PHOTO: Hendrik Holler (Wosa Library).

Volatile thiols in wine are important compounds contributing to the varietal character of especially Sauvignon blanc wine.

 

Origin of volatile thiols

The volatile thiols (4MMP and 3MH) are not present in grapes and are only released/formed from precursors to their volatile and pleasant smelling form during alcoholic fermentation. 3MHA (an acetate ester) is formed from 3MH through yeast-driven acetylation and does not normally exceed the concentration of 3MH in the final wine.4 During the 1990’s and 2000’s research regarding participation of the cysteine (Cys-3MH) and glutathione (Glut-3MH) bound precursors came to the fore.5 Since then a substantial amount of work has been done to understand these precursors. In follow up ferments it has been conclusively proven that these conjugated precursors are, in fact, not the main precursors and in reality only contribute to a fraction of the formed volatile thiols.6 Recently other similar precursors, such as Glut-3MH-Al and Glut-3MH-SO3, have also been identified,7 but still only account for a small percentage of volatile thiols formed during fermentation. Other conjugated compounds are also being investigated and can add to the pool of precursors.

The conjugated precursors are not released linearly and a low conversion rate when using commercial wine yeast was observed, as well as an almost complete lack of correlation between the bound precursor concentrations and the final thiol concentrations in wine.8 Even with this information widely known and accepted, these conjugated precursors are still constantly referred to as being the main source of volatile thiols in wine, even though the statement remains unsupported. The presence of these precursors does, however, excite the scientific community due to the enormous untapped potential of these bound thiols. Discovering a method to release the volatile thiols from these conjugated precursors could result in huge volatile thiol concentrations in the corresponding wine. Needless to say these precursors (although not the main source of the volatile form) still enjoys a significant amount of attention.

Other pathways for the formation of volatile thiols are being investigated and showing much more promising results. These pathways include the contribution of H2S (or another -SH moiety) as a sulphur donor in combination with a C6 compound, such as (E)-2-hexenal or (E)-2-hexenol, during the first few hours of fermentation.9 This is a more direct pathway for the formation of 3MH. However, this reaction requires the presence of yeast activity as tests have shown that the reaction does not take place in un-inoculated media.

C6 compounds, such as hexenal and hexenol, are formed from C18 polyunsaturated fatty acids in plant cell membranes. This reaction takes place rapidly during the damaging of plant cells due to the oxidation of the polyunsaturated fatty acids in the presence of oxygen and specific enzymes. Grape damage can occur due to grape processing methods, such as mechanical harvesting and crushing, or even fungal infection. Why does the plant/berry produce these C6 compounds in “stress” situations? Evolutionary speaking the reaction makes great sense: The C6 compounds have anti-microbial properties and this reaction (which takes place in berry damaging situations) will lead to the production of toxic protecting compounds (in this case the C6 compounds) to minimise the potential damage caused by the specific “threat”. As a result the (somewhat damaged) grape must have elevated concentrations of these protecting C6 compounds. The problem with these elevated levels is that it can become toxic to the plant itself, as well as the inoculated yeast. Glutathione, already naturally present in the grape must, can react with the C6 compounds to render the non-toxic Glut-3MH precursor, in this way protecting the plant from elevated levels of the C6 compounds. Interesting that a thiol precursor is formed during the detoxification process. In a similar manner yeast that is inoculated into the (somewhat damaged) grape must will disable the toxic compounds in its own way by converting for example hexenol to hexanol and by possibly adding compounds containing the antioxidant -SH group for toxic deactivation, thereby resulting in precursor/volatile thiol formation. So the plant and/or yeast will use mechanisms to convert the toxic substances formed during berry damage to an inactive form resulting in an odorous product.

