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Diacetyl is a natural by-product of secondary or malolactic fermentation. It is more formally referred to as vicinal diketone or 2,3-butanedione. Its molecular formula is C4H6O2.

At low levels in alcoholic beverages, it contributes a slipperiness to the feel of the beer or wine in the mouth. As levels increase, it imparts a buttery or butterscotch flavor.

Beer sometimes undergoes a diacetyl rest, which entails waiting two or three days after fermentation is complete, to allow the yeast to absorb the diacetyl it produced earlier in the fermentation cycle. The makers of some wines, such as chardonnay, deliberately promote the production of diacetyl because of the feel it imparts.