Effects of soybean protein isolate − polyphenol conjugate formation on the protein structure and emulsifying properties: Protein − polyphenol emulsification performance in the presence of chitosan

Herein, we investigated the influence of covalent complexation between (−)-epigallocatechin gallate (EGCG) and soy protein isolate (SPI) on the structural and emulsifying properties of the protein as well as the effect of chitosan (CS) on the stability of emulsions formed by the covalent conjugates. The formation of the covalent complex of SPI and EGCG was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Three-dimensional fluorescence spectroscopy and circular dichroism spectroscopy were conducted to analyze the changes in the protein structure at different EGCG concentrations (0.005, 0.015, and 0.025 wt%). The addition of EGCG changed the tertiary conformation and secondary structure of the protein—unfolding of the protein polypeptide chain, decrease in the β-strand content, and increase in α-helix and random coil contents were observed. After covalent bonding of SPI with EGCG (0.015 wt %), the complex exhibited optimal solubility and emulsifying properties. SPI–EGCG–CS emulsions were prepared by layer-by-layer electrostatic assembly at pH 5.0. The effect of environmental stress on the stability of the CS–loaded SPI–EGCG (0.015 wt%) emulsion oil droplets was studied by freezing (−20 ℃, pH 5) and thermal treatment (90 ℃, 30 min, pH 5). The stability of the emulsion was improved when the CS concentration was >0.25 wt%; this improvement was attributed to the increase in electrostatic force and steric hindrance among droplets and the formation of a relatively thick biopolymer interface on the surface of oil droplets. In addition, the rheological properties showed that thicker interface layers and network structures caused deviations from the flow behavior of emulsions. These results indicate that SPI − EGCG conjugates can be used as food additives and for the construction of the lipophilic active ingredient delivery system.