Compared to the non-covalent complex, the covalent complex between black soybean protein isolate (BSPI) and chlorogenic acid (CA) was found to have higher polyphenol-binding capacity. Fluorescence spectroscopy and circular dichroism (CD) spectroscopy were used to analyze the effect of CA cross-linking on BSPI structure. It was found that after the addition of CA, the secondary and tertiary structures of BSPI were changed. The proportion of α-helix decreased significantly and the proportion of random coil increased significantly. The protein structure became more loose and disordered, and this effect was more obvious for the covalent complex than the non-covalent one. Crosslinking with CA greatly improved the emulsifying capacity and antioxidant activity of BSPI. Nanoemulsions prepared with the BSPI-CA conjugates had smaller mean particle sizes and higher absolute values of zeta-potential than those prepared with native BSPI. The storage stability and oxidative stability of nanoemulsions with the BSPI-CA conjugates were enhanced significantly, and the covalent conjugate was more beneficial to the basic properties and stability of nanoemulsions.

In this study, a covalent conjugate between ferulic acid (FA) and soybean protein isolate (SPI) was prepared by free radical method and was used to prepare gluconolactone (GDL)-induced emulsion gels. The effects of covalent binding to FA on SPI structure, emulsion properties and emulsion gel characteristics were investigated. The optimum concentration of FA was determined as 150 μmol/g protein based on intermolecular forces, textural properties, and water-holding capacity of SPI-FA (SFA) stabilized emulsion gels. Under this condition, spectral analysis showed that FA had a fluorescence quenching effect on SPI, and after covalent binding to FA, a decrease in the β-folded content and an increase in the α-helix, β-turn and random coil contents of SPI appeared. The absolute value of zeta potential and interfacial protein content of SFA stabilized emulsions increased, and the mean particle size and apparent viscosity decreased. The final storage modulus (G’) of SFA stabilized emulsion gels increased, and the changes in relaxation times and peak ratios observed in low-field nuclear magnetic resonance (NMR) measurements indicated that the SFA stabilized emulsion gels had better hydration properties. Moreover, they had a more uniform and dense porous network structure. These results show that SPI covalently bound to 150 μmol/g protein of FA is valuable in the preparation of emulsion gels.