The Science of Volatile Acidity

Jessica Dupuy. https://daily.sevenfifty.com/

Jessica Dupuy est une rédactrice spécialisée dans le vin, les spiritueux et la gastronomie basée à Austin, au Texas. Elle a notamment travaillé pour Texas Monthly, le magazine Imbibe, le magazine Wine Enthusiast, le Sommelier Journal et le magazine The Tasting Panel, ainsi que pour la Guild of Sommeliers. Sommelière certifiée, spécialiste certifiée du vin et spécialiste certifiée des spiritueux, elle est titulaire du diplôme en vins du Wine & Spirits Education Trust. Dupuy garde son palais affûté grâce aux voyages, à la lecture et à des dégustations sans fin.

All wines have some volatile acidity, which can add to a wine’s complexity at low levels. At what point does it become a fault, and how can winemakers manage it?

Volatile acidity, commonly referred to as VA, is like the Jekyll and Hyde of the wine world. In small, controlled amounts, it can lend complexity and intrigue to a wine’s profile. But once it reaches a certain level, VA can overpower a wine with sharp aromas of vinegar, or a jarring whiff of nail polish remover. 

In wine, VA refers to the measure of a wine’s volatile, or gaseous, acids. According to The Oxford Companion to Wine, all wines have some level of VA, and there are a handful of different volatile acids that can derive from the fermentation process. However, the chemistry behind VA’s dual personality centers on two key components: acetic acid and ethyl acetate. While they’re distinct compounds—one an acid, the other an ester—they’re so intertwined in their creation and sensory impact that they’re often discussed as a package deal.

How, exactly, does VA form in wine, and what steps can winemakers take to harness its complexity while keeping its flaws in check? Here, SevenFifty Dailyexplores the science behind volatile acidity, from its chemical formation to the tools winemakers use to manage it.

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The Chemistry of Volatile Acidity 

Acetic acid is the more straightforward component, and is commonly associated with vinegar’s sour tang. It’s primarily produced by acetic acid bacteria like Acetobacter and Gluconobacter, which convert ethanol (alcohol) into acetic acid when oxygen is present. Yeast metabolism can also produce acetic acid as a byproduct during fermentation. It’s the sharp, tangy contributor to the VA equation, providing a zippy, mouth-puckering acidity.

Ethyl acetate is the more aromatic component. This compound is an ester, known for its bold, volatile aroma that’s often compared to nail polish remover. Ethyl acetate forms when acetic acid reacts with ethanol, a process called esterification. Microbial activity, particularly from certain yeast strains or bacteria, can speed up this reaction. Together, acetic acid and ethyl acetate create the sensory impression of VA, with one dominating the taste and the other leading the charge aromatically.

This interconnectedness is further complicated as acetic acid gives rise to ethyl acetate, making it difficult to separate their roles. Both are volatile, meaning they evaporate easily and contribute to the wine’s aroma, and both are products of the same microbial processes. While acetic acid is usually the main culprit in high VA levels, ethyl acetate amplifies the perception, making it impossible to ignore.

Measuring Volatile Acidity

When it comes to detecting these compounds in wine, there’s a clear line between subtle complexity and obvious fault. Within legal limits (typically between 1.2 and 1.5 grams per liter), it can shift from a potential flaw to a nuanced contributor to the wine’s overall profile. PJ Alaimo, Ph.D., a professor and chair of the chemistry department at Seattle University, provides some concrete numbers: “The sensory threshold for most people for acetic acid is around 0.72 grams per liter,” he says. “Below this amount, it is not easily detected. The threshold for most people for [ethyl acetate] is around 0.12 grams per liter.”

Measuring these compounds requires different approaches. According to Alaimo, “Acetic acid is measured by a process called steam distillation, which separates the volatile acids from the rest of the wine. A titration is then done to measure how acidic the distillate is.” But ethyl acetate is determined differently: “Ethyl acetate cannot be measured this way, in contrast to what many textbooks say.” Instead, winemakers turn to gas chromatography, a sophisticated technique that separates volatile molecules.

The intimate connection between these components tells a story of chemical partnership. As Gus Zhu, a chemist and the author of Behind the Glass: The Chemical and Sensorial Terroir of Wine Tasting, explains, “Acetic acid is only made by bacteria, Acetobacter, which is everywhere in our air. Ethyl acetate is made by yeast—when fermentations become compromised or poorly managed, particular yeast strains can generate excessive ethyl acetate, amplifying the nail polish remover aroma that many associate with VA.”

