Hop Genetic Resources
Humulus Genetic Resources
by Jeanine S. DeNoma
USDA ARS National Clonal Germplasm Repository,
(Freshly Harvested Hop Cones in Dryer)
Use and Importance of Crop
Hops (Humulus lupulus L.) are commercially important as an essential flavoring in beer. Glands on the hop cone contain many unique compounds, making the chemistry of the hop plant extremely complex. The compounds most important to the brewing industry are the bitter acids, which give beer its characteristically bitter flavor, and the essential oils, which give beer its "hoppy" aroma.
United States and World Hop Production
According to data from the Food and Agricultural Organization (FAO) of the United Nations, over 109,000 metric tons (Mt) of hops are produced worldwide today. Hops are grown on 76,000 hectares in 30 countries (FAO, 1998). The ten countries leading in hop production, based on FAO records for 1998, were: Germany (34,200 Mt), USA (27010 Mt), China (13,000 Mt), Czech Republic (7,415 Mt), United Kingdom (5,200 Mt) Slovenia (4,134 Mt), Poland (2,736 Mt), Australia (2,500 Mt), Spain (2,000 Mt) and Ukraine (1,353 Mt).
The United States produces about 25% of the worldís hops, and is second only to Germany in worldwide production. In 1998 the US produced 27,010 metric tons of hops on 14,830 hectares, according to data from the FAO. Germany, the world leader in hop production, harvested 34,200 metric tons of hops on 19,789 hectares in 1998.
According to USDA agricultural statistics, US hop growers harvested 74.87 million pounds (34,033 Mt) of hops in 1997, the last year for which complete statistics are available. This harvest was valued at 117.9 million dollars. About a third (11,324 Mt) of this production was exported and the remainder was for domestic use. Additionally, the US imported 5,792 Mt of hops, of which 4,106 Mt came from Germany.
Among the hop growing states in the US, Washington leads in both acreage and total production. In 1997 Washington produced 55,816,000 pounds (25, 371 Mt) on 31,080 acres; Oregon produced 13,572,000 pounds (6169 Mt) on 8,352 acres; and Idaho produced 5,484,100 pounds (2493 Mt) on 3,870 acres. US yields average around 1,700 pounds per acre. The average price per pound received by growers from 1995 to 1997 has ranged from $1.71 in 1995 to $1.59 per pound in 1997.
Historical Uses for Hop
Over the years the hop has had many uses besides brewing. These have included:
- medicinal and herbal preparations
- preserving yeast cultures in bread making
- fiber for cloth and paper
- a dye for cinnamon brown
- hop pillows, used for their soporific effect.
Prior to their use in beer, hops were cultivated as early as the 8th century by French and German monks for medicinal purposes. Even earlier, the hop was apparently eaten as a vegetable; Burgess (1964) writes, "It is mentioned, in the first century A.D., by Pliny in his Natural History, as being used as a salad."Most recently the sugar industry has researched utilizing hopsí bacteriostatic properties in sugar processingóthe hop acids retard gram-positive bacteria. Plant nurseries sell many ornamental varieties for the home gardener. None of these uses, however, has achieved the economic importance of their use for making beer.
History of Hop Use in Beer
Hops were probably first added to beer as a preservative by Germans in the twelfth century. The bitter hop flavor was not particularly well liked, but brewers used hops, despite the taste, because its bacteriostatic property kept drink from spoiling and allowed manufacturers to transport and market their beers further from its production site. This may have eventually led to early large-scale brewing (Burgess, 1964).
History of Hop Cultivation
Hop cultivation probably began in Eastern Europe around Bohemia, Slovenia and Bavaria before the 8th century, and from there spread to the rest of Europe (Neve, 1991). Commercial hop production did not begin in England until the early 1500s when cultivation practices were adopted from Flemish growers (Burgess, 1964). As Europeans began colonizing the new world, they carried the tradition of hop cultivation with them. English settlers introduced hops into the Southern Hemisphere in South Africa, New Zealand and Australia in the early 1800s. As the popularity of hopped beer and ales spread, the Japanese began cultivating American and German hop varieties around 1876.
