Volatile Acidity (VA) — Acetic Acid & Ethyl Acetate (Vinegar / Nail Polish)

Volatile acidity (VA) refers to the fraction of a wine's total acidity composed of volatile, or steam-distillable, acids. The primary volatile acid is acetic acid (CH3COOH), the compound that defines vinegar, which delivers a sharp, pungent aroma when present above sensory threshold. Closely linked is ethyl acetate (CH3COOC2H5), an ester formed when acetic acid reacts with ethanol; it smells of nail polish remover or solvent glue and is detectable at even lower concentrations than acetic acid itself. Other minor volatile acids include propionic, butyric, and formic acids, but acetic acid dominates and is the compound regulated by law. Unlike fixed acidity (tartaric, malic, and citric acids), which remain stable in solution, VA compounds are volatile and contribute directly to the nose of the wine. All wines contain some VA naturally; the average acetic acid level in a fresh dry table wine is below 400 mg/L, and it typically falls in the range of 0.36-0.48 g/L even in clean, sound examples.

• Acetic acid (vinegar character) and ethyl acetate (solvent/nail polish character) are the two primary sensory markers of VA
• VA differs from fixed acidity: acetic acid evaporates easily and registers directly on the nose, while tartaric acid remains non-volatile in solution
• US legal limits: 1.4 g/L for reds, 1.2 g/L for whites; EU limits: 1.2 g/L for reds, 1.08 g/L for whites and rosés (wines above 12.5% ABV)
• All sound wines contain VA at low levels, typically 0.36-0.48 g/L, well below the sensory detection threshold of 0.6-0.9 g/L

The primary driver of elevated VA is acetic acid bacteria (AAB), a group of Gram-negative, obligate aerobic bacteria belonging to the family Acetobacteraceae. The two genera of greatest concern in winemaking are Acetobacter and Gluconobacter. Acetobacter aceti and Acetobacter pasteurianus are the most common species isolated from wine, and Acetobacter species preferentially oxidize ethanol as a carbon source; Gluconobacter species are more active on glucose and sugars and are especially prevalent on damaged or botrytis-infected grapes before fermentation begins. The biochemical pathway is: ethanol is oxidized to acetaldehyde, and acetaldehyde is then oxidized to acetic acid. Because AAB are strict aerobes, they require oxygen to proliferate and carry out this oxidation. The optimum growth temperature for both genera is 25-30 degrees Celsius, meaning warm cellars and warm fermentation environments dramatically accelerate risk. Beyond AAB, Saccharomyces yeast and lactic acid bacteria (including Oenococcus oeni during malolactic fermentation) both produce small amounts of acetic acid as normal metabolic byproducts, which is why VA is present even in clean wines. Spoilage yeasts such as Brettanomyces are also documented acetic acid producers.

• Acetobacter and Gluconobacter are the two AAB genera of winemaking importance; Acetobacter aceti and A. pasteurianus most commonly cause spoilage in finished wine
• The oxidation pathway is: ethanol to acetaldehyde to acetic acid; both steps require oxygen, so anaerobic conditions halt bacterial activity
• Optimal AAB growth temperature is 25-30 degrees Celsius; cooler cellars meaningfully slow multiplication and risk
• Saccharomyces, lactic acid bacteria, and Brettanomyces all produce acetic acid to varying degrees, contributing to baseline VA even in clean wines

Because VA is far easier to prevent than to remediate, winemakers organize their defenses around three pillars: fruit quality, oxygen exclusion, and sulfur dioxide (SO2) management. Starting with sound, undamaged fruit is essential; Gluconobacter populations on healthy grapes are low, below 100 colony-forming units per gram, but on damaged or botrytis-infected fruit they can reach over 100,000 cfu/g, delivering a high AAB load directly to the fermentation vessel. SO2 additions at crush, typically adjusted by pH and fruit condition, provide both antimicrobial and antioxidant protection; AAB are obligate aerobes, so SO2 combined with rigorous exclusion of oxygen creates a hostile environment for them. During fermentation, CO2 produced by active yeast effectively blankets the fermenting mass, which is why VA risk is highest before fermentation is fully underway and again during post-fermentation aging. In barrel aging, consistent topping (ouillage) eliminates headspace and prevents AAB from colonizing the wine-air interface. Cellar temperature management is also critical; keeping the cellar cool slows AAB growth significantly. Inoculating with commercially selected Saccharomyces strains, which are chosen in part for low VA production, reduces the risk compared with spontaneous fermentations.

