Volatile Acidity (VA) — Acetic Acid & Ethyl Acetate

Volatile acidity encompasses the steam-distillable acids present in wine, with acetic acid (CH3COOH) accounting for the overwhelming majority of the volatile acid fraction. Other minor contributors include formic, butyric, and propionic acids. Ethyl acetate (CH3COOC2H5), an ester produced when acetic acid combines with ethanol through esterification, is technically distinct from VA itself but is so closely tied to it in origin and sensory impact that the two are routinely discussed together. Unlike fixed acids such as tartaric, malic, and lactic acid, these volatile compounds evaporate readily, rising into the headspace of a glass and making themselves detectable by aroma long before they dominate the palate. In the US, VA is reported in grams per liter standardized to acetic acid equivalents; in Europe, measurements are often expressed in sulfuric acid equivalents, so laboratory reports from different regions require careful comparison.

• Acetic acid: the primary volatile acid in wine, producing vinegar character above ~0.6–0.9 g/L
• Ethyl acetate: formed by esterification of acetic acid and ethanol; nail polish remover aroma detectable at ~100–200 mg/L
• Other minor volatile acids include formic (pungent), butyric (rancid butter), and propionic (fatty) acids
• VA and fixed acidity together constitute a wine's total acidity; only VA is removed by steam distillation

VA is almost entirely a product of microbial activity. Acetic acid bacteria, principally from the genera Acetobacter and Gluconobacter, are the primary culprits: Acetobacter oxidizes ethanol to acetic acid under aerobic conditions, while Gluconobacter converts glucose to acetic acid and is particularly active on damaged fruit in the vineyard before harvest. Because these bacteria are obligately aerobic, oxygen availability is the single greatest risk factor for VA accumulation. During healthy, vigorous alcoholic fermentation, the continuous production of CO2 displaces oxygen and keeps acetic acid bacteria in check. Risk increases sharply during the pre-fermentation period, at the end of fermentation when CO2 production slows, and throughout barrel aging when small amounts of oxygen enter through stave pores. Lactic acid bacteria, including those responsible for malolactic fermentation, can also generate acetic acid from residual sugar if conditions are poorly controlled. Brettanomyces yeast is another notable VA contributor during aging. In 1857, Louis Pasteur identified acetic acid bacteria as the cause of wine souring, establishing the scientific foundation for modern cellar hygiene.

• Acetobacter oxidizes ethanol to acetic acid; Gluconobacter preferentially converts glucose to acetic acid
• Acetic acid bacteria are obligately aerobic: CO2 from active fermentation suppresses their growth naturally
• Lactic acid bacteria can produce acetic acid from residual sugar, particularly under stressed or contaminated conditions
• Brettanomyces yeast is also a significant VA producer during barrel aging

Detecting VA by aroma requires a calibrated palate, because acetic acid itself has a relatively high sensory threshold of 0.6–0.9 g/L depending on wine style and the individual taster. Lighter-bodied, lower-alcohol wines reveal VA at lower concentrations, while higher sugar or alcohol levels can mask it. Ethyl acetate is far easier to detect, with a threshold of roughly 100–200 mg/L, and its nail polish remover character often alerts tasters to a VA problem before the vinegar note of acetic acid becomes obvious. On the palate, elevated VA creates a sharp, biting quality beyond normal acidity and can amplify the perception of tannins and fixed acids. Concentrations above approximately 0.7 g/L are widely considered indicative of spoilage, and at 1.4 g/L wine takes on the unequivocal odor and flavor of vinegar. While trace VA at background levels of 0.3–0.5 g/L is normal and can contribute complexity, the line between character and fault is crossed well below the legal limits set by regulators.

