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Volatile Thiols

Sulfur containing compounds have high aroma impact, including key varietal flavors. Avery Heelan, 2015

Thiols, or the sulphur-containing analog of an alcohol, can have a variety of aromas and smells. For general purposes, they can be divided into two groups; those with negative smells and those that contribute positively to a wine.  Certain sulphur containing compounds are those that take one a rotten egg smell, which come from the formation of H2S by wine yeast fermentation.  Although the formation of H2S is undesirable, it is very common in wine and there are many efforts to combat this aroma.  In addition to the production of rotten egg aromas, secondary reductive odors such as cooked vegetables, onion and cabbage are formed from sulphur containing compounds such as thioacetic acid esters and mercaptans due to too low redox potential of the wine (Brajkovich et al., 2005).  The second category of sulphur containing compounds are those that contribute positively to wine, named volatile thiols.

 

Volatile thiols are most prevalent in Sauvignon blanc, specifically New Zealand style, giving it the distinctive aroma profile which has given designation and respect to this variety and grape growing region. The volatile thiols responsible for these aromas include:

3-Mercaptohexan-1-ol (3MH) (C6H14OS)

Aroma: Grapefruit, passion fruit, gooseberry, guava

Range in wines (ng/L): 26–18,000

Odor threshold (ng/L)
: 0.8

3-mercaptohexylacetate (3MHA) (C8H16O2S)

Aroma: Passion fruit, grapefruit, box tree, gooseberry, guava

Range in wines (ng/L): 0–2500

Odor threshold (ng/L)
: 60

4-methyl-4-mercaptopentan-2-one (4MMP) (C6H12OS)

Aroma: Box tree, passion fruit, broom, black current

Range in wines (ng/L): 4-40

Odor threshold (ng/L)
: 4.2

4-mercapto-4-methylpentan-2-ol (4MMPOH) (C6H14OS)

Aroma: box tree, broom flower; cat pee

Range in wines (ng/L): 0-40

Odor threshold (ng/L)
: 0.8

Coetzee et al, 2012; Tominaga et al, 1998; Ribéreau‐Gayon et al, 2006; Lund et al, 2009

 

Although these volatile thiols are character-impacting compounds in Sauvignon blanc, they are not unique to this cultivar and have been found in other varieties such as Riesling, Colombard, Semillon, Cabernet Sauvignon and Merlot (Coetzee et al, 2012).  For Sauvignon blanc, the contribution of these thiols to varietal aroma is quite significant as the levels in wine usually exceed the threshold of detection. Although volatile thiols as a group encompass a variety of compounds, these four are amongst the most significant aromas in Sauvignon blanc.

Unlike most aroma compounds found in wine, volatile thiols are unique in the fact that they exist in trace amounts in the berries.  Aside from a small amount of 3MH (approximately 100 ng/L) found at harvest, the other compounds are virtually nonexistent in grape juice (Coetzee et al., 2012). This formation throughout fermentation as a yeast byproduct has been the focus of considerable amounts of research. From this, it has been identified that direct precursors become cysteinlated or glutathionylated for each compound during fermentation.  These precursors include S-3-(hexan-1-ol)-L-cysteine (Cys-3MH), S-4-(4-methylpentan-2- one)-L-cysteine (Cys-4MMP), [S-3-(hexan-1-ol)- glutathione (Glut-3MH) and S-4-(4-methylpentan-2-one)-glutathione (Glut-4MMP) for 3MH and 4MMP.  The formation of these precursors is still not entirely understood chemically.

 

The pathway of formation for these aromatic precursors is speculated to involve four important steps: enzymatic oxidation, metabolic processing of unsaturated fatty acids, cysteinlated or glutathionylated conjugation to aldehydes, and a β-lyase cleavage during alcoholic fermentation to release the aromatic compound.  Each of the steps within this pathway are known to contribute to aromatic potential independently, but as one established pathway, have yet to be validated for the formation for these compounds.  In addition, these aromas can be manipulated by certain viticultural practices or winemaking decisions such as: machine harvesting, skin contact (precursors are found in concentrations up to eight times higher in the skins than pulp), low temperature fermentations, the use of certain yeast strains, earlier harvest dates (precursor concentrations are shown to peak at a semi-ripe stage prior to grape maturity), and a number of other practices which may contribute to the enhancement of these aromas in a final wine.

References

Brajkovich, M. T., N.; Peron, G.; Lund, C. M.; Dykes, S. I.; Kilmartin, P. A.; Nicolau, L, Effect of screwcap and cork closures on SO2 levels and aromas in a Sauvignon Blanc wine. Journal of agricultural and food chemistry 2005, 53, 10006-10011.

Coetzee, C. D. T., W.J., A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International 2012, 45, 287-298.

Lund, C. M. T., M. K.; Benkwitz, F.; Wohler, M. W.; Triggs, C. M.; Gardner, R.; Heymann, H.; Nicolau, L., New Zealand Sauvignon blanc Distinct Flavor Characteristics: Sensory, Chemical, and Consumer Aspects. American Journal of Enology and Viticulture 2009, 60, 1-12.

Ribéreau‐Gayon, P., Glories, Y., Maujean, A., Dubourdieu, D., Handbook of Enology: The chemistry of wine stabilization and treatments. John Wiley & Sons Ltd, Chichester.: 2006.

Tominaga, T. P. d. G., C.; Dubourdieu, D., A New Type of Flavor Precursors in Vitis vinifera L. cv. Sauvignon Blanc- S-Cysteine Conjugates. Journal of agricultural and food chemistry 1998, 46, 5215-5219.

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