Diacetyl is the volatile compound behind the rich, buttery aroma of butter, cultured dairy products and some wines and beers, produced by lactic acid bacteria during fermentation. At low levels it is a prized flavour note; at high levels it becomes an off-flavour, notably in lager beers. Food manufacturers add it to margarines and popcorn flavourings, though inhalation risks for factory workers have led to strict exposure controls and reformulation efforts. Proper ventilation and monitoring protect staff while the flavour continues to delight consumers.