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How are Volatile Sulphur Compounds formed?

The appearance of volatile sulphur compounds in wine is a complex, multifaceted chemical and biochemical process.1 Volatile sulphur compounds have been shown to derive directly from grapes, from the fermentation of grapes, and from chemical reactions in must and wine.2 Unlike most aroma compounds found in wine, they can exist in trace amounts in the grapes themselves,3 but they are much more likely to be produced by yeasts during fermentation, via a number of pathways that include the reduction of sulphur, sulphate, and sulphite as well as the degradation of sulphur containing amino acids.1

Sulphur can accumulate in must as a result of its use as a pesticide or fungicide in the vineyard, and the highly reductive conditions present during fermentation can facilitate its conversion into hydrogen sulphide.1,4 Sulphate and sulphite, which is used in the winemaking process as an antioxidant and antimicrobial agent, can also be present in the must and can also lead to the production of hydrogen sulphide during fermentation. But by far the main mechanism for the formation of volatile sulphur compounds is the degradation of sulphur containing amino acids by yeasts, as this leads to the production of both hydrogen sulphide as well as the higher sulphide compounds (e.g. dimethyl disulphide) and the mercaptans (e.g. methane thiol).

Yeasts have been shown to produce volatile sulphur compounds from the sulphur containing amino acids methionine, cysteine, and S-methylethionine.5 Nitrogen plays a key role in the formation of volatile sulphur compounds from these amino acids during fermentation. Yeasts need nitrogen to survive, and if they cannot find enough of it in the must, they will use amino acids as a nitrogen source. Unfortunately, in addition to nitrogen, some of these amino acids will also contain sulphur (the main ones being cysteine and methionine), and in metabolising these sulphur-containing amino acids to extract nitrogen, the yeasts will also form volatile sulphur compounds.

Chemical reactions in must and finished wine constitute the most complicated mechanism for the formation of volatile sulphur compounds. These reactions, which are even the subject of some disagreement, might more aptly be classified as transformations. As was mentioned in the article on sulphur taint, the volatile sulphur compounds are like chemical chameleons that can morph from one compound into another depending on the chemical environment they are in, and it is in this way that some of them first appear. The mercaptans, for instance, are readily oxidised into disulphides in the presence of oxygen, peroxide, or radiation (although this can occur in the absence of these catalysts as well). The disulphides, on the other hand, are readily reduced back into mercaptans if SO2 is present, especially under acid conditions (see the role of SO2 in wine), as the SO2 will eventually cleave the disulphide bond and leave a mercaptan behind.

The reactivity of the volatile sulphur compounds and the wealth of potential reactants present in must and wine make it very difficult to tell how a particular volatile sulphur compound was originally formed. Even if this were to be known, it would be impossible to eradicate them completely. That said, reductive winemaking conditions will certainly encourage their formation, and their emergence can indeed be curtailed by careful winemaking.

Check back soon for an article on the role of screwcaps in the formation of volatile sulphur compounds after bottling.

References

  1. Wine Chemistry and Biochemistry, M. V. Moreno-Arribas and C. Polo, Springer-Verlag New York, 2009, Chapter 8D.
  2. Evolution of Volatile Sulfur Compounds during Wine Fermentation, M. I. Kinzurik, M. Herbst-Johnstone, R. C. Gardner and B. Fedrizzi, Journal of Agricultural and Food Chemistry, 2015.
  3. http://wineserver.ucdavis.edu/industry/enology/fermentation_management/wine/off_characters.html
  4. Kinetics of the ethanethiol and diethyl disulphide interconversion in wine-like solutions, R. A. Bobet, A. C. Noble and R. B. Boulton, Journal of Agricultural and Food Chemistry, 1990.

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