Fault
Acetic acid
Vinegar character produced by acetic acid bacteria, which need oxygen — so it always points to air ingress.
Also called vinegar, vinegary.
What it tastes and smells like
Responsible compound: acetic acid.
Where it may be expected
A measured acetic edge is part of the character of Flemish aged sours and some lambic, contributing to the balsamic comparison those beers earn. Vinegar in anything else is a fault, and even in these styles it is a matter of degree — dominant acetic character indicates a barrel that went too far.
Intensity and context decide. In the styles below a restrained version of this can be part of the character; the same character running past that is still a fault there.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- American wild ale — A broad New World category of mixed-fermentation and barrel-aged sour beers, often fruited, with no fixed base style.
- Flanders red ale — A deep red sour ale matured for years in oak foeders, balancing sharp acidity against dark fruit and a vinous depth.
- Fruit lambic — Lambic refermented on whole fruit — traditionally sour cherry or raspberry — producing a dry, tart, intensely fruity beer.
- Gueuze — A blend of young and old lambic, refermented in bottle — intensely complex, bone dry and Champagne-like in its carbonation.
- Old ale — A matured English strong ale with dried fruit, treacle and often a trace of oxidative or wild character from age.
- Oud Bruin — An East Flanders aged brown ale, maltier and less sharply acidic than Flanders red, with raisin and caramel depth.
If you are drinking it
A vinegary beer outside the Flemish sour styles has been exposed to oxygen and contaminated. It is harmless but unpleasant.
Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.
What causes it
| Cause | Stage | How often | Who can act |
|---|---|---|---|
| Acetobacter converting ethanol to acetic acid. It is strictly aerobic, so its presence always indicates oxygen reaching the beer. | conditioning | most common | brewer |
| Slow oxygen ingress through barrel staves during long wood ageing. | conditioning | common | brewer |
If you are brewing it
- Brewer: Eliminate oxygen ingress: keep vessels full, purge headspace with CO₂, and manage barrel topping-up during long maturation. (prevents)
Where in the process this comes from
Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.
- Barrel ageing — Mature beer in wooden barrels for the wood and spirit character they contribute, or for the microflora they harbour and the slow oxygen exchange they allow.
- Cask conditioning — Complete fermentation and carbonation inside the cask the beer will be served from, so it arrives at the drinker alive.
- Cellar conditioning — Receive, stillage, vent, fine and temperature-manage cask beer in the pub so it reaches the drinker in condition.
Often confused with
- Unintended acidity — sharp sourness, tart
- Infection — unexpected sourness, barnyard
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
Lactic acid reads as clean and yoghurt-like while acetic acid reads as sharp and vinegary, which is why a gose and a Flanders red are both sour but do not taste alike, and why acetic character in a cask ale indicates air ingress rather than intent.
Backed by a source
Sources: Tasting Beer (2nd edition) — print; The Oxford Companion to Beer — print