How to Perform a Proper Diacetyl Rest

One of the things we love most about being homebrewers is having control over not just the finish and the feel of a brew, but the flavor. And whether you’re a dedicated tinkerer, forever modifying the secret recipe for your homebrew with herbs and spices, or a straightforward type who loves the bitter crispness that comes with dry hopping your beer, one flavor you might not want in your beer comes from the chemical diacetyl.

In homebrewing, as in life, patience is a virtue, and to keep this rogue agent of flavor on a tight leash, you’ll need to master what’s known as a diacetyl rest. This rest is more important for some beer styles than others, but it’s particularly important for lagers.

The key to protecting your lager’s flavor is first knowing what diacetyl is and what it does, and then how to counter it with the stationary phase, also known as the diacetyl rest.

What is Diacetyl?

Famous for producing a rich, buttery flavor, diacetyl is one of more than 500 chemical compounds produced when yeast ferments your favorite brew. It’s a ketone, a type of organic compound that’s created when alcohol is oxidized—for example, by yeast during fermentation. Those same yeasts can naturally process diacetyl and convert it into compounds that are relatively flavorless to humans, but it takes time.

How to Perform a Proper Diacetyl Rest

Time and temperature are your allies in this battle, with longer times and higher temperatures improving the performance of your yeast as they break down the diacetyl. Here’s how to perform a diacetyl rest:

• Begin when your wort’s specific gravity is within 2 to 5 points of its terminal gravity, or the final gravity of your finished lager.

• As your primary fermentation draws to a close, plan on a two-day diacetyl rest (or longer) to allow the yeast to do their work.

• For the final two days of fermentation, raise the temperature of your wort to between 65°and 68°F, which will increase yeast activity and help polish off any remaining diacetyl. You can raise the temperature by moving it to a warmer location and letting it warm up on its own, using a warm-water bath, or with a carboy warming jacket or carboy heater.

After two days, you can test your wort and, if it’s where you want it to be, bottle it or rack it for lagering in cold storage. If it’s not, continue the rest until you’re satisfied.