on structural and antioxidant capacities of soy protein isolate under multiple freeze-thaw cycles.

Multiple freeze-thaw (F-T) treatments could change a protein structure and affect its physicochemical activities. In this work, soy protein isolate (SPI) was subjected to multiple F-T treatments, and the changes in its physicochemical and functional properties were investigated. The three-dimensional fluorescence spectroscopy indicated that F-T treatments changed the structure of SPI, including an increase in surface hydrophobicity. Fourier transform infrared spectroscopy showed that SPI underwent denaturation, unfolding and aggregation due to the interchange of sulfhydryl-disulfide bonds and the exposure of hydrophobic groups. Correspondingly, the particle size of SPI increased significantly and the protein precipitation rate also increased from 16.69%/25.33% to 52.52%/55.79% after nine F-T treatments. The F-T treated SPI had a higher antioxidant capacity. Results indicate that F-T treatments may be used as a strategy to ameliorate preparation methods and improve functional characteristics of SPI, and suggest that multiple F-T treatment is an alternative way to recover soy proteins.