Sulfur Dioxide and Glutathione Alter the Outcome of Microoxygenation

Cabernet Sauvignon wines with low and high levels of SO₂ and glutathione (GSH) were treated with microoxygenation (MOx) in a 23 L pilot experiment. Treatment generally increased O₂, aldehydes, and derived products while decreasing anthocyanins, vanillin reactive flavonoids, and SO₂. During the treatment, when free SO₂ was depleted in the low GSH wine, dissolved oxygen levels collapsed, with a concomitant increase of acetaldehyde, pyranoanthocyanins, polymeric pigments, and acetaldehyde acetals. This outcome indicates a possible acceleration of the Fenton oxidation of ethanol and other oxidation reactions by way of direct free radical reactions with oxygen. In wines containing high levels of GSH, anthocyanins were protected, revealing a protective effect for GSH for the first time. However, the protection was only partial, and while GSH may be effective in preventing the loss of volatile thiols, its use does not prevent color stabilization in red wines. Because both SO₂ and GSH are able to modulate the reactions initiated by MOx, but have somewhat dissimilar reactions, it may be possible to manage oxidation outcomes by choosing one or the other during wine processing and aging. Because SO₂ and O₂ levels are related to large differences in MOx reaction rates, those levels are candidate indicators of the rate of MOx oxidation. Alternatively, the levels of acetaldehyde acetals may be useful indicators of the cumulative extent of oxidation under MOx conditions.