Studies in wine and model systems have established that iron is an essential catalyst that mediates the reaction of polyphenols with oxygen. This investigation examined how this metal exerts its action. When wine is protected from air, iron exists in its reduced ferrous state, Fe(II), which is rapidly oxidized to the ferric state, Fe(III), on exposure to oxygen. This rapid transformation is observed when Fe(II) is added to model wine saturated with aerial oxygen, but the reaction slows to a very slow rate before completion due to the inhibitory action of Fe(III). 4-Methylcatechol was found not to be oxidized by Fe(III) or as Fe(II) was reacting with oxygen. It was apparent, therefore, that the catechol did not react with intermediate oxygen radicals. Consequently, it is proposed that hydroperoxyl radicals are not produced in wine conditions and a revised mechanism for the reaction of Fe(II) with oxygen to produce hydrogen peroxide is proposed. However, sulfite addition, which is known to promote catechol oxidation, resulted in rapid Fe(III) reduction and attainment of an Fe(III)/Fe(II) redox equilibrium. Benzenesulfinic acid, which does not react with oxygen, produced the same effect and it is proposed that nucleophiles, which react rapidly with quinones, allow the oxidation of catechols to proceed. Examination of the Fenton reaction showed that the reaction of Fe(II) with hydrogen peroxide was rapid and resulted in the uptake of oxygen. A comparison of the rates of Fe(II) oxidation and Fe(III) reduction in the presence of different ligands showed a dependence on reduction potentials.
- ©2013 by the American Society for Enology and Viticulture