Attempts have been made to grow hops in latitudes below 35O, although, for the most part, these attempts have failed because of the hopís requirements for a dormancy period and long days for adequate flowering. India first imported hop cultivation into Kashmir in the 1880s, although this early attempt was unsuccessful. Neve (1991) reported that recently India has grown most of its own hops, although in 1998 the FAO did not list India among the 30 hop producing countries. Cape Providence, South Africa has successful commercial hop-yards; the FAO reported it produced 270 metric tons in 1998. Both Kashmir and Cape Providence are located at 34O latitude. Many other countries in lower latitudes, including Mexico, Kenya and Burma, to name only a few, have attempted to grow hops, generally without success. In some cases, these counties have experimented with extending day-length with artificial lightning, but this has usually proven too costly.
History of US Production
Early settlers to North America could pick native wild hops in the woods around their settlements. Dutch settlers, however, chose to import dried hops from their homeland. English settlers imported cuttings from England and in 1629 the Massachusetts Company began growing hops commercially. In time, Massachusetts, Connecticut, Vermont, New Hampshire and Maine all became early hop growing areas.
By the early 1800s hop growing had declined in the northern New England states. New York, which had better soil and was close to the large population centers, became the main hop growing region. By 1880 New York produced 21 million pounds of hops annually. Shortly thereafter, however, hop production in this region began to decline. Powdery mildew first hit New York in 1909, although sulfur fungicides brought it under control. Then in the late 1920s downy mildew devastated eastern hop yards. At the same time, Prohibition disrupted the hop growing and brewing industries.
(Baling Dried Hop Cones)
But already the main hop producing region had shifted again, this time to the Pacific Northwest (PNW). Hop growing began in the PNW in the 1850s and by the turn of the century both Oregon and California produced more hops than New York. Powdery Mildew was unknown in the region and downy mildew, while present, did not present a threat to the crop. Early western farmers were helped by the construction of the trans continental railroad, which allowed them to transport their hops to the breweries in the mid-west and east. During Prohibition hops were primarily grown for export. When the Prohibition was lifted in 1933 hop acreage rapidly increased. Oregon, Washington and California lead in hop production. Idaho joined the other three western hop producing states in the 1950s. Following World War II, the US was the largest hop producing country in the world, and much of what was produced was exported. Today most hops are grown for the domestic brewing industry, although about 12,000 Mt of hops are still exported.
Humulus is indigenous to the Northern Hemisphere, including the continents of Europe, Asia, and North America. The genus likely originated in China, where all three species of the genus occur. Distinct populations then developed from isolated plants dispersed to America and Europe.
Humulus is one of two genera in the Cannabinaceae family. Burgess, in his 1964 book on hops, identified the genus as composed of two species, Humulus lupulus and Humulus japonicus. In 1978, Ernest Small conducted a comprehensive study of Humulus taxonomy, examining 783 herbarium specimens. From this survey, he identified a third species, Humulus yunnanensis, commonly misidentified as H. lupulus.
Only a few specimens of H. yunnanensis are available and few reports appear in the literature. Neve (1991) described it as a perennial growing at high altitudes around 25ON in the Yunnan providence in southern China. "Since it grows at lower latitudes than other members of the genus it could have potential as breeding material to extend the areas in which hops could be grown commercially," wrote Neve. From his observations of the few available herbarium specimens, Small (1978) described H. yunnanensis as clearly distinct from either H. lupulus or H. japonicus, but resembling H. japonicus in many respects.
Humulus japonicus is native to Japan, Taiwan and China and has been introduced into North America. Small (1998) described it as having general adaptability to a wide range of geographical locations due to its aggressive growth, durable and easily transportable seeds, and "general purpose genotype." H. lupulus, and probably H. yunnanensis, on the other hand, are more narrowly adapted and have propagation features such as rhizomes and less durable seeds that limit their dispersal to the local area around the mother plant.
