Soy protein interactions with polyphenols: Structural and functional changes in natural and cationized forms

Herein, cationic soy protein (NSPI) was synthesized by grafting Ethylenediamine (EDA) onto soy protein isolate (SPI), and protein–gallic acid (GA) complexes were formed by mixing NSPI with GA in various ratios. We assessed the structure, particle size, thermal stability, emulsifying ability, and antioxidant capacity of NSPI and complexes. Results show that grafting with EDA introduced a positive charge to SPI and resulted in a uniform particle size, and enhanced thermal stability, emulsifying ability, and antioxidant capacity. In addition, NSPI presented more amino groups and stronger interactions with GA compared to SPI. EDA and GA synergistically increased the flexibility of SPI, reducing the α-helix content and increasing the random coil content. Moreover, the interactions between SPI, NSPI, and GA were static, and hydrophobic and electrostatic between GA and SPI and NSPI, respectively. Grafting SPI with EDA improved functionality and interactions with GA, implying that NSPI-GA complexes may function as emulsifiers and antioxidants.