Lactic Acid Bacteria in Sour Beer Production
Lactic acid bacteria make beer sour. Here is how Lactobacillus and Pediococcus differ, and how to kettle sour step by step with target pH, temperature and time.
In this post
The short answer: lactic acid bacteria, mainly Lactobacillus and Pediococcus, turn sugar into lactic acid and make beer sour. The quickest method is kettle souring: pitch Lactobacillus into unhopped wort at about 95 to 115°F (35 to 46°C), depending on the strain, hold it until the pH drops to about 3.2 to 3.5, usually in 1 to 3 days, then boil to kill the bacteria before fermenting as normal. Hops inhibit Lactobacillus, so sour before you add them.
| Lactobacillus | Pediococcus | |
|---|---|---|
| Sourness | Clean, yogurt-like | Sharper, more intense |
| Speed | Fast when warm | Slow, over months |
| Hop tolerance | Low | Higher |
| Side effects | Few | Can make diacetyl and a ropy texture that Brettanomyces later cleans up |
| Used in | Kettle sours, Berliner weisse, gose | Barrel-aged and mixed-fermentation sours |
Kettle souring, step by step
- Mash as normal and collect the wort. Keep the recipe simple: pale malt and some wheat is typical for a Berliner weisse or gose.
- Bring the wort to a boil for a few minutes, with no hops. This kills the bacteria that live on grain, so only your culture does the souring.
- Chill to your culture’s temperature, about 100°F (38°C) for most.
- Pre-acidify to about pH 4.5 with food-grade lactic acid. That keeps out the bacteria that make rotten, vomit-like flavours while the Lactobacillus gets going.
- Pitch the Lactobacillus.
- Keep oxygen out. Lid on, headspace purged with CO₂; some brewers lay plastic wrap right on the wort. Don’t recirculate while it sours.
- Hold the temperature and check the pH once or twice a day with a calibrated pH meter.
- At pH 3.2–3.5, boil. That kills the bacteria. Add your hops now, if any, then chill.
- Pitch a healthy, full-size pitch of clean yeast and ferment as normal. Very acidic wort can slow some yeasts down.
Because the boil kills everything before the wort reaches the fermenter, kettle souring is the lowest-risk way to make a sour at home. More on reading and adjusting acidity in our guide to beer pH.
Which culture to use
| Culture | Examples | Use it for |
|---|---|---|
| Lactobacillus | WLP677 (L. delbrueckii), WLP672 (L. brevis), LalBrew WildBrew Sour Pitch (L. plantarum) | Kettle sours. Each strain has its own best temperature: check the pack. |
| Pediococcus | WLP661 (P. damnosus) | Long mixed fermentations. Always pair it with Brett. |
| Mixed blends | Wyeast 3763 Roeselare Ale Blend, Wyeast 3278 Belgian Lambic Blend, WLP655 Belgian Sour Mix I | Flanders red, lambic-style and barrel sours |
| Souring yeast | LalBrew WildBrew Philly Sour (Lachancea) | A yeast that makes lactic acid itself: sour beer with no bacteria at all |
Souring in the fermenter
The slow, traditional way is to let the bacteria work alongside the yeast, usually with Brett, for months. Pediococcus handles hops, so these beers can be hopped normally. Pedio alone leaves diacetyl and a slick, ropy texture; Brett cleans both up with time, which is why the two go together. Expect 6 to 12 months or more. Read more about Brettanomyces and wild and mixed fermentation.
Live bacteria in the fermenter means gear to keep separate. Stainless cleans and sanitizes normally. Soft parts don’t: keep a labelled set of tri-clamp gaskets and silicone tubing just for sours.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| pH barely drops | Hops in the wort, too cool, or a small or old culture | Sour unhopped wort, hold the culture’s temperature, pitch fresh bacteria |
| Smells of vomit, rot or baby diapers | Other bacteria took over | Dump it. Next time pre-acidify to pH 4.5, keep oxygen out and hold temperature |
| Yeast slow to start after souring | Very low pH | Pitch more healthy yeast; don’t sour much below pH 3.2 |
| Buttery | Diacetyl from Pediococcus | Give the Brett time to clean it up |
| Slick, oily, stringy | Ropy phase from Pediococcus | Wait: it usually clears as the Brett works |
Frequently asked questions
Is kettle souring safe?
Yes. Pre-acidifying keeps unwanted bacteria out, and the boil afterwards kills everything. If it smells rotten or like vomit, don’t try to save it.
Can I sour with yogurt or grain?
Some brewers do, but you get whatever came with it, including bacteria you don’t want. A lab culture is more predictable and costs little compared with a dumped batch.
Do I need a pH meter?
For kettle souring, yes. A calibrated meter tells you when to stop; strips aren’t precise enough in this range.
What’s the difference between a kettle sour and a mixed-fermentation sour?
A kettle sour is soured in a day or two and then boiled, so it tastes clean and sharp. A mixed-fermentation sour keeps its bacteria and Brett for months, and tastes more complex and funky.
Related guides
Beer pH, wild fermentation, spontaneous fermentation, fruit beers, cleaning and sanitizing a fermenter.