fermentation
Spontaneous fermentation
Allow ambient microorganisms to inoculate cooling wort rather than pitching a culture, producing lambic and its derivatives.
What is actually happening
- Cooled overnight in a coolship. Hot wort is run into a very shallow open pan so it cools quickly over one night, taking up organisms from the air and the fabric of the brewery. This is done in the cooler months, because summer air carries a different population.
- Into barrel. The inoculated wort goes to oak. The barrels are not sterile and are part of the culture rather than merely a container.
- A succession, over years. Different organisms dominate in turn — first the ones that ferment the simple sugars, then those that work slowly through what is left. One to three years, not weeks.
Wort is run into a coolship and left overnight, exposed to the air of the brewery and its surroundings. What settles into it is a succession of organisms rather than one: enterobacteria first, then Saccharomyces, then Pediococcus, then Brettanomyces over months and years.
It is seasonal because the air is. Summer carries the wrong organisms in the wrong proportions, so traditional producers brew only in the cooler months.
Typical conditions
| Duration | 12–36 months |
|---|---|
| Scale | A commercial-scale operation |
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.
- Mousy taint — An unpleasant taint of mixed-fermentation beer, characteristically tasted rather than smelled and arriving late on the finish.
Styles where this step is decisive
- Lambic
Spontaneously fermented Belgian wheat beer, matured in oak for years — funky, acidic, dry and unlike any other beer.
- Gueuze
A blend of young and old lambic, refermented in bottle — intensely complex, bone dry and Champagne-like in its carbonation.
- Fruit lambic
Lambic refermented on whole fruit — traditionally sour cherry or raspberry — producing a dry, tart, intensely fruity beer.
- Sahti
A Finnish farmhouse ale filtered through juniper branches and fermented with baking yeast — banana, juniper and no hops.
Related steps and concepts
- Coolship inoculation — The step that performs the inoculation.
- Turbid mashing — The mash that feeds the long fermentation.
- Mixed fermentation — The pitched-culture equivalent.
Also at the fermentation stage
Biotransformation · Bottom fermentation · Diacetyl rest · High-gravity brewing · Hop creep · Mixed fermentation · Open fermentation · Pitching · Pressure fermentation · Primary fermentation · Top fermentation · Yeast harvesting
See the whole process in order on how beer is made.
Last reviewed 2026-08-16.