Protein-Polyphenol Covalent Bonding: Mechanisms and Bioactivities in Food Systems

Proteins and polyphenols are widespread in food systems and often interact during processing and storage, affecting protein structure, functionality, and nutrition. This review examines the formation and bioactivities of covalent protein–polyphenol conjugates. These conjugates form in foods mainly when polyphenols oxidize to quinones and react with nucleophilic amino acid residues via Michael addition or condensation, producing stable conjugates with altered properties. Recent studies show that conjugation generally enhances protein antioxidant capacity, though it is typically lower than that of free polyphenols. Conjugation can also improve polyphenol stability during digestion and potentially increase bioavailability, although results are system-dependent and sometimes inconsistent. Emerging evidence also highlights potential anti-inflammatory, anti-allergic, and anticancer properties of these conjugates due to structural changes. Despite their potential, gaps remain in understanding in vivo behavior, structure–function relationships, and the role of non-covalent interactions. Future research should focus on metabolic fate, optimized conjugation, and validated health effects.