mashing
Kettle souring
Sour wort with Lactobacillus before the boil, then boil to kill the bacteria and ferment clean — producing lactic acidity in days rather than years.
Also called quick souring, sour worting.
What is actually happening
- Cool the wort and purge the oxygen. Wort is brought down to around 40 °C — warm enough for Lactobacillus, too warm for brewing yeast — and the headspace is purged. Oxygen at this stage is what turns a clean lactic sourness into vinegar.
- Pitch Lactobacillus and hold. Held at 35–45 °C for one to three days while the pH falls to a target, typically 3.2–3.5. The brewer is watching a number rather than a clock: souring stops when the pH is right, not when the time is up.
- Boil, which kills the bacteria. An ordinary boil follows, and it ends the souring permanently. This is the step a mixed-fermentation beer never gets, and the reason a brewery making clean beer can run this on shared equipment.
- Ferment clean, package finished. Brewing yeast takes it from there, and nothing acid-producing reaches the fermenter or the bottle. The beer is complete at packaging — which is why a kettle sour has a short, fresh window and is not a cellaring candidate.
Wort is cooled to around 40 °C, purged of oxygen, inoculated with Lactobacillus and held until the pH drops to the target, typically 3.2–3.5. It is then boiled normally, which kills the bacteria.
Because the boil follows the souring, no live souring culture reaches the fermenter or the package. This is what makes the technique safe to run in a brewery that also makes clean beer, and it is why a kettle-soured gose is a fresh beer with a short window rather than a cellaring candidate.
Typical conditions
| Temperature | 35–45°C |
|---|---|
| Duration | 1–3 days |
| 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.
- Unintended acidity — Lactic sourness in a style that should not have it — a fault of intent rather than of the organism responsible.
- Infection — Unintended microbial growth — which is a fault only because it was unintended, since the same organisms define lambic.
Styles where this step is decisive
- Gose
A lightly sour German wheat beer with added salt and coriander — tart, savoury and unusually thirst-quenching.
- Berliner Weisse
A very low-strength sour wheat beer: sharply lactic, bone dry, lightly sparkling and enormously refreshing.
- Kettle sour
Beer soured with Lactobacillus in the kettle over hours rather than months, then boiled and fermented normally — usually fruited.
- Fruit lambic
Lambic refermented on whole fruit — traditionally sour cherry or raspberry — producing a dry, tart, intensely fruity beer.
Questions
Is a kettle-soured beer worth ageing?
Generally not. The acidity is stable, but nothing is alive in the package to develop anything further, and the delicate aromatics that make the style enjoyable fade. This is the practical difference between a kettle sour and a lambic: one is finished at packaging, the other is not.
Related steps and concepts
- Mixed fermentation — The slow alternative that leaves live culture.
- Lactobacillus — The organism used.
Also at the mashing stage
Decoction mash · Infusion mash · Mash pH adjustment · Mash-out · Mashing · Sour mashing · Step mash · Turbid mashing · Water treatment
See the whole process in order on how beer is made.
Last reviewed 2026-08-16.