PT - JOURNAL ARTICLE AU - John C. Danilewicz TI - Toward Understanding the Mechanism of Wine Oxidation AID - 10.5344/ajev.2021.21008 DP - 2021 Jun 08 TA - American Journal of Enology and Viticulture PG - ajev.2021.21008 4099 - http://www.ajevonline.org/content/early/2021/06/03/ajev.2021.21008.short 4100 - http://www.ajevonline.org/content/early/2021/06/03/ajev.2021.21008.full AB - The electronic configuration of oxygen (O2) does not allow it to react directly with wine reductants such as polyphenols. It relies on the catalytic intervention of iron (Fe), which redox cycles between its ferrous (Fe(II)) and ferric (Fe(III)) states. O2 oxidizes Fe(II) to Fe(III) and Fe(III) then oxidizes polyphenols. Low concentrations of Copper (Cu) accelerate oxidation, while nucleophiles, especially sulfite, promote polyphenol oxidation. Fe exists mainly as Fe(II) in wine that is protected from air, but the Fe(III):Fe(II) concentration ratio increases immediately on air exposure, stabilizing at varying speeds and values. To better understand the process, the oxidation of Fe(II) in air-saturated model wine, and Fe(III) reduction by a catechol under nitrogen in model wine, were examined separately. The Fe(III) produced when Fe(II) reacted with O2 slows the reaction. As in wine, it was important to include sulfite to remove the intermediate hydrogen peroxide, which also oxidizes Fe(II). The reaction was pseudosecond-order in Fe(II), indicating that the transfer of both electrons to O2 is rate determining. Similarly, when Fe(III) was reduced by the catechol, the Fe(II) produced inhibited the reaction, which overall followed a pseudosecond-order rate law in Fe(III). The rate of Fe(II) oxidation proved slower than the rate of Fe(III) reduction, but when the reaction occurred together, as in wine oxidation, Fe(III) and Fe(II) concentrations equilibrated such that their rates equalized, which under the conditions studied, occurred at 32% Fe(III). This equilibrium was quickly attained as in red wine. These additional findings on the oxidative process should help to explain the relationship between wine composition, redox state and Fe(III):Fe(II) concentration ratios.