Deactivation of riboflavin triplet-excited state by phenolic antioxidants: mechanism behind protective effects in photooxidation of milk-based beverages

The water-soluble plant polyphenols rutin, (+)-catechin, and (-)-epigallocatechin gallate were each found to be efficient quenchers for the triplet-excited state of riboflavin in aqueous solution using nano-second laser flash photolysis. The deactivation followed second-order kinetics with rates close to the diffusion control: (-)-epigallocatechin gallate (1.70 +/- 0.2x10(9) l mol(-1) s(-1)), (+)-catechin (1.44 +/- 0.04x10(9) l mol(-1) s(-1)), and rutin (9.7 +/- 0.2x10(8) l mol(-1) s(-1)) at 25 degrees C and pH = 6.8. No synergetic effects for mixtures of the phenolic compounds were found. Hydrogen atom transfer is calculated to be more exergonic than electron transfer, suggesting the hydrogen atom transfer mechanism to be operative. The efficient deactivation by the plant polyphenols makes this type of compounds of importance as protectors against light-induced oxidation in flavored milk-based beverages.