Reactions of plant polyphenols in foods: Impact of molecular structure

Plant polyphenols are widely distributed in foods and beverages, either naturally present in plant-based foods or added as plant extracts due to their multifaceted functional and bioactive properties, especially as antioxidants that retard rancidity caused by lipid oxidation. More than 8000 phenolic compounds with different molecular structure exist in nature, but only a limited range of polyphenol-containing plant extracts are commercially available as food ingredients, e.g. from rosemary, green tea, and grapes. Polyphenols react with a wide range of food components, including proteins, peptides, amino acids, lipid oxidation products, intermediates of Maillard reactions, and compounds formed during thermal degradation of carbohydrates. The reaction mechanisms are very different for some of these reactions, and the molecular structure determines how efficiently a given polyphenol is involved in these reactions. This review will describe the chemical mechanisms for reactions of polyphenols with food components, and illustrate how choice of polyphenols with specific molecular structure can be used to control undesired reactions in foods. A discussion is included on other characteristics of polyphenols that need to be taken into consideration in order to avoid the introduction of new unwanted quality changes in foods. Polyphenols with o-diphenol structure oxidize in foods and the resulting o-quinone reacts fast with nucleophilic amino acid residues, especially with thiol groups, and may change protein functional properties. Polyphenols with hydroxyl groups in meta-position will favor reaction with carbonyls, but these types of reactions are slower than reactions between quinones and proteins.