Formation and fate of sulfur compounds associated with negative attributes in wine
The occurrence of volatile sulfur compounds (VSCs) is influenced by factors including yeast selection and fermentation conditions; the nature and quantity of precursor compounds; the availability or absence of oxygen at different points of the winemaking process; and the availability and speciation of transition metal ions such as copper. As defence against the formation of undesirable VSCs, especially hydrogen sulfide, winemakers typically rely on monitoring must-nitrogen, additions of diammonium phosphate and copper, and choice of closure and its oxygen transfer rate. However, despite these commonly used management practices, post-bottling VSC release and sulfur-related off-flavours, can still be observed in some wines.
In previous AWRI research, a number of obvious sulfur-containing precursors (thioacetates, s-methyl methionine, cysteine, DMSO, glutathione, methionine, DMDS and methionol) have been assessed for their role in releasing VSCs, and results indicate that many of these do not play a significant role in post-bottling VSC release. This suggests that other precursors are present which are yet to be identified. Sufficient observational experiments have been established across a range of wines, to show that the accumulation patterns of VSCs are relatively consistent post-bottling, but that they differ significantly between red wine and white wine. It remains to be established whether the accumulation patterns observed are driven only by release from precursors or, more likely, a combination of release and subsequent loss of VSCs by incorporation into other chemical forms (e.g. binding into quinones, or the formation of compounds such as disulfides or polysulfanes). In addition to the de-novo formation of VSCs, their accumulation might also reflect their stability and reactions causing their ‘loss’ in wine. Thus there remains a knowledge gap relating to novel precursors and their degradation mechanisms, which will be addressed by this project.
This project will also characterise the role of yeast in forming precursors for VSCs, and the biochemical pathways that affect VSC formation. The yeast biochemical pathways associated with release of hydrogen sulfide from cysteine have been established. However, recent work indicates that sulfate, for example, is a key sulfur source incorporated into ethanethiol, ethylthioacetate and diethyl disulfide, but, interestingly, not into methanethiol, methyl thioacetate or DMS. Research by international teams has also indicated that some vineyard interventions such as foliar spraying, have an impact on VSC formation, as does the addition of compounds like glutathione to must. Hence different classes of VSC are likely being derived from different pathways at different points of production, and therefore understanding VSC accumulation from fermentation through to bottle-aged wine, requires a multidisciplinary approach.
In summary, the project will develop an in-depth understanding of:
- the main precursors to sensorially important VSCs, with a focus on hydrogen sulfide, methanethiol, thioacetates, disulfides, polysulfanes and possibly DMS, phenylmethanethiol and others
- the metabolic and chemical pathways that lead to their formation and loss after formation
- the chemical and environmental factors which lead to otherwise innocuous sulfur-based compounds being converted to those that have a significant sensorial impact in wine.
Evaluating the formation of sulfur traps and latent sources of sulfur compounds
Hydrogen sulfide (H2S) can have a highly detrimental effect on wine quality, imparting aromas of ‘rotten egg’ and ‘sewage’ when present at concentrations above its aroma threshold (1.1 to 1.6 μg/L). Aerative remediation strategies, such as macro-oxygenation and racking/splashing techniques are successfully used by winemakers to remove H2S from wine. The mechanisms by which aerative treatments are thought to remove H2S include straightforward volatilisation of H2S or the formation of less detrimental oxidation products. Sulfhydryl compounds may also add to quinones produced when polyphenols are oxidised, and these adducts are presumed to be stable and unable to release H2S once bound (‘sulfur traps’). Aerative remediation techniques may also produce latent sources of H2S (compounds that can release H2S at a later time, such as diorganopolysulfanes).
To investigate the chemical mechanism through which aerative remediation strategies remove H2S, an experiment was designed to investigate the compounds formed when different remediation strategies were applied to a synthetic wine with ‘reduced’ character. Ferments were conducted in chemically defined grape juice media and treated with either aerative sparging (macro-oxygenation) or copper fining at the time when H2S was generated by the yeast in the ferments. A trace amount of a glutathione-based polysulfane was produced in wines treated with copper fining, as well as in wines treated with macro-oxygenation, and this compound may act as a latent source of H2S during storage. Larger concentrations of polyphenolic-sulfhydryl adducts (‘sulfur traps’) were produced in the wines treated with aerative sparging (Figure 17).
Influence of yeast strain on volatile sulfur compound formation
Commercial yeast manufacturers offer a wide range of yeast strains, but little information is known about their ability to modulate the formation of important VSCs such as benzyl mercaptan (associated with ‘struck flint’ character) in wine. During fermentation of a synthetic wine, it was shown that the formation of benzyl mercaptan was dependent on the metabolic activity of the wine yeast. The amount of benzyl mercaptan produced depended on the strain used, and correlated with the amount of H2S produced by the yeast during fermentation (Figure 18). These results suggest that H2S might be one of the precursors necessary for the formation of benzyl mercaptan.
The Ehrlich pathway is a major pathway describing amino acid catabolism and it explains the breakdown of methionine into methionol and methional. In a laboratory-scale experiment using synthetic wine, it was shown that yeast strains with an active Ehrlich pathway produced lower levels of the undesirable compounds methanethiol, methylthioacetate and methional. These strains could be chosen by winemakers to decrease the formation of off-flavours in wine.