Free radical grafting of whey protein isolate with tea polyphenol: Synthesis and changes in structural and functional properties

Free radical grafting is a green and expeditious tool that was used to modify whey protein isolate (WPI) to synthesize WPI-(−)-epigallocatechin-3-gallate (EGCG) and WPI-catechin (C) antioxidant emulsifier and improve its functional and interfacial properties. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF–MS) were used to verify the covalent bonding between polyphenols and WPI. Results revealed that at least two EGCG molecules and three catechin molecules were grafted onto WPI. Meanwhile, the grafting of tea polyphenols, especially EGCG, with WPI decreased their fluorescence intensity; α-helix content; and free amino, sulfhydryl, and tyrosine group contents. A lycopene nanoemulsion constructed with the WPI-EGCG conjugate had smaller changes in size and ζ-potential than that constructed with WPI during 30-day storage, which was attributed to its enhanced emulsifying activity, emulsification stability, and ability to reduce the interfacial tension of conjugates. The improvement in oxidation resistance and interfacial adsorption layer thickness helped the WPI-EGCG conjugate to significantly reduce the lycopene degradation rate during storage. Thus, WPI modified with polyphenol by free radical grafting can be used to construct an emulsion delivery system for functional nutrients with a stable interface and high retention rate.