Bottom fermentation

What is actually happening

S. pastorianus is a hybrid carrying cold tolerance from S. eubayanus, which was not found in the wild until 2011. That tolerance is what made cold fermentation in Bavarian cellars possible at all.

At 8–12 °C ester and phenol production is minimal. The trade-off is that fermentation takes considerably longer and that there is nowhere for a process fault to hide — which is why lager is often described as the hardest beer to brew well.

Sulphur compounds are produced in quantity during cold fermentation and are normal at this stage; they leave during lagering.

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.

• Diacetyl — A buttery compound with a very low threshold — normally a sign of rushed fermentation, but traditional at trace levels in some Czech lagers.
• Excess sulphur — Sulphur compounds from fermentation — normal and transient in young lager, a fault when they persist.
• DMS — A cooked-vegetable note from a malt precursor that a vigorous open boil should have driven off.

Styles where this step is decisive

• Munich Helles4.7–5.4% ABV

  Bavaria’s everyday pale lager: soft, bready and malt-forward, with just enough hop to balance and nothing to distract.

• German Pils4.4–5.4% ABV

  Pale, dry and firmly bitter, with a crisp finish and a spicy-floral noble hop character — Germany’s answer to the Bohemian original.

• Schwarzbier4.4–5.4% ABV

  Nearly black but remarkably light on the palate: smooth roast, clean lager finish and none of the heaviness the colour suggests.

Related steps and concepts

• Lagering — The cold maturation that must follow.
• Diacetyl rest — The step that prevents a butterscotch note.
• Top fermentation — The warm-fermented counterpart.