The Winemaker’s Balancing Act

The formation of VA depends on a complex litany of factors that impact the winemaking process. Oxygen exposure plays a lead role, as acetic acid bacteria flourish in its presence, spinning ethanol into acetic acid. Meanwhile, elements like Brettanomyces, wild yeasts, and other spoilage organisms can amplify VA levels, especially when conditions like high residual sugar or poor hygiene are involved.

In the trenches of daily winemaking, VA is less of a fault and more of a constant companion that needs careful watching. Bree Stock, MW, the winemaker at Limited Addition Wines in Oregon, puts it plainly. “It’s almost impossible not to have any volatility in wine, even in very conventionally made wines,” she says, adding that VA exists on a spectrum, where those low levels can contribute a “lift” or “brightness” to wine. “But it starts to get tangy, like lemon sherbet or lemon drop candy, and then moves into nail polish remover or even a hair salon aroma. With VA, there’s a fine balance: it can add intrigue and complexity to certain wines, but it can just as quickly become a flaw when left unchecked.”

Managing Volatile Acidity

The story of VA doesn’t begin in the cellar—it begins in the vineyard, where grape health is key. When disease or damage strikes, especially from Botrytis cinerea, the fruit becomes a perfect host for microbial activity. 

The 2022 Napa Valley harvest offers a perfect case study of these challenges. Both Stock and winemaker James McCeney of Patz & Hall pointed to this vintage as an example of how Mother Nature can throw a curveball during the growing season. 

“The hard heat weakened the grape skins, and the bee activity increased, which created a breeding ground for Acetobactor in the vineyards,” says McCeney. “In some cases, you could smell a cluster and think, ‘That smells like vinegar—this is bad.’”

Once the grapes reach the cellar, oxygen exposure becomes a primary concern. If Acetobacter gets too much air, whether through neglected barrel maintenance, loose tank seals, or limited sulfur dioxide use, it will become predominant.

The first line of defense? Sulfur dioxide. “Bacteria, in general, are very intolerant to sulfur dioxide,” McCeney explains. “That’s why people add sulfur to the crusher.” 

Temperature also becomes critical. “If my fermenter is sitting at 65 degrees for too long in Oregon, it’s more likely to develop VA,” Stock shares. Her solution? Carefully raising temperatures to encourage healthy yeast activity and carbon dioxide production—nature’s own VA bodyguard.

McCeney emphasizes this point, noting that temperatures in the mid-70s to 80s can be “highly beneficial to Acetobacter,” which thrives in warm conditions. 

Other contributing elements include pH levels (low pH plays defense against spoilage organisms, while high pH provides a breeding ground), residual sugar (food for Acetobacter in oxygen-rich environments), and basic cellar hygiene. Even the barrels themselves play a dual role—essential for controlled oxygen exposure, but potential troublemakers if not maintained properly.

Caution is also advised after malolactic fermentation. “During malolactic, you’re topping your barrels and still have some microbial protection from sulfur dioxide,” McCeney explains. “But once malolactic concludes and you lose that antimicrobial effect, that’s your next biggest window of danger.”

Moving Beyond Stereotypes

In the world of wine education, VA has become an element that everyone remembers, but often misunderstands. It’s frequently associated with Italian wines, but Chris Tanghe, MS, the director of education for GuildSomm and owner of Walter’s Wine Shop in Seattle, suggests it’s time for a script rewrite.

“A lot of people teach it that way, but the problem is, it creates an unfair association with the identity of Italian wine,” Tanghe explains. While VA might have a recurring role in traditional Italian reds like Barolo or Amarone della Valpolicella, he emphasizes that it’s more of a global performer. “It’s simply just the result of a winemaking process and the conditions in a given winery—or even a single barrel,” he says.

The Italian connection stems from traditional winemaking practices in regions like Piedmont and Veneto. “For instance, in Barolo, post-fermentation macerations can last a month or longer, exposing the wine to air and increasing the potential for Acetobacter activity,” Tanghe notes. 

Combined with long aging in large oak casks and minimal intervention techniques, these methods created conditions where VA could flourish. This association was particularly strong 30 years ago, when less hygienic winemaking practices in Italy often led to higher levels of VA—but this doesn’t make it inherently Italian.

Volatile acidity may not always be a desirable element in wine, but when managed with precision by a skilled winemaker, it can enhance a wine’s complexity and character. However, if left uncontrolled, its impact can dominate, leading to a sensory imbalance that detracts from the wine’s overall quality.