Hops are the female flowers (also called seed cones or strobiles) of the hop plant, Humulus lupulus. They are used primarily as a flavoring and stability agent in beer, to which they impart a bitter, tangy flavor, though hops are also used for various purposes in other beverages and herbal medicine.
In the Middle Ages, beers tended to be of a very low alcohol content (small beer) and were commonly consumed as a safer alternative to untreated water. Each village tended to have one or more small breweries with a barley field and a hop garden in close vicinity. Early documents include mention of a hop garden in the will of Charlemagne's father, Pepin III. However, the first documented use of hops in beer as a flavoring agent is from the 11th century. Before this period, brewers used a wide variety of bitter herbs and flowers, including dandelion, burdock root, marigold, horehound (the German name for horehound means "mountain hops"), ground ivy, and heather.
Hops are used extensively in brewing for their antibacterial effect that favors the activity of brewer's yeast over less desirable microorganisms and for many purported benefits, including balancing the sweetness of the malt with bitterness, contributing a variety of desirable flavors and aromas. Historically, traditional herb combinations for beers were believed to have been abandoned when beers made with hops were noticed to be less prone to spoilage.
The hop plant is a vigorous, climbing, herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden (nomenclature in the South of England), or hop yard (in the West Country and U.S.) when grown commercially. Many different varieties of hops are grown by farmers around the world, with different types being used for particular styles of beer.
The first documented hop cultivation was in 736, in the Hallertau region of present-day Germany, although the first mention of the use of hops in brewing in that country was 1079. However in a will of Pepin the Short, the father of Charlemagne, 768 hop gardens were left to the Cloister of Saint-Denis. Not until the 13th century did hops begin to start threatening the use of gruit for flavoring. Gruit was used when taxes were levied by the nobility on hops. Whichever was taxed made the brewer then quickly switch to the other. In Britain, hopped beer was first imported from Holland around 1400, yet hops were condemned in 1519 as a "wicked and pernicious weed". In 1471, Norwich, England, banned use of the plant in the brewing of ale ("beer" was the name for fermented malt liquors bittered with hops; only in recent times are the words often used as synonyms).
Hops were imported from France, Holland and Germany and naturally import duty was raised on those; it was not until 1524 that hops were first grown in the southeast of England (Kent) when they were introduced as an agricultural crop by Dutch farmers. Therefore, in the hop industry there were many words which originally were Dutch words, such as oast house, which is derived from the Dutch word eest huis which means "drying house"; scuppet, a large wooden spade used on the hop floor to turn the hops into the hanging pocket or bale, which is derived from the Dutch word schop. Hops were then grown as far north as Aberdeen near breweries for infrastructure convenience. It was another century before hop cultivation began in the present-day United States, in 1629 by English and Dutch farmers.
Hops production is concentrated in moist temperate climates, with much of the world's production occurring near the 48th parallel north. Hop plants prefer the same soils as potatoes. The leading potato-growing states in the United States are also major hops-producing areas. However, not all potato-growing areas can produce good hops naturally. Soils in the Maritime Provinces of Canada lack the boron that hops prefer, for example. Historically, hops were not grown in Ireland, but were imported from England. In 1752 more than 500 tons of English hops were imported through Dublin alone.
Important production centers today are the Hallertau in Germany which, in 2006, had more hop-growing area than any other country on Earth, the Yakima (Washington) and Willamette (Oregon) valleys, and western Canyon County, Idaho (including the communities of Parma, Wilder, Greenleaf, and Notus). The principal production centers in the UK are in Kent which produces Kent Goldings hops, Herefordshire and Worcestershire. Essentially all of the harvested hops are used in beer making.
|Hop producing country||2010 hop output in tonnes (t)|
|New Zealand||830 |
||This article may require cleanup to meet Wikipedia's quality standards. The specific problem is: <This section needs to be re-written for easier reading>. (June 2013)|
Cultivation and harvest
Although hops are grown in most of the continental U.S. states and Canadian Provinces, cultivation of hops for commercial production requires a particular environment. As hops are a climbing plant, they are trained to grow up trellises made from strings or wires that support the plants and allow them significantly greater growth with the same sunlight profile. Energy that would have been required to build structural cells is also freed for crop growth.
