Thiols—more precisely, volatile thiols or volatile sulfur compounds—are an interesting and diverse group of aroma compounds. They are most associated with the tropical and citrus aromas in Sauvignon Blanc, though thiols are also responsible for flint and mineral aromas in certain classic wines, as well as unpleasant aromas like sewage and rotten cabbage that can make appearances in the glass.
While controversial aromas that are commonly associated with reduction, such as hydrogen sulfide (H2S), are volatile thiols, they function differently than varietal thiols, which originate in the grape. Just as monoterpenes are often associated with Muscat, tropical thiol compounds are most frequently associated with the Sauvignon Blanc grape. However, thiols are also important to the character of Riesling, Gewürztraminer, Sémillon, Pinot Gris, Pinot Blanc, and many others. Passionfruit, guava, grapefruit, and boxwood are characteristic aromas associated with numerous thiols.
The names of the thiols themselves are usually abbreviated, as they’re particularly cumbersome; some of the most common are 3SH for sulfanylhexan-1-ol, 3SHA for 3-sulfanylhexyl acetate, and 4MSPfor 4-methyl-4-sulfanylpentan-2-one. These names might surprise even those who are familiar with thiols; the International Union of Pure and Applied Chemistry has recently decided to use “sulfanyl” instead of “mercapto” in thiol nomenclature.
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Thiols are present in red grapes and wines as well, but much less research has been done and their contribution isn’t entirely clear. The Australian Wine Research Institute article “Do Varietal Thiols Matter in Red Wine?” states that what would be perceptible levels of tropical fruit in whites, appear only to boost “red fruit” in some red wines.
But how does viticulture and vinification impact varietal thiols, and which compounds impact the perception of thiols in wine? SevenFifty Daily spoke with research scientists and winemakers to find out.
In the Vineyard
There are myriad ways in which thiol concentration can be affected during the growing season. According to Marlize Bekker, Ph.D., the principal research scientist at the Australian Wine Research Institute, various studies have shown that heat shock and UV radiation treatments to grapes increase thiol precursor concentrations. This means that “leaf plucking is a simple way to increase thiol precursors in Sauvignon Blanc,” she says. Thiol precursor concentration peaks in grapes in a “semi-ripe stage” before maturity, says Avery Heelan in a 2015 Waterhouse Lab article, so picking earlier may result in higher concentrations.
For the Grenache rosé and Sauvignon Blanc at Simon Family Estate in Napa, California, winemaker Maayan Koschitzky works to get direct light on the grapes, but only early in the season. “We open some of the leaves early, and later on, when lateral shoots are pushing, we don’t remove them, as to create extra cover,” he says. For the rosé, Koschitzky feels this encourages “dominant aromas of grapefruit and other tropical fruit, but being careful not to create a wine that is one-dimensional.” He feels the later-season shade is important. “With the warmer conditions we are experiencing, we want to have nice natural shade on clusters [in the hot months],” Koschitzky notes.
Though he notes that it’s difficult to directly correlate irrigation with thiol levels, “for irrigation, we push toward deeper irrigation, with bigger gaps between waterings—or dry farming depending on the soils—which seems to assist with increasing thiols in fermentation,” says Koschitzky. He is also experimenting with foliar nitrogen application—the application of nitrogen to vine leaves—around veraison, another vineyard strategy that can increase thiol precursor concentration in grapes.
Botrytis infection in grapes also boosts thiol concentration in the resulting must. For example, in some Sauternes wines, 3SH concentration reaches extreme levels: up to 5000 nanograms per liter (for reference, the compound has a sensory threshold of 60 nanograms per liter). It’s also stable enough to remain that high for decades, according to the Handbook of Enology, Volume 2: The Chemistry of Wine Stabilization and Treatments. Machine harvesting can also increase thiol concentration by four-fold compared to hand picking, according to a 2017 Australian Journal of Grape and Wine Research article.
In the Winery
Like most wine aroma compounds, varietal thiols are not yet aromatic while they are still in grapes, as they are bound to other molecules (specifically amino acids and tripeptides in this case). These compounds are released in different quantities during fermentation, depending on yeast strains and other factors.
