Formation and fate of positive and negative sulfur compounds
Volatile sulfur compounds (VSCs) can contribute both positive and negative attributes to wines, and their control in a winery environment is an important avenue to increasing wine value either by increasing positive sensory attributes or through the reduction of negative characters. The occurrence of VSCs can be 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 availability and speciation of transition metal ions such as copper.
Winemakers are familiar with the practice of adding copper sulfate to wines that exhibit ‘rotten egg’ or other ‘reduced’ characters when in tank. Many winemakers have also seen these ‘reduced’ characters disappear from their wines in the short term, only to see them return at a later date, sometimes after bottling. By exploring the chemistry of VSC formation and the important role played by metals, these common winemaking observations can be better understood, potentially leading to recommendations for ways to reduce the risk of undesirable ‘reduced’ aromas.
Origin of volatile sulfur compounds
It is generally considered that yeast metabolism is the underlying source of most sulfur-based volatile compounds in wine, either through the production of precursors or of the compounds themselves. Fermentations conducted using 100 yeast strains in a synthetic grape juice and a red must as part of the AWRI’s yeast research were analysed for low molecular weight sulfur compounds and volatile thiols. The analytical results, combined with genomic data on these yeast strains, revealed that the potential of a particular strain to release the volatile thiols 3MH and 4MMP from cysteine-bound precursors correlates with certain alleles (versions) of the gene IRC7. The majority (75%) of yeast strains analysed were classified as low thiol releasers, and it was established that the cause for this phenotype was the presence of inactivating mutations in the IRC7 gene.
Managing the nature of copper in wine
It has commonly been thought that when sulfides in wine interact with copper, they simply precipitate out and are removed from the wine through racking and filtration. Recent work by collaborators at Charles Sturt University has shown, however, that this is not necessarily the case. This finding, along with increased understanding of the risks of residual copper species in wine, has made it important to gain a more thorough understanding of the nature and role of copper sulfides formed in wine.
Many wine compounds have the capacity to bind copper, including organic acids (such as tartaric acid), tannins, polyphenols, and VSCs. The binding of copper to these various compounds may affect the particle size of the copper-complexes, which in turn may affect the binding sites of copper that are available to catalyse the formation of VSCs. To investigate this, the impacts of different ratios of H2S and copper in model wine were studied using nanoparticle tracking analysis (NTA).
A decrease in the mean particle diameter was seen as the ratio of H2S to copper increased. Furthermore, the copper-tartrate complex concentration increased with H2S addition. At equimolar concentrations of H2S and copper the particle size was smaller than in the other treatments, suggesting strong and uniform binding of H2S and copper that prevented copper from further interacting with other compounds to produce larger particles. As the ratio of copper was increased in relation to H2S, the particle size increased, suggesting that the unbound copper was available to interact with other compounds (in this case tartaric acid) to form larger particles. Further studies are needed to determine whether the smaller particle size is associated with lower catalytic ability, that is if all the binding sites of copper are occupied and prevented from further interaction with other wine compounds.
The impact of pH on complex formation was also investigated. Lower pH decreased copper-tartrate complex size and lower particle concentrations were measured when compared to copper-tartrate complexes produced at higher pH. The difference in particle size and concentration of copper-tartrate complexes suggests that the type of copper-tartrate complexes that are produced at varying pH levels may affect the binding sites of copper that are available to either catalyse the formation of VSCs such as H2S, or quench the thiols produced to form copper sulfide complexes. Current work has shown that wine pH, copper additions and the interaction between pH and copper significantly influence VSC formation in Chardonnay and Shiraz wines post-bottling.
The chemical speciation of copper in wine
A collaborative project with Charles Sturt University (CSU) investigated the speciation of the copper found in a range of 52 commercially available wines. Using anodic stripping voltammetry Dr Andrew Clark and co-workers at CSU were able to show that levels of electrochemically labile copper as low as 25 µg/L correlated with the inhibition of free H2S. While confirming the ability of copper to bind H2S, the study also showed that in the vast majority of the wines tested the copper was present in non-labile forms and was not available to inhibit the formation of free H2S. This correlates with earlier findings that residual copper in wine rarely serves to inhibit the formation of volatile sulfur compounds and in many cases appears to facilitate it.
The impact of timing of copper additions
Copper additions made immediately before bottling were found to lead to significant increases in H2S 12 months after packaging in comparison to equivalent additions made directly after fermentation. Interestingly, these differences did not become evident until the 12-month time point, with earlier time points showing essentially equivalent results for both treatments. This work highlights the weakness of traditional benchtop trials used to determine copper additions, where only the immediate effects are assessed and not the long-term impacts in-bottle.
Methods for managing the impacts of copper
While the risks of residual copper in packaged wine are becoming increasingly apparent, copper is still an important tool for winemakers dealing with volatile sulfur characters. For this reason it is important to identify ways to manage residual copper after it has been added to wine. One possibility is to bind copper to chelating compounds that completely remove it from the wine matrix or chemically isolate it so that it can no longer participate in the formation of reductive characters. To investigate this, a range of possible chelating agents were trialled in wine. A commercial polyphenol extract was found to suppress H2S formation at the 12-month time point in comparison to controls. As seen in the trial on the timing of copper additions, this suppression did not become evident until the 12-month time point, suggesting an interesting interaction of polyphenols, metals and other wine components occurring over time in bottle. Ethylene diamine tetraacetic acid, a common chelation agent used in food production, also demonstrated significant effects both at equimolar concentrations (to copper and iron) and in excess. The results suggest that differing metal chelation environments can be present and that these can have a significant effect on the mechanism of H2S generation.
Toni Garcia Cordente