Diacetyl

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Diacetyl is a compound that is formed during fermentation by the yeast, or by certain bacterial infections. It has a distinctive butter-like or butterscotch flavor. This flavor is easily reproduced with a few drops of artificial butter flavoring in a pint. It is a desirable flavor in some styles at low to moderate levels, but is usually unacceptable at higher levels where the compound may even be described as rancid. Common causes include insufficient oxygenation prior to fermentation, long lag time, and poor yeast health. Typically, diacetyl is re-absorbed by the yeast in the weeks following active fermentation. When scheduled by the brewer, this re-absorption by the yeast is usually done at a slightly higher temperature than fermentation occurred, and is known as a diacetyl rest.