Sour mashing

What is actually happening

Malt carries lactic acid bacteria on its husk. Held at around 45 °C without oxygen, they multiply and produce lactic acid, and the mash sours over a day or two with nothing added.

It is the older route to acidity and the less controllable one. What grows is whatever was on the grain, so alongside the lactic acid there is a real chance of the sulphurous and faecal notes that give sour mashing its reputation — the technique is remembered as smelling awful because it frequently did.

Kettle souring replaced it by taking the same idea and controlling the population: boil first, cool, pitch a known Lactobacillus culture, then boil again. Same acid, chosen organism.

BeerHQ keeps both because they are not the same process and the difference is exactly what a reader asking about sour beer needs to understand.

Typical conditions

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.
• Excess sulphur — Sulphur compounds from fermentation — normal and transient in young lager, a fault when they persist.

Styles where this step is decisive

• Berliner Weisse2.8–3.8% ABV

  A very low-strength sour wheat beer: sharply lactic, bone dry, lightly sparkling and enormously refreshing.

• Gose4–5% ABV

  A lightly sour German wheat beer with added salt and coriander — tart, savoury and unusually thirst-quenching.

• Kentucky Common4–5.5% ABV

  A quick-turnaround dark American beer brewed largely from maize, often lightly soured, extinct since Prohibition.

Related steps and concepts

• Kettle souring — The controlled technique that replaced it.
• Mashing — The step it extends.