Male and female flowers of the hop plant usually develop on separate plants (that is, the plant is dioecious), although fertile monoecious individuals appear occasionally. Because viable seeds are undesirable for brewing beer, only female plants are grown in hop fields, thus preventing pollination. Female plants are propagated vegetatively, and male plants are culled, if plants are grown from seeds. The flowers from the female plants are used to flavor beer.
Hop plants are planted in rows about six to eight feet apart. Each spring the roots send forth new bines. that are started up strings from the ground to an overhead trellis. The cones grow high on the bine, and in the past, these cones were picked by hand. Harvesting of hops became much more efficient with the invention of the mechanical hops separator, patented by Emil Horst in 1909.
Harvest comes near the end of summer when the bines are pulled down and the flowers are taken to a hop house or oast house for drying. Hop houses are two-story buildings, of which the upper story has a slatted floor covered with burlap. Here the flowers are poured out and raked even. A heating unit on the lower floor is used to dry the hops. When dry, the hops are moved to a press, a sturdy box with a plunger. Two long pieces of burlap are laid into the hop press at right angles, the hops are poured in and compressed into bales.
Hop cones contain different oils, such as lupulin, a yellowish, waxy substance, an oleoresin, that imparts flavor and aroma to beer. Lupulin contains lupulone and humulone, which possess antibiotic properties, suppressing bacterial growth favoring brewer's yeast to grow. After lupulin has been extracted in the brewing process the papery cones are discarded.
Emil Clemens Horst
||It has been suggested that this section be split into a new article titled Emil Clemens Horst. (Discuss) Proposed since July 2014.|
E. Clemens Horst (18 March 1867 – 24 May 1940) was a major figure in the cultivation, harvest and sale of hops in the U.S.A. He emigrated to the U.S. as a child in 1874 from Tuttlingen, Baden-Württemberg, Germany. In the mid 1880s he purchased a small plot of land just east of Daniel P. Durst, the first Bear River hop grower, as site of his first hop farm. He soon bought out two other hop growers, Hugh Roddan and Joseph M.C.Jasper. He married in 1898, and moved to San Francisco in 1902. By 1904 he was supplying Oregon hops to the Guinness Brewery in Ireland. By 1912, Horst owned the largest number of acres of hops under cultivation in the world with offices in Chicago, New York and London. Horst Company was headquartered at 150 Pine Street, San Francisco, California. Prior to World War II, Horst Company was the only exporter of hops to Germany.
The cultivation of hops requires rich flood plains. One of Horst’s oldest and most productive hop ranches was the site of Campus Commons in Sacramento along the American River. Another nearby ranch of Horst's was based in Horstville, a company town nearby Wheatland, California in the Bear River flood plain. It has rich loamy soil, that geologists even named after Horst, like Horst Sandy Loam and Horst Silt Loam.
Horstville consisted of a post office, dining hall, company store and a tent city for seasonal workers. In 1886, labor shortages and threats of a strike by white workers had become serious; hop growers founded the California Hop Growers’ Association, and hired Chinese workers. By 1898 Horst had become so big that ten hop drying kilns ran daily. Simultaneously hop labor camp conditions had become "unspeakably bad throughout the state and the pay was equally abysmal" so that by 1899 the Horst Brothers ranch's 300 white and a number of Japanese employees quit and went on strike for more pay. To recruit workers Horst launched advertising campaigns not only by making false promises but advertised for at least twice as many workers than they actually needed, with the goal to play one group off against the other, as he admitted before the U.S. Immigration Commission  Hop production peaked between 1912 and 1916 and plummeted thereafter. On 3 August 1913 the Durst Ranch became the site of the Wheatland Hop Riot. Until 2009, when Google released a digitized version of the 1916 Commission on Industrial Relations, the Horstville ranch was often confused with the neighboring Durst Ranch.
Horst revolutionized the processing of hops with the invention of his mechanical hop separator, patented 1909 in Elk Grove, California. It picked the hops while discarding the bine and leaves. It produced 25 bales of hops in one day, while an experienced worker picked just two bales in a week, allowing him to bypass the hassles of hiring seasonal labor. Horst continuously invented new hop related machinery (1884-1924) holding at least 14 patents. The harvesting process was further developed in 1939 (US Patent #2139046)  by his son-in-law, Edouard Thys, founder of Thys Company which, among other industrial parts, manufactured the parts for the portable hop picking machine.
