Effects of catechin types found in tea polyphenols on the structural and functional properties of soybean protein isolate–catechin covalent complexes

Soy protein isolate (SPI) has useful functional properties, such as gelation, emulsification, and foaming. However, SPI is usually dense and has a unique spherical structure, which hinders its practical application. In this study, the effects of three typical catechins (epigallocatechin gallate (EGCG), epigallocatechin (EGC), and epicatechin (EC)) on the structure and functional properties of soy protein isolate (SPI) under alkali conditions were investigated. Compared with the native SPI, the fluorescence intensity of SPI–EC, SPI–EGC, and SPI–EGCG complexes decreased, while their fluorescence quenching rate and binding sites increased, indicating the formation of covalent complexes. The SPI–EGCG complex has a higher fluorescence quenching rate and more binding sites than those of SPI–EC and SPI–EGCG complexes. The emulsifying activity and stability of the SPI–EGCG complex were 1.16 m2/g and 147 min, respectively, higher than those of latter two complexes. Hence, the functional property of SPI and the stability of catechins could be improved by the covalent interaction between the two. This study provides new insights into the relationship between the type of polyphenol and the structure of SPI.