TY - JOUR T1 - Mechanism of Autoxidation of Polyphenols and Participation of Sulfite in Wine: Key Role of Iron JF - American Journal of Enology and Viticulture JO - Am. J. Enol. Vitic. DO - 10.5344/ajev.2011.10105 SP - ajev.2011.10105 AU - John C. Danilewicz Y1 - 2011/05/02 UR - http://www.ajevonline.org/content/early/2011/05/02/ajev.2011.10105.abstract N2 - The involvement of Fe in the reaction of wine with O2 was deduced at least 80 years ago. It was observed that when wine was exposed to air, some O2 reacted rapidly, part to oxidize the Fe to the ferric state and 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 re-examined in model wine to see 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, presumable 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’ is 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 is found to increase in the order (+)-catechin< caffeic acid< (−)-epicatechin < gallic and 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 shown to be oxidized extremely slowly. The initial faster phase of oxidation was also observed in a white wine. ER -