H2S is thought to be the main -SH source used by the yeast to inactivate the C6 compounds. However, this hypothesis does have some defects due to the fact that the C6 compounds are consumed by the yeast within 24 hours after inoculation and H2S production by the yeast is delayed until well after all the C6 compounds have been converted. Thus, C6 uptake and H2S formation does not coincide. The possibility of another sulphur donor could perhaps explain the natural formation of volatile thiols. However, again a huge untapped potential remains in that large amounts of C6 compounds are present during early fermentation stages that can all be converted to volatile thiols if the correct sulphur donor is available right there and then. A patent is currently pending to allow the bubbling of H2S through the grape must during early stages of fermentation to facilitate the reaction of the C6 compounds with H2S. This addition is currently illegal, but a few interesting wines would probably be produced if and when approved. In a study where H2S was added to the must before fermentation an unprecedented amount of 257 µg/ℓ of 3MH and 35 µg/ℓ 3MHA were present in the finished wines. These concentrations are up to 38 times the amount found in “high thiol” wines. However, this treatment would most certainly raise some eyebrows due to the question of the “naturality” of the product and the authenticity of the wine of origin. Hexenol and hexenal formation is also not particular to Sauvignon blanc or other “thiol containing cultivars” for that matter. They can occur in musts from all grape varieties. Wine writers will presumably have a field day with the subject debating both sides of the argument.

If the bubbling of H2S is approved, the application should be done with great caution due to the unpleasant sensory effect unreacted H2S can have on wine aroma. Using a more natural approach by encouraging the earlier formation of H2S by yeast or delaying the disappearance of C6 compounds could have the same effect. The extent to which this happens in a commercial setup clearly needs further study.

The presence of elemental sulphur can also have an effect on the thiol concentration of wines. Where increased amounts of elemental sulphur were added to Sauvignon blanc grape must an increase in the volatile thiols were also observed. However, these increases were also accompanied with increases in reductive compounds, such as H2S and methyl-thiol, as well as corresponding reductive aroma attributes.9

These mechanisms seem promising, however, the exact origin of the volatile thiols in wine continues to elude the scientific community at present. The race to expose the pathway involved in volatile thiol formation is a constant pressure point in the research community due to the implications it will have on production of all wine varieties, but especially Sauvignon blanc wines. The secret to this mechanism will unlock huge potential and a wealth of possibilities. Other than controlling thiol production in the winery, it will also provide viticulturists and winemakers with the tools needed to predict the thiol potential of grapes in the vineyards. However, until then, the procedures known to increase volatile thiol levels in wine should be exploited in order to ensure maximum thiol production (if that is what the winemaker is going for).

 

References

  1. Swiegers, J.H., Willmott, R., Hill-Ling, A., Capone, D.L., Pardon, K.H., Elsey, G.M., Howell, K.S., De Barros Lopes, M.A., Sefton, M.A. & Lilly, M. et al., 2006. Modulation of volatile thiol and ester aromas by modified wine yeast. Dev. Food Sci. 43(C), 113 – 116.
  2. Coetzee, C. & Du Toit, W.J., 2012. A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Res. Int. 45(1), 287 – 298.
  3. Subileau, M., Schneider, R., Salmon, J.M. & Degryse, E., 2008. New insights on 3-mercaptohexanol (3MH) biogenesis in Sauvignon blanc wines: Cys-3MH and (E)-hexen-2-al are not the major precursors. J. Agric. Food Chem. 56(19), 9230 – 9235.
  4. Thibon, C., Böcker, C., Shinkaruk, S., Moine, V., Darriet, P. & Dubourdieu, D., 2016. Identification of S-3-(hexanal)-glutathione and its bisulfite adduct in grape juice from Vitis vinifera L. cv. Sauvignon blanc as new potential precursors of 3SH. Food Chem. 199, 711 – 719.
  5. Pinu, F.R., Jouanneau, S., Nicolau, L., Gardner, R.C. & Villas-Boas, S.G., 2012. Concentrations of the volatile thiol 3-mercaptohexanol in Sauvignon blanc wines: No correlation with juice precursors. Am. J. Enol. Vitic. 63(3), 407 – 412.
  6. Harsch, M.J., Benkwitz, F., Frost, A., Colonna-Ceccaldi, B. & Gardner, R.C., 2013. New precursor of 3-mercaptohexan-1-ol in grape juice: Thiol-forming potential and kinetics during early stages of must fermentation. J. Agric. Food Chem. 61(15), 3703 – 3713.

 

– Originally written for the IGWS. For more information, contact Carien Coetzee at carien@vinlab.com.

 

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