Loss and formation of malodorous volatile sulfhydryl compounds during wine storage.

Volatile sulfur compounds (VSCs), particularly low molecular weight sulfhydryls like hydrogen sulfide (H2S) and methanethiol (MeSH), are often observed in wines with sulfurous off-aromas. Recent work has shown both H2S and MeSH can increase up to a few µM (> 40 µg/L) during anoxic storage, but the identity of the latent sources of these sulfhydryls is still disputed. This review critically evaluates the latent precursors and pathways likely to be responsible for the loss and formation of these sulfhydryls during wine storage based on the existing enology literature as well as studies from food chemistry, geochemistry, biochemistry, and synthetic chemistry. We propose that three precursor classes have sufficient concentration and metastability to serve as latent sulfhydryl precursors in wine: 1) transition metal-sulfhydryl complexes, particularly those formed following Cu(II) addition, which are released under anoxic conditions through an unknown mechanism; 2) asymmetric disulfides, polysulfanes, and (di)organopolysulfanes formed through transition-metal mediated oxidation (e.g., Cu(II)) of sulfhydryls or pesticide degradation, and released through sulfitolysis, metal-catalyzed thiol-disulfide exchange or related reactions; 3) S-alkylthioacetates, primarily formed during fermentation, and releasable hydrolytically. Some evidence also exists for S-amino acids serving as precursors. Based on these findings, we propose a "decision tree" approach to choosing appropriate strategies for managing wines with sulfurous off-aromas.

Liberation of Hydrogen Sulfide from Dicysteinyl Polysulfanes in Model Wine.

Diorganopolysulfanes can be generated when hydrogen sulfide (H2S) and thiols are oxidized in the presence of Cu(II) under conditions usually aimed at removing H2S from wine. This work sought to understand if polysulfanes could act as latent sources of H2S during postbottling storage. The stability of the polysulfanes formed in situ in model wine containing cysteine, H2S, and transition metals was dependent both on the number of sulfur linking atoms (Sn) and on the presence of a reducing agent, such as sulfur dioxide or ascorbic acid. A polysulfane containing three linking sulfur atoms was the most stable, with 84% of the relative initial amount remaining in solution after six months, compared to polysulfanes containing four or more linking sulfur atoms that decomposed rapidly, with 26% remaining after six months. Importantly, sulfur dioxide was associated with the rapid degradation of polysulfanes and subsequent liberation of H2S. Three cysteine- S-sulfonates were also tentatively identified, which gives insight into the possible release mechanisms involved with H2S reappearance.

Copper(II)-Mediated Hydrogen Sulfide and Thiol Oxidation to Disulfides and Organic Polysulfanes and Their Reductive Cleavage in Wine: Mechanistic Elucidation and Potential Applications.

Fermentation-derived volatile sulfur compounds (VSCs) are undesirable in wine and are often remediated in a process known as copper fining. In the present study, the addition of Cu(II) to model and real wine systems containing hydrogen sulfide (H2S) and thiols provided evidence for the generation of disulfides (disulfanes) and organic polysulfanes. Cu(II) fining of a white wine spiked with glutathione, H2S, and methanethiol (MeSH) resulted in the generation of MeSH-glutathione disulfide and trisulfane. In the present study, the mechanisms underlying the interaction of H2S and thiols with Cu(II) is discussed, and a prospective diagnostic test for releasing volatile sulfur compounds from their nonvolatile forms in wine is investigated. This test utilized a combination of reducing agents, metal chelators, and low-oxygen conditions to promote the release of H2S and MeSH, at levels above their reported sensory thresholds, from red and white wines that were otherwise free of sulfidic off-odors at the time of addition.

Removal of fumonisin B1 and B2 from model solutions and red wine using polymeric substances.

