fermentation
Open fermentation
Ferment in a vessel open to the air, allowing krausen to be skimmed and yeast to be cropped from the surface.
What is actually happening
The carbon dioxide blanket produced by an active fermentation sits over the beer and protects it, so the contamination risk is lower than it sounds — though it is not zero and the room has to be clean.
Skimming the brown scum from the krausen removes bitter, harsh hop resins and dead cells, which is a real quality intervention rather than a ritual.
The vessel geometry differs too: open fermenters are wide and shallow, which changes hydrostatic pressure and yeast behaviour compared with a tall cylindroconical.
What goes wrong here
Risks rather than certainties. This step does not cause these faults; getting it wrong can.
- Infection — Unintended microbial growth — which is a fault only because it was unintended, since the same organisms define lambic.
Styles where this step is decisive
- Ordinary bitter
The everyday British cask ale: low in strength, gently bitter, and built to be drunk in pints over a whole evening.
- Kölsch
Cologne’s pale hybrid beer: ale-fermented then lagered, giving lager clarity and crispness with a whisper of fruit.
- Altbier
Düsseldorf’s copper-coloured hybrid: ale-fermented and cold-conditioned, with firm bitterness over nutty, toasty malt.
- Best bitter
A step up from ordinary bitter in strength and depth, with more malt body while staying firmly sessionable.
- Strong bitter
The strongest of the bitters, with pronounced malt richness, firm bitterness and noticeable fruit from warm fermentation.
- Dark mild
A low-strength dark British ale, gently sweet and nutty, built for drinking in quantity after physical work.
- Weissbier
Bavarian wheat beer defined by its yeast: pronounced banana and clove over a soft, cloudy, highly carbonated body.
- Saison
Dry, peppery and highly carbonated, with a bracing finish — the most refreshing thing strong beer does.
Related steps and concepts
- Coolship inoculation — The most extreme form of open vessel.
- Krausen — What gets skimmed.
Also at the fermentation stage
Biotransformation · Bottom fermentation · Diacetyl rest · High-gravity brewing · Hop creep · Mixed fermentation · Pitching · Pressure fermentation · Primary fermentation · Spontaneous fermentation · Top fermentation · Yeast harvesting
See the whole process in order on how beer is made.
Last reviewed 2026-08-16.