compound
Diacetyl
A buttery, butterscotch compound with a very low flavour threshold — a fault almost everywhere, and traditional at low levels in some Czech lagers.
The full record
Diacetyl has its own catalogue entry with the detail this page summarises — mechanism, temperatures and what goes wrong.
Diacetyl — A buttery compound with a very low threshold — normally a sign of rushed fermentation, but traditional at trace levels in some Czech lagers.
Detectable by many tasters below 0.1 mg/L, which is why it is one of the most commonly identified off-flavours. It also contributes a slick, coating mouthfeel that some drinkers notice before the flavour.
Two quite different origins: normal fermentation by-product that healthy yeast reabsorbs, or bacterial contamination — chiefly Pediococcus — which produces it continuously and cannot be rested out.