High Quality Content by WIKIPEDIA articles! Diacetyl (IUPAC systematic name: butanedione or butane-2,3-dione) is a natural byproduct of fermentation. It is a vicinal diketone (two C=O groups, side-by-side) with the molecular formula C4H6O2. Diacetyl occurs naturally in alcoholic beverages and is added to some foods to impart a buttery flavor.