Diacetyl arises naturally as a byproduct of fermentation. At low levels, diacetyl contributes a slipperiness to the feel of the alcoholic beverage in the mouth. As levels increase, it imparts a buttery or butterscotch flavor. It is generally considered a fault at too high of a level in beer or distillates. The simplest way to reduce diacetyl is to increase fermentation temperatures near the end - called a Diacetyl Rest. The remaining yeast remove it by storing it in the yeast cell where it cannot be tasted.