Within Humulus lupulus, Small (1978) identified five taxonomic subspecies based on morphological properties and geographical location:
- H. lupulus var. lupulus
- H. lupulus var. cordifolius
- H. lupulus var. neomexicanus
- H. lupulus var. pubescens
- H. lupulus var. lupuloides
Crosses among all taxonomic varieties produce fertile offspring (Neve, 1991). H. lupulus var. lupulus originated in Europe but has been introduced into North America and Japan through cultivation. H. lupulus var. cordifolius is found in Asia and Japan. The remaining three subspecies are indigenous to North America: H. lupulus var. neomexicanus is found in the southwestern regions of North America and has clearly defined morphological distinctions. H. lupulus var. pubescens appears throughout the Midwestern United States and also is well defined morphologically. H. lupulus var. lupuloides is found primarily in the eastern United States and Canada and throughout the northern midwest. According to Small, lupuloides overlaps geographically with the other botanical hop varieties and "intergrades with the other three varieties populating North America, wherever it is sympatric with them" (Small, 1978).
The World Conservation Center 1997 IUCN Red List of Threatened Plants has no Humulus species noted as threatened or endangered.
Economically Important Taxa
All commercial hops are of the species Humulus lupulus. Most cultivars were originally derived from European-type hops, although North American germplasm appears in the pedigree of some cultivars. Most notable was the first report in 1919 by Professor Salmon of Wye College, England of the hybridization a wild female hop from Manitoba, Canada with traditional English hops to increase the soft resin content in seedlings. From this cross Salmon released Brewers Gold, Northern Brewer and Bullion. Brewers Gold is included in the pedigree of many, many new varieties released from breeding programs around the world.
Traditional North American cultivars were noted for their high resin content and strong aroma. The American cultivars Late Cluster, which probably originated in New England, and Early Cluster, which originated in Oregon around 1908, were almost certainly selections from accidental hybridizations between European types and indigenous plants (Brooks et al., 1961). The cultivar Comet was derived from a cross with native North American germplasm from Utah. Comet remained in commercial production for only a few years due to "an objectionable aroma," a common problem associated with the use of wild American germplasm.
In a 1980 study on the relationship between wild hops and named cultivars, Small reported, "European cultivars proved to be very similar to indigenous wild European Humulus lupulus and almost certainly are derived from the wild plants. Japanese cultivars seem to be imported European stock which have been introgressed by indigenous wild Japanese H. lupulus. Similarly, American cultivars appear to be of hybrid origin, apparently between European cultivars and one of the three major indigenous varieties of wild North American plants."
All three species in the genus Humulus are herbaceous vines with hooked climbing hairs to grasp and twine clockwise up available climbing structures. All species are dioecious, having separate male and female plants, although there are exceptions to this and individual monoecious plants are sometimes found, as noted below. Male and female plants are easily distinguished during flowering, but no other morphological differences identify the sex of a plant. Flowers are wind pollinated and the female inflorescence develops into a "strobile," commonly called a "cone."
As noted above, there are few reports about H. yunnanensis. When Small (1978) encountered the first herbarium specimen, he considered the possibility that it might be a cross between H. lupulus and H. japonicus. However, these two species have different chromosome numbers and there are no reports of successful crossing between them. Staining indicated 98% of the H. yunnanensis pollen was viable (Small, 1978). There are no reports of chromosome number for this species.
H. yunnanensis is known to be a perennial and, like the other species in this genus, it is dioecious. Its leaves are less lobed and its cones larger than those of H. japonicus; however, writes Small (1978), "In most respects H. yunnanensis resembles H. japonicus more than it does H. lupulus." Although commonly misidentified as H. lupulus in herbarium samples, Small notes the morphology, especially the stiffness of the climbing hairs, pubescent leaf surfaces, small leaf glands, distribution of the trichomes, and large pollen grains, all clearly distinguish it as a separate species from H. lupulus. H. yunnanensis has only a few glands on the bracteoles of the cone and has no value for brewing.
H. japonicus is a dioecious annual native to Asia where it is widespread as an aggressive weed. It has large, seven-lobed leaves, long internodes, and extremely strong hooked climbing hairs. Although it is reportedly used as an ornamental, primarily for hedges, its aggressive nature and particularly strong climbing hairs make it unpleasant to handle and potentially an invasive weed. The female inflorescence is smaller than the cone of H. lupulus and there are few glands on either its leaves or cones, making it of no value to the brewing industry.
