Mechanism of Interaction of Polyphenols, Oxygen, and Sulfur Dioxide in Model Wine and Wine

The interaction of oxygen, sulfur dioxide, and 4-methylcatechol (4-MeC) was studied in a model wine containing catalytic concentrations of iron and copper in order to provide further evidence that when a catechol and oxygen interact, hydrogen peroxide and a quinone are formed, both of which react with SO₂. The aerial oxidation of the catechol in the presence of benzenesulfinic acid (BSA) slowly produced the BSA-quinone adduct in high yield. It was also quickly prepared by adding ferric chloride, demonstrating that the quinone is cleanly produced in this model wine and that the catechol is rapidly oxidized by Fe(III) ions. This reaction is important in the catalytic function of the metal. The oxygen and SO₂ molar reaction ratio was 1:2, which is consistent with one mole equivalent of SO₂ reacting with hydrogen peroxide and a second with the quinone. When BSA was added to the system to trap the quinone the ratio was reduced to 1:1. The rate of reaction of oxygen and SO₂ increased with catechol concentration. However, the rate of reaction of oxygen was also markedly accelerated by SO₂ and by BSA, and it is proposed that substances that react with quinones accelerate catechol autoxidation. When 4-MeC was oxidized in the presence of SO₂, ~38% of the quinone that was formed reacted with bisulfite to produce the sulfonic acid adduct and most of the remainder was reduced back to the catechol. The O₂/SO₂ molar reaction ratio in two red wines was 1:~1.7, suggesting that some nucleophilic substances may be competing with bisulfite for quinones. The rate of reaction of oxygen was also accelerated by SO₂ in red wine.