Abstract: In order to improve the emulsifying properties of soybean protein isolate (SPI) and prevent oxidative degradation of (?)-epigallocatechin gallate (EGCG), the effect of individual and combined alkali and ultrasonic treatments on the structural and emulsifying properties of SPI and its interaction with EGCG was investigated. Fourier transform infrared spectroscopy and fluorescence spectroscopy showed that both individual treatments could promote the unfolding of protein structure, decrease the α-helix relative content in the secondary structure of the protein (P < 0.05) and increase the random coil relative content (P < 0.05). Compared with the control group, the combined treatment group showed a decrease in the α-helix relative content of 13.23% and an increase in the random coil relative content of 8.44%, resulting in the exposure of more hydrophobic amino acid residues. Both alkali and ultrasonic treatments significantly increased the surface hydrophobicity, sulfhydryl content, emulsifying activity and emulsion stability of SPI (P < 0.05), and a better effect was observed when they were combined. Fluorescence quenching analysis revealed that the treated SPI had a stronger binding affinity to EGCG, blocking the degradation of EGCG. Besides, the treated SPI could protect EGCG from being oxidized with the combined treatment being more effective than either treatment alone. Together, this study confirmed that a combination of ultrasonic and alkali treatment can improve the emulsifying properties of SPI, making it a potential protective carrier for functional substances.

Key words: soybean protein isolate; (?)-epigallocatechin gallate; ultrasound treatment; alkali treatment; emulsifying properties