The Wonder of Wild Fermentation: A Journey into Natural Yeasts
A wild beer is fermented partly or entirely by microbes other than clean brewer’s yeast. Here are the microbes involved, where to get them, what each tastes like, and how to try it without infecting your other beers.
In this post
The short answer: a wild beer is fermented, in part or entirely, by microbes other than clean brewer’s yeast: wild yeast such as Brettanomyces and bacteria such as Lactobacillus and Pediococcus. They can come from a lab culture, the dregs of a bottle, a house culture you keep going, fruit, barrels, or the air itself (spontaneous fermentation, as in lambic). It’s slow, often months to years, and gives sour, funky, fruity and earthy flavours.
| Microbe | What it does | Flavour |
|---|---|---|
| Wild Saccharomyces | Most of the alcohol, early on | Fruity, spicy, sometimes phenolic |
| Brettanomyces | Slowly ferments sugars other yeast leave behind | Funky, barnyard, leather, tropical fruit; very dry |
| Lactobacillus | Makes lactic acid | Clean, yogurt-like sourness |
| Pediococcus | Makes lactic acid; can make diacetyl and a ropy texture early on | Sharper sourness |
| Acetobacter | Turns alcohol into acetic acid when oxygen gets in | Vinegar; usually a fault |
Keep separate soft parts (hoses, gaskets, plastic fermenters) for wild beers. Stainless steel cleans fully and can be shared once it’s properly cleaned and sanitized.
Wild, mixed, spontaneous, kettle sour: what the words mean
| Term | What it means | Example |
|---|---|---|
| Wild | Any beer fermented with Brett, bacteria or other non-brewer’s yeast, from any source | Brett saison, farmhouse ale with a house culture |
| Mixed fermentation | Brewer’s yeast, Brett and bacteria working together, usually pitched on purpose | Flanders red |
| Spontaneous | Nothing pitched: microbes come from the air and the barrels | Lambic, gueuze. See spontaneous fermentation |
| Kettle sour | Lactobacillus sours the wort, then a boil kills it before clean yeast goes in | Many modern Berliner weisse and gose. See kettle souring |
| 100% Brett | Brett as the only yeast | Dry, fruity pale ales |
In the BJCP guidelines, most of these beers sit in Category 23 (European Sour Ale) and Category 28 (American Wild Ale). See our overview of the BJCP style guidelines.
Where to get wild microbes
| Source | What you get | Predictability |
|---|---|---|
| Lab culture | Single Brett or bacteria strains, or blends such as Wyeast 3763 Roeselare Ale Blend, Wyeast 3278 Belgian Lambic Blend or WLP655 Belgian Sour Mix I | High |
| Bottle dregs | Whatever survived in an unpasteurized sour or Brett beer; pour the last bit into a small starter | Medium |
| House culture | Re-pitch the yeast cake from a wild beer you liked, or keep a solera: draw off part of the beer and top up with fresh wort | Medium, and it changes over time |
| Captured culture | Yeast from your own air, fruit or flowers (below) | Low: test it small first |
| Spontaneous | A whole batch left open overnight | Lowest |
How to capture your own yeast
- Make a small, weak wort, about 1.030–1.040, with a little hop bitterness. The hops hold back most Lactobacillus, which gives yeast the upper hand. Boil it and cool it.
- Split it between several sanitized jars.
- Leave them open overnight outdoors, near fruit trees or flowers, or drop in a few unwashed berries or blossoms.
- Close them with airlocks and wait a few days to two weeks.
- Smell every jar. Keep the ones that smell clean, fruity, bready or pleasantly funky. Dump anything that smells of vomit, rot, nail polish or strong vinegar, and anything with fuzzy mould.
- Step the best jar up to a bigger starter, taste the fermented starter, and only then pitch a full batch.
What to expect while it ferments
- A pellicle. A white, wrinkled or bubbly skin on top is normal in wild beers. Leave it alone. Fuzzy, raised spots in green, blue or black are mould: dump the batch.
- Time. Primary looks normal, then the gravity keeps creeping down for months as Brett and bacteria work. Package only when the gravity is stable over a few weeks, or bottles can over-carbonate.
- Temperature. A steady room temperature is fine. Avoid big swings and heat.
- Oxygen. Keep the fermenter sealed and the headspace small. Too much air turns wild beer into vinegar.
Keeping wild microbes out of your clean beers
Stainless fermenters, racking arms and fittings clean and sanitize fully, so the same stainless tank can make a wild beer and then a clean lager. Soft parts hold microbes in tiny scratches and pores. Keep a labelled set just for wild beers: tri-clamp gaskets, silicone tubing, siphons, bottling wands and any plastic fermenter. Wood that has held a wild beer stays wild. More in our guide to cleaning and sanitizing a fermenter.
Frequently asked questions
Is wild beer safe to drink?
Yes, once it has fermented. Beer’s alcohol, acidity and hops keep out the microbes that make people sick. Anything that smells rotten or grows fuzzy mould gets dumped, not tasted.
Is kveik a wild yeast?
No. Kveik is traditional Norwegian farmhouse yeast, a Saccharomyces culture passed down for generations. It ferments fast, even warm, without Brett funk or sourness. See our kveik guide.
How long does a wild beer take?
A 100% Brett beer can be ready in weeks. Mixed-fermentation sours usually take 6 to 12 months, and spontaneous beers a year or more.
Can I use my stainless fermenter for wild beers?
Yes. Clean and sanitize it properly afterwards and swap in your wild-only gaskets and hoses while the wild beer is in it.
Related guides
Spontaneous fermentation, Brettanomyces, lactic acid bacteria, barrel fermentation, fruit beers.