Protein-polyphenol complexation vs. conjugation: A review on mechanisms, functional differences, and antioxidant-emulsifier roles

The interaction between proteins and polyphenols has gained attention for improving the stability and physicochemical properties of colloidal systems. This review focuses on recent advances in their interaction mechanisms and functional impacts. Protein-polyphenol assemblies are formed through covalent bonding (i.e., conjugation) or non-covalent bonding (i.e., complexation). These interactions may lead to distinct functionalities and properties influenced by environmental conditions. The conjugation is usually initiated by the oxidation of polyphenols through alkaline treatment, free radicals, etc., while complexation occurs even under milder conditions, such as neutral pH and room temperature, resulting in weaker and reversible interactions. Despite the significant increase in the number of studies on protein-polyphenol interactions in recent years, there is no clear consensus in the previous literature on whether covalent or non-covalent bonding provides better stability under different processing conditions. In emulsion-based food systems, protein-polyphenol assemblies can function dually as antioxidant-emulsifiers, where proteins provide the emulsifying ability, and polyphenols contribute antioxidant activity. This effect is due to the positioning of these assemblies in the oil-water interface. Understanding the distinct roles and conditions of protein-polyphenol interactions is crucial for optimizing their use in designing stable, functional colloidal systems, particularly in emulsion-based foods.