One of the things we love most about being homebrewers is having control over not only the finish and feel of a brew, but also the flavor. Whether you are a dedicated tinkerer, constantly modifying the secret recipe for your homebrew with herbs and spices, or a straightforward type who enjoys the bitter crispness that comes with dry hopping your beer, there is one flavor you may not want in your beer – diacetyl.
In homebrewing, as well as in life, patience is a virtue. To keep this rogue flavor agent under control, you need to master what is known as a diacetyl rest. This rest is more important for certain beer styles than others, but it is particularly crucial for lagers.
The key to preserving the flavor of your lager lies in understanding what diacetyl is and what it does, and then knowing how to counter it with the stationary phase, also known as the diacetyl rest.
What is Diacetyl?
Famous for creating a rich, buttery flavor, diacetyl is one of over 500 chemical compounds produced when yeast ferments your favorite brew. It is a ketone, a type of organic compound formed when alcohol is oxidized, such as during fermentation by yeast. These same yeasts can naturally process diacetyl and convert it into compounds that are relatively flavorless to humans, but this takes time.
How to Perform a Proper Diacetyl Rest
Time and temperature are your allies in this battle, as longer durations and higher temperatures enhance the performance of your yeast in breaking down diacetyl. Here is how to perform a diacetyl rest:
Begin the rest 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 nears completion, plan for a two-day diacetyl rest, or even longer, to allow the yeast to work their magic.
During the final two days of fermentation, raise the temperature of your wort to between 65°F and 68°F. This will increase yeast activity and help eliminate any remaining diacetyl. You can accomplish this by moving the container to a warmer location and allowing it to warm naturally, using a warm-water bath, or by utilizing a carboy warming jacket or carboy heater.
After two days, you can test your wort. If it has reached the desired state, you can bottle it or rack it for cold storage. If not, continue the rest until you are satisfied.