A major beer flavour produced by the effect of heat on the a acids from hops. These humulone compounds are isomerised by boiling and converted to iso a acids which taste bitter. Bitterness levels are measured in IBU (International Bitterness Units) where 1 bitterness unit = 1 mg of iso a acid per litre. Beers generally have bitterness levels between 20 and 50 IBU.