The Science Behind Reductive Aromas in Wine

Volatile sulfur compounds are polarizing aromas that can smell like anything from rotten eggs to struck match and beyond. How do they occur during viticulture and vinification—and are we thinking about them all wrong?

Oenologist checking the wine properties before closing barrels
These divisive aromas may lend more benefits to wine than originally assumed. Photo credit: iStock.

One of today’s most polarizing topics in wine is reduction. Are reductive aromas faults to be eradicated? A lovely aspect of wine character? Does it depend on the intensity or the eye of the beholder? Or is there a broader way to think about reduction altogether?

Winemaking practices in the late 20th century sought to avoid and undo reduction, while in the 21st century, there is an increasing population that welcomes reductive character. Some have moved beyond the binary “like or dislike” question, and begun to consider what reduction means about a wine, and how to manage or encourage it in ways that can increase a wine’s lifespan.

Speaking chemically, reduction is the opposite of oxidation. In wine, the term is often used in a colloquial sense to refer to the presence of myriad volatile sulfur compounds (VSCs) that tend to result from the absence of oxygen. This article considers the branch of VSCs often considered to be off-aromas, or “defects.” Varietal thiols—such as the various tropical aromas in Sauvignon Blanc—are also VSCs, but are for the most part created in the grape and are not the type of VSCs considered in this article. 

There are many reductive compounds that can affect wine aroma, but some of the most important are hydrogen sulfide (H2S, which often smells like rotten eggs), mercaptans (like methanethiol and ethanethiol, which smell like anything from cabbage and burnt rubber to pleasant earthiness), and dimethyl disulfide (which may smell like truffles, blackcurrant, canned corn, or molasses). 

VSCs, in addition to their own aromas, affect the expression of other aromas in wines. “Reductive aromas mask delicate fruit and confectionary aromas, and reductive wines seem less fruity or floral,” says Marlize Bekker, Ph.D., a senior lecturer in food chemistry at the University of Queensland in Australia. “Certain reductive wines may also be wrongly assessed as having smoke taint, when in fact they may have high ‘flint’ aroma.”

With an understanding of the incredibly complex (and often unknown) factors that result in how any given aromatic compound is actually expressed, it’s easy to see how these aromas can be so controversial.

Yeast’s Role in Creating Reductive Aromas

A major source of VSCs is yeast metabolism of certain nitrogen-containing compounds during fermentation, which can occur when grape must has low yeast assimilable nitrogen (YAN), or nitrogen compounds in forms that yeast can utilize, which are critical for their functioning. There are many causes of low YAN in must, such as nutrient-poor soils and vine stress. 

Though adding nitrogen to soils can help boost YAN, changing soils’ nitrogen levels can impact a wine’s structure in other ways. “Keeping nitrogen levels lower in the vineyard can result in a higher level of anthocyanins,” says Bruce Zoecklein, Ph.D., a professor emeritus of enology at Virginia Tech. He points to Burgundy, a region where wines can be reductive but winemakers still keep vineyard nitrogen levels low. “Pinot Noir isn’t overly endowed with color, so you don’t want to jeopardize that by boosting soil nitrogen,” he says.

Many winemakers test YAN levels and use various products to raise YAN levels in the must to an optimal amount. Diammonium phosphate (DAP) is a common solution as it is cheap, though there are other options as well.

Headshots of Marlize Bekker and Seth Long
From left to right: Marlize Bekker, Ph.D., a senior lecturer in food chemistry at the University of Queensland in Australia (photo courtesy of Marlize Bekker); and Seth Long, the owner and winemaker of Morgen Long Winery (photo courtesy of Seth Long).

However, it’s not clear-cut, according to Dr. Zoecklein. “There’s only about a 70 percent correlation between having a low YAN and the production of VSCs,” he says. “If it were 100 percent, adding nitrogen would solve this issue. When you measure YAN, you’re measuring ammonia and FAN (free amino nitrogen) amino acids … It seems the important component here is not simply the total, but the qualitative nature of the YAN—specifically, the particular mix of ammonia and these FAN amino acids.” Zoecklein notes that a must may have a very low YAN, but the mix is such that the must ferments without creating VSCs.

Trying to optimize YAN levels can lead to its own problems. Every yeast strain has different nutritional requirements and varying potential for the production of VSCs, and “certain yeast strains cannot utilize DAP,” says Dr. Bekker. “This leads to reductive faults even when the juice has been supplemented.” 

