TY - JOUR T1 - Redox Cycling of Iron: Effects of Chemical Composition on Reaction Rates with Phenols and Oxygen in Model Wine JF - American Journal of Enology and Viticulture JO - Am J Enol Vitic. DO - 10.5344/ajev.2021.20024 SP - ajev.2021.20024 AU - Thi H. Nguyen AU - Andrew L. Waterhouse Y1 - 2021/03/01 UR - http://www.ajevonline.org/content/early/2021/02/22/ajev.2021.20024.abstract N2 - Wine oxidation is mediated by the redox cycling of iron between two oxidation states: iron(II) is oxidized by oxygen and iron(III) is reduced by phenols. The effects of phenolic structure, pH, and copper on the rates of these reactions were evaluated in model wine. In the absence of a nucleophile, pyrogallol exhibited greater reactivity with iron(III) than 4-methylcatechol, though both compounds ultimately required aid from the nucleophile benzenesulfinic acid for unrestricted reduction of iron(III) to occur, illustrating the differential structure-dependent reactivities of phenols and the importance of nucleophiles to oxidation. It was hypothesized the rate of oxygen consumption would depend on the rate at which iron(II) is recycled from iron(III), though this was not found to be the case: while the rate of iron(III) reduction by 4-methylcatechol in the presence of benzenesulfinic acid decreased with higher pH, the opposite was observed for the rate of oxygen consumption. Furthermore, copper had no effect on the rate of iron(III) reduction, but significantly increased the rate of oxygen consumption, indicating the two reactions do not necessarily occur synchronously despite being coupled through iron. Pseudo-first order rate constants for oxygen consumption were much lower than those for iron(III) reduction, except when nucleophiles are absent, unlikely in wine, suggesting iron(II) oxidation is the rate-determining reaction for the wine oxidation pathway. Therefore, the rate at which wine ages is likely limited not by chemical composition, but by oxygen ingress, though the overall capacity of wine for oxidation may still depend on constituent phenols and nucleophiles, thus a method to assess these factors would be of interest. ER -