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

Abstract

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.

Hydrogen sulfide in physiology and pathogenesis of bacteria and viruses

“…Interestingly, a wine‐like model was used to study the reaction mechanism of metals with H 2 S. Sulfidic off‐odors encountered during wine production are due to the presence of H 2 S and low‐molecular‐weight thiols . These off‐odors are usually removed in a process called Cu fining, wherein Cu (II) is added to selectively and rapidly form ∼1.4:1 H 2 S/Cu and ∼2:1 thiol/Cu complexes, resulting in oxidation of H 2 S and reduction of Cu (II) to Cu (I) . The CuS precipitate formed is than subsequently removed from the wine by racking and/or filtration .…”

“…These off‐odors are usually removed in a process called Cu fining, wherein Cu (II) is added to selectively and rapidly form ∼1.4:1 H 2 S/Cu and ∼2:1 thiol/Cu complexes, resulting in oxidation of H 2 S and reduction of Cu (II) to Cu (I) . The CuS precipitate formed is than subsequently removed from the wine by racking and/or filtration .…”

The effects of copper fining on the wine content in sulfur off-odors and on their evolution during accelerated anoxic storage

“…In addition, recent reports demonstrate that the interaction between copper and H 2 S and wine mercaptans is more complex than a simple precipitation equilibrium. Apparently, sulfide and mercaptans form complexes with copper in which there is a charge transfer so that copper is reduced to Cu(I) and S(-II) is oxidized to S(-I) (Kreitman, Danilewicz, Jeffery, & Elias, 2016a). These species can further coordinate, release disulfides and form large clusters some of which could remain in solution.…”

Methanogenic pathways and δ13C values from swine manure with a cavity ring‐down spectrometer: Ammonia cross‐interference and carbon isotope labeling

“…An MFC was used to continuously dose 100 mL N 2 min –1 through the headspace of a DURAN flask (Hounisen, Skanderborg, Denmark) containing 100 mL of pre‐incubated (6 months) swine manure at room temperature. The outlet gas flowed into a 200 mL aqueous 0.1 M CuCl 2 solution (CAS 7447‐39‐4, Sigma Aldrich, St. Louis, MO, USA), which converts H 2 S and methanethiol (CH 3 SH) into CuS . The gas stream was then directed through a 90 cm Nafion tube with inner diameter of 1.1 mm and a constant counter‐current air flow rate of ~ 3 L min −1 .…”

“…An MFC was used to continuously dose 100 mL N 2 min - which converts H 2 S and methanethiol (CH 3 SH) into CuS. 33,34 The gas stream was then directed through a 90 cm Nafion tube with inner diameter of 1.1 mm and a constant counter-current air flow rate of3 L min −1 . Nafion is a fluorocarbon polymer with sulfonic acid groups and removes gases that can function as bases, including water vapor.…”