A distinctive flavour present in some beers and resulting from the chemical conversion of acetolactate metabolic intermediate during fermentation. Levels rise with active yeast growth but decline during maturation when yeast reabsorb diacetyl and reduce it to 2,3 butanediol. Many breweries control diacetyl to non perceptible levels, below 0.2 mg per litre, but higher levels may be acceptable in some beers, particularly British and Scotch ales.