Review of Reaction Mechanisms of Oxygen and Proposed Intermediate Reduction Products in Wine: Central Role of Iron and Copper

The chemical, biochemical, and enological literature has been broadly surveyed to identify the reaction mechanisms of oxygen and of its intermediate reduction products that should apply to wine. The reduction potentials of redox couples derived from wine polyphenols and oxygen, as well as that of the Fe3+/Fe2+ couple, have been calculated for wine conditions and form the basis for discussing how these redox systems are likely to interact. Values obtained for wine quinone/catechol couples agree well with those reported for wine-model conditions. Catechol derivatives are oxidized sequentially to semiquinones and quinones, while oxygen is reduced in turn to hydroperoxyl radicals and hydrogen peroxide. The whole process is mediated by redox cycling of the Fe3+/Fe2+ couple, which is made possible by the lowering of its reduction potential by coordination of Fe3+ to hydroxy acids. Hydrogen peroxide is then further reduced by Fe2+ in the Fenton reaction to produce hydroxyl radicals, which oxidize saturated hydroxy compounds. Intermediate radicals may react with oxygen, providing an additional pathway for its reduction. Thus, both ferric and ferrous ions, which are present in wine, perform an important catalytic function. The antioxidant activity of bisulfite is largely restricted to its reaction with hydrogen peroxide. Direct reaction of sulfur dioxide with oxygen, which is a radical chain process, is prevented by the radical scavenging activity of polyphenols.