Fermentation
Diacetyl Rest
Diacetyl, a vicinal diketone (VDK), is commonly formed as part of the beer fermentation process. Its aroma and flavor in beer presents as a buttery or butterscotch when present at above sensory threshold levels (0.1ppm). Diacetyl is also known to exhibit a slick or slippery mouthfeel in beer. Factors that influence the diacetyl levels in beer are yeast strain selection, fermentation temperature, aeration, and maturation time. Diacetyl production can also be affected by insufficient nutrients. If FAN levels are low, the yeast will need to adjust their metabolism to create more amino acids, specifically in this case, the amino acid valine.
Timing of Diacetyl Rest
To help ensure the reabsorption of diacetyl into the yeast cell, a diacetyl rest can be performed. A diacetyl rest is when the fermentation temperature is raised a couple of degrees when the gravity is a few degrees Plato from terminal. Diacetyl rests can be performed in either ale or lager ferments but are recommended especially for lagers or cooler fermentation profiles. To perform a diacetyl rest, raise the fermentation temperature several degrees (typically 16–20°C, 61–68°F) late fermentation.
Forced Diacetyl Test
Forced diacetyl testing is a cheap and effective means to assess your brew before it is crash chilled. It enables brewers to use sensory evaluation to determine whether a beer has undergone sufficient diacetyl reduction prior to chilling.
Factors Influencing Diacetyl Reduction
During fermentation the yeast has a great capacity to remove diacetyl; however, toward the end of fermentation there may not be enough active yeast to remove diacetyl.
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