Elusive Chemistry of Hydrogen Sulfide and Mercaptans in Wine

This paper summarizes, discusses, and complements recent findings about the fate of H₂S and methanethiol (MeSH) during wine storage. Analytical assays to determine free volatile sulfur compounds (VSCs) and brine-releasable (BR-) VSCs in combination with accelerated reductive (AR) aging and micro-oxygenation (MOX) assays allow characterizing the different categories of species able to produce H₂S and MeSH and the processes of interconversion. Each wine seems to contain a specific total amount of H₂S and MeSH distributed into free, metal-complexed, and oxidized forms (di and polysulfides) interconnected through reversible redox equilibria whose external expression is wine redox potential. Oxidation transforms all mercaptans likely into nonvolatile disulfides and hydrodisulfides. In anoxia, these molecules are spontaneously and quantitatively reduced back. The concomitant accumulation of major wine thiols would provoke complex dissociation and the release of free H₂S and MeSH. Additionally, total amounts can increase due to the metal-catalyzed desulfhydration of cysteine and methionine.