Reaction of Oxygen and Sulfite in Wine

When model wines that contain polyphenols are oxidized, hydrogen peroxide and quinones are produced. Sulfur dioxide reacts with the hydrogen peroxide, preventing ethanol oxidation by way of the Fenton reaction, and in the case of (+)-catechin, sulfite reduces the quinone near quantitatively back to the catechol. Consequently, the O-2:SO2 molar reaction ratio is close to 1: 2 in ideal experimental conditions. Here, eight wines (three red and five white wines) were studied to investigate whether this ratio might be similar in practice, so as to assess how effective SO2 might be as an antioxidant in real wine. The reaction ratio was found to be decreased down to 1: 1.7 in most wines. To determine the reason for this decrease, a white wine was treated with a large amount of benzenesulfinic acid. This substance reacts very efficiently with quinones and would therefore prevent their interaction with sulfite. The molar reaction ratio was then reduced to 1: 1, as has been previously observed in model wine. This result was taken to indicate that sulfite is fully effective in removing hydrogen peroxide and that the reduction in the molar reaction ratio from the theoretical 1: 2 ratio was due to limited interaction with polyphenol oxidation products. Two white wines, which were found to be rapidly oxidized with much reduced O-2:SO2 molar reaction ratios, were found to contain ascorbic acid. The effect of adding ascorbic acid to a white wine on the reaction of oxygen was therefore also examined.