Diacetyl Rest: What It Is, When to Do It, Temperature and How Long
Diacetyl is the butterscotch flavour yeast make early and clean up late, if you let them. When to start the rest, how warm, how long, how to test for it, and why some beers never lose it.
In this post
Diacetyl is the compound that makes beer taste like butter, butterscotch or movie popcorn. Every fermentation makes some. A healthy fermentation also removes it, but only if the yeast are warm, active and still in the beer when the time comes. The diacetyl rest is the step that makes sure they are.
This post answers the four questions people actually search for: what a diacetyl rest is, when to do it, what temperature, and how long. Then how to test whether it worked.
What diacetyl is and where it comes from
During the growth phase of fermentation yeast leak a precursor, alpha-acetolactate, into the beer. Outside the cell it slowly converts to diacetyl on its own, faster when warm. Later, yeast take diacetyl back in and reduce it to acetoin and then 2,3-butanediol, both of which are flavourless at normal levels. So diacetyl rises early, peaks around the middle of fermentation, and falls at the end.
The problem is that the precursor keeps converting to diacetyl for days after fermentation looks finished. If the yeast have gone dormant, dropped out, or been crashed cold before that happens, the diacetyl stays. A beer that tasted clean at packaging turns buttery in the keg two weeks later. Brewers call it a diacetyl bomb.
What a diacetyl rest is
A diacetyl rest is a period of two to three days near the end of fermentation when the beer is held warm enough, and long enough, for the yeast to reabsorb the diacetyl and convert the remaining precursor. For lagers that means raising the temperature. For ales it usually means not lowering it yet.
When to do it
Start the rest when fermentation is about two-thirds to three-quarters done: 3 to 5 gravity points above your expected final gravity. For a 1.050 lager finishing at 1.010, that is around 1.018 to 1.020, typically day 5 to 7 at 50°F. Yeast are still active and in suspension, which is exactly what you want.
Starting later, after terminal gravity, still works but takes longer, because fewer yeast are in suspension. Starting earlier, before half attenuation, risks pushing ester production up in a lager. If you are unsure, a hydrometer or refractometer reading every day from day four tells you exactly where you are.
The short version: when the beer is 3 to 5 points from done, warm it up and hold for 2 to 3 days after it reaches terminal gravity. Then crash.
What temperature
| Beer | Fermentation temperature | Diacetyl rest temperature |
|---|---|---|
| Lagers (pilsner, helles, märzen, bock) | 48 to 55°F | Raise to 60 to 65°F |
| Kölsch, cream ale, other clean hybrids | 58 to 62°F | Raise to 64 to 68°F |
| Ales (pale ale, IPA, stout) | 64 to 68°F | Hold, or raise 2 to 4°F |
| Pressure-fermented lagers | 62 to 68°F under 10 to 15 psi | Hold at fermentation temperature |
Raise the temperature over a day rather than in an hour; yeast do not like shocks in either direction. On a jacketed fermenter with a controller that is just a new setpoint. In a fridge, turn the controller up and let the room do the rest.
How long
Two to three days after the beer reaches terminal gravity. Most of the reabsorption happens in the first 48 hours at 65°F. A big or underpitched beer may need four or five days. There is no harm in going longer as long as the beer is sealed from oxygen; the yeast are not producing anything new at this point.
The forced diacetyl test
The only way to know the rest is done is to test, because the precursor is flavourless until it converts. The test forces that conversion in a sample:
- Draw two samples from the tank into two glasses. Cover both.
- Heat one to about 140°F (60°C) for 15 to 20 minutes. A water bath or a microwave in short bursts works. Keep it covered so aromas do not escape.
- Cool the heated sample back to the temperature of the other one.
- Smell and taste both. If the heated sample has butterscotch the unheated one lacks, there is still precursor in the beer. Keep the rest going and test again in 24 hours.
When both samples taste the same, the beer is clean and you can crash it.
Why some beers never lose it
- Underpitching. Too few yeast make more diacetyl and have less capacity to remove it. The single most common cause, especially in lagers. Pitch double what you would for an ale.
- Crashing too early. Cold yeast stop working. If you crash at terminal gravity without a rest, whatever precursor is left converts in the keg.
- Stressed yeast. Poor oxygenation at pitch, old yeast, or a big temperature drop mid-ferment all leave yeast unable to finish.
- Racking off the yeast too soon. Transferring to a secondary before the rest takes the cleanup crew out of the beer.
- Infection. Pediococcus and some lactobacillus produce diacetyl in packaged beer, often with a slick mouthfeel and a slow souring. If a beer that tested clean turns buttery weeks later and does not recover, suspect sanitation, especially in tap lines.
Diacetyl rests in a jacketed fermenter
All of the above assumes you can change the beer's temperature on demand and hold it there. That is the job of a jacketed Unitank with a controller and solenoid: type 65 into the controller and the jacket does the rest, then type 33 three days later. The glycol chiller that makes the crash possible is the same one that holds a lager at 50°F in July.
If you ferment under pressure, the rest is simpler still: the beer is already at 62 to 68°F, so you hold rather than raise, then crash with the spunding valve still set so no air is pulled in. The pressure fermentation guide has the full method.
Quick answers
Do ales need a diacetyl rest?
They need the same thing: two or three days at fermentation temperature after terminal gravity before you crash. Because ales are already warm, nobody calls it a rest, but skipping it causes the same buttery beer.
Can I do the rest after the cold crash?
Only by warming the beer back up, which undoes the crash and may not wake the yeast. Do it first.
Is a little diacetyl ever wanted?
A trace is part of some English ales and Czech lagers. Beyond a trace it is a fault in every style.
Does the rest raise esters in my lager?
Not if you start it after most of the fermentation is done. Esters form during growth, which is over by the time you warm it up.
Next in the lager process: lagering, temperatures and how long, and how to cold crash without pulling in air.