Better known as diacetyl, this volatile diketone is the molecule behind the rich aroma of butter, fresh popcorn and cultured dairy products. Lactic acid bacteria generate it during fermentation by decarboxylating alpha-acetolactate, and tiny concentrations define the character of buttermilk, sour cream and fine Chardonnay. Flavour houses use it widely, but it carries a serious occupational caveat: prolonged inhalation of concentrated diacetyl vapour in flavour factories has caused bronchiolitis obliterans, a severe lung disease, in workers. Handling is therefore strictly controlled, and many manufacturers now formulate with safer substitutes or encapsulated forms.