Winge (1929), Ono (1955) and Sinoto (1929) examined the chromosomes of H. japonicus and determined it is a diploid species with a basic chromosome number of x=8. The female has the chromosome complement of 2n=2x=16. The male carries an extra chromosome and has the complement 2n=2x=17.
Humulus lupulus is a perennial that regrows each spring from the rhizomes of an underground rootstock. In the wild it spreads through underground rhizomes and by seed. Small (1978) separated H. lupulus into five taxonomic varieties based primarily on leaf characteristics, including pubescence, leaf hairs, leaf lobes and number of glands. European-type hops are less pubescent, have fewer glands, and have heart-shaped leaves with few lobes. Brewers have long identified traditional European hops as milder and more aromatic than those from North America. Haunold et al. (1993) reported native North American hops had "deeply lobed leaves with five or seven lobes predominating." Small also noted the heavily pubescent and deeply lobed leaves of the native American hop varieties. Early English hop breeders noted the high resin content of the North American hop, describing it as rich with glands and "buttery" to touch.
In commercial hop production, hop vines are trained to climb strings suspended from 25 foot or higher trellises. The vines clasp the strings with strong hooked hairs and twine clockwise to the top of the trellis. Lateral arms develop at the nodes, producing flowers at their terminal buds. At harvest the vines are cut down and the cones stripped off, but the crown remains in the ground as the source for the next yearís growth. Crowns can remain productive for years and are generally only removed when a hop yard is either removed from production or replaced with a new variety. Hops require long summer days to flower and produce good cone yields. Most commercial production, therefore, occurs in countries located between the 30th and 50th parallels.
Only the cones of the female plants are used by the brewing industry. Mature cones are covered with tiny glands that hold the sticky yellow resin containing the compounds used by the brewing industry. Cones range in size from about one to ten centimeters in length with papery green bracts and bracteoles. The resin glands coat the bracteoles and seeds of the cones.
Although the hop plant is normally dioecious, monoecious plants are occasionally observed. Diploid monoecious plants are rare; in most cases these monoecious plants are found to be polyploids. Haunold (1991) wrote, "In monoecious diploids, only one sex is functional and no authenticated case of functional bisexuality has ever been reported." In a 1993 paper on native North American hops, however, Haunold et al. reported finding a high percentage of monoecious types in some wild North American populations, including monoecious plants that produce both seed and fertile pollen. Monoecious plants are rarely found among the European types used in cultivated hop yards or in breeding material, as most breeders select against it to avoid the subsequent loss of yield potential.
Male hop plants are useful as pollinators in commercial hop yards to stimulate cone development and produce seeded hops. In most parts of the world, however, seeds are considered undesirable. Most hops are grown unseeded and vigorous efforts are made to remove all sources of hop pollen near commercial yards. Males are essential, however, in hop breeding programs to develop new varieties through controlled hybridization.
Humulus lupulus is a diploid with a chromosome compliment 2n=2x=20. Tetraploids are occasionally found occurring naturally in wild or breeding populations. Tetraploids are also produced artificially by doubling the diploid chromosome set by treatment with colchicine. Tetraploids can be crossed with diploids, resulting in sterile triploids (2n=3x=30) that produce few seeds. Triploid plants usually grow more vigorously than either diploid or tetraploid plants, and triploid varieties are useful for producing seedless hops. Triploid males are useful in the production of some diploid varieties to stimulate cone growth and increase yield without producing seed.
Vegetative or asexual propagation
Because H. lupulus is dioecious, it is highly heterozygous; therefore, seedling populations are highly variable. Even seeds collected from a superb variety will fail to produce a plant with the brewing value of its mother plant. For this reason, commercial plant material is propagated vegetatively either from rhizomes or softwood cuttings. Rhizomes, or underground shoots, have several buds at each node and can be pruned from the crown of the mother plant, cut into pieces, and planted in either the field or greenhouse. Softwood cuttings are taken from the stem, with each piece including one node with two leaves and about 5 to 8 cm of stem below the node. Rooting is enhanced by dipping the lower portion of the stems in a rooting solution of 1000 ppm indolebutyric acid and 1000 ppm boric acid before planting it into light soil and placing it in a mist chamber for one to two weeks.