The need for massed labor at harvest time meant hop-growing had a big social impact. Around the world, the labor-intensive harvesting work involved large numbers of migrant workers who would travel for the annual hop harvest. Whole families would partake and live in hoppers' huts, with even the smallest children helping in the fields. The final chapters of W. Somerset Maugham's Of Human Bondage contain a vivid description of London families participating in this annual hops harvest. In England, many of those picking hops in Kent were from eastern areas of London. This provided a break from urban conditions that was spent in the countryside. People also came from Birmingham and other Midlands cities to pick hops in the Malvern area of Worcestershire. Some photographs have been preserved.
Particularly in Kent, because of a shortage of small-denomination coin of the realm, many growers issued their own currency to those doing the labor. In some cases, the coins issued were adorned with fanciful hops images, making them quite beautiful.
In the US, Prohibition had a major impact on hops productions, but remnants of this significant industry in West and Northwest are still noticeable in the form of old hop kilns that survive throughout Sonoma County, among others. Florian Dauenhauer of Santa Rosa in Sonoma County, became a manufacturer of hop-harvesting machines in 1940, in part because of the hop industry's importance to the county. Ironically, this mechanization helped destroy the local industry by enabling large-scale mechanized production, which moved to larger farms in other areas. Dauenhauer Manufacturing remains a current producer of hop harvesting machines.
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 favor the exclusive activity of brewing yeast in the fermentation of beer. Alpha acids such as isohumulone are responsible for the bitter flavor in the beer.
Beta acids do not isomerize during the boil of wort, and have a negligible effect on beer taste. Instead, they contribute to beer's 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-flavors of rotten vegetables or cooked corn.
The effect of hops on the finished beer varies by type and use, though there are two main hop types: bittering and aroma. Bittering hops have higher concentrations of alpha acids, and are responsible for the large majority of the bitter flavor of a beer. European (so-called "noble") hops typically average 5–9% alpha acids by weight (AABW), and the newer American cultivars typically range from 8–19% AABW. Aroma hops usually have a lower concentration of alpha acids (~5%) and are the primary contributors of hop aroma and (nonbitter) flavor. Bittering hops are boiled for a longer period of time, typically 60–90 minutes, to maximize the isomerization of the alpha acids. They often have inferior aromatic properties, as the aromatic compounds evaporate off during the boil.
The degree of bitterness imparted by hops depends on the degree to which alpha acids (AAs) are isomerized during the boil, and the impact of a given amount of hops is specified in International Bitterness Units (IBUs). Unboiled hops are only mildly bitter. On the other hand, the nonbitter flavor and aroma of hops come from the essential oils, which evaporate during the boil.
Aroma hops are typically added to the wort later to prevent the evaporation of the essential oils, to impart "hop taste" (if during the final 30 minutes of boil) or "hop aroma" (if during the final 10 minutes, or less, of boil). Aroma hops are often added after the wort has cooled and while the beer ferments, a technique known as "dry hopping", which contributes to the hop aroma. The three major components of the essential oil of hops are myrcene, humulene, and caryophyllene, which comprise about 60–80% of the oil for most hop varieties. Farnesene is a major component in some hops. The composition of hop essential oils can differ a lot between varieties and between years in the same variety. About 250 components of essential oils have been identified. 22 of these are known to have significant influence on the flavor and aroma.
Today, a substantial amount of "dual-use" hops are used, as well. These have high concentrations of alpha acids and good aromatic properties. These can be added to the boil at any time, depending on the desired effect.
Flavors and aromas are described appreciatively using terms which include "grassy", "floral", "citrus", "spicy", "piney", "lemony", "grapefruit", and "earthy". Many pale lagers have fairly low hop influence, while lagers marketed as Pilsener or brewed in the Czech Republic may have noticeable noble hop aroma. Certain ales (particularly the highly hopped style known as India Pale Ale, or IPA) can have high levels of hop bitterness.
Brewers may use software tools to control the bittering levels in the boil and adjust recipes to account for a change in the hop bill or seasonal variations in the crop that may lead to the need to compensate for a difference in alpha acid contribution. Data may be shared with other brewers via BeerXML allowing the reproduction of a recipe allowing for differences in hop availability.