“I think the key is to do everything possible to protect the juice and wine from oxygen. These precursors are highly oxidizable and once they’re gone, they’re gone.” – Ernst Storm, Storm Wines
However, rather than simply releasing the varietal thiols that exist in the grape, in at least one unique instance, different yeast strains can convert one important thiol into another—changing the structure and aroma of the molecule itself. Different yeasts convert varying, often significant, proportions of a must’s 3SH into 3SHA, according to a 2006 Australian and New Zealand Wine Journal article. This is an interesting variable for the terroir expression and wine authenticity conversation.
It’s because of this special relationship of yeast and these two prominent thiols that Ernst Storm, the owner of Storm Wines in Santa Barbara County, California, chooses to inoculate his Sauvignon Blanc even though all of his other wines are allowed to ferment spontaneously. “We did a lot of experiments in the winery regarding reductive winemaking, the use of SO2, and how certain yeast strains ferment in these different treatments,” he says. “We got the same results when we tasted the wines three years in a row. The choice of yeast was easy.”
Tools for Modifying Thiols
Another important consideration for thiols and winemaking is oxygen exposure. For his Sauvignon Blanc, “I think the key is to do everything possible to protect the juice and wine from oxygen,” says Storm. “These precursors are highly oxidizable and once they’re gone, they’re gone.” Interestingly, in working with low oxygen to protect grape-derived thiols, one may create other, “reductive” thiols (particularly), which may be desirable or not, depending on their intensity and who’s tasting.
Sulfur dioxide (SO2) can be a tool to preserve thiol concentrations. “SO2 not only limits free oxygen in wine, but also inactivates enzymes and quinones (an oxidative reaction product of polyphenols) that would otherwise decrease thiol concentrations,” says Dr. Bekker.
However, she also notes that “trace amounts of metals, such as copper and iron can also lead to the oxidation of thiols.” Because copper is so reactive with sulfur compounds (like thiols), winemakers may use it to decrease H2S levels in wine—knowing it carries a risk of impacting the wine’s overall aroma.
Fermenting white wines in contact with their skins also boosts thiol intensity, as most thiol precursor compounds are in the grape skins. Fermenting on the skins can increase thiol concentration up to eight times in wines. Thiol sensory thresholds are especially low, which may be an important reason for the intense, often exaggerated fruit aromas in many skin-fermented whites.
Yeast strains may also affect an interesting thiol called phenylmethanethiol (PMT, previously known as benzenemethanethiol or benzylmercaptan), which is associated with the flinty, struck match, or gun-smoke aroma in some Burgundies and cooler-climate Australian Chardonnays. Though it’s “thought to be produced during barrel-ageing … the formation pathway of PMT is not completely understood,” says Bekker. ”PMT shows strong varietal dependence, which suggests that its formation may be more complex than other oak-derived sulfur compounds.” A 2022 study by Bekker and her colleagues also showed that certain yeasts increased PMT concentration in model wines.
Perception of Thiols in Wine
How humans perceive aroma compounds—including thiols—in wine is complex. “The perception of tropical thiol aroma is not only related to the concentrations of thiols in wine, but is affected by many other compounds in wine,” says Bekker. “Perception of tropical aromas can be masked by … acetic acid [volatile acidity], as stated in ‘The Role of Potent Thiols in Chardonnay Wine Aroma.’ High concentrations of fruity esters or H2S may have similar masking effects.” In addition to other wine components potentially masking thiol perception, she notes that “studies have shown that different combinations of esters and thiols can produce different expressions of tropical fruit” aromas, according to a January 2023 Heliyon article.
Thiols are an interesting, sensitive, and unique class of aroma compounds. They can leave a definite signature on many wines, in particular in Sauvignon Blanc. They require particular ways of being handled in the winery if they are to be expressed as the winemaker envisions.
Dispatch
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Alex Russan, based in Los Angeles, is a former winemaker, importer, and sherry bottler. He writes about viticulture, enology, tasting and the nature of wine
