Vinegar notes in beer and the acetic acid bacteria behind them

What vinegar tastes like in beer

The note is easy to spot once you know it: sharp, sour and pungent, like malt vinegar on chips. It catches the back of the nose before you sip. On the palate it bites rather than refreshes and it often comes with a solvent edge of nail polish from ethyl acetate, which tends to travel with it.

That sharpness is acetic acid, the same acid that makes vinegar. Lactic acid, the main acid in most sour beers, is softer and rounder, closer to yoghurt. If a sour beer tastes clean and tangy, that is lactic. If it stings, there is acetic in it too.

The bacteria behind it

Vinegar is made by acetic acid bacteria, a group that includes Acetobacter and Gluconobacter. They take ethanol and oxygen and turn them into acetic acid. That is exactly how vinegar is made from wine or cider and it is why vinegar in beer is nearly always a story about air.

These bacteria are everywhere: on fruit, in the air and especially around fruit flies, which carry them from place to place. They tolerate alcohol and acidity well, so normal beer offers them no protection. What it does offer is a lack of oxygen. Sealed under carbon dioxide, a beer gives them very little to work with. Let air in and they wake up.

Where air gets in

Most vinegar problems trace back to one of these:

• Draught lines and taps. Lines that are not cleaned on a regular cycle build up a film where bacteria live. A tap left dripping and sticky between services becomes a home for fruit flies.
• Air-driven dispense. A keg pushed with an air pump rather than carbon dioxide will start to sour within a day or two in warm weather.
• Casks. Traditional cask ale draws air in as it is served, which is why a cask has a short life once opened.
• Open fermentation and poor seals. Open fermenters, loose lids and badly sealed barrels let air and fruit flies in.
• Half-empty bottles. An opened growler or a swing top left half full in the fridge for a week.

Heat speeds the whole thing up. A keg left at 30 °C in an Indian summer turns far faster than one kept in a cold room. Our guide to draught line cleaning covers the cleaning cycle that keeps bars clear of it.

When acetic is welcome

Some styles carry a measured acetic note on purpose. Flanders red and oud bruin, aged for months in large oak vessels, often show a light, wine-like acetic tang alongside fruit and caramel. Some lambics have it too. In those beers a small amount adds lift and complexity. Too much and even a Flanders red turns harsh.

The rule of thumb for drinkers: if a beer is sold as a clean lager, pale ale, IPA or stout, vinegar is a fault. If it is a deliberately aged sour from a style with acetic character, judge the balance. A gentle edge can be right. Stinging sharpness is not.

Telling vinegar from other sour notes

Vinegar is often confused with lactic sourness and with the nail polish smell of ethyl acetate. A quick way to separate them:

• Lactic. Clean, soft, yoghurt-like. Common in Berliner weisse and gose.
• Acetic. Sharp, pungent, malt vinegar. Catches the nose.
• Ethyl acetate. Sweet solvent, nail polish, pear drops. Often comes with acetic but is a separate compound, covered in our guide to nail polish notes from ethyl acetate.

If you are unsure, smell a teaspoon of malt vinegar diluted in a glass of water, then smell the beer. The match is usually obvious.

What to do about it

At home or in a bar, an acetic beer cannot be fixed. Send it back, or pour it away if it is your own bottle. For a bar, a run of vinegary pints from one tap means the line needs cleaning and the coupler and tap need stripping. For a brewery, the search runs through anything that lets air reach finished beer: barrels, hoses, fittings and dispense.

Brewers check for acetic acid bacteria by plating samples on suitable media and incubating them with air present. A growth on those plates from finished beer tells the brewery where its oxygen problem is.

Your first step: dilute a teaspoon of malt vinegar in a glass of water and smell it. That one reference will let you name acetic notes in beer for good.