Fault
Acetaldehyde
A green-apple note indicating beer served before fermentation finished — the classic sign of a rushed product.
Also called green apple, green beer.
What it tastes and smells like
Responsible compound: acetaldehyde.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- Belgian single — The low-strength pale beer brewed for the monastery’s own table, dry and hoppy with full Belgian yeast character.
- Dunkelweizen — A dark Bavarian wheat beer where banana and clove meet bread crust and light caramel from Munich malt.
- Kölsch — Cologne’s pale hybrid beer: ale-fermented then lagered, giving lager clarity and crispness with a whisper of fruit.
- Kristallweizen — A filtered Weissbier: the same banana-and-clove character in a brilliantly clear beer with a lighter, crisper body.
- Munich Helles — Bavaria’s everyday pale lager: soft, bready and malt-forward, with just enough hop to balance and nothing to distract.
- Roggenbier — A German rye beer, spicy and full-bodied, fermented with Weissbier yeast for banana and clove alongside rye’s pepper.
- Weissbier — Bavarian wheat beer defined by its yeast: pronounced banana and clove over a soft, cloudy, highly carbonated body.
- Weizenbock — A strong wheat beer combining Weissbier yeast character with bock malt depth — banana and clove over dark fruit and bread.
If you are drinking it
A sharp green-apple note usually means the beer was released too young. It is harmless, and in a bottle-conditioned beer it will often diminish with a few more weeks in the bottle.
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 |
|---|---|---|---|
| Packaging or serving before yeast has converted acetaldehyde through to ethanol. It is a normal intermediate, not a contaminant, and time usually fixes it. | conditioning | most common | brewer |
| Filtering or crashing the yeast out before it completed the clean-up phase. | conditioning | common | brewer |
| Severe oxidation can convert ethanol back to acetaldehyde, so a very stale beer can develop it in the package. | consumer storage | occasional | consumer |
If you are brewing it
- Brewer: Allow fermentation to complete fully and give the yeast time in contact with the beer before cooling or filtering. (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.
- Conditioning — Mature beer after primary fermentation so flavours settle, harsh young compounds are cleared and carbonation reaches equilibrium.
- Primary fermentation — Convert wort sugars into ethanol, carbon dioxide and the esters, phenols and higher alcohols that give beer much of its character.
- Wort oxygenation — Dissolve oxygen into cooled wort before pitching, because yeast needs it to build the cell membranes required for healthy growth.
Often confused with
- Oxidation — wet cardboard, paper
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.
Acetaldehyde is a normal intermediate in alcoholic fermentation that yeast converts onward to ethanol, so its presence in finished beer generally indicates the beer left the yeast before fermentation and clean-up were complete.
Backed by a source
Sources: Yeast: The Practical Guide to Beer Fermentation — print; Beer: Tap into the Art and Science of Brewing — print
Unlike most off-flavours, acetaldehyde is often recoverable: left in contact with healthy yeast at a reasonable temperature, the compound is generally consumed.
Backed by a source
Source: Yeast: The Practical Guide to Beer Fermentation — print