Abstract
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.
Highlights
Wine oxidation is mediated by the redox cycling of iron between two oxidation states: the oxidation of iron(II) by oxygen and reduction of iron(III) by phenols
Factors influencing the rate of the redox reaction between iron(III) and phenols were investigated for the first time in wine conditions. 4-Methylcatechol and pyrogallol, models for ortho-dihydroxy and 1,2,3-trihydroxy phenols, respectively, differed in their ability to reduce iron(III), illustrating how reactivity can vary with molecular structure
The model nucleophile benzenesulfinic acid was required for thermodynamically unrestricted reduction of iron(III) to occur with either phenolic compound, indicating wine oxidation may be limited by the availability of nucleophiles, and their exhaustion may preclude further reactions
Summary
Wine oxidation is mediated by the redox cycling of iron between two oxidation states: the oxidation of iron(II) by oxygen and reduction of iron(III) by phenols. Iron speciation studies using excess oxygen to drive the oxidation of iron(II) showed that in most cases, the iron(II):iron(III) ratio stabilized before complete oxidation to iron(III) occurred (Danilewicz 2016, 2018, Nguyen and Waterhouse 2019); wine phenols should be able to reduce iron(III) at an observable rate that at least matches that of oxidation to effectively maintain a non-zero level of iron(II). Reduction potentials vary among different phenolic compounds (Kilmartin et al 2001, 2002, Danilewicz 2003, 2012), and studies conducted in non-wine matrices have demonstrated that the structure of phenolic compounds can affect their ability to complex with and reduce iron(III) (Moran et al 1997, Mira et al 2002)
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