|Common Hop plant (Humulus lupulus)|
Humulus, hop, is a small genus of flowering plants in the family Cannabaceae, which also includes cannabis. The hop is native to temperate regions of the Northern Hemisphere. The species H. lupulus is the main flavour ingredient in many types of beer, and as such is widely cultivated for use by the brewing industry.
Although frequently referred to as the hop "vine", it is technically a bine; unlike vines, which use tendrils, suckers, and other appendages for attaching themselves, bines have stout stems with stiff hairs to aid in climbing. It is a perennial herbaceous plant which sends up new shoots in early spring and dies back to the cold-hardy rhizome in autumn. Hop shoots grow very rapidly, and at the peak of growth can grow 20 to 50 centimetres (8 to 20 in) per week. Hop bines climb by wrapping clockwise (except for Humulus japonicus) around anything within reach, and individual bines typically grow between 2 to 15 metres (7 to 50 ft) depending on what is available to grow on. The leaves are opposite, with a 7 to 12 cm (2.8 to 4.7 in) leafstalk and a heart-shaped, fan-lobed blade 12 to 25 cm (4.7 to 9.8 in) long and broad; the edges are coarsely toothed. When the hop bines run out of material to climb, horizontal shoots sprout between the leaves of the main stem to form a network of stems wound round each other.
Male and female flowers of the hop plant develop on separate plants (dioecious). Female plants, which produce the hop flowers used in brewing beer, are often propagated vegetatively and grown in the absence of male plants. This prevents pollination and the development of viable seeds, which are sometimes considered undesirable for brewing beer owing to the potential for off-flavors arising from the introduction of fatty acids from the seeds.
There are three species, one with five varieties:
- Humulus japonicus (syn. H. scandens). Asian Hop. Leaves with 5–7 lobes. Eastern Asia.
- Humulus lupulus. Common Hop. Leaves with 3–5 lobes. Europe, western Asia, North America.
- Humulus lupulus var. lupulus. Europe, western Asia.
- Humulus lupulus var. cordifolius. Eastern Asia.
- Humulus lupulus var. lupuloides (syn. H. americanus). Eastern North America.
- Humulus lupulus var. neomexicanus. Western North America.
- Humulus lupulus var. pubescens. Midwest North America.
- Humulus yunnanensis. Yunnan Hop. Leaves with 3–5 lobes, densely hairy below. Southeast Asia (endemic in Yunnan, China).
Hop applications 
In pharmacy lupulus is the designation of hop. The dried catkins, commonly referred to as hop cones, of the female plant of H. lupulus are used to prepare infusion of hop, tincture of hop, and extract of hop.
Hop chemistry 
The characteristic bitterness imparted by the addition of hops to the brewing process is mainly due to the presence of the bitter acids, which are prenylated acylphloroglucinol derivatives. Bitter acids are divided into the alpha-acids, with humulone the major compound, and the beta-acids, with lupulone the major compound. Alpha-acids isomerize during the brewing process to form iso-alpha acids, which themselves have a bitter taste. Hops also contain xanthohumol, a prenylated chalcone compound, that shows cytoprotective and other health-promoting activities 
Plants in the genus Humulus also produce terpenophenolic metabolites.
- Interactive Agricultural Ecological Atlas of Russia and Neighboring Countries. Economic Plants and their Diseases, Pests and Weeds. Humulus lupulus.
- "Hop". Collier's New Encyclopedia. 1921.
- Verzele M, De Keukeleire D: Chemistry and analysis of hop and beer bitter acids. New York: Elsevier; 1991.
- Jaskula et al (2008). A kinetic study on the isomerization of hop α-acids. J. Agric. Food Chem. 56 (15), 6408–6415.
- Stevens JF, Page JE. (2004). Xanthohumol and related prenylflavonoids from hops and beer: to your good health! Phytochemistry 65, 1317–1330.
- Chapter eight: Biosynthesis of terpenophenolic metabolites in hop and cannabis. Jonathan E. Page and Jana Nagel, Recent Advances in Phytochemistry, 2006, Volume 40, Pages 179–210, doi:10.1016/S0079-9920(06)80042-0
- Lee W. Janson, Ph. D.; Brew Chem 101; Storey Publishing; ISBN 0-88266-940-0 (paperback, 1996)
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