Lessons from the AWITC: The Aromatic Thiols

Low YAN Levels: 2014 Harvest Update
December 26, 2014
The Copper Dilemma and Volatile Sulphur Compounds
December 27, 2014


At the AWITC technical conference in July, we attended an aroma and flavour compound workshop where we were given the opportunity to familiarize ourselves with a range of wine-related, good-bad-and-ugly aroma compounds. In a previous release we discussed the different compounds that contribute to green character in wine. This time we focus on one of the important fermentation aromas, the aromatic thiols.

These are sulphur-containing compounds, and are related by their chemistry to the negative ‘reductive aromas’ previously discussed. The most important fermentation thiols include:

  • 3MHA: 3-mercaptohexylacetate. Passion fruit, gooseberry, guava and other tropical fruit aromas at lower levels, sweaty at higher levels. Sensory perception threshold 4ng/L
  • 3MH: 3-mercaptohexanol. Passion fruit, grapefruit and general citrus aromas. Sensory perception threshold 60ng/L
  • 4MMP: 4-mercapto-4-methylpentan-2-one. Box tree, broom, blackcurrant and cat urine aromas. Sensory perception threshold 0.8-3.0ng/L

These aromas contribute significantly to the aroma of Sauvignon blanc, but also form part of the fruit aromas of Cabernet Sauvignon, Merlot, Shiraz and Grenache, as well as other white varieties such as Chenin blanc, Riesling, Pinot gris and Gewurztraminer.

Fermentation thiols have their precursors in the grape and are released into wine to a greater or lesser degree depending on the way the grapes and juice are handled, both in the vineyard and the winery.

The 4MMP precursors develop earlier in the grape than those of 3MH and 3MHA. Thus timing of harvest can influence the relative concentration of the individual thiols in wine, and thus which of the aromas will be more dominant. Earlier harvesting favours higher concentrations of 4MMP in the resulting wine and the potential dominance of boxtree aromas, while later harvesting favours 3MH and 3MHA and the potential dominance of tropical and citrus aromas.

4MMP is found equally in skin and pulp while 3MH and 3MHA are found mainly near the skins. Thus skin damage e.g. by mechanical harvester, or during crushing may proportionately increase the levels of the 3MH and 3MHA precursors in the juice. The same is true for skin contact and the use of extraction enzymes. This proportionate increase in 3MH and 3MHA precursors may proportionately increase the levels of their thiols in the wine.

Yeast strain selection can significantly affect the concentration and relative proportion of the individual thiols. Many commercial strains have been specifically isolated to enhance thiol release into wine. Relatively higher fermentation temperatures may also favour the release of thiols.

Reductive processing and maturation conditions will favour the preservation of thiols in wine.

Note that, because they are chemically related to the volatile sulphur compounds, any CuSO4 fining will remove these desirable aromatics from wine.