Fumonisins are a group of mycotoxins found in various foods whose consumption is known to be harmful for human health. In this study, we evaluated the ability of three polymers (Polyvinylpolypyrrolidone, PVPP; a resin of N-vinyl-2-pyrrolidinone with ethylene glycol dimethacrylate and triallyl isocyanurate, PVP-DEGMA-TAIC; and poly(acrylamide-co-ethylene glycol-dimethacrylate), PA-EGDMA) to remove fumonisin B1 (FB1) and fumonisin B2 (FB2) from model solutions and red wine. Various polymer concentrations (1, 5 and 10mgmL-1) and contact times (2, 8 and 24h) were tested, with all polymers exhibiting fumonisin removal capacities (monitored by LC-MS). The impact of all polymers on polyphenol removal was also assessed. PA-EGDMA showed to be the most promising polymer, removing 71% and 95% of FB1, and FB2, respectively, with only a 22.2% reduction in total phenolics.

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.

Reaction Mechanisms of Metals with Hydrogen Sulfide and Thiols in Model Wine. Part 2: Iron- and Copper-Catalyzed Oxidation.

Sulfidic off-odors arising during wine production are frequently removed by Cu(II) fining. In part 1 of this study ( 10.1021/acs.jafc.6b00641 ), the reaction of H2S and thiols with Cu(II) was examined; however, the interaction of iron and copper is also known to play an important synergistic role in mediating non-enzymatic wine oxidation. The interaction of these two metals in the oxidation of H2S and thiols (cysteine, 3-sulfanylhexan-1-ol, and 6-sulfanylhexan-1-ol) was therefore examined under wine-like conditions. H2S and thiols (300 µM) were reacted with Fe(III) (100 or 200 µM) alone and in combination with Cu(II) (25 or 50 µM), and concentrations of H2S and thiols, oxygen, and acetaldehyde were monitored over time. H2S and thiols were shown to be slowly oxidized in the presence of Fe(III) alone and were not bound to Fe(III) under model wine conditions. However, Cu(II) added to model wine containing Fe(III) was quickly reduced by H2S and thiols to form Cu(I) complexes, which then rapidly reduced Fe(III) to Fe(II). Oxidation of Fe(II) in the presence of oxygen regenerated Fe(III) and completed the iron redox cycle. In addition, sulfur-derived oxidation products were observed, and the formation of organic polysulfanes was demonstrated.

Effect of metal chelators on the oxidative stability of model wine.

Oxidation is a major problem with respect to wine quality, and winemakers have few tools at their disposal to control it. In this study, the effect of exogenous Fe(II) (bipyridine; Ferrozine) and Fe(III) chelators (ethylenediaminetetraacetic acid, EDTA; phytic acid) on nonenzymatic wine oxidation was examined. The ability of these chelators to affect the formation of 1-hydroxyethyl radicals (1-HER) and acetaldehyde was measured using a spin trapping technique with electron paramagnetic resonance (EPR) and by HPLC-PDA, respectively. The chelators were then investigated for their ability to prevent the oxidative loss of an important aroma-active thiol, 3-mercaptohexan-1-ol (3MH). The Fe(II)-specific chelators were more effective than the Fe(III) chelators with respect to 1-HER inhibition during the early stages of oxidation and significantly reduced oxidation markers compared to a control during the study. However, although the addition of Fe(III) chelators was less effective or even showed an initial pro-oxidant activity, the Fe(III) chelators proved to be more effective antioxidants compared to Fe(II) chelators after 8 days of accelerated oxidation. In addition, it is shown for the first time that Fe(II) and Fe(III) chelators can significantly inhibit the oxidative loss of 3MH in model wine.

Investigation of ethyl radical quenching by phenolics and thiols in model wine.

In the present study, the reaction between 1-hydroxyethyl radicals (1-HER) and various wine-related phenolics and thiols, including gallic acid, caffeic acid, ferulic acid, 3-mercaptohexan-1-ol (3MH), cysteine (Cys), and glutathione (GSH), was studied using competitive spin trapping with electron paramagnetic resonance (EPR) and mass spectrometry. Previous studies have reported several important reactions occurring between quinones and other wine components, but the fate of 1-HER within the context of wine oxidation is less understood. Furthermore, the ability of these compounds to prevent formation of acetaldehyde, a known nonenzymatic oxidation product of ethanol, was measured. The hydroxycinnamic acids and thiol compounds tested at 5 mM concentrations significantly inhibited spin adduct formation, indicating their reactivity toward 1-HER. In addition, we confirm that loss of 3MH under model wine conditions is due to quinone trapping as well as 1-HER-induced oxidation.