fermentation
Diacetyl rest
Raise a fermentation’s temperature near its end so the yeast reabsorbs and reduces the diacetyl it produced earlier.
What is actually happening
During growth, yeast excretes a precursor which spontaneously converts to diacetyl outside the cell. Healthy yeast then takes the diacetyl back up and reduces it to compounds with a far higher flavour threshold.
That clean-up is slow at lager temperature. Raising the beer to 16–20 °C for a couple of days once fermentation is nearly complete accelerates it, and only then is the beer crashed for lagering.
Cooling before the rest is complete is precisely how a butterscotch note gets locked into a finished lager.
Typical conditions
| Temperature | 16–20°C |
|---|---|
| Duration | 2–4 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.
- Diacetyl — A buttery compound with a very low threshold — normally a sign of rushed fermentation, but traditional at trace levels in some Czech lagers.
Styles where this step is decisive
- German Pils
Pale, dry and firmly bitter, with a crisp finish and a spicy-floral noble hop character — Germany’s answer to the Bohemian original.
- Munich Helles
Bavaria’s everyday pale lager: soft, bready and malt-forward, with just enough hop to balance and nothing to distract.
- Czech premium pale lager
The original pale lager: rich bready malt, generous Saaz hopping and a rounded bitterness that soft Bohemian water makes possible.
- Czech pale lager
The Czech everyday lager: lower in strength than the premium version but keeping the soft bready malt and generous Saaz hopping.
- Munich Dunkel
Deep brown Bavarian lager led by rich Munich malt — bready and gently chocolatey, without real roast bitterness.
- Schwarzbier
Nearly black but remarkably light on the palate: smooth roast, clean lager finish and none of the heaviness the colour suggests.
- Märzen
A rich amber lager brewed in March for autumn drinking, with deep toasty malt and a clean, firm finish.
- Festbier
The golden, stronger lager actually served at modern Oktoberfest — malt-forward and smooth, but pale rather than amber.
- Vienna lager
An amber lager built on gently kilned Vienna malt: toasty and lightly sweet, but finishing clean and dry.
- Dortmunder Export
A pale lager sitting between Helles and Pils: fuller and firmer than the first, softer and less bitter than the second.
- Bock
A strong, dark, malt-dominated lager with deep bread-crust and toasty character and only enough hop to keep it from being sweet.
- Doppelbock
Intensely malty strong lager, dense with bread crust, caramel and dark fruit, and remarkably clean for its strength.
- Helles Bock
A pale, strong lager with the body of a bock but a brighter, more hop-balanced character — traditionally a spring beer.
- Kellerbier
Unfiltered, gently carbonated lager served young from the cellar, with a soft yeasty fullness filtration removes.
- Czech amber lager
A Bohemian amber lager balancing caramel malt against Saaz hop character, sitting between the pale and dark Czech beers.
- Czech dark lager
A dark Bohemian lager richer and sweeter than its German counterparts, with caramel and light roast over a soft body.
- Baltic porter
A strong dark lager descended from exported English porter: deep roast and dark fruit, remarkably smooth for its strength.
Related steps and concepts
- Bottom fermentation — The fermentation this concludes.
- Lagering — What follows it.
Also at the fermentation stage
Biotransformation · Bottom fermentation · High-gravity brewing · Hop creep · Mixed fermentation · Open fermentation · Pitching · Pressure fermentation · Primary fermentation · Spontaneous fermentation · Top fermentation · Yeast harvesting
See the whole process in order on how beer is made.
Last reviewed 2026-08-16.
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
Raising a fermentation to 16–20 °C near its end accelerates yeast reuptake and reduction of diacetyl, which is slow at lager fermentation temperatures and is interrupted entirely if the beer is crashed cold too early.
Backed by a source
Sources: Yeast: The Practical Guide to Beer Fermentation — print; Technology Brewing and Malting — print