fermentation
Mixed fermentation
Ferment with a deliberately chosen combination of organisms — Saccharomyces with Brettanomyces, Lactobacillus and/or Pediococcus — over months or years.
What is actually happening
Saccharomyces does the primary fermentation. Lactobacillus and Pediococcus then produce lactic acid over subsequent months, and Brettanomyces slowly consumes carbohydrates the others cannot, developing the barnyard and stone-fruit character over a much longer timescale.
The Pediococcus phase produces a viscous "ropy" stage that alarms brewers new to the technique. It resolves as Brettanomyces breaks the material down; it is a stage, not a failure.
The organisms are difficult to remove from wood and soft plant components, which is why breweries doing this work usually keep separate equipment.
Typical conditions
| Duration | 6–36 months |
|---|---|
| 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.
- 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
- Flanders red ale
A deep red sour ale matured for years in oak foeders, balancing sharp acidity against dark fruit and a vinous depth.
- Oud Bruin
An East Flanders aged brown ale, maltier and less sharply acidic than Flanders red, with raisin and caramel depth.
- American wild ale
A broad New World category of mixed-fermentation and barrel-aged sour beers, often fruited, with no fixed base style.
- Gueuze
A blend of young and old lambic, refermented in bottle — intensely complex, bone dry and Champagne-like in its carbonation.
- Lambic
Spontaneously fermented Belgian wheat beer, matured in oak for years — funky, acidic, dry and unlike any other beer.
- Fruit lambic
Lambic refermented on whole fruit — traditionally sour cherry or raspberry — producing a dry, tart, intensely fruity beer.
- Kettle sour
Beer soured with Lactobacillus in the kettle over hours rather than months, then boiled and fermented normally — usually fruited.
Related steps and concepts
- Spontaneous fermentation — The uncontrolled equivalent.
- Barrel ageing — Where it usually happens.
- Brettanomyces — The organism that defines the late character.
Also at the fermentation stage
Biotransformation · Bottom fermentation · Diacetyl rest · High-gravity brewing · Hop creep · Open 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.