Glossary
Diacetyl
A compound tasting of butter or butterscotch, produced during fermentation and normally reabsorbed by healthy yeast before packaging.
Pronounced dye-ASS-uh-teel.
Also called butterscotch, buttery, vdk, vicinal diketone.
The full record
Diacetyl has its own catalogue entry with the detail this page summarises — causes, thresholds, related styles and evidence.
Diacetyl — A buttery compound with a very low threshold — normally a sign of rushed fermentation, but traditional at trace levels in some Czech lagers.
Yeast produces a precursor during its growth phase, which converts to diacetyl outside the cell; healthy yeast then takes it back up and reduces it to something flavourless. A diacetyl problem is usually a fermentation-management problem — cooling too early, cropping yeast too soon, or unhealthy yeast — rather than a contamination problem, though Pediococcus and Lactobacillus also produce it.
At high levels it is unmistakably a fault, slick and buttery with a coating mouthfeel. At low levels it is traditional and expected in some Czech pale lagers and some Yorkshire and Scottish ales, where it contributes a rounded, buttery softness that the brewers concerned are not trying to remove.
It is also one of the few beer compounds with a genuine occupational-health literature, arising from inhalation exposure in food manufacturing rather than from drinking beer.
Commonly misunderstood
Frequently reported as a universal fault. A trace is part of the house character of several respected traditional beers; it is the level and the style that decide.
Related
- Diacetyl rest — The step that clears it.
- Diacetyl — The fault record, with thresholds and causes.
- Hop creep — A modern cause of it reappearing after packaging.
More vocabulary
Acetaldehyde · Autolysis · Bottle bomb · DMS · Gushing · Lightstrike · Oxidation · Staling
See every term on the glossary index.
Last reviewed 2026-08-16.