Effects of Radio Frequency Heating Treatment on Structure Changes of Soy Protein Isolate for Protein Modification

The effects of radio frequency (RF) heating treatments with different final temperatures (70, 80, and 90 °C) and electrode gaps (120, 160, and 200 mm) on the structural characteristics of soy protein isolate (SPI) dispersion were investigated. The results showed that RF heating significantly influenced free sulfhydryl groups and surface hydrophobicity of SPI. Free sulfhydryl groups increased with the increase of final temperature. The hydrophobicity of the RF-heated sample was higher than the original SPI without RF treatment. The highest hydrophobicity of the RF-heated SPI was found with electrode gap of 200 mm at 90 °C. RF heating treatment resulted in the reduction of ultraviolet absorption of SPI indicating the change of three-dimensional positions of soy protein but did not modify the protein primary structure of SPI. The Fourier transform infrared spectroscopy showed that hydration of SPI was decreased by RF heating. The self-reassembly from random coil structure to β-sheet structure suggested that RF heating treatment can change the secondary structure of soy protein to be more orderly.