A diacetyl rest is used when making lagers and ales. After a beer has fermented to near final gravity the beer is raised from fermenting temperature to a higher temperature roughly 3-4 degrees Fahrenheit above the original fermentation temperature and allowed to sit for two-four days. The purpose of this higher temperature rest is to allow the yeast to reabsorb their diacetyl that is naturally produced during fermentation. While yeast will absorb diacetyl at colder temperatures as well, the absorption happens much faster at warmer temperatures when the metabolic activity of the yeast is speed up. This quick absorption of diacetyl may help to lower the overall necessary aging time and help make a cleaner tasting beer.
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