Top fermentation

What is actually happening

Ale yeast works fastest between about 18 and 22 °C, and most strains rise with the krausen — historically allowing brewers to crop yeast from the surface for the next brew.

Warm fermentation raises ester production sharply. The same strain at 18 °C and 24 °C produces recognisably different beer, which is why fermentation temperature control matters more to an ale than almost any recipe decision.

Typical conditions

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.

• Fusel alcohol — Harsh, solvent-like alcohol heat from fermentation run too warm or under stress — distinct from the pleasant warmth of a strong beer.
• Solvent — A solvent-like ester that reads as pear drop at low levels and as nail varnish remover at high ones.

Styles where this step is decisive

• Ordinary bitter3.2–3.8% ABV

  The everyday British cask ale: low in strength, gently bitter, and built to be drunk in pints over a whole evening.

• American pale ale4.5–6.2% ABV

  The beer that defined American craft brewing: clean pale malt under bright citrus and pine hop character, balanced rather than extreme.

• Weissbier4.3–5.6% ABV

  Bavarian wheat beer defined by its yeast: pronounced banana and clove over a soft, cloudy, highly carbonated body.

Related steps and concepts

• Bottom fermentation — The cold-fermented counterpart.
• Ester — What warm fermentation produces more of.