Hops contain humulone, isohumulone and humulene which are bitter-tasting compounds. It also contains the natural phenols xanthohumol, isoxanthohumol and the most estrogenic phytoestrogen known, 8-prenylnaringenin.
|This section needs additional citations for verification. (July 2014)|
There are many different varieties of hops used in brewing today. Historically, hops varietals were identified by geography (such as Hallertau, Spalt, and Tettanag from Germany), by the farmer who is recognized as first cultivating them (such as Goldings or Fuggles from England), or by their growing habit (e. g., Oregon Cluster).
Around 1900, a number of institutions began to experiment with breeding specific hop varieties. The breeding program at Wye College in Wye, Kent was started in 1904 and rose to prominence through the work of Prof. E. S. Salmon. Salmon released Brewer's Gold and Brewer's Favorite for commercial cultivation in 1934, and went on to release more than two dozen new cultivars before his death in 1959. Brewer's Gold has become the ancestor of the bulk of new hop releases around the world since its release.
Wye College continued its breeding program and again received attention in the 1970s, when Dr. Ray A. Neve released Wye Target, Wye Challenger, Wye Northdown, Wye Saxon and Wye Yeoman. More recently, Wye College and its successor institution Wye Hops Ltd., have focused on breeding the first dwarf hop varieties, which are easier to pick by machine and far more economical to grow. Wye College have also been responsible for breeding hop varieties that will grow with only 12 hours of daily light for the South African hop farmers. Wye College was closed in 2009 but the legacy of their hop breeding programs, particularly that of the dwarf varieties, is continuing as already the U.S.A. private and public breeding programs are using their stock material.
Particular hop varieties are associated with beer regions and styles, for example pale lagers are usually brewed with European (often German, Polish or Czech) noble hop varieties such as Saaz, Hallertau and Strissel Spalt. British ales use hop varieties such as Fuggles, Goldings and W.G.V. North American beers often use Cascade hops, Columbus hops, Centennial hops, Willamette, Amarillo hops and about forty more varieties as the U.S.A. have lately been the more significant breeders of new hop varieties, including dwarf hop varieties.
The term "noble hops" traditionally refers to varieties of hops which are low in bitterness and high in aroma. They are the European cultivars or races Hallertau, Tettnanger, Spalt, and Saaz. They are not bred as "modern" hop varieties but they are wild hops found and named for a specific region or city in which they were first found or by the farmer which found them or first propagated them. They contain high amounts of the hop oil humulene and low amounts of alpha acids cohumulone and adhumulone, as well as lower amounts of the harsher-tasting beta acids lupulone, colupulone, and adlupulone.
Their low relative bitterness but strong aroma are often distinguishing characteristics of European-style lager beer, such as Pilsener, Dunkel, and Oktoberfest/Märzen. In beer, they are considered aroma hops (as opposed to bittering hops); see Pilsner Urquell as a classic example of the Bohemian Pilsener style, which showcases noble hops.
As with grapes, the location where hops is grown affects the hops' characteristics. Much as Dortmunder beer may within the EU be labelled "Dortmunder" only if it has been brewed in Dortmund, noble hops may officially be considered "noble" only if they were grown in the areas for which the hop varieties (races) were named.
English noble varieties are Fuggle, East Kent Goldings and Goldings. They are characterized through analysis as having an alpha:beta ratio of 1:1, low alpha-acid levels (2–5%) with a low cohumulone content, low myrcene in the hop oil, high humulene in the oil, a ratio of humulene:caryophyllene above three, and poor storability resulting in them being more prone to oxidation. In reality, this means they have a relatively consistent bittering potential as they age, due to beta-acid oxidation, and a flavour that improves as they age during periods of poor storage.
- Hallertau or Hallertauer–The original German lager hop; named after Hallertau or Holledau region in central Bavaria. Due to susceptibility to crop disease, it was largely replaced by Hersbrucker in the 1970s and 1980s. (Alpha acid 3.5–5.5% / beta acid 3–4%)
- Žatec–Noble hop used extensively in Bohemia to flavor pale Czech lagers such as Pilsner Urquell. Soft aroma and bitterness. (Alpha acid 3–4.5% /Beta acid 3–4.5%)
- Spalt–Traditional German noble hop from the Spalter region south of Nuremberg. With a delicate, spicy aroma. (Alpha acid 4–5% / beta acid 4–5%)
- Tettnang–Comes from Tettnang, a small town in southern Baden-Württemberg in Germany. The region produces significant quantities of hops, and ships them to breweries throughout the world. Noble German dual-use hop used in European pale lagers, sometimes with Hallertau. Soft bitterness. (Alpha acid 3.5–5.5% / beta acid 3.5–5.5%)
In addition to beer, hops are also used in herbal teas and in soft drinks. These soft drinks include Julmust (a carbonated beverage similar to soda that is popular in Sweden during December), Malta (a Latin American soft drink) and kvass.