• Acetic acid sensory threshold: 0.6–0.9 g/L, highly variable by wine style and taster sensitivity
• Ethyl acetate sensory threshold: approximately 100–200 mg/L; often the first VA warning signal
• At ~0.7 g/L, VA is widely regarded as a spoilage fault; at 1.4 g/L wine resembles vinegar
• VA magnifies the perception of tannins and fixed acidity on the palate

VA accumulates at multiple stages of the winemaking process. Compromised or physically damaged fruit arriving at harvest already harbors elevated populations of acetic acid bacteria and wild yeasts that produce VA. During alcoholic fermentation, yeast stress from nutrient depletion, very low or very high fermentation temperatures, or high sugar loads can increase VA output. In barrel aging, inadequate topping allows headspace to develop, providing both surface area and oxygen that accelerate Acetobacter activity. Oak barrels also contribute a small, chemically driven increment of 0.06–0.12 g/L per year through acid hydrolysis of hemicellulose. Prevention centers on four practices: meticulous cellar sanitation, appropriate SO2 management to inhibit acetic acid bacteria and wild yeasts, rigorous oxygen exclusion through barrel topping and inert gas blanketing, and the use of sound fruit free of mold or insect damage. Once a wine exceeds roughly 0.7 g/L, options are limited. Blending with a clean, low-VA wine after sterile filtration can dilute VA below the sensory threshold in milder cases. For wines at or above 0.7 g/L, reverse osmosis is the only reliable technical means of physically reducing acetic acid concentration.

• Risk factors: damaged or moldy fruit, stressed fermentation, inadequate SO2, oxygen exposure, poor sanitation
• Barrel aging adds 0.06–0.12 g/L per year even in well-managed cellars via hemicellulose hydrolysis
• Prevention: SO2 management, oxygen exclusion, sound fruit selection, temperature control, sanitation
• Remediation: blending with sterile-filtered low-VA wine (mild cases) or reverse osmosis (severe cases)

Most major wine-producing nations set legal VA ceilings well above the sensory detection threshold, acknowledging that some wines naturally accumulate higher levels during extended aging. In the United States, the TTB sets the limit at 1.4 g/L for red table wines and 1.2 g/L for white table wines, with elevated allowances of 1.7 g/L and 1.5 g/L respectively for late-harvest wines produced from grapes picked above 28 Brix. International legal limits in most producing countries typically fall between 1.07 and 1.5 g/L depending on wine type, with whites held to stricter limits than reds because VA is more sensorially prominent in lighter wines. Fortified wines and intentionally oxidative styles tolerate naturally higher VA as a consequence of their production methods. Analytically, VA is traditionally measured by steam distillation using a Cash Still, followed by titration with sodium hydroxide. Enzymatic analysis and gas chromatography are also widely used today. Ethyl acetate, contrary to some older references, cannot be accurately measured by steam distillation and requires gas chromatography for precise quantification.

• US TTB limits: 1.4 g/L for red table wines, 1.2 g/L for white table wines (higher for late-harvest wines)
• International limits generally range from 1.07 to 1.5 g/L; whites are held to stricter limits than reds
• Traditional analysis: Cash Still steam distillation followed by sodium hydroxide titration
• Ethyl acetate requires gas chromatography for accurate measurement, not steam distillation

Not all VA is unwelcome. At concentrations below the sensory threshold, acetic acid is a normal fermentation by-product and can add a subtle lift and complexity to wine. Acetic acid is also an important precursor to the acetate esters that contribute much of a young wine's fresh, fruity character. The natural wine movement has brought renewed attention to VA tolerance, with some producers and their advocates viewing modest VA as a marker of minimal intervention rather than a defect. However, the scientific and professional wine community broadly defines VA above approximately 0.7 g/L as a spoilage fault, regardless of philosophical intent. Importantly, VA interacts with other wine components: higher sugar and alcohol levels mask it, while leaner, lower-alcohol wines expose it at lower concentrations. The distinction between fault and character ultimately depends on concentration, wine style, and context, but objective analytical measurement remains the most reliable arbiter.

• Background VA of 0.3–0.5 g/L is normal in sound wine and can contribute positive complexity
• Acetic acid is a precursor to fruity acetate esters that are desirable in young wine
• VA is more perceptible and problematic in lean, low-sugar, low-alcohol wine styles
• Professional consensus: VA above approximately 0.7 g/L constitutes a spoilage fault across all wine traditions