Flavanols react preferentially with quinones through an electron transfer reaction, stimulating rather than preventing wine browning.

Wine oxidation changes the chemistry, sensory profile and color of wines. In wine oxidation, phenolics are oxidized to quinones and these reactive compounds can be quenched by sacrificial nucleophiles, such as the A-ring on flavanoids, preventing oxidative damage from the loss of desirable flavor molecules. The "catechol" B-ring on flavanoids, in contrast, can be oxidized by quinones through electron transfer reactions that lead to flavanoid quinones, precursors of browning products. Here we compared the rate of flavanoids reacting by either nucleophilic quinone quenching, or by electron transfer to generate flavanoid quinones. Our approach is based on mathematical modeling of reaction data to derive the rate constants of reactions of A-ring quenching vs B-ring electron transfer with caffeic acid quinone, by fitting the predicted loss of precursors and the appearance of products (or derivatives) with experimental data collected by LC/MS. The rate constant of the electron transfer reaction of caffeic acid quinone towards 4-methyl-catechol was fast (k4MC  = 3.43E-2 mLmol-1sec-1) but nucleophilic reactions with afzelechin (kAfz  = 2.53E-3 mLmol-1sec-1) or malvidin-3-glucoside (kMal  = 5.34E-3 mLmol-1sec-1), were much slower. No reaction was detected between caffeic acid quinone and isorhamnetin. Additionally, the electron transfer reaction rate of catechin and caffeic acid quinone was much faster at pH 7 (1.22E-02 mLmol-1sec-1) vs pH 3.5 (1.79E-03 mLmol-1sec-1). These results help explain why the reaction of catechin and caffeic acid quinone favors the formation of browning products, and more so at higher pH values. Furthermore, bisulfite reacted with quinones faster than the electron transfer reactions, preventing the browning observed in the reaction of catechin with caffeic acid quinone in the absence of bisulfite.