As the female cone-shaped flowers (
hops) are used to preserve and flavor
beer, the species is widely cultivated for the
brewing industry.[2]
Description
Humulus lupulus is a
perennialherbaceous plant up to 10 metres (33 feet) tall, living up to 20 years.[3] It has simple
leaves with 3–5 deep lobes that can be opposite or alternate.[4] The species is triggered by the longer summer days to flower,[5] usually around July or August in the Northern Hemisphere.[6] The plant is
dioecious, with male and female flowers on separate plants. The fragrant flowers are
wind-pollinated.[7] The staminate (male) flowers do not have petals, while the pistillate (female) flowers have petals enveloping the fruit. The female flower cones (or
strobili) are known as
hops.[3] The fruit is an
achene, meaning that it is dry and does not split open at maturity.[4] The achene is surrounded by tepals and lupulin-secreting glands are concentrated on the fruit.[8][9]
The species is sometimes described as a
bine rather than a vine because it has stiff downward facing hairs that provide stability and allow it to climb.[3]
Hops are unique for containing
secondary metabolites,
flavonoids,
oils, and
polyphenols that impact the flavor of the products they are common in, such as
beer.[10] The bitter flavors in hops can be accounted for by acids composed of
prenylatedpolyketides (a group of secondary metabolites), which highly impact the taste of hop-based products.[11] Multiple genes have been identified as factors in the expression of taste including
O-methyltransferase1, geranyl diphosphate
synthase, and
chalcone synthase.
Genomic analyses have shown evidence that the intervention of humans in the selection process of the hop over the thousands of years it has been cultivated have provided noticeable enhancements in aroma and bitterness as well as selection of varieties with high yield rates.[12]
Flowering, growth, and stress response
Predicted genes in
homologous primary contigs have been identified as accounting for various traits
expressed via variation in the growth, flowering, and stress responses in the plant. These homologous primary contigs correspond to regions with large amounts of
sequence variation.
Genes in the hop that contain higher rates of
sequence divergence in homologous primary contigs (overlapping
DNA sequences inherited by a common ancestor) have been attributed to the expression of flowering, growth and responses to (both
abiotic and
biotic) stress in the plant. The responses to stress are thought to manifest in the distinct differences and difficulties in the cultivation processes between geographically popular varieties of the hop plant.[13] Outside environmental stress, such as changes in temperature and water availability has also been shown to significantly alter the
transcriptome and incite reductions in genes known to be involved in the synthesis of
secondary metabolites (including bitter acids), which are
organic compounds produced that do not impact development or reproduction of hops. Environmental stress has also been shown to reduce
expression of the
valerophenonesynthasegene, which is known to be an essential genetic component in the regulation of bitter acid production. This shows that impacts of outside stress on H. lupulus likely has a direct implication of the expression of the bitter flavor that remains an essential component of the popularity of the plant.[10]
Humulus lupulus extract is antimicrobial, an activity which has been exploited in the manufacture of natural deodorant.[16]
Spent H. lupulus extract has also been shