Diacetyl is an organic compound belonging to the chemical group known as ketones. It is a fermentation by-product that may lend buttery or butterscotch notes to beer. This is considered an off-flavor in excessive amounts in any beer, however it is considered an off-flavor in most lagers in any amount. Several factors can affect diacetyl levels in beer; these include fermentation temperature, aeration level, yeast strain, and bacterial contamination. Learn more about Diacetyl.