[size=75]From Wikipedia, the free encyclopedia [/size]
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 © 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. The hop plant is a vigorous climber, usually grown up strings in a field called a hop garden or hop yard.
The first documented instance of hop cultivation was in 736, in the Hallertau region of present-day Germany (which, in 2006, had more hop-growing area than any other country in the world), although the first mention of the use of hops in brewing was in 1079. Hops were introduced to British beers in the early 15th century, and hop cultivation began in the present-day United States in 1629.
Until mechanisation (in the late 1960s for the UK), the need for massed labour at harvest time meant hop-growing had a big social impact. Many of those hop picking in Kent were Eastenders, for whom the annual migration meant not just money in the family pocket but a welcome break from the grime and smoke of London. Whole families would come down on special trains and live in hoppers’ huts and gradients for most of September, even the smallest children helping in the fields.
Today, the principal production centres for the UK are in Kent (which produces Kent Golding hops) and Worcestershire.  Other important production areas include Washington’s Yakima Valley and Oregon’s Willamette Valley in the USA. Belgium, Germany, and the Czech Republic are also important centers of the hop industry.
Hops have to be dried in an oast before they can be used in the brewing process. Hop resins are composed of two main acids: alpha and beta acids.
Alpha acids have a mild antibiotic/bacteriostatic effect against Gram-positive bacteria, and favour the exclusive activity of brewing yeast in the fermentation of beer.
Beta acids do not isomerise during the boil of wort, and have a negligible effect on beer flavour. Instead they contribute to beer’s bitter aroma, and high beta acid hop varieties are often added at the end of the wort boil for aroma. Beta acids may oxidize into compounds that can give beer off-flavours of rotten vegetables or cooked corn.