Glossary
Acetaldehyde
A compound tasting of green apple or fresh-cut pumpkin, an intermediate in fermentation that healthy yeast normally converts onward into ethanol.
Pronounced ass-uh-TAL-duh-hide.
Also called green apple, green beer.
The full record
Acetaldehyde has its own catalogue entry with the detail this page summarises — causes, thresholds, related styles and evidence.
Acetaldehyde — A green-apple note indicating beer served before fermentation finished — the classic sign of a rushed product.
Its presence usually means the beer was taken off the yeast too early, or that fermentation was rushed or unhealthy. The traditional term "green beer" is literal: the beer has not finished.
It is often fixable rather than fatal. Left in contact with healthy yeast at a reasonable temperature, the compound is usually consumed.
Related
- Conditioning — The period during which it clears.
- Acetaldehyde — The fault record.
More vocabulary
Autolysis · Bottle bomb · Diacetyl · DMS · Gushing · Lightstrike · Oxidation · Staling
See every term on the glossary index.
Last reviewed 2026-08-16.