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 Daily explores the science behind volatile acidity, from its chemical formation to the tools winemakers use to manage it.
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.”
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