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The Mercaptans

The Mercaptans: Ethanethiol and Methanethiol

The thiols are the most foul smelling of the volatile sulphur compounds. They are the sulphur analogues of alcohols, as indicated by the word thiol, which is a combination of the word thion, Greek for sulphur, and the word alcohol. They are also known as mercaptans, from the Latin mercurius captan (capturing mercury), because they bond very strongly with mercury. The two most common thiols are ethane thiol and methane thiol.

Methanethiol is a very flammable substance which is naturally present in the blood and brain of humans and animals, in plant tissues, and in certain foods like nuts and cheeses. Methanethiol has a distinctive putrid smell, and it is one of the main chemicals responsible for bad breath and flatus. In the chemical industry, it is intentionally added to butane and propane to impart a noticeable smell to these normally odorless and potentially dangerous fuels. It is also used as an “olfactory alarm system” for emergencies in mines, and as a precursor in the manufacture of animal feed, pesticides, and petroleum. Most interestingly, methanethiol is produced as a byproduct when humans digest asparagus, and is hence partially responsible for “asparagus pee” (which all humans produce but not all humans can smell). Aromas associated with methanethiol include rotten cabbage, cooked cabbage, burnt rubber, stagnant water, and rotten eggs.

Ethanethiol is more volatile than ethanol, it has an “infamous” odour, and technically speaking, it is toxic, although it is certainly not harmful to humans at the low concentrations found in wine. Like methanethiol, ethanethiol is used as an “olfactory alarm system” for emergencies in mines, and it is intentionally added to butane and propane to impart a noticeable smell to these fuels. Aromas associated with ethanethiol include rotten cabbage, burnt cabbage, garlic, onion, and aromas described as sweaty, fecal, and putrid.

Ethanethiol can be synthesised in several different ways. Commercially, it is most often obtained from the reaction of hydrogen sulphide with ethylene over a catalyst, but it can also be obtained from the reaction of hydrogen sulphide with ethanol, by using an acidic solid catalyst such as aluminium oxide. It has been suggested that this same reaction between hydrogen sulphide and ethanol leads to the formation of ethanethiol in wine, but although some authors confirm it,5 others say it has not yet been demonstrated.6 Given the fact that commercially, this reaction occurs at 300˚ C and in the presence of a catalyst, it seems unlikely that this would occur spontaneously in wine.

Several authors have also suggested that ethanethiol can be formed in wine by the reaction between hydrogen sulphide and acetaldehyde, but this has never been demonstrated.3,6 In fact, it has been shown that it is ethanedithiol, another less well-known mercaptan, which is formed.2 Instead, the main mechanism for the production of both ethanethiol and methanethiol in wine is the degradation of sulphur containing amino acids by yeasts during fermentation (see How are Volatile Sulphur Compounds formed?).

Once formed, the thiols are susceptible to oxidation, and will readily oxidise into the higher sulphides, with ethanethiol forming diethyl disulphide and methanethiol forming dimethyl disulphide, especially in the presence of oxygen or radiation (specifically UV light). The higher sulphides, in turn, are susceptible to reduction, and they will readily reduce back into the thiols they came from if conditions are reductive and acidic enough. At wine’s pH, conditions are not acidic enough for these transformations to occur rapidly, but the presence of sulphite ions which results from the addition of SO2 makes conditions reductive enough to accelerate the process considerably. Hence, even if great care is taken to limit the presence of ethanethiol and methanethiol in newly fermented wine, if dimethyl disulphide and diethyl disulphide are present and SO2 is added, e.g. after malolactic fermentation, the wine will once again be left continuing thiols. Some winemakers believe that the level of SO2 used in the winemaking process should come down, especially with wines bottled under screwcap (check back soon for an article on Screwcaps and Sulphur Taint)

References

  1. Volatile Sulfur Compounds: Incidence and factors affecting their formation in Califomian Wines, A. C. Noble, R. B. Boulton and L. F. Bisson,
  2. http://wineserver.ucdavis.edu/industry/enology/fermentation_management/wine/off_characters.html
  3. 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.
  4. Wine Chemistry and Biochemistry, M. V. Moreno-Arribas and C. Polo, Springer-Verlag New York, 2009, Chapter 10.
  5. Handbook of Enology, The Chemistry of Wine: Stabilization and Treatments, P. Ribéreau-Gayon, Y. Glories, A. Maureen and D. Dubourdieu, John Wiley & Sons, 2006
  6. Biology of Microorganisms on Grapes, in Must and in Wine, H. König, G. Unden and J. Fröhlich, Springer Science & Business Media, 2009.
  7. R. B. Boulton, personal communication, September 2015.

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