Some speculate that DAP encourages yeast to produce generic aromatic compounds, obscuring what would normally come from a fermentation. Additionally, any YAN not consumed by yeast during fermentation can later be used as a food source for other microbes, such as Brettanomyces, or cause other sensory problems. As there is no way to determine how much YAN will be consumed in any given ferment—especially if fermenting spontaneously—adding DAP is a risky game for winemakers to play: attempting to solve one problem, but increasing risk of others.

“Two remedial things that winemakers often use to remove VSCs are aeration and copper,” says Zoecklein. “Many winemakers, if getting too concerned with sulfur compounds, will simply try to aerate the wine—with the assumption that the majority of the odor is H2S, which is volatile and you can get out easy—but if it’s not H2S, you can oxidize mercaptans to disulfides, and that can cause more problems.” 

Depending on which VSCs are present, copper and ascorbic acid may be employed, but copper doesn’t discriminate between VSCs, removing attractive varietal thiols as well. “We’ve moved beyond these practices,” says Zoecklein, “and it’s more about trying to learn how to tolerate these compounds in the short term, because the amount of oxygen you get in, say, a barrel, may be enough to mute them, change their ionization (changing their sensory attributes), or to mask them.”

Creating Volatile Sulfur Compounds in the Vineyard and Cellar

But there are myriad other factors in the viticulture and vinification process that contribute to reductive aromas in wine, starting with the vineyard. Reduction can occur when elemental sulfur, an especially important tool in organic vineyards, is sprayed too late in the season. “If sulfur residues remain on the skin of the grapes, it will be converted to H2S by the yeast during fermentation,” says Bekker. “Similarly, in very dry years, sulfur residues may remain on grapes if they are not washed off by the rain.”

This can lead to flinty aromas, which are often attributed to terroir in regions such as Burgundy, where late-season spraying can be necessary due to disease pressure. Bekker notes that H2S “is a precursor to phenylethanethiol, the compound associated with ‘gun flint’ aroma.” However, there is no definitive word on where all flint character comes from, and Bekker notes that most viticulturalists and winemakers take extra care to apply fungicides within the recommended withholding periods, so she believes this to occur rarely.

“That really heavy flinty character happens when it happens, it doesn’t when it doesn’t,” says Seth Long, the owner and winemaker of Morgen Long Winery, which strictly focuses on Willamette Valley Chardonnay. “It does seem to happen more frequently with organic vineyards, but I don’t know if it’s because they use more elemental sulfur.” There are even rumors that some New World winemakers have intentionally sprayed late in the season to create that flint aroma in their wines. 

“I want the core; the thing that is the most impervious to oxidation. I want my fermentations done sooner, rather than later, to capture some of the reduction that happens in the barrel when yeast get stressed because of the high heat and finishing fast.” – Seth Long, Morgen Long Winery

Fermentation temperature also plays a role. “Temperature fluctuations during winemaking that lead to faster ferments can also lead to the formation of reductive aromas produced by stressed yeast,” says Bekker.

Long, who formerly interned with Domaine de Montille in Meursault and prefers reductive aromas in his wines, leverages this fact. “I ferment hot,” he says. “I want my fermentations done sooner, rather than later, to capture some of the reduction that happens in the barrel when yeast get stressed because of the high heat and finishing fast.” Along with other techniques, the heat helps Long get rid of more easily oxidizable compounds in his wine early—therefore, there is less to oxidize once bottled. 

“I want the core; the thing that is the most impervious to oxidation,” he says. “When you get rid of the stuff that oxidizes easily, the wine is something that’s harder, it’s closer to indestructible.” 

The Impact of Reductive Strength on Wine Longevity

It’s clear that reduction may result from certain practices winemakers use, but why do Long—and other winemakers—actively seek reductive character itself in their wines?

On one hand, it can simply be an aesthetic choice: many appreciate reductive character in their wines. Long is fascinated by VSC aromas in his wines, and he also feels that the volatile sulfides in his wines work well with new oak. “I think it marries better—the right quantities [of VSCs] with the right oak structure around it,” he says. “I don’t think H2S works as well when there isn’t new oak, and oak is a really important component to Chardonnay in general.”

But reductive character can also indicate that a wine has higher reductive strength, and therefore, capacity to age. “The reductive strength of a wine is its ability to take up oxygen without oxidizing,” says Zoecklein. “It’s influenced primarily by phenols and somewhat by lees and other components.” If a wine can absorb more oxygen without oxidizing, it can survive, age, and evolve for longer.

To understand how reductive aromas and reductive strength relate to a wine’s longevity, it’s important to have an understanding of oxidation-reduction—or redox—potential. This is an electrochemically complicated process, and an area of understanding where much remains unknown. 

