Effect of Hydrodynamic Cavitation Treatment on the Interaction between Soy Protein Isolate (SPI) and Catechin and Structure and Functional Properties of SPI-Catechin Conjugates

In this study, to understand the effect of hydrodynamic cavitation (HC) treatment on the interaction between catechin and soy protein isolate (SPI) and the structural and functional properties of the conjugates, we explored the changes in binding between catechin and SPI as well as the ultraviolet (UV) absorption and fluorescence spectra, free sulfhydryl and amino group contents, surface hydrophobicity, average particle size, zeta potential, secondary structure, and antioxidant properties of the formed conjugates before and after HC treatment. The results showed that HC treatment could promote the interaction between catechin and SPI. At 2.0 mg/mL catechin concentration, HC treatment could increase the amount of catechin bound to SPI from (21.82 ± 0.18) to (62.55 ± 0.36) mg/g. Compared with the untreated sample, HC treatment enhanced the UV absorption intensity, weakened the fluorescence intensity, increased the average particle size, decreased the absolute value of the zeta potential and surface hydrophobicity, and reduced the free sulfhydryl and amino group contents. Additionally, HC treatment caused an increase in α-helix, β-turn, and random coil relative contents and a decrease in β-sheet relative content. Furthermore, after HC treatment, the antioxidant activity of the catechin-SPI conjugates was significantly enhanced. For the conjugate formed at 2.0 mg/mL catechin concentration, the 1,1-diphenyl-2-picrylhydrayl (DPPH) free radical scavenging capacity increased from (48.64 ± 1.24)% to (84.72 ± 0.12)%, the 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical scavenging capacity from (35.60 ± 1.21)% to (75.51 ± 0.79)%, and the ferric reducing power from 0.81 ± 0.02 to 1.52 ± 0.05. It can be seen that HC treatment can promote the combination of catechin and SPI, change the structural properties of the resulting conjugates, and enhance its antioxidant activity.