Formation and fate of sulfur compounds associated with negative attributes in wines
Abstract
Volatile sulfur compounds (VSCs) are important contributors to wine style, as they impart both varietal aromas and aromas associated with ‘reductive’ aromas, which account for approximately 30% of all wine faults and have significant economic implications. Many aspects of the formation and fate of VSCs were investigated in this project, including chemical and biochemical formation pathways, new analytical methods and the development of novel smart surfaces. Each aspect contributed to improved understanding of how VSCs are produced, how to modulate their formation through winemaking practices, and ultimately, how to remediate unwanted VSCs.
Summary
Volatile sulfur compounds are a group of compounds that contribute significantly to wine aroma. These compounds are associated with important varietal character and stylistic expression through their contribution to ‘passionfruit’, ‘tropical’, ‘blackcurrant’ and ‘struck flint’ aromas. The latter is associated with premium aroma character present in high-quality white and sparkling wines. VSCs are also associated with negative ‘reductive’ aromas, such as ‘rotten egg’, ‘rubber’, ‘sewage’ and ‘canned corn’. The management of ‘reductive’ off-aromas is one of the main concerns for the wine sector, as these aroma faults make up to one-third of all faults in wine and result in economic losses (Goode et al. 2008).
This project aimed to develop an in-depth understanding of the role of compounds suspected to be the main precursors of sensorially important VSCs, with a focus on negative sensory characters deriving from hydrogen sulfide (H2S), methanethiol (MeSH), methylthioacetate (MeSAc), and the generally positive ‘tropical’ sensory characteristics of 3-sulfanylhexan-1-ol (3-SH or 3-MH) and 3-sulfanylhexyl acetate (3-SHA or 3-MHA). The project was also focused addressing the knowledge gap surrounding phenylmethanethiol (PMT), also known as benzyl mercaptan, a compound that imparts ‘struck flint’ aroma, which is associated with premium sensory character in white and sparkling wines. The project sought to understand the metabolic and chemical pathways that lead to the formation of these sulfur compounds, and the chemical and environmental switches that lead to otherwise innocuous sulfur-based compounds being converted to those that have a significant sensory impact. This knowledge can be implemented to provide practical advice to the wine sector for the development of practical strategies to control the expression of positive and negative VSCs in wine.
To be able to study these highly volatile and reactive sulfur compounds and evaluate their speciation precursors and other latent forms of VSCs, six methods were developed that allowed for the quantification of 34S-labelled and 32S-non labelled VSCs, their precursors and breakdown products. These methods were critical in supporting experiments elucidating the biochemical and chemical formation pathways of VSCs, as well as determining the role of copper and VSC speciation in the perception of ‘reductive’ aromas.
A significant finding of this project was establishing the importance of polysulfides as latent sources of ‘reductive’ aromas in wine post-bottling. The project demonstrated their stability and susceptibility to decomposition and liberation of H2S when exposed to sulfur dioxide or ascorbic acid, two reducing agents that are commonly used in the winemaking process. This work addressed the knowledge gap of identifying latent sources of VSCs that are responsible for the post-bottling formation of VSCs. Follow-up studies investigated conditions that would promote the formation of polysulfides or promote the formation of irreversible polyphenolic adducts with VSCs. These experiments aimed to elucidate the mechanisms that drive the success of aerative remediation techniques, such as splashy racking and macro-oxygenation. These techniques have previously been considered to pose a risk of producing latent sources of VSCs, such as disulfide or polysulfide species. Experiments in this project showed that stable oxidation reaction products with polyphenolic compounds were preferentially produced after macro-oxygenation and that ‘reductive’ aromas were permanently removed from the experimental wines. This work identified the mechanism that drives the long-term effectiveness of macro-oxygenation in removing unwanted VSCs that had been observed in previous AWRI studies.
This project also identified genetic pathways that favour the formation of VSCs. Genes that play a pivotal role in H2S, 3-SH and 4-methyl-4-sulfanylpentan-2-one (4-MSP or 4-MMP) expression, as well as pathways that modulate MeSH, MeSAc and methional formation, were identified. The expression of tropical aroma in red wine was also investigated, and strains were identified that were high producers of the desirable tropical sulfhydryl compound 3-SH, as well as low producers of the undesirable VSCs, MeSH, MeSAc and methional. This work provided evidence for yeast strain selection to modulate the formation of VSCs and provided information to winemakers to aid their selection of yeast strains to control the expression of positive or negative flavour profiles of their wine.
Commercial yeast manufacturers offer a wide range of yeast strains, but little is known about their ability to modulate the formation of PMT. This compound is associated with ‘struck flint’ character, which is a sought-after aroma character for certain white wine styles. This project addressed the large knowledge gap regarding yeast selection and winemaking conditions that promote the formation of PMT, as well as factors that affect the preservation of PMT once it has been produced. This work identified yeast strains that are high PMT producers, identified the dependence of PMT formation on the presence and timing of H2S production, the important role of benzaldehyde as a precursor to PMT, and demonstrated the critical role of nitrogen levels in the fermentation media on PMT formation by yeast. The studies also demonstrated that oxygen and elevated concentrations of copper and iron negatively affected PMT formation by decreasing concentrations of its precursor, H2S. Factors that influence the preservation of PMT, such as residual metal concentrations and elevated SO2 levels, were also identified.
To understand the prevalence of PMT in Australian wine and establish consumer preference for wines that contain ‘struck flint’ aroma, a targeted survey of PMT was conducted in a range of commercially available white wines. This enabled the natural variability of PMT in wine to be determined, as well as the relationship between increasing concentrations of PMT and the transition from ‘flint’ aroma to ‘sulfurous’/‘plastic’ aroma at higher concentrations. An important finding of this trial was that both PMT and 2-furanmethanethiol (2-FMT) were associated with ‘struck flint’ aroma. A follow-up odorant addition study showed that 2-FMT exerted a 20 times larger effect on ‘struck flint’ aroma than PMT in Chardonnay wine.
Winemakers use various remediation strategies to manage ‘reductive’ aroma formation; however, the relative effectiveness of these techniques has not previously been evaluated. An experiment was designed to evaluate the effectiveness of five commonly used ‘reductive’ aroma remediation strategies to provide the wine sector with clear guidelines and practical advice on which remediation strategy would be best suited for their requirements. This work demonstrated that macro-oxygenation during fermentation was the most effective strategy over a period of 12 months for ameliorating ‘reductive’ character when compared to diammonium phosphate (DAP) addition, copper fining and lees addition. Macro-oxygenation also produced wines with increased ‘red fruit’ aroma and flavour, compared to the control. The use of tannin products in removing the appearance of ‘reductive’ faults was also evaluated, following anecdotal evidence that oenological tannin products can ‘freshen’ up ‘reductive’ wine. Experiments were performed to explore the effectiveness of this treatment and the mechanism by which it removes VSCs, with results showing that H2S, MeSH, and ethanethiol (EtSH) were effectively removed from the wine through the formation of oxidation reaction products with tannins.
A novel smart technology was developed which was successful in removing up to 45% of free H2S. The smart surface was also more effective in removing free MeSH compared to copper fining. This technology has the potential to overcome the limitations of copper addition currently used to remove unpleasant ‘reductive’ aromas.