fermentation
Primary fermentation
Convert wort sugars into ethanol, carbon dioxide and the esters, phenols and higher alcohols that give beer much of its character.
What is actually happening
Yeast first consumes the simple sugars, then maltose, and finally maltotriose if the strain can. Dextrins are left untouched, which is why mash temperature sets the floor on final gravity.
Temperature is the brewer’s main control over flavour. Warmer fermentation produces more esters and fusel alcohols; cooler fermentation suppresses both, which is why lagers taste clean and expose the malt and hops with nothing to hide behind.
The rate is not constant. A vigorous phase marked by high krausen is followed by a slower tail in which the yeast finishes the remaining sugar and cleans up intermediates such as acetaldehyde and diacetyl. Ending the fermentation during that tail is one of the most common causes of a green-tasting beer.
Typical conditions
| Temperature | 8–24°C |
|---|---|
| Duration | 3–14 days |
| Scale | Used at both homebrew and commercial scale |
Ranges, not targets. These describe what is usual across the styles and breweries that use this step, and a value outside the range is a decision rather than an error.
What goes wrong here
Risks rather than certainties. This step does not cause these faults; getting it wrong can.
- Acetaldehyde — A green-apple note indicating beer served before fermentation finished — the classic sign of a rushed product.
- Diacetyl — A buttery compound with a very low threshold — normally a sign of rushed fermentation, but traditional at trace levels in some Czech lagers.
- Fusel alcohol — Harsh, solvent-like alcohol heat from fermentation run too warm or under stress — distinct from the pleasant warmth of a strong beer.
- Stalled fermentation — Fermentation stopping well short of terminal gravity — leaving a sweet, underdeveloped beer and a package-pressure risk.
- Soapy — A soapy note from fatty acids released by yeast left too long on the trub, or from detergent residue in glassware.
Related steps and concepts
- Conditioning — The maturation that follows.
- Top fermentation — The ale variant.
- Bottom fermentation — The lager variant.
- Krausen — The foam marking its most active phase.
Also at the fermentation stage
Biotransformation · Bottom fermentation · Diacetyl rest · High-gravity brewing · Hop creep · Mixed fermentation · Open fermentation · Pitching · Pressure fermentation · Spontaneous fermentation · Top fermentation · Yeast harvesting
See the whole process in order on how beer is made.
Last reviewed 2026-08-16.