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 (the premier source of information about the natural world for fifteen hundred years), 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.
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.
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 oxidize and oxidized beta acids form sulfur compounds such as DMS (dimethyl sulfide) that can give beer off-flavours of rotten vegetables or cooked corn.
The flavour imparted by hops varies by type and use: hops boiled with the beer (known as "bittering hops") produce bitterness, while hops added to beer later impart some degree of "hop flavour" (if during the final 10 minutes of boil) or "hop aroma" (if during the final 3 minutes, or less, of boil) and a lesser degree of bitterness. Adding hops after the wort has cooled and the beer has fermented is known as "dry hopping", and adds hop aroma, but no bitterness. The degree of bitterness imparted by hops depends on the degree to which otherwise insoluble alpha acids (AAs) are isomerized during the boil, and the impact of a given amount of hops is specified in International Bitterness Units. Unboiled hops are only mildly bitter.
Flavours and aromas are described appreciatively using terms which include "grassy", "floral", "citrus", "spicy", and "earthy". Most of the common commercial lagers have fairly low hop influence, while true pilseners should have noticeable noble hop aroma and certain ales (particularly the highly-hopped style known as India Pale Ale, or IPA) can have high levels of bitterness.
The term Noble hops traditionally refers to four varieties of hop which are low in bitterness and high in aroma. They are the central European cultivars, 'Hallertauer Mittelfrüh', 'Tettnanger', 'Spalter', and 'Saaz'. They are each named for a specific region or city in which they were first grown or primarily grown. 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 Pilsener style, which showcases Noble hops.
As with grapes, land where the hops were grown affects the hops' characteristics. Much like Dortmunder beer may only within the EU be labelled "Dortmunder" if it has been brewed in Dortmund, Noble hops may only officially be considered "Noble" if they were grown in the areas for which the hops varieties were named.
Particular hop varieties are associated with beer styles, for example pale lagers are usually brewed with European (and often Czech and/or German) hop varieties such as Saaz, Hallertau and Strissel Spalt. English ales use hop varieties such as Fuggle, Golding and Bullion. North American varieties include Cascade, Columbia, Willamette and Amarillo.
List of hop varieties
The term 'Noble Hop' is a traditional designation for hops grown in four areas within Bavaria or Bohemia, something like the French appelleations for grapes & wine. Historically, these regions produced superior quality hops, particularly well suited for continential European style beers. Hops grown outside these regions cannot be 'Noble Hops' but nonetheless may be excellent hops.
Hallertau - The original German lager hop; 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%)
Saaz – 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, with a delicate, spicy aroma. (Alpha acid 4–5% / beta acid 4–5%)
Tettnang – 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%)
Other hop varieties
Admiral – An English bittering hop used in some English ales. (Alpha acid: 13.5–16%)
Ahtanum – An American aroma-type variety developed by Yakima Chief Ranches, similar to Cascade. (Alpha acid: 5.7–6.3% / beta acid: 5.0–6.5%)
VGX001 (Amarillo®) – Popular American mid-range alpha variety developed by Virgil Gamache Farms in late 20th century. (Alpha acid: 8–11% / beta acid: 6–7% )
Brewer's Gold – British bittering hop developed in 1919. Both Brewer's Gold and Bullion are seedlings of BB1 (found wild in Manitoba). Many modern high alpha hops were developed from Brewer's Gold. (Alpha acid 7.1–11.3% / beta acid 3.3-6.1% / cohumulone 36-45%)
Cascade – Very successful and well-established American aroma hop developed by Oregon State University's breeding program in 1956 from Fuggle and Serebrianker (a Russian variety), but not released for cultivation until 1972. Piney, citrusy, and quite assertive. Aroma of Sierra Nevada Pale. One of the "Three Cs" along with Centennial and Columbus. (Alpha acid: 4.5–6.0% / beta acid: 5.0–7.0% )
Centennial – American aroma-type variety bred in 1974 and released in 1990. Similar to Cascade and Chinook. One of the "Three Cs" along with Cascade and Columbus. (Alpha acid: 9.5–11.5% / beta acid: 4.0–5.0%)
Challenger – English hop with fresh pine notes. Introduced in 1972. (Alpha acid 6.5–8.5% / beta acid 4–4.5%)
Chinook – American cross between Petham Golding and a USDA-selected male. Typical American citric pine hop with notable grapefruit and pineapple flavours. (Alpha acid 12.0–14.0% / beta acid 3.0–4.0%)
Cluster - Originated from mass selection of the Cluster hop, which is an old American cultivar. It is suggested that they arose from hybridization of varieties, imported by Dutch and English settlers and indigenous male hops. (Alpha acid: 5.5–8.5% / beta acid: 4.5–5.5%) Also known as Golden Cluster, used as the sole bittering hop in the iconic Queensland, Australia beer XXXX Gold and XXXX Bitter.
