Among the myriad components that make up wine, very few have entirely positive or negative roles to play. Take volatile thiols, for instance, a family of molecules also known as mercaptans that are sulfur-containing compounds—specifically, they all include a sulfhydryl group (-SH). Aromas associated with mercaptans are either reduction-like (burnt rubber, cabbage, flint, smoke) or fruity (grapefruit, passion fruit).
Thiols are found in many wines, though the compounds present may differ between them. For example, Sauvignon Blanc possesses 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH), and 3-mercaptohexan-1-ol acetate (3MHA), which are the primary compounds that contribute to characteristic Sauvignon Blanc aromas. 4MMP is known to contribute box tree, passion fruit, and black currant aromas; 3MH contributes grapefruit, gooseberry, passion fruit, and guava aromas; and 3MHA contributes grapefruit, gooseberry, sweet passion fruit, box tree, and guava aromas.
The perception thresholds for these compounds vary, though all are considered to be relatively low. Specifically, these thresholds have been found to be 0.8ng/L, 60ng/L, and 4.2ng/L, for 4MMP, 3MH, and 3MHA, respectively.
While these compounds impart pleasant aromas in wines, others can be problematic. For instance, during fermentation, hydrogen sulfide (H2S) can form and interact with benzaldehyde to produce methyl mercaptan, giving off less-than-desirable aromas like garlic, cabbage, and cooked vegetables. The threshold for methyl mercaptan is very low as well—0.02 parts per billion can be enough to make it detectable in water. Other compounds, like 2-methyl 3-furanethiol or benzenemethane thiol, can give smoky aromas that can have more variable effects, depending on concentration and interactions with other aromas.
It isn’t completely understood how thiols form in wine, though there are some solid theories. These compounds aren’t present in grapes, or only in trace amounts. The winemaking process is what brings them out—fruity thiols have precursors in the grapes that are revealed through fermentation, while reductive/smoky mercaptans seem to form during the entire vinification process.
One possible mechanism relates to nitrogen availability during fermentation. Basically, yeasts need a good nitrogen supply to go through fermentation. If there is a good nitrogen supply, the yeasts should be happy and the finished wine should be pleasant. Reductive thiols can form and become problematic when the nitrogen source for the yeasts is limited. If there isn’t enough nitrogen, the yeasts will start to use the nitrogen tied up in the amino acid cysteine, which possesses a sulfur group. When the yeasts break down the cysteine for nitrogen, the sulfur group is released and can interact with other compounds in the wine, yielding H2S and the off-aromas associated with H2S buildup.
A new study found that even the yeast strain is important in determining thiol/mercaptan formation during fermentation. Other studies have shown that yeast strain can also affect the development of 3MH and 3MHA in wines, as the yeasts reveal the precursors to give fruity notes.
Another mechanism for the formation of reductive or smoky mercaptans is thought to occur during aging. In order for this formation to occur, it’s thought there may exist an odorless precursor or multiple precursors that react over time with other compounds in the wine under a reduced oxygen environment to form these undesirable mercaptans. Studies have shown that significant increases in methyl mercaptan can occur in bottled wine, especially with lower post-bottling oxygen exposure.
On the other hand, volatile thiols such as 3MH and 3MHA have been shown to decrease over time, particularly when exposed to more oxidative conditions. There are a couple ways that can happen. First, they can oxidize easily when exposed to oxygen and iron, and second, they can react with other compounds in the wine like o-quinones and phenolic compounds, resulting in the formation of non-volatile compounds that would effectively reduce the recognizable varietal character of that wine. It has also been shown that factors like oxygen exposure, copper, or glutathione additions can greatly affect the longer-term presence of thiols like 3MH in bottled wine.
Some have speculated that mercaptans may form via odorless complexes with metal cations. Specifically, winemakers will often use copper salts to remove mercaptans from wine. Some evidence supports the idea that it isn’t possible to remove all the mercaptans from the wine and there remains some copper-mercaptan complexes in the solution that react in the bottle over time to release the mercaptans into an odorous form.
While the mechanisms still remain relatively unknown, we know more about the production of mercaptans in wine than ever. Still, more research is needed to fully understand the mechanisms and how to optimize mercaptans levels in wine. So what can you do as a winemaker? Monitoring nitrogen levels and/or selecting a yeast strain that is known to yield lower H2S (and the subsequent mercaptan products produced from reactions involving H2S in the fermenting and aging wine) should help reduce H2S formation and encourage the formation of fruity thiols. Finally, carefully controlling the level of oxygen exposure during aging can also help maintain a proper balance of varietal aromatic characteristics in the wine. Too little oxygen can set up an environment favorable to runaway mercaptan formation, while too much oxygen opens up a whole other can of worms. Choosing the appropriate closure for your wine can also help reduce the chances of developing off odors caused by certain mercaptans while maximizing the desired aromatic characteristics associated with other volatile thiols.
Coetzee, C. 2014. Oxidation treatments affecting Sauvignon blanc wine sensory and chemical composition (Doctoral dissertation). Stellenbosch University.
Ferreira, V., Bueno, M., Franco-Luesma, E., Culleré, L., and Fernández-Zurbano, P. 2014. Key changes in wine aroma active compounds during bottle storage of Spanish red wines under different oxygen levels. Journal of Agricultural and Food Chemistry 62: 10015-10027.
Franco-Luesma, E., and Ferreira, V. 2014. Quantitative analysis of free and bonded forms of Volatile Sulfur Compounds in wine. Basic methodologies and evidences showing the existence of reversible cation-complexed forms. Journal of Chromatography A (1539): 8-15.
Patrignani, F., Chinnici, F., Serrazanetti, D.I., Vernocchi, P., Ndagijimana, M., Riponi, C., and Lanciotti, R. 2016. Production of Volatile and Sulfur Compounds by 10 Saccharomyces cerevisiae Strains Inoculated in Trebbiano Must. Frontiers in Microbiology 7:243.
Maurizio Ugliano,*,† Jean-Baptiste Dieval,† Tracey E. Siebert,§ Mariola Kwiatkowski,§,⊥ Olav Aagaard,†Stéphane Vidal,† and Elizabeth J. Waters, Oxygen Consumption and Development of Volatile Sulfur Compounds during Bottle Aging of Two Shiraz Wines. Influence of Pre- and Postbottling Controlled Oxygen Exposure, Journal of Agricultural and Food Chemistry, 2012 Sep 5;60(35):8561-70.
Ugliano M1, Kwiatkowski M, Vidal S, Capone D, Siebert T, Dieval JB, Aagaard O, Waters EJ., Evolution of 3-mercaptohexanol, hydrogen sulfide, and methyl mercaptan during bottle storage of Sauvignon blanc wines. Effect of glutathione, copper, oxygen exposure, and closure-derived oxygen. Journal of Agricultural and Food Chemistry, 2011 Mar 23;59(6):2564-72