Diacetylrest at 22 C (72 F)

This may sound crazy, but I commonly hold the “Diacetyl rest” at 22 C (72 F) for my lagers. When reading through some German brewing papers I notices that some of them referred to a fermentation schedule where the lager was fermented at 8 C (46 F) for about a week and after that the beer temperature was raised to 22 C for 1-2 days before the beer was crash cooled to near freezing for cold conditioning.

In my brewing it takes a bit longer than a week for the beer to be done with primary fermentation. It also doesn’t reach final gravity as fast as it was shown in those papers. To make maters worse, fermenting the last few fermentable sugars until the attenuation is close enough to the attenuation limit always tends to take longer than I want it to.

To speed things up I started holding the “Diacetyl rest” rest around 20 C. I intentionally put diacetyl rest in quotes since for most of us the primary benefit of this rest is not diacetyl reduction but speeding up yeast’s consumption of those last fermentable sugars.I prefer to call this rest a maturation rest and may hold it for up to a week, if the beer is still too far away from its attenuation target. (If you don’t know how to determine final extract or gravity target of the beer, check out the Fast Ferment Test.)

Warm fermentation and lagers doesn’t seem to go together, but don’t worry. Low fermentation temperatures are only needed during primary fermentation when the yeast is actively growing and its metabolism shows activity along many different pathways that can leak excessive esters and higher alcohols if the temperature is too high. Once the yeast is done growing and all nitrogen sources have been assimilated, the fermentation temperature can be raised without the risk of creating the flavor compounds that are generally associated with high temperature fermentations.

keg with heating pad and temperature sensor

keg with heating pad and temperature sensor

I do these maturation rests in kegs for a number of reasons. First, I can easily heat them with a heating pad controlled by a temperature controller. Second, I can use this fermentation to carbonate the beer. Lastly, the yeast will consume all the oxygen that is introduced during racking.

The image above shows how the heating pad (you’ll need one that doesn’t have an automatic shut-off) is attached to the bottom of the keg. At the top of the keg I attach the temperature sensor, covered with a piece of foam.

When the beer is racked to the keg, I make sure plenty of yeast is transferred as well. The kegs have a shortened dip tube, which allows for transferring the beer and leaving behind the yeast sediment later.

Finally the whole thing is wrapped in a blanket for further insulation:

Keg wrapped in a blanket

Keg wrapped in a blanket

A pressure gauge is attached to monitor the pressure build-up and allow controlled CO2 release.

8 thoughts on “Diacetylrest at 22 C (72 F)

    • I have the heating pad set at the lowest setting. I never really checked the temperature progression, next time I try to remember to do that, but from fleeting glances at the temperature controller I guess that it takes maybe 12 hours for the temperature to rise from 10 C to 22 C.

  1. This is my standard schedule for fermenting lagers as well. It seems that most professional brewers I talk to have no reservations about raising a lager to 20°C or more.

    Now I just need to get spunding valves for my home kegs and I’ll be set…

    • I have tried building a spunding valve using the cheap adjustable pressure valves, but I found that they slowly leak CO2. So far I have not looked for better quality ones that don’t leak.

      • For my lager I currently have fermenting, I may just rack it to a keg once it’s <1°P above where the FFT finishes. The only risk would be that if it gets contaminated it would over-pressurize, but the PRV on the keg should keep it from getting dangerous.

  2. At what point in the fermentation do you raise the temperature to 20C for the maturation rest? Are you looking for 1-2P above the FFT value?

    • yes, about 1-2 Plato above the FFT value. For a regular strength (~12 Plato) lager this is about 10 days after pitching the yeast. The point when the temp is raised is not all that critical. Given that I’ll remove most of the yeast by transferring the beer I do want the beer to be fairly close to its final attenuation, though.

  3. I pretty much do the same thing, raising to 18C when fermentationn is nearly done – about 10 days – until terminal gravity. I do this in the primary fermenter by turning up the controller and introducing a heat source in the temperature controlled chest freezer. After that I do things differently, slowing dropping the temperature by 1C per day and then holding at -2C for a few days. At that point I keg, filter and carbonate. The whole process takes exactly five weeks for a standard lager or pilsner style beer.

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