Seedlings of H. lupulus are used to originate populations for selection in breeding programs. Once a superior seedling has been identified, it is clonally propagated to maintain its unique qualities. Hop seeds require a cold period to germinate. Haunold and Zimmermann (1972) obtained 80% germination using the following procedure. Seeds were sterilized in 2 parts 95% ethanol, 4 parts bleach, and 10 parts tap water for five minutes, rinsed thoroughly, then placed in a petri dish on moist blotter paper. The seeds and blotter paper were moistened with 20 ppm Captan fungicide and refrigerated for six to eight weeks. Following the cold treatment, dishes were transferred to a growth chamber at 25 C with 8-hour days. Germinated seedlings were then transferred to light soil in the greenhouse.
Present USDA Germplasm Activities
The United States national Humulus germplasm collection is housed at the USDA/ARS National Clonal Germplasm Repository (NCGR) in Corvallis, Oregon. This collection is comprised of:
- field collection of 587 accessions
- greenhouse collection of 68 virus-tested accessions
- tissue culture collection of 86 accession, most virus-tested
- seed collection of 22 accessions
- hop virus collection of 9 plants with all major US hop viruses represented
- cryo-preserved collection currently being established.
The NCGR field collection is kept in conjunction with Oregon State University and an active hop breeding program. The NCGR greenhouse collection of virus-tested plants was establish from rhizomes from the field, processed through heat therapy and tissue cultured meristems, and reestablished into the greenhouse. The tissue culture collection is also primarily composed of these virus-tested plants, but includes additional wild American clones. The tissue culture collection also composes the basis from which a cryopreserved collection is being developed.
Crop Vulnerability Status
Small (1981) reported finding considerable variation among different samples of some named cultivars. He noted that "environmental differences and misidentification may explain some of this variation. However, it is probable that a range of germ plasm is encompassed today in most hop cultivars, despite the usual asexual mode of reproduction. Somatic mutations, and pollination of a cultivar by foreign germ plasm followed by chance incorporation of the seedlings into hop plantations, would widen the range of variation found within given cultivars."
- Barth, Heinrich J., Christiane Klinke, and Claus Schmidt (1994) The Hop Atlas: The History and Geography of the Cultivated Plant. Joh. Barth & Sohn, Nuremberg, Germany.
- Brooks, S.N. (1962) Effectiveness of selection within Fuggle hops (Humulus lupulus L.). Crop Science 2:5 10.
- Burgess, A.H. (1964) Hops: Botany, Cultivation, and Utilization. Interscience Publishers Inc. New York.
- Haunold, Alfred (1972) Polyploid breeding with hop Humulus lupulus L. Technical Quarterly of Masters Brewers Association of America 9(1):36-40.
- Haunold, Alfred and Charles E. Zimmermann (1974) Pollen collection, crossing and seed germination of hop. Crop Science 14:774-76.
- Haunold, Alfred (1991) Cytology and Cytogenetics of Hops. In: Chromosome Engineering in Plants: Genetics, Breeding, Evolution, Part B. T.Tsuchiya and P.K. Gupta, Editors. Elsevier Science Publishers B.V. Amsterdam.
- Haunold, A., G.B. Nickerson, U. Gampert, P.A. Whitney, and R.O. Hampton (1993) Agronomic and quality characteristics of native North American hops. Am Soc Brew Chem. 51(3):133-137.
- Neve, R.A. (1986) Centenary Review: Hop breeding worldwide - its aims and achievements. J. Inst. Brewing 92:21-24.
- Neve, R.A. (1991) Hops. Chapman and Hall, New York.
- Small, Ernest (1978) A numerical and nomenclatural analysis of morpho-geographical taxa of Humulus. Systematic Botany 3(1):37-76.
- Small, Ernest (1980) The relationships of hop cultivars and wild variants of Humulus lupulus. Can. J. Bot. 58:676-686.
- Small, Ernest (1981) A numerical analysis of morpho-geographic groups of cultivars of Humulus lupulus based on samples of cones. Can. J. Bot. 59:311-324.