Effect of Covalent Conjugation with Polyphenols by Free Radical Method on Gel Properties of Soybean Protein-Stabilized Emulsion

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.