Beer Fermentation
Diacetyl Rest
Employing high temperatures at the end of primary fermentation are for reducing diacetyl and 2,3-pentanedione (collectively referred to as vicinal diketones or VDKs). This procedure is known as a diacetyl rest. VDKs, which are assimilated by yeast toward the end of fermentation, are responsible for off-flavors in beer. The diacetyl rest reinvigorates the yeast culture so that it metabolizes those byproducts-such as diacetyl and 2,3-pentanedione-that are excreted early in the fermentation, thereby removing them from solution. Depending upon the yeast type, the medium, and the physical environment, this process is variable in time and temperature.
Employing a diacetyl rest is particularly important when producing beers with high adjunct fraction due to the high levels of diacetyl produced (11). It is also important in brewing lagers, as they do not have a heavy flavor impression. The technique reduces the time needed to achieve a product similar to beer brewed when using the more intensive cold-lagering process. With its use, lagering may require only 7 to 14 days to achieve the same clarity and flavor stability that would only be expected after 5 to 7 weeks with traditional lagering (42).
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