For most of brewing history, sourness was a risk rather than a goal. Today, tart and funky beers are some of the most exciting things in the glass — and some of the most rewarding to brew. There are two broad routes: the fast, tidy kettle sour, and the slow, unpredictable world of mixed fermentation. Both are doable at home, provided you respect the microbes.
Meet the Microbes
| Microbe | What It Is | What It Brings | Speed |
|---|---|---|---|
| Saccharomyces | Regular brewer’s yeast | Alcohol, CO₂, fruity esters | Days |
| Lactobacillus | Lactic acid bacteria | Clean, yoghurt-like tartness | Hours to days |
| Pediococcus | Lactic acid bacteria | Deeper sourness; can make beer temporarily “ropy” or buttery | Months |
| Brettanomyces | Wild yeast | Funk, leather, hay, tropical fruit; ferments leftover sugars | Months to years |
| Acetobacter | Acetic acid bacteria | Vinegar — usually a flaw | Whenever oxygen gets in |
Most Lactobacillus strains are sensitive to hop bitterness, which is why sour wort usually starts out with little or no hops.
Route 1: The Kettle Sour
Kettle souring sours the wort before the main fermentation, then boils the bacteria away. The result is a clean, bright tartness in days rather than months — think Berliner weisse or gose.
Step by Step
- Brew a simple, low-gravity wort. Funkwerks, writing for the American Homebrewers Association, suggests around 1.040 and a scrupulously clean kettle, since even traces of hop acids can stall some strains.
- Give it a short boil. About 10–15 minutes sterilises the wort before souring.
- Pre-acidify. Drop the pH to roughly 4.0–4.5 with food-grade lactic acid. This gives Lactobacillus a head start and discourages unwanted bacteria.
- Cool to the strain’s preferred temperature. Many cultures like somewhere around 32–49 °C; some work as low as about 29 °C. Check your supplier’s guidance.
- Pitch a healthy culture. A starter of a few hundred millilitres speeds things along.
- Keep oxygen out. Purge the headspace with CO₂ and seal the kettle; some homebrewers also lay cling film directly on the wort surface.
- Wait and measure. Most strains get there in 24–48 hours. Craft Beer & Brewing suggests aiming for a final pH of about 3.0–3.7 and tasting as you go.
- Boil again, properly. A full boil kills the bacteria and is when you add hops.
- Ferment as normal — with a generous pitch of yeast, because acidic wort is a tough environment.
Kettle-Sour Pitfalls
- A warm, un-acidified wort left open can grow the wrong bacteria, bringing cheesy, rotten or vomit-like aromas. If it smells foul, dump it.
- Skipping the second boil means live bacteria will reach your fermenter and packaging.
- Buy a decent pH meter; tasting alone is guesswork.
Route 2: Mixed Fermentation
Mixed fermentation lets several organisms share the job. According to the Milk The Funk wiki, a common approach is to let Saccharomyces ferment for a few days, then add Brettanomyces, Lactobacillus and Pediococcus — though plenty of brewers pitch everything at once or simply add the dregs of an unpasteurised sour beer.
What to expect:
- Time — traditional complex sours often take nine to eighteen months; quicker versions can be ready in a couple of months
- A pellicle — a strange skin on the surface is normal and shows the microbes are working
- Oxygen control — a little is unavoidable, but too much invites vinegar and solvent-like notes, so fill vessels well and avoid permeable plastic for long ageing
- Patience before packaging — the wiki advises waiting until gravity stays stable across several readings over a month or two
What About Spontaneous Fermentation?
Belgian lambic brewers cool wort overnight in open vessels and let local microbes move in. It is possible to try this at home, but results vary enormously. Bottle dregs from commercial sours are a much more reliable way to capture a complex culture.
Keeping Your Clean Beers Clean
Cross-contamination is the big worry for homebrewers who also make crisp lagers and IPAs. In Craft Beer & Brewing, Dave Carpenter points out that Brettanomyces is easy to spread and hard to evict once it gets into scratched plastic.
- Stainless steel and glass can be shared safely if properly cleaned and sanitised
- Plastic buckets, tubing, gaskets, bottling wands and auto-siphons should be dedicated to sour and wild batches
- Label the “funky” kit clearly, perhaps with a coloured tape
- Clean immediately after use and replace scratched parts
Bottle-Bomb Warning
This is the serious safety bit. Brettanomyces and Pediococcus keep nibbling sugars that ordinary yeast leaves behind, so a beer that seems finished can keep producing CO₂ in the bottle for months.
- Package only when gravity has been steady for weeks, not days
- Use thick, pressure-rated bottles — Champagne-style or heavy Belgian bottles
- Store conditioning bottles in a closed box, away from people and pets
- If bottles gush or feel dangerously firm, chill them straight away and open them wearing eye protection and gloves
Legal Note
Homebrewing laws vary by country and region, so check what is allowed where you live. And whatever you brew, ferment it — never distill at home, which is illegal in many places and genuinely dangerous.
Key Takeaways
- Kettle sours are fast and clean: acidify, sour, boil, then ferment
- Mixed fermentations are slow, complex and need patience
- Dedicate plastic gear to funky batches to protect your clean beers
- Wild microbes keep working, so wait for stable gravity and use strong bottles