Reaction Mechanisms of Oxygen and Sulfite in Red Wine

Abstract

Studies on the aerial oxidation of a simple catechol and also (+)-catechin in model wine have shown that the SO₂:O₂ molar reaction ratio is 2:1. One mole equivalent of sulfur dioxide (SO₂) reacts with the hydrogen peroxide (H₂O₂) that is produced, and the second with the quinone. However, more recent investigations in real wine have found that these ratios can be much lower, suggesting that SO₂ may be a much less effective antioxidant in practice. This study was therefore undertaken to examine the aerial oxidation of six red wines, looking closely at the uptake of O₂ and SO₂ at different starting SO₂ concentrations. Low ratios were indeed found in some wines initially, but as oxidation proceeded, the ratio increased and finally, given sufficient time, total ratios approached or were ~2:1. The effect was accentuated with low starting SO₂ concentrations when its availability was limited. It has been proposed that simple quinones are reduced by first adding sulfite, and that this adduct then hydrolyzes to release catechol and sulfate. On the basis of this mechanism, it is proposed that real wine contains polyphenols that form more stable initial adducts, which decompose more slowly. Consequently, SO₂ is released from these initial intermediate adducts when total SO₂ concentration is determined and so appears not to have reacted, resulting in low ratios. Given sufficient time, all the adduct decomposes, releasing sulfate, and the ratio increases to 2:1. It is further proposed that SO₂ still reacts with H₂O₂, as in model wines. The apparent molar reaction ratio therefore should not fall below 1:1, and ratios lower than this were not found here, contrary to previous reports.