Hops are also used in herbal medicine in a way similar to valerian, as a treatment for anxiety, restlessness, and insomnia. A pillow filled with hops is a popular folk remedy for sleeplessness, and animal research has shown a sedative effect. The relaxing effect of hops may be due, in part, to the specific chemical component dimethylvinyl carbinol. Hops tend to be unstable when exposed to light or air and lose their potency after a few months' storage.
Dermatitis sometimes results from harvesting hops. Although few cases require medical treatment, an estimated 3% of the workers suffer some type of skin lesions on the face, hands, and legs. Hops are toxic to dogs.
- Gruit, an old-fashioned herb mixture used for bittering and flavoring beer, popular before the extensive use of hops
- Mugwort, a herb historically used as a bitter in beer production
- Oast house, a building designed for drying hops
- Rhamnus prinoides, a plant whose leaves are used in the Ethiopian variety of mead called tej
- "University of Minnesota Libraries: The Transfer of Knowledge. Hops-''Humulus lupulus''". Lib.umn.edu. 2008-05-13. Retrieved 2012-05-20.
- Jackson, Michael (1988). The New World World Guide to Beer. Philadelphia, Pennsylvania: Running Press. p. 18. ISBN 0-89471-884-3.
- "A short history of hops". 2009-11-20. Retrieved 2010-02-07.
- "Understanding Beer - A Broad Overview of Brewing, Tasting and Analyzing Beer - October 12th, 2006, Beer & Brewing, The Brewing Process". Jongriffin.com. Retrieved 2012-05-20.
- F. G. Priest; Iain Campbell (2003). Brewing microbiology. Springer. pp. 5. ISBN 978-0-306-47288-6.
- Corran, H.S. (23 Jan 1975). Purchase Used: A History of Brewing. Vermont Canada: David and Charles PLC. p. 303. ISBN 0-7153-6735-8.
- Unger, Richard W. (2004). Beer in the Middle Ages and Renaissance. University of Pennsylvania Press. p. 100.
- Bamforth, Charles W. (1998). Beer: tap into the art and science of brewing. Plenum Press. p. 245. ISBN 0-306-45797-0.
- [dead link]
- "The London magazine, 1752", page 332
- Summary of Reports: Nürnberg, Germany, 14 November 2006, INTERNATIONAL HOP GROWERS’ CONVENTION: Economic Committee
- "NCGR-Corvallis Humulus Genetic Resources". Ars-grin.gov. Retrieved 2012-05-20.
- Norman Moss, A Fancy to Worcesters, Agricultural research Service, U.S. Department of Agriculture
- "Herefordshire Through Time - Welcome". Smr.herefordshire.gov.uk. Retrieved 2012-05-24.
- Food and Agriculture Organization of the United Nations (FAO). FAOSTAT. Rome, Italy: Food and Agriculture Organization of the United Nations (FAO).
- "2009 New Zealand Hop Harvest spread over six weeks.". nzhops.co.nz. Retrieved 2013-05-22.
- "Humulus lupulus L. common hop". USDA Plants database. Retrieved 2013-09-13.
- "Interactive Agricultural Ecological Atlas of Russia and Neighboring Countries. Economic Plants and their Diseases, Pests and Weeds. ''Humulus lupulus''". Agroatlas.ru. Retrieved 2012-05-20.
- Andrew, Sewalish. "Hops: Anatomy and Chemistry 101". Retrieved 2013-09-13.
- "Sacramento History Online". Sacramentohistory.org. Retrieved 2014-08-09.
- "Images of Our Past". Elkgrovehistoricalsociety.com. 1940-05-24. Retrieved 2013-12-28.
- Lytle 1998:68-69. in http://www.wheatland.ca.gov/userfiles/file/Appendix%20N.pdf p2.
