Elusive Chemistry of Hydrogen Sulfide and Mercaptans in Wine

Abstract

This paper summarizes, discusses, and complements recent findings about the fate of HS 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 HS and MeSH and the processes of interconversion. Each wine seems to contain a specific total amount of HS 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 HS and MeSH. Additionally, total amounts can increase due to the metal-catalyzed desulfhydration of cysteine and methionine.

Simplified measures of copper fractions in wine: colorimetric and filtration‐based approaches

“…Extensive research has been conducted on the role of Cu during reductive ageing of wine (Bekker et al 2016, 2018, Vela et al 2017). The reductive character of wine is associated with the sensory detection of lower molecular mass sulfur compounds in wine for which hydrogen sulfide and methanethiol tend to be the dominant compounds (Smith et al 2015, Ferreira et al 2018, Kreitman et al 2019). Many studies have shown that while Cu is added to wine by winemakers to remove sulfidic‐off odours, increased Cu addition to wine at bottling can lead to an increase in hydrogen sulfide concentration during wine ageing in many cases (Ugliano et al 2011, Viviers et al 2013, Vela et al 2017).…”

“…Past studies on the impact of Cu on volatile sulfur compounds during bottle ageing have tended to measure a combined fraction of both forms of hydrogen sulfide (Ugliano et al 2011, Viviers et al 2013) rather than the different forms (Vela et al 2017). The free hydrogen sulfide form is more relevant to that detected during wine sensory evaluation, while the brine‐releasable form is related to involatile metal sulfide complexes (Ferreira et al 2018). To date, only one study has investigated the links between the concentration of the different Cu forms with the free form of hydrogen sulfide (Kontoudakis et al 2019b).…”

Gas chromatographic-sulfur chemiluminescent detector procedures for the simultaneous determination of free forms of volatile sulfur compounds including sulfur dioxide and for the determination of their metal-complexed forms

“…Differences between model and real wines and between real wines should be related to the different types of structures and interactions in which H2S is involved in each wine. Recent evidences suggest that in the particular case of copper, nanoparticles of different sizes can be formed [41], and that the fraction of H2S in free forms depends on the wine redox potential [12,13]. Regarding incubation times, at 70C the maxima signals were in all cases obtained after 15-25 minutes, and were stable at least up to 35 min (data not shown).…”

“…interconnected via chemical equilibria: free forms, metal-complexed forms and mixed disulfides and polysulfanes, some of which act as oxidized precursors [12,13]. Only free forms are odor active.…”

Increasing the Efficiency and Accuracy of Labile Cu Measurement in Wine with Screen-Printed Electrodes

“…A wine with significant concentrations of hydrogen sulfide and methanethiol is sensorially labeled as "reduced", and this is more likely to occur in conditions of low oxygen, as found during bottle aging in screw capped bottles. The role of Cu in this process is complex as it can decrease the volatile component of hydrogen sulfide and thiol compounds when added to wine during wine production but can also lead to an increase in the total concentrations of these thiol compounds when the wine is stored in low oxygen conditions [3][4][5].…”