Insights into the effects of combined caffeic acid and glucose covalent modification on the conformational and functional properties of two milk proteins

Two ternary conjugates (WPI–Ca-Glc con and CS-Ca-Glc con) were constructed by radical grafting and glycosylation techniques with caffeic acid (Ca), glucose (Glc), and whey protein isolate (WPI)/casein (CS) as raw materials, and their conformational and functional properties were investigated. The results indicated that the glycosylation degrees of WPI-Ca-Glc con and CS-Ca-Glc con were 34.46% and 23.21%, respectively. The glycosylation and polyphenol modification induced the unfolding of both proteins, which was manifested as α-helix unwinding and intense fluorescence quenching. After the covalent modification with Ca and Glc, the antioxidant capacity, emulsifying activity, and emulsifying stability of CS were found to have enhanced by 16.62 times, 1.65 times, and 1.38 times, respectively. The structural and functional changes observed in the proteins are believed to be the result of the covalent binding between Ca and WPI/CS via free radical-mediated reactions and the glycosylation reaction between the WPI/CS-Ca conjugates and Glc. This study has provided new insights into the structural and functional relationships of polyphenol-protein-saccharide ternary conjugates, which possess potential applications as novel antioxidants and emulsifiers.