a by-product of yeast during fermentation, it is normally re-absorbed during the secondary fermentation. mutation of yeast can produce respiratory deficient cells which have lost their ability to reduce the diacetyl to more innocuous compounds. another cause is the gram positive bacterium, pediococcus damnosus and other lactic acid bacteria in cooled beer, young beer, and aging beer. note that the aroma/taste produced by all of these causes is indistinguishable.