The rate of oxidation of SO₂ was studied in a wine-model system under aerial oxygen saturation conditions to gain further insight into its mechanism of action in wine. When SO₂ was studied alone, no significant oxidation was observed unless iron and copper were introduced. When these metals were added a slow oxidation was observed and bound SO₂ also increased to a small but significant extent. These results are consistent with a radical chain mechanism initiated by metal catalysis, in which powerful oxidizing radicals, capable of oxidizing ethanol to acetaldehyde, are produced. This increase in bound SO₂ is prevented by 4-methylcatechol (4-MeC) in keeping with the known ability of polyphenols to scavenge these intermediate radicals and thus to inhibit SO₂ autoxidation, which consequently should not occur in wine. When 4-MeC was introduced at a concentration that simulated the reducing capacity of red wine, again no significant SO₂ oxidation was observed without addition of iron and copper. Had the catechol been oxidized, hydrogen peroxide would have been generated and reacted with the SO₂. In the presence of both metals the rate of SO₂ oxidation was markedly increased compared to SO₂ alone and then was dependent on the concentration of the catechol. These results demonstrate the crucial importance of metals in allowing polyphenol oxidation and that the rate of SO₂ consumption is dependent on the rate of catechol oxidation. When iron and copper were added separately, only a modest increase in rate of catechol oxidation resulted. However, when combined, marked synergism was observed and the rate then became very sensitive to copper concentration. It is proposed that copper, by interacting with oxygen, facilitates redox cycling of iron. Exposure of a red wine to the conditions used in this study produced similar results regarding SO₂ oxidation to those observed.

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