Hops are the female flower of the perennial climbing vine, Humulus lupulus. The vine grows in almost any moist, sunny area, and can reach lengths of over 40 feet. The flowers themselves are green in color, with dust-like yellow lupulin glands, which contain the bittering and flavoring compounds prized by brewers. Most commercially produced hops in the United States come from the Pacific Northwest, and they also are widely grown in Britain, Germany (naturally) and Australia. The cones, as the flowers are usually called, are generally dried before use.
Hops are an essential ingredient in beer. Their first major contribution is to provide bitterness to balance the sweetness of the malt. The bittering compounds in hops are called alpha acids, five of which have been isolated:
humulone
cohumulone
adhumulone
prehumulone
posthumulone
These acids require an extended period of boiling to isomerize and make them soluble in the wort (unfermented beer). They are highly subject to oxidation, which will reduce their bittering power, making freshness an important consideration when selecting hops.
The second major characteristic that hops bring to beer is their unmistakeable flavor and aroma, which cannot be duplicated with other ingredients. The aroma and flavoring compounds are essential oils, and more that 250 have been identified. Unlike the bittering compounds, the essential oils are destroyed or evaporated by extended boiling, so flavoring hops are always added during the last few minutes of the boil. Sometimes dry hops are added to the fermenter to provide a distinctive aroma.
Hops produced for the brewing industry are generally packaged in one of three ways: whole hops, hop plugs, or pelleted hops. Whole hops are the whole dried cones, packaged without much treatment. Whole hops have a shorter shelf life than plugs or pellets. Plugs are whole flowers which have been pressed into small disklike plugs of about 1/2 oz. each. They are less perishible than whole hops and take up far less room. Pellets are rapidly becoming the most popular form, though some say that the processing they receive changes their character. Pellets are made by grinding whole hop cones into a fine powder and then pressing into tiny pellets which resemble rabbit food. They are the least perishable and most compact form of hops available.
Many different varieties of hops are used in varying quantities for different kinds of beers. Some varieties of hops are used mainly for their bittering power, while more fragrant and delicate varieties are used for flavoring and aroma. Beer aficionados can often tell from the aroma which variety of hop has been used a particular brew. Most American light lagers are very lightly hopped for bitterness and have almost no hop aroma. British pale ale falls somewhere in the middle of the spectrum for both bitterness and "hoppiness", the characteristic aroma and flavor of hops. India pale ale lies at the high end for both characteristics.