Volatile acidity in wine: legal limits, where it comes from and how to keep it down

Volatile acidity is the one analysis every wine carries from the first week of fermentation to its last day in bottle. It is also the number that cannot be taken back: there is no addition that removes acetic acid once it is there. A wine that climbs from 0.4 to 0.9 g/L over a winter of ageing has lost something that blending, fining or time will not return. This guide covers what VA measures, where the limits sit, where it comes from and the routine that keeps it flat.

What volatile acidity measures

VA is the sum of the acids that distil with steam — in practice more than 90% acetic acid, the acid of vinegar. It is reported as acetic acid (g/L) in most of the world, sometimes as sulfuric acid (g/L H₂SO₄) in older French-style bulletins, or in milliequivalents per litre (meq/L), the unit EU law uses. The conversions are simple: 1 meq/L = 0.06 g/L acetic acid = 0.049 g/L H₂SO₄.

A small amount is normal. Yeast produce 0.2–0.4 g/L during a healthy fermentation, and malolactic fermentation adds a little more. The problem starts when it keeps rising after the wine is dry.

Legal limits and the sensory threshold

• EU (Regulation 2019/934): 18 meq/L for white and rosé (≈ 1.08 g/L acetic acid), 20 meq/L for red (≈ 1.20 g/L). Some aged wines and sweet wines from botrytised grapes have derogations.
• United States (27 CFR 4.21): 1.2 g/L for white table wine and 1.4 g/L for red, expressed as acetic acid and excluding SO₂.
• Sensory: most tasters pick acetic acid somewhere between 0.6 and 0.9 g/L, earlier in light whites and later in big reds. Ethyl acetate — the nail-varnish note that usually travels with acetic acid — often makes the wine look faulty before the acetic acid number does.

Treat the legal limit as the wall, not the target. A practical alarm sits around 0.6 g/L in a young wine: anything climbing past that before bottling needs a cause found, not just another reading.

The four usual sources

• Acetic acid bacteria (Acetobacter, Gluconobacter) — they need oxygen. Ullage in barrels and tanks, a film on the surface, loose bungs and long gaps between toppings are the classic route, and the VA rises slowly across the ageing months.
• Lactic acid bacteria — heterofermentative strains turn residual sugar into acetic acid. A stuck or sluggish alcoholic fermentation that lets malolactic start with sugar still present ("piqûre lactique") can add several tenths of a gram in days.
• Stressed yeast — very high-sugar musts, nitrogen deficiency and temperature shocks push yeast to make more acetic acid. Stuck fermentations are the usual suspect.
• Damaged fruit — sour rot, hail damage and wasp-split berries arrive already loaded with acetic bacteria and ethyl acetate. Some of the VA is in the grapes before the first pump-over.

A cellar routine that keeps VA flat

• Sort at reception: leave out sour-rot bunches and measure VA on the must of suspect lots, so you know the starting point.
• Run a clean alcoholic fermentation: enough yeast-assimilable nitrogen, steady temperature, and no long gap between pressing and inoculation.
• Decide the sequence before the wine is dry, so malolactic starts only when sugar is gone (or co-inoculate with a strain chosen for it).
• Hold molecular SO₂ once fermentations are over — around 0.5–0.8 mg/L — because SO₂ is what keeps acetic bacteria quiet. At high pH that means much more free SO₂ than habit suggests.
• Top barrels and tanks on a schedule, keep headspace under inert gas, and check bungs and valves.
• Measure VA on every lot at a fixed rhythm (monthly during ageing is a good default) and look at the trend, not only the last value.

When it is already high

First stop the climb: top up, add SO₂ to a protective molecular level, cool the wine and, if the lees are suspect, rack it into a clean vessel. Then treat what is there. Blending with a low-VA wine lowers the number but also commits the clean wine; check the rules for your appellation before blending a lot that is near or over the limit. Membrane treatments (reverse osmosis combined with an ion exchange step) can remove acetic acid, but they are specialist services and their legal status depends on the market — confirm with your authority and your lab before planning on them.

VA rarely jumps without warning; it creeps, lot by lot, between readings nobody compares. In GrapeFlow every analysis is recorded against the lot, so the VA curve of each wine sits next to its SO₂ additions, toppings and rackings — and a rising lot shows up before it reaches the limit.

These are general guidelines. Confirm limits for your wine category and market with the official text, and base decisions on your own lab results.