A sharp, acid and often acrid taste. Sounds unpleasant but it is a valuable part of the taste of most beers and is required to balance the sweet malty flavours. Bitterness can be added to beer by the use of hops and roasted malts.
Beer derives its bitter taste from bitter acids present in hops that undergo isomerization during the brewing process.
Hop cones contain the bitter alpha and beta acids; n-, co-, and ad- humulones and lupulones respectively. The humulones in particular are widely used in the brewing process to provide bitterness to beer. Boiled hop in the wort causes the largely insoluble alpha acids to convert to more bitter and more soluble iso-alpha acids, the primary bittering compounds.
Hops and hop extracts used in the wort boiling process contain three main alpha-acids (humulone, cohumulone, adhumulone) and three beta-acids (lupulone, colupulone, adlupulone). During wort boil, six iso-alpha-acids are formed from the three hop alpha-acids.