Disulfides

Mercaptans such as methanethiol (methyl mercaptan) and ethanethiol (ethyl mercaptan) can be rapidly oxidized to dimethyldisulfide (DMDS) and diethyldisulfide (DEDS), respectively. The aroma of DMDS is described as ‘onions’ and ‘cooked cabbage’. The aroma of DEDS is described as ‘burnt rubber’ and ‘garlic’.

Mercaptans can oxidize to form disulfides when exposed to oxygen. These new compounds smell like garlic, canned asparagus, burnt rubber, and onion, and are almost impossible to eliminate. The chemical change induced by the oxidation from mercaptan to disulfides increases their sensory threshold and changes their ability to bind to copper. Therefore, while mercaptans react with copper, their oxidized form doesn’t. Disulfides can be reduced back to mercaptans, then bound with copper. This is the main concept of using ascorbic acid in combination with copper sulphate or copper citrate as a treatment. Disulfides are first reduced with the addition of 50 mg/L or more of ascorbic acid, immediately followed by an appropriate addition of copper. This reaction can take a couple months and it is important to make sure that free SO2 levels are adequate before adding ascorbic acid, which can increase the potential for wine oxidation.

For more info about how to remove this compound from wine or other beverages, see Hydrogen sulfide screening and Hydrogen sulfide removal