Further evidence is presented that Fe in association with Cu is an essential catalyst in oxidative processes of wine. The model polyphenol, 4-methylcatechol (4-MeC), is shown not to react with oxygen at a significant rate without addition of these metals in model wine. Similarly, sulfite autoxidation, which is a radical chain reaction, is demonstrated to dependent on these metals. However, the free radical scavenging activity of polyphenols prevents free radical chain propagation, so that sulfite cannot react with oxygen directly but with the hydrogen peroxide that is produced when polyphenols are oxidized. The Fe-catalyzed autoxidation of (+)-catechin is markedly accelerated by Cu, which it is proposed facilitates the redox cycling of Fe. The addition of these metals to wine accelerates the reaction with oxygen, while their removal with potassium ferrocyanide slows oxidation and in white wine can prevent it entirely. In the autoxidation of (+)-catechin in model wine, the rate of reaction of SO2 depends on (+)-catechin concentration and is faster when (−)-epicatechin is oxidized. The dependence of the rate of reaction of SO2 on the catechol is further evidence that SO2 is reacting with the hydrogen peroxide it produces. However, by measuring the changes in concentration of (+)-catechin and (−)-epicatechin when oxidized, it is evident that sulfite largely reduces the quinones back to the original catechol. With (+)-catechin, sulfite and benzenesulfinic acid markedly accelerate oxygen consumption, indicating that the rate of oxidation of polyphenols depends on nucleophiles, which by reacting with quinones displace the reversible catechol-iron redox interaction to drive the overall process forward.
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