Mechanism of Autoxidation of Polyphenols and Participation of Sulfite in Wine: Key Role of Iron

Abstract

The involvement of iron (Fe) in the reaction of wine with oxygen was deduced at least 80 years ago. It was observed that when wine was exposed to air, some O₂ reacted rapidly, in part to oxidize the Fe to the ferric state and in part to generate what were thought to be unstable peroxides. It was proposed that it was these intermediate oxidants that oxidized wine polyphenols. In this present work the oxidation of (+)-catechin was reexamined in model wine to determine if different phases of oxidation could be observed. In the absence of sulfite, after an initial uptake of O₂ very little further reaction occurred. This initial O₂ uptake increased with increasing concentrations of Fe(II) and Cu(II), indicating that Fe was the initial reactant and furthermore was absent when Fe was added as Fe(III). In contrast, in the presence of sulfite, the oxidation of (+)-catechin was markedly accelerated and, again, an initial more rapid phase of oxidation, presumably due to the metals, was discernable. With (+)-catechin, insufficient O₂ to oxidize all the Fe was initially taken up and formation of other oxidants such as peroxides was unlikely. Further evidence is presented that nucleophiles such as sulfite accelerate oxidation by reacting with quinones. The rate of oxidation in the presence of sulfite was found to increase in the order (+)-catechin< caffeic acid< (−)-epicatechin < gallic. It is proposed that the rate of oxidation is determined by the relative reactivity of their respective quinones. In the absence of sulfite, these additional phenols were also oxidized extremely slowly. The initial faster phase of oxidation was also observed in a white wine.