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.
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