Role of pectin degree of esterification in the structural assembly and oxidative stability of WPI–citrus Pectin–EGCG conjugates stabilized emulsion

Protein–polysaccharide–polyphenol ternary conjugates are promising emulsifiers for improving the oxidative stability of food emulsions. However, the relationship between antioxidant loading and interfacial effectiveness remains unclear. In this study, citrus pectins with different degrees of esterification (DE; 25%, 55%, and 77%) were used to construct whey protein isolate (WPI)–pectin–epigallocatechin gallate (EGCG) conjugates, and their structure–function relationships were systematically investigated. Molecular docking and covalent grafting assays revealed that high-DE pectin (77CP) exhibited the strongest affinity for WPI (−2.12 kcal/mol) via hydrophobic interactions, leading to the highest EGCG loading. Paradoxically, in oil-in-water emulsions, conjugates prepared with low-DE pectin (25CP) demonstrated superior oxidative stability, effectively suppressing the formation of hydroperoxides and thiobarbituric acid-reactive substances during storage. Multiscale structural analysis indicated that although 77CP enabled high antioxidant loading, its dense and thick interfacial layer induced steric masking, limiting the accessibility of EGCG to lipid radicals. In contrast, 25CP formed a thinner and more ordered interface that maximized antioxidant exposure. Furthermore, 25CP provided a synergistic defense mechanism combining potent metal chelation (via abundant carboxyl groups) with strong electrostatic repulsion. These results indicate that the performance of antioxidant emulsifiers depends on antioxidant loading, as well as interfacial architecture and antioxidant accessibility. This study provides a useful framework for designing protein–polysaccharide conjugates for oxidation-resistant emulsion systems.