In a highly oversimplified sense, the redox potential of a wine can be thought of as its potential at a given moment to oxidize or reduce compounds in that wine, as measured in volts. A wine with a low redox potential will be disposed to reduce compounds in that wine, giving it a larger buffer against oxidation, while a wine with a high redox potential will oxidize compounds in that wine, and be more prone to oxidation.

This relates to reductive aromas because the type, concentration, and volatility (and thus “smellability”) of the particular VSCs in a wine at any given moment are relative to its redox potential. As a wine’s redox potential shifts, so will the chemical structures of VSCs in that wine, and therefore, the character and intensity of reductive aromas in that wine will change. A wine with a low redox potential will have VSCs that are particularly volatile and intense, while at higher redox potentials, VSC chemical structures change in ways that render many of them non-volatile, so they don’t impact wine aroma. 

A wine’s redox potential inevitably changes over time; once bottled, it should slowly increase over the years. A low redox potential is also thought to be why many young wines may seem aromatically closed—as reduction masks varietal aromas—but open up over time as redox potential increases and reductive aromas are less prominent. Changes in redox potential also impact the form of other potentially aromatic compounds in a wine, increasing or diminishing them, which may also contribute to the perception of young wines being closed.

All wines start off with a low redox potential due to yeast because redox potential is lowered when wines have low dissolved oxygen—as happens during fermentation, due to yeasts’ consumption of oxygen. So, yeasts not only build VSCs—they temporarily create an environment that encourages their expression and intensity.

Considering all of this, it might be time to change the thinking around early reductive aromas, as they may be an indication of a greater reductive strength in a wine. “It used to be we’d want to jump on these ‘problem’ compounds (if you want to call them that) right out of the fermenter,” says Zoecklein, “but most winemakers now realize that by doing that—say, by excessively aerating—they may be raising redox potential and probably negatively impacting longevity, depending on the nature of the wine.”

For wines intended to drink young, this may not be a big consideration—and getting rid of “off-aromas” may make more sense—but for wines made to age, it’s critically important. “People now realize that there is a correlation between the production of these compounds that used to be considered detrimental and a low redox potential, and that low redox potential adds longevity,” says Zoecklein. “It’s because of this that a low redox potential is so important.”

Steaming wine barrels
A better understanding of a wine’s redox potential can influence a winemaker’s decision to get rid of reductive aromas. Photo courtesy of Seth Long.

Building Reductive Strength in Wines

In red winemaking, one way winemakers can increase a wine’s reductive strength—and longevity—by exposing it to oxygen (in certain quantities) very early in its life. “A young red wine that’s been exposed to oxygen generally becomes much more resistant to later oxygen exposure,” says Zoecklien.

Although the concept of increasing reduction by adding oxygen seems counterintuitive, “some phenols have a unique ability to bind or react with oxygen, and while doing so, bind with other compounds—including anthocyanins and sulfur-derived compounds—and react in such a way that allows the wine to pick up even more oxygen,” says Zoecklien, reducing a wine further. This—along with other practices that increase reductive strength—may cause reductive odors early in a red’s life, but it will also allow it to live longer, aging and developing over longer periods than it otherwise would have. As the wine ages and its redox potential slowly increases, reductive character should diminish. As Clark Smith puts it in Postmodern Winemaking, “I used to consider sulfides a defect. Now I worry about young reds that don’t have them.”  

While reductive strength can be built in reds from their phenol content, in whites it generally cannot, so reductive strength and aging potential in whites must come from other avenues. Lees aging is a particularly important consideration for a white wine’s ability to age, as wines can pick up reductive strength (and character) while aging on the lees, particularly when gross lees are retained and when lees are not stirred, allowing them to compact. 

“Lees adsorb oxygen, and not just while the wine’s on the lees, but once removed as well,” says Zoecklein. “The macromolecules that are released into the wine during autolysis can also adsorb oxygen over time and keep a wine in a semi-reductive state.” Reds can benefit from lees aging too, but only after color has stabilized, as prior to this, lees can adsorb both oxygen and anthocyanins—both critical components in building reductive strength in reds.

Reductive aromas themselves do not increase longevity, and not all reduction is indicative of aging potential. However, by avoiding practices that can lead to reduction, or by trying to remove reduction, a winemaker may inadvertently shorten a wine’s life.

Reduction is one of the more complicated areas for understanding not only wine aromatics, but a wine’s structure, mechanics, and trajectory. It’s one of the more prominent examples of wine appreciation that requires us to go beyond simple aesthetic likes and dislikes, and requires an appreciation of what aromas mean about a wine. What smells off to one person today, may indicate a wine’s ability to evolve in the future in a way that it could not, if those “off” aromas had been precluded in its youth.


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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

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