Reaction Mechanisms of Metals with Hydrogen Sulfide and Thiols in Model Wine. Part 1: Copper-Catalyzed Oxidation

Sulfidic off-odors as a result of hydrogen sulfide (H₂S) and low-molecular-weight thiols are commonly encountered in wine production. These odors are usually removed by the process of Cu(II) fining, a process that remains poorly understood. The present study aims to elucidate the underlying mechanisms by which Cu(II) interacts with H₂S and thiol compounds (RSH) under wine-like conditions. Copper complex formation was monitored along with H₂S, thiol, oxygen, and acetaldehyde concentrations after the addition of Cu(II) (50 or 100 μM) to air-saturated model wine solutions containing H₂S, cysteine, 6-sulfanylhexan-1-ol, or 3-sulfanylhexan-1-ol (300 μM each). The presence of H₂S and thiols in excess to Cu(II) led to the rapid formation of ∼1.4:1 H₂S/Cu and ∼2:1 thiol/Cu complexes, resulting in the oxidation of H₂S and thiols and reduction of Cu(II) to Cu(I), which reacted with oxygen. H₂S was observed to initially oxidize rather than form insoluble copper sulfide. The proposed reaction mechanisms provide insight into the extent to which H₂S can be selectively removed in the presence of thiols in wine.