Evaluation of structural and functional properties of protein–EGCG complexes and their ability of stabilizing a model β-carotene emulsion

The objectives of the present study were to first covalently modify different bovine milk proteins (α-lactalbumin, β-lactoglobulin, lactoferrin and sodium caseinate) using (−)-epigallocatechin-3-gallate (EGCG), then compare structural and functional properties between covalent and non-covalent protein–EGCG complexes and finally test the complexes with greater antioxidant potential in stabilizing a model β-carotene emulsion. Covalent modification of milk proteins with EGCG was testified by a reduction of free amino groups and free sulfhydryl groups as well as matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOF-MS). Furthermore, the proposed schematic formation pathway showed that milk proteins formed covalent complexes with EGCG dimers. Then structural and functional properties of covalent and non-covalent protein–EGCG complexes were analyzed. Covalent protein–EGCG complexes had higher denaturation temperatures than non-covalent ones. Besides, covalent protein–EGCG complexes exhibited much stronger antioxidant activity than the same amount of non-covalent ones. A comparison of interfacial concentration fraction of protein and EGCG in emulsions revealed that covalent protein–EGCG complexes exhibited better interfacial adsorption behavior and greater antioxidant potential than non-covalent ones in emulsion systems. Thus, research was then focused on testing covalent protein–EGCG complexes in a model β-carotene emulsion. Covalent protein–EGCG complexes significantly enhanced chemical stability of β-carotene in emulsions against heat treatment and ultraviolet (UV) light exposure. Given both physical and chemical stability of emulsions, the overall ability of covalent protein–EGCG complexes in stabilizing a model β-carotene emulsion followed the order: sodium caseinate > β-lactoglobulin > lactoferrin > α-lactalbumin.