Volatile Acidity in Fermented Foods

Acetic acid (CH₃COOH) is produced by two main microbial pathways. First, heterofermentative lactic acid bacteria (Lactobacillus, Leuconostoc) produce acetic acid as a byproduct of pentose sugar fermentation alongside lactic acid and CO₂ — a normal and desirable process in sourdough fermentation and some cheese aging. Second, acetic acid bacteria (Acetobacter, Gluconobacter) oxidize ethanol to acetic acid in the presence of oxygen: C₂H₅OH + O₂ → CH₃COOH + H₂O. This pathway is responsible for wine spoilage (Acetobacter infects wine exposed to air) and is deliberately harnessed in traditional vinegar production (Orléans process, submerged acetator). In wine, legal VA limits in the EU are 1.08 g/L (white/rosé) and 1.20 g/L (red); sensory threshold for most tasters is 0.6–0.8 g/L, though botrytized sweet wines naturally produce higher VA that is masked by sugar. Acetic acid vapor (pKa = 4.76) is partially ionized at wine pH (3.2–3.8), but the undissociated form is volatile and sensory-active — a highly acidic environment keeps more acid in its volatile, olfactorily active form. Ethyl acetate (the ester of acetic acid and ethanol, formed during fermentation) often co-occurs and has a sharp, nail-polish remover note at concentrations above ~150 mg/L, compounding the perception of VA defect.

• In wine, VA above threshold is a legal and commercial defect — wines may be rejected at competition or customs; understanding the microbial cause allows targeted cellar interventions.
• In sourdough, the balance between acetic and lactic acid determines whether the crumb is sharply sour or mildly tangy; warm, wet fermentation favors lactic; cool, stiff fermentation favors acetic.
• In vinegar production, VA is the product; understanding the Acetobacter pathway and oxygen supply allows precise control of acidity and flavor complexity in artisan vinegar.
• High VA in fermented hot sauce or fermented vegetables signals aerobic contamination rather than healthy anaerobic LAB fermentation.

1. 1In winemaking, minimize oxygen exposure during racking and storage — cover wines tightly, use inert gas (argon, nitrogen, CO₂) for headspace, and add SO₂ to inhibit Acetobacter.
2. 2To favor acetic acid (complex, sharp sourdough character), use a stiff dough (low hydration), ferment at cooler temperatures (18–21 °C), and allow longer, slower fermentation.
3. 3To favor lactic acid (mild, yogurt-like sourdough character), use a wetter dough, ferment warm (26–28 °C), and use more frequent refreshments (shorter cycles).
4. 4Use Titrets or enzymatic assay kits to measure wine VA quantitatively rather than relying on palate alone — sensory threshold varies by 50% between tasters.
5. 5In fermented chili or vegetable ferments, use fully submerged anaerobic vessels (airlocks, weights) to exclude oxygen and prevent Acetobacter from converting alcohol to acetic acid.
6. 6For traditional sherry-style wines, controlled VA development is desired — the solera system exposes wine to controlled oxygen, allowing limited Acetobacter contribution as a flavor component.

• Sharp, vinegary aroma detectable on the nose of wine before tasting — the hallmark of elevated VA from acetic acid vapor.
• Nail-polish remover or solvent note (ethyl acetate) that often accompanies high VA in wine, amplifying the perception of defect.
• In sourdough, a sharp, punchy sour note on the nose and a clean acidic bite on the palate versus a more rounded, creamy sourness — the former indicates acetic acid dominance.
• A burning or stinging sensation at the back of the throat in wine — acetic acid at high concentration has this effect.

• Leaving wine vessels with significant headspace (air gap) during storage, creating ideal Acetobacter conditions.
• Not monitoring SO₂ levels in wine and allowing free SO₂ to fall below the protective threshold (typically >25 mg/L molecular SO₂ at the wine's pH).
• Attributing all sour flavor in sourdough to lactic acid; failing to consider that VA (acetic) character can be deliberately cultivated or undesirably high depending on fermentation conditions.
• Confusing high VA defect with high total acidity — a very tart wine with low VA is a style choice; a wine with moderate total acid but high VA is a spoilage issue.
• Using low-quality or damaged corks that allow micro-oxidation, slowly raising VA in bottled wine over years.

• Lactic Acid Fermentation

  The primary acid pathway in most fermented foods; acetic acid is the secondary product of heterofermentative LAB and competes with lactic as the sourdough flavor axis.

• Residual Sugar & Perceived Sweetness in Beverages

  Residual sugar in wine is the substrate for Acetobacter VA production if oxygen is present; the sugar-acid relationship also governs how VA is perceived sensorially.

• Microbial Spoilage vs. Controlled Fermentation

  VA is the paradigm case of the spoilage-vs.-complexity spectrum: the same molecule in trace amounts is complexity (traditional vinegar, botrytis wine), in excess is defect.

• Maillard Reaction

  Indirectly related: the flavor complexity of aged balsamic vinegar results from both high VA and Maillard products formed during concentration and aging.

1. 1.Pascal Ribéreau-Gayon et al., Handbook of Enology Vol. 1 (Wiley, 2006)
2. 2.Harold McGee, On Food and Cooking (Scribner, 2004)
3. 3.Marco Gobbetti & Rudi Vogel, eds., Handbook on Sourdough Biotechnology (Springer, 2013)
4. 4.Roger Boulton et al., Principles and Practices of Winemaking (Springer, 1999)

Confidence: high