Diacetyl Rest
One of the most common flaws in the beers brewed by homebrewers who are new to the hobby, particularly in lagers, is diacetyl.
Diacetyl, which has a taste and aroma of butter or butterscotch, is naturally present in all beer during fermentation. Also known as 2,3 butanedione, diacetyl is produced through a chemical reaction out- side of the yeast cell when the compound alpha-acetolactate is oxidized by metal ions or dissolved oxygen. It can also be produced from bacteria, most notably Pediococcus and Lactobacilli.
To avoid diacetyl through bacterial contamination, a consistent cleaning and sanitation routine must be followed. Reducing the naturally forming diacetyl is best done by limiting the beer’s contact with oxygen after the start of fermentation (since oxygen causes alpha-acetolactate to convert to diacetyl) and a technique called diacetyl rest — that is, allowing the beer to sit on the yeast a couple of days (or up to a week for lagers) at the end of fermentation and prior to cold crashing, bottling or kegging. At the end of fermentation the yeast will absorb diacetyl into the cell and reduce it enzymatically to 2,3-butanediol. Note that if you plan to rack your homebrew to a secondary fermenter prior to bottling or kegging, the diacetyl rest should be done in the primary fermenter to ensure there is enough yeast to absorb the diacetyl.
The diacetyl rest should be done at the same temperatures as fermentation for ales to ensure a full conversion of alpha-acetolactate to diacetyl, and with enough yeast present to reduce the diacetyl. The diacetyl rest for lagers should be done at similar temperatures as ales, so that means once fermentation is almost complete you will need to bring the temperature up to around 65 °F (18 °C) and hold it there for a few days. This warmer temperature speeds up the conversion of αalpha-acetolactate to diacetyl and makes the yeast more active so it is in a better state to convert the diacetyl. Afterwards, gradually allow the beer to cool to the desired lagering temperature. Warming a lager for the diacetyl rest will speed up the process, and since most homebrewers have that ability it is recommended. It is worth noting that most commercial breweries have no way of warming their lagers up to an ale’s fermentation temperature prior to cooling and therefore do a diacetyl rest at cooler temperatures — this works but is a slower process.
The diacetyl rest period should begin just prior to the end of active fermentation to ensure that there is still active yeast suspended in the beer to convert the diacetyl. This rest period should begin when your beer is two to five specific gravity points away from the target terminal gravity (or waiting until the bubbles in your airlock have slowed to once a minute or so and the yeast head has collapsed back into the beer).
A diacetyl rest is just as important with ales if you do not want the diacetyl taste in your beer (it is worth mentioning that some brewers like a little diacetyl in certain styles like English bitters or Scotch ales and believe that the buttery notes add complexity). Even if you don’t notice the taste of diacetyl right after fermentation, without a rest period your ale could still contain high levels of alpha-acetolactate, which may still be converted to diacetyl later on. Once you have removed the yeast there is no way to get rid of the diacetyl, so you don’t want to take that risk.
The rest period will extend the time it takes until you have a finished product, but a little rest and built up anticipation never hurt anybody.