Columbus – A high yielding, high alpha acid American bittering hop. Also known by the trade name Tomahawk. One of the "Three Cs" along with Cascade and Centennial. (Alpha acid 14–17%)
Crystal – An American triploid variety developed in 1993 from Hallertau, Cascade, Brewer's Gold and Early Green. Quite aromatic, fruity. (Alpha acid 3.5–5.5% / beta acid 4.5–6.5%)
Eroica – A strongly flavoured bittering hop used in wheat beers. (Alpha acid 9–12%)
First Gold – English dwarf hop. A cross-pollination of Whitbread Golding variety and a dwarf male. It is like a spicier Golding, with a higher alpha and slightly richer bitterness. (Alpha acid 6.5–8.5% / beta acid 3–4%)
Fuggles - Main English hop developed late 19th century. Considered by some to be less refined than Goldings, others prefer its juicier, more woody character.(Alpha acid 4–5.5% / Beta acid 2–3%)
Galena - American bittering hop developed from Brewer's Gold by open pollination in the state of Idaho. Has a moderate bitterness despite its high alpha content. (Alpha acid 12–14% / beta acid 7–9%)
Glacier - Low-cohumulone American Fuggle descendant. Mild bittering and soft, fruity character with hints of apricot and pear. (Alpha acid 5.5% / beta acid 8.2%)
Goldings - The traditional and very popular English aroma hop. Developed in 1790. Soft, earthy, vaguely farm-like aroma. Widely cultivated. Called East Kent Goldings if grown in East Kent, Kent Goldings if grown in mid-Kent, and Goldings if grown elsewhere. (Alpha acid 4–5.5% / beta acid 2–3.5%)
Greenburg - American Hop found in southern Idaho. Fruity flavor with a touch of woody flavors. Used mainly in microbrews (Alpha acid 5.2% / beta acid 7.2%)
Hersbrucker - Noble hop used in German pale lagers. Noted for grass and hay aroma. (Alpha acid 3–5.5% / beta acid 4–5.5%)
Herald - An English aroma and bittering hop; sister of Pioneer. (Alpha acid 11-13% / Beta acid 4.8-5.5%)
Horizon - American high alpha cross made in Oregon in 1970. Horizon and Nugget share a common parent (#65009). Soft bitterness. (Alpha acid 11–13% / beta acid 6.5–8.5% / cohumulone 17-21%)
Liberty - American cross between Hallertauer Mittlefrüh and downy mildew resistant male, developed in 1983. (Alpha acid 3.0–5.0% / beta acid 3.0–4.0%)
Lublin - The grassy, hay-like signature of Polish lagers. The bitterness is slightly harsher than noble varieties, but the aroma is a little bit softer.
Magnum - A bittering/aroma type cultivar, bred in 1980 at Huell, the German Hop Research Instititute, from the American variety Galena and the German male 75/5/3. (Alpha acid: 10.0–12.6% / beta acid: 5.0–7.0%)
Millennium - Bittering variety, bred from Nugget and with similar characteristics. (Alpha acid 15.5% / beta acid 4.8%)
Mount Hood - Soft American variety developed from Hallertau. Frequently used in styles that require only a subtle hop aroma. (Alpha acid 5.0–8.0% / beta acid 5.0–7.5%)
Nelson Sauvin - A new variety developed in Nelson, New Zealand. Named with more than a nod towards the Sauvignon Blanc grape, this hop produces unique fruity characteristics reminiscent of white wine. (Alpha acid 12-14% / beta acid 7-8%)
Newport - Recently developed American high-alpha bittering hop. (Alpha acid 10–17%)
Northdown - Dual purpose hop in England developed in 1970s, with a Northern Brewer-like bitterness, and soft aroma. (Alpha acid 7.5–9.5% / beta acid 5–5.5%)
Northern Brewer – Developed in England in 1934 from a cross between a female hop of wild American parentage and an English male. Grown in Europe and America as a dual-purpose hop, but the aroma is mellow, so is mainly used for bittering in combination with other hops. (Alpha acid 8–10% / beta acid 3–5%)
Nugget – Bittering hop. (Alpha acid 12–14% / beta acid 4–6%)
Pacific Gem – High alpha bittering hop from New Zealand. Most are organic. Pleasant woody flavour and berry aroma. . (Alpha acid 14–16% / beta acid 8–8.4%)
Palisade– Fairly recent American cross of Tettnager and open pollenation resulting in a moderate alpha hop with good aroma characteristics. (Alpha acid 6-10% / beta acid 4-7%)
Perle – German dual-purpose hop, with floral, spicy aroma. Often used in combination with other hops. (Alpha acid 7–9.5% / beta acid 4–5%)
Pioneer - English hop; a sister of Herald. A clean, soft and rounded bitterness with a recognisable English aroma. (Alpha acid 8-10% / beta acid 3.5-4%)
Pride of Ringwood - Infamous Australian hop. First used in 1965 when it was the highest alpha acid hop in the world. Used extensively in Australian pale ales and lagers. (Alpha acid 7–10% / beta acid 4–6%)
Progress – Higher alpha English hop developed in the 1960s as a replacement for Fuggles. Often used with Goldings. (Alpha acid 5–7% / beta acid 2–2.5%)
Santiam – American floral aroma hop with mid-range alpha acid. (Alpha acid 5–7% / beta acid 6–8%). Pedigree includes Tettnang (mother), Hallertau Mittelfrüh (grandmother) and Cascade (great grandmother).