• Reject damaged, moldy, or split fruit at harvest; Gluconobacter populations increase by several orders of magnitude on compromised berries
• SO2 at crush, combined with anaerobic storage conditions, is the most effective and practical preventive measure against AAB
• Active CO2 during fermentation physically excludes oxygen; VA risk is highest before fermentation is fully underway and during barrel aging
• Regular barrel topping (ouillage) eliminates headspace and removes the surface oxygen interface where AAB thrive

Modern commercial winemaking treats detectible VA as a fault, and competition judges typically penalize it. However, the relationship between VA and wine quality is more nuanced in fine and natural wine contexts. Some winemakers seek a low, barely detectable level of acetic acid because it can add perceived lift and aromatic complexity at sub-threshold concentrations. In the natural wine movement, low-intervention producers who use minimal or no added SO2 and rely on wild yeast fermentation accept a higher statistical probability of elevated VA as a trade-off for avoiding additives; critics and supporters debate whether this is intentional style or winemaking imprecision. Fine aged wines, including mature red Burgundy and aged Loire Chenin Blanc, can accumulate modest VA over long bottle aging as a byproduct of slow microbial activity, and experienced tasters often perceive this as tertiary complexity rather than a fault. The key distinction is context and degree: at or near threshold, VA in a structured aged wine can coexist with complexity, while the same level in a young, fresh white is almost always perceived as a defect. The natural wine debate has prompted certification organizations including WSET to revisit how contextual faults are assessed.

• Sub-threshold VA can add lift and complexity; the consensus is that it must not be distinctly perceptible to be considered beneficial
• Natural and low-SO2 producers accept higher VA risk as a trade-off for minimal intervention; opinions in the trade are divided on whether this is style or fault
• Aged fine wines may show elevated VA as a tertiary aging character, particularly in long-aged reds and oxidatively aged whites
• The sensory context matters: VA that reads as fault in a fresh white may blend into complexity in a structured, aged red

Volatile acidity is assessed both analytically and sensorially. The traditional and legally recognized analytical method is steam distillation using a Cash Still, where volatile acids are distilled from the wine sample and the distillate is titrated with sodium hydroxide; a correction for SO2 interference is applied. More modern approaches include gas chromatography for individual volatile acid separation, HPLC, and enzymatic assay kits that specifically measure acetic acid. It is important to note that steam distillation measures total volatile acidity including all steam-distillable acids, while enzymatic kits measure acetic acid specifically, so results may differ. Sensorially, acetic acid presents as a sharp, prickling vinegar aroma at threshold (approximately 0.6-0.9 g/L), while ethyl acetate manifests at much lower concentrations (100-200 mg/L) as nail polish remover or solvent on the nose. Ethyl acetate is therefore the more useful early sensory warning indicator of developing VA problems; detecting it on the nose at an early stage allows winemakers to intervene before acetic acid climbs further. VA also magnifies the perception of tannins and fixed acidity, which can make wines seem sharper and harder on the palate than their analytical acidity alone would predict.

• Analytical standard: Cash Still steam distillation followed by titration with sodium hydroxide, corrected for SO2 interference
• Gas chromatography, HPLC, and enzymatic kits (measuring acetic acid specifically) are modern alternatives used in commercial wineries
• Sensory: ethyl acetate (nail polish, solvent) is detectable at 100-200 mg/L, making it a useful early warning sign before acetic acid climbs to perceptible levels
• VA magnifies the perception of tannins and fixed acidity; wines with elevated VA often taste sharper and more astringent than their chemistry alone explains

Once VA has developed in tank or barrel, options for meaningful correction are limited, and prevention is always the superior strategy. For wines with lower VA still below the sensory threshold, the primary action is to arrest further development: high SO2 additions, strict anaerobic storage, and if necessary sterile filtration to remove residual AAB populations will stabilize the wine and prevent further acetic acid production. For wines with elevated VA above threshold, the two main remediation routes are blending and reverse osmosis. Blending a high-VA lot with a sound, low-VA wine can dilute acetic acid below the sensory threshold; the high-VA wine should be sterile filtered before blending to avoid cross-contaminating the clean lot. Reverse osmosis (RO) can selectively remove acetic acid from wine via ion-selective columns that bind acetic acid from the RO permeate, after which the treated permeate is returned to the wine. RO can reduce VA by a meaningful percentage, but the process requires specialized equipment and careful management; as UC Davis enologist Roger Boulton has noted, there are limited independent published studies on the precise sensory effects. Premium producers generally prefer to declassify VA-compromised lots rather than apply remediation techniques that conflict with quality positioning.

• Below sensory threshold: high SO2 additions and anaerobic storage stabilize VA and prevent further development; sterile filtration removes residual bacteria
• Blending with clean, low-VA wine dilutes acetic acid below sensory threshold; sterile filter the high-VA lot first to protect the clean wine
• Reverse osmosis uses ion-selective columns to selectively bind and remove acetic acid from the RO permeate, then returns cleaned permeate to the wine
• Premium producers generally declassify VA-compromised lots rather than remediate; prevention through hygiene and oxygen management remains the only reliable strategy