Definition of Alpha acid unit

Hops are a flower used primarily as a flavouring and stability agent in beer, as well as in herbal medicine. The first documented use in beer is from the eleventh century. Hops come from the flowers of Humulus lupulus, originally named by Pliny the Elder in his Naturalis Historia, and contain several characteristics very favourable to beer: (a) hops contribute a bitterness that balances the sweetness of the malt, (b) hops can contribute aromas that are flowery, citrus, fruity or herbal, and (c) hops have an antibiotic effect that favors the activity of brewer's yeast over less desirable microorganisms. While hop plants are grown by farmers all around the world in many different varieties, there is no major commercial use for hops other than in beer, although hops are an ingredient in Julmust, a carbonated beverage similar to cola soda that is popular in Sweden during December.