Saphire - A new breed of hop that is starting to replace the Hallertauer Mittlefrueh variety, which has become more and more susceptable to diasease and pests. Shares many of the Hallertaur Mittlefrueh characteristics and is very well suited as an aroma hop. This hop is distinguished by a sweet and clean citrus aroma that has a hint of tangerine. (Alpha acid 2-4.5% / Beta acid 4-7%)
Satus - A bittering-type cultivar of recent origin. (Alpha acid: 12.5–14.0% / beta acid: 8.5–9.0%)
Select - German disease-resistant Hallertauer and Spalt pale lager variety developed in early 1990s. (Alpha acid 4–6% / beta acid 3.5–4.5%)
Simcoe - American high alpha variety released in 2000. Distinctive passionfruit flavour and aroma where Simcoe hops are utilised late in the boil (ie, boiled for between 20 and 0 minutes). (Alpha acid 12–14% / beta acid 4–5%)
Sterling - American floral hop released in 1998. A cross between Saaz and Mount Hood in character but easier to grow. (Alpha acid 6–9% / beta acid 4–6%)
Strisselspalt - French aroma hop from Alsace, used mostly in pale lagers. Similar to Herbrucker. (Alpha acid 3–5% / beta acid 3–5.5%)
Styrian Goldings - Slovenian variant of Fuggles. Used in English ales and Belgian strong ales amongst others. (Alpha acid: 4.5–6.0% / beta acid: 2.5–3.5%)
Target - Dusty, earthy English mid-to-high alpha hop bred from Kent Goldings. (Alpha acid 9.5–12.5% / beta acid 5–5.5%)
Tomahawk - Bittering hop. Same as Columbus. (Alpha acid: 14–18% / beta acid: 4.5–5.8%)
Tradition - Bred in 1991 from Hallertau Mittlefrüh by the Hull Hop Research Institute in Germany for resistance to disease. Grassy like Hallertau, but easier to grow. (Alpha acid 5–7% / beta acid 4–5%)
Ultra - A triploid aroma-type cultivar, originated in 1983 from a cross between the colchicine-induced tetraploid Hallertau mf (USDA 21397) and the diploid Saazer-derived male genotype (USDA 21237m). Ultra is the half-sister to Mount Hood, Liberty and Crystal. Its genetic composition is 4/6 Hallertau mf, 1/6 Saazer, and 1/6 unknown. This cultivar was released for commercial production in March, 1995. (Alpha acid: 4.5–5.0% / beta acid: 3.6–4.7%)
Vanguard – American aroma cross developed from Hallertau in 1982. (Alpha acid 5.5%–6% / beta acid 6–7%)
Warrior - New American bittering hop, popular with growers and brewers. (Alpha acid 15–17% / beta acid 4.5–5.5%)
Willamette – Popular American development in 1976 of the English Fuggle. Mild aroma hop, with an herbal, sometimes gently fruity character. (Alpha acid 4.0–6.0% / beta acid 3.0–4.0%)
Zeus – American aromatic high-alpha hop with noticeable bitterness. Similar, if not identical, to Columbus/Tomahawk. (Alpha acid 15.0%)
- ↑ Secundus, Gaius Plinius (77). "Naturalis Historia". Pliny the Elder. Retrieved on 26-Jan-2007.
- ↑ Corran, H.S. (23-Jan-1975). Purchase Used: A History of Brewing. Vermont Canada: David and Charles PLC. pp. 303. ISBN-10:0715367358. http://www.abebooks.com/servlet/BookDetailsPL?bi=592008163&isbn=0715367358 Purchase Used:.
- ↑ 
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3. University of Vermont Extension System Department of Plant and Soil Science Growing Hops in New England - COH 27 Leonard P. Perry, Extension Associate Professor