- Pacific Rural Press, 3 September 1898 in http://www.wheatland.ca.gov/userfiles/file/Appendix%20N.pdf p.29
- Street, R.S. Beasts of the Field: A Narrative History of California Farmworkers, 2004, 1769-1913. Stanford University Press. in http://www.wheatland.ca.gov/userfiles/file/Appendix%20N.pdf p.16
- "Hop separator - Edouard, Thys". Freepatentsonline.com. 1938-12-06. Retrieved 2013-10-10.
- "Hops Film". Sacramentohistory.org. Retrieved 2013-10-10.
- "Connie's Homepage - Hop Picking in Kent". Btinternet.com. Archived from the original on 2012-07-21. Retrieved 2012-05-20.[dead link]
- "George Orwell: Hop-picking". Theorwellprize.co.uk. Retrieved 2012-05-20.
- Smith, Keith. Around Malvern in old photographs.. Alan Sutton Publishing, Gloucester. ISBN 0-86299-587-6.
- "Charles Levett Hop Tokens, 60 Bushels Denomination, The Fitzwilliam Museum, fitzmuseum.cam.ack.uk". Fitzmuseum.cam.ac.uk. Retrieved 2012-05-20.
- Lebaron, Gaye (2008-06-29). "Hops, once king of county's crops, helped put region on map". Press Democrat. Retrieved 4 September 2012.
- Hough, James S (1991). The Biotechnology of Malting and Brewing. ISBN 978-0-521-39553-3. Retrieved 2009-03-02.
- Aguilera, Elizabeth (10 September 2008). "Hop harvest yields hip beer for brewer". Denver Post.
- Underwood, Kristin. It's Harvest Time at the Sierra Nevada Brewery. Treehugger. 6 August 2009. Retrieved 20 March 2011.
- Rabin, Dan; Forget, Carl (1998-10-01). The Dictionary of Beer and Brewing - Google Book Search. books.google.co.uk. ISBN 978-1-57958-078-0. Retrieved 2009-03-02.
- Palmer, John (2032). How to Brew. Boulder, CO: Brewers Publications. pp. 41–44. ISBN 0-937381-88-8.
- Nikolic, D; Li, Y; Chadwick, LR; Grubjesic, S; Schwab, P; Metz, P; Van Breemen, RB (2004). "Metabolism of 8-prenylnaringenin, a potent phytoestrogen from hops (Humulus lupulus), by human liver microsomes". Drug metabolism and disposition: the biological fate of chemicals 32 (2): 272–9. doi:10.1124/dmd.32.2.272. PMID 14744951.
- Capper, Allison; Darby, Peter (March 24, 2014). "What makes British Hops Unique in the world of Hop Growing?". British Hop Association. Retrieved July 4, 2014.
- "History of Hops". British Hop Association. Retrieved July 19, 2014.
- "Hop growers union of the Czech Republic". Czhops.cz. Retrieved 2012-05-20.
- "Hop Chemistry: Homebrew Science". Byo.com. 2000-04-28. Retrieved 2012-05-20.
- Plants for a Future: Humulus lupulus Plants for a Future. Retrieved 4 September 2012.
- Franco L, Sánchez C, Bravo R, Rodriguez A, Barriga C, Juánez JC (June 2012). "The sedative effects of hops (Humulus lupulus), a component of beer, on the activity/rest rhythm". Acta Physiologica Hungarica 99 (2): 133–9. doi:10.1556/APhysiol.99.2012.2.6. PMID 22849837.
- "Hops: Humulus lupulus". Retrieved 14 February 09.
- Bourne, Edmund J. (132). "Natural Relief for Anxiety".
- Bowe J, Li XF, Kinsey-Jones J, et al. (November 2006). "The hop phytoestrogen, 8-prenylnaringenin, reverses the ovariectomy-induced rise in skin temperature in an animal model of menopausal hot flushes". The Journal of Endocrinology 191 (2): 399–405. doi:10.1677/joe.1.06919. PMC 1635969. PMID 17088409.
- Keiler AM, Zierau O, Kretzschmar G (May 2013). "Hop extracts and hop substances in treatment of menopausal complaints". Planta Medica 79 (7): 576–9. doi:10.1055/s-0032-1328330. PMID 23512496.
- "Purdue University: Center for New Crops and Plant Products. ''Humulus lupulus'' L". Hort.purdue.edu. 1998-01-07. Retrieved 2012-05-20.
- "Animal Poison Control Center. Hops". ASPCA. Retrieved 2012-05-20.
|Wikimedia Commons has media related to Humulus lupulus.|