What Are Acetic Acid Bacteria?
Acetic acid bacteria (AAB) are aerobic bacteria — primarily Acetobacter and Gluconobacter species — that oxidize ethanol into acetic acid (vinegar) and ethyl acetate (nail polish remover smell). They're everywhere in the environment: on grape skins, in the air, on winery equipment, and in the presence of fruit flies.
Unlike most wine spoilage organisms, AAB require oxygen to cause damage. That single dependency is your greatest lever for control.
How AAB Damage Wine
The primary damage pathway is the conversion of ethanol to acetic acid:
- Acetic acid — the sharp, sour "vinegar" aroma and taste. Legal limits in most markets are 1.2–1.4 g/L for reds, 1.1–1.2 g/L for whites. Above threshold, the wine is unsalable.
- Ethyl acetate — formed when acetic acid reacts with ethanol. Smells like nail polish remover or model glue. Detectable at concentrations as low as 50–150 mg/L depending on the taster.
Small AAB populations are common and manageable. Large populations — given oxygen exposure — can spoil a tank in days.
When AAB Strike: High-Risk Windows
AAB problems cluster around predictable moments in the production timeline:
- Harvest and crush — damaged, botrytized, or shriveled fruit arrives with high AAB loads. Gluconobacter is especially active on raisined grapes.
- During fermentation — cap management exposes the cap surface to air. Inconsistent punch-down or pump-over frequency creates oxygen pockets.
- Post-fermentation and racking — headspace in partially full tanks is the classic setup for AAB colonization.
- Barrel aging — barrels filled below the bung line are especially vulnerable. Evaporation creates headspace. Weekly top-offs are not optional.
- Bottling — dissolved oxygen pickup during bottling, combined with low free SO₂, can trigger AAB activity in the bottle.
Prevention: The AAB Control Stack
Controlling AAB is not a single intervention — it's a stack of overlapping defenses. Lose one layer and the others usually hold. Lose two or three and the bacteria win.
1. Sulfur Dioxide (SO₂)
Free SO₂ is your primary chemical defense. Molecular SO₂ — the active antimicrobial fraction — is pH-dependent. At wine pH, you typically need 20–30 mg/L free SO₂ to maintain 0.5–0.8 mg/L molecular SO₂, which is inhibitory to most spoilage organisms including AAB.
Check free SO₂ every 2–4 weeks during aging. Don't let it drop below your target, especially in warm storage conditions.
2. Oxygen Exclusion
Since AAB are obligate aerobes, oxygen exclusion is the most direct control lever:
- Top off barrels and tanks weekly. Use inert gas (nitrogen or argon) to fill headspace.
- Keep airlocks filled. Check them every racking.
- Minimize splash racking. Use closed-loop transfers when possible.
- Store barrels bung-to-side (bung on the 3 o'clock position) to keep the bung wet.
3. Temperature Management
AAB are mesophilic — they thrive between 25–30°C (77–86°F). Cellar temperatures above 18°C (64°F) accelerate their activity significantly. During summer months, check tanks and barrels more frequently and prioritize cooling if storage temps are climbing.
4. Sanitation
AAB establish biofilms on equipment surfaces. Regular cleaning with hot water, caustic, and peracetic acid breaks down biofilm. Pay particular attention to hoses, fittings, pump heads, and any surface that contacts wine in open air.
Fruit flies carry AAB. Control fly populations in the cellar aggressively during harvest and fermentation.
Detecting AAB Early
The earlier you catch AAB, the more options you have:
- Smell — the first sign is often a faint vinegar or nail polish note. Smell samples at each racking, not just at formal evaluations.
- Volatile acidity (VA) testing — test VA at each racking during aging. Rising VA between rackings with no obvious cause points to AAB activity. A cash still or enzymatic test kit works fine for small-scale monitoring.
- Free SO₂ drop — AAB activity can accelerate SO₂ depletion. An unexplained SO₂ drop between additions is worth investigating.
What to Do If AAB Are Already Active
If you detect active AAB spoilage, your response depends on severity:
- Early / low VA (<0.8 g/L) — correct headspace, top up SO₂ to target, reduce cellar temperature. The wine is likely salvageable. Monitor weekly.
- Moderate VA (0.8–1.2 g/L) — same as above, but the wine needs close monitoring and may require blending to dilute VA below threshold. Evaluate honestly.
- High VA (>1.2 g/L) — the wine is at or beyond legal limits for many markets. Options narrow to: distillation, vinegar production, or disposal. There is no reliable way to chemically reduce VA in finished wine at scale without significant flavor impact.
Reverse osmosis VA reduction is available through custom crush facilities and service labs — it's expensive but effective for wine with commercial potential that has tipped past threshold.
Build AAB Monitoring Into Your Routine
Small winemakers who track VA and free SO₂ systematically rarely lose wine to AAB. The spoilage isn't subtle — it announces itself through smell and chemistry long before it becomes irreversible. The failure mode is almost always neglect: barrels not topped, SO₂ not checked, tanks not smelled.
Build a simple monitoring schedule. Check every barrel and tank at least monthly. Track VA and free SO₂ in a log so you can see trends before they become emergencies.
AAB don't surprise winemakers who are paying attention. They only surprise the ones who weren't.