Effects of covalent interactions and gel characteristics on soy protein-tannic acid conjugates prepared under alkaline conditions

The effects of covalent cross-linking of soy protein isolate (SPI-12%) with tannic acid (TA-29,58,88,117,146 μmol/g SPI) in an alkaline environment on the structure, antioxidant activity and gelation characteristics of SPI-TA conjugates were found to improve all properties. After alkali treatment, the tryptophan content in the conjugate decreased and the covalent binding rate increased. The addition of TA initiated irreversible SPI modifications, as shown by sodium dodecyl sulfate−polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and thermogravimetry. The texture analysis of the gel showed that hardness and elasticity were optimal at pH 11, and the rheological properties of the storage modulus (G′) and loss modulus (G″) increased with higher TA concentrations. Analysis of the microstructure indicates that an increase in TA concentration results in a more compact network structure. Our results indicate that pH-dependent interactions between SPI and TA modify the formation of the gel in a way that improves the rheology and textural properties of the gels.