Diacetyl Rests, and whether you should do one?

Diacetyl is one of the common enemies of the homebrewer.  A normal byproduct of beer during the fermentation stage, Diacetyl imparts a buttery or butterscotch flavor at higher concentrations, and can overpower an otherwise good beer.

In a previous post, we talked about a Cold Crash; bringing the fermentation temperatures down to near freezing to sink the yeast and clear the beer.  Because Diacetyl is reduced at a slower pace at lower temperatures, if you cold crash too soon you may be stuck with Diacetyl off flavors that did not have a chance to be removed.  So before the cold crash, it's important to have a Diacetyl rest.

A Diacetyl rest allows adequate time for the Diacetyl to be reduced during fermentation -- preferably at the end of the fermentation right before the cold crash.  By allowing the temperature of the fermentor to remain higher for a day or two before the cold crash, the  Diacetyl should be reduced.

For ales, a temperature increase to about 65-70 degrees F at the end of fermentation should be fine to reduce the Diacetyl -- which is right at the normal recommended fermentation range anyway.  So you probably need do nothing to accomplish this with ales.

Lagers, however, present a problem, since the normal fermentation temperatures for lagers range from 45-55 degrees F.  An increase to about 65-70 degrees F for two days near or at the end of fermentation will vastly reduce Diacetyl.