Effects of flexibility and surface hydrophobicity on emulsifying properties: Ultrasound-treated soybean protein isolate

The structure and physicochemical properties of a protein have significant effects on its molecular flexibility. Herein, different flexible protein molecules are obtained after ultrasound treatment at different power conditions (0, 200, 400, and 600 W), and the structural and functional changes in the soybean protein isolate (SPI) are determined. The correlation between flexibility (F), surface hydrophobicity (H0), and emulsifying properties is established. It is demonstrated that ultrasound decreases the α-helical content and increases the β-sheet content of the SPI, thereby increasing its flexibility and sulfhydryl content, and unfolding its tertiary structure. The SPI affords the best solubility, turbidity, surface hydrophobicity, emulsifying activity (EA), and emulsifying stability (ES) when subjected to an ultrasound power of 400 W. The correlation coefficients between flexibility and emulsification performance (0.938 (F:EA) and 0.958 (F:ES)) are significantly higher than those between the surface hydrophobicity and emulsification performance (0.772 (H0:EA) and 0.883 (H0:ES)). Therefore, the conformational flexibility of SPI, rather than its surface hydrophobicity, determines its emulsification properties. These findings provide a theoretical basis for understanding the emulsification mechanism of ultrasound-modified proteins, which is important in the food industry.