Toward Understanding the Mechanism of Wine Oxidation

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 accelerate oxidation, and nucleophiles, especially sulfite, promote polyphenol oxidation. In wine that is protected from air, Fe exists mainly as Fe(II), but the Fe(III):Fe(II) concentration ratio increases immediately on air exposure, stabilizing at varying speeds and values. The oxidation of Fe(II) in air-saturated model wine and the reduction of Fe(III) by a catechol under nitrogen in model wine were examined separately to better understand the oxidative process. 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 was slower than the rate of Fe(III) reduction, but when the reactions occurred together, as in wine oxidation, Fe(III) and Fe(II) concentrations equilibrated such that their rates equalized. Under the conditions studied, this occurred at 32% Fe(III). This equilibrium was attained quickly, as is the case in red wine. These findings on the oxidative process should help explain the relationships between wine composition, redox state, and Fe(III):Fe(II) concentration ratios.