In addition to carrying out the bio-deacidification of wine, malolactic (ML) bacteria influence aroma and flavour through various mechanisms, including the production of volatile grape- and yeast derived metabolites. In wine, one of those volatile compounds – diacetyl – has important stylistic implications. This diketone, also known as 2,3-butanedione, is associated with the “buttery” character of wine and is formed as an intermediate metabolite in the reductive decarboxylation of pyruvic acid to 2,3-butanediol.The formation and degradation of diacetyl is closely linked to the growth of such ML bacteria as Oenococcus oeni and the metabolism of sugar, malic acid and citric acid. Yeasts are also able to synthesize diacetyl during alcoholic fermentation (AF). However, most of this diacetyl is further metabolized to acetoin and 2,3-butanediol. This issue of Winemaking Update will review winemaking practices and the latest findings to help modulate diacetyl content in wine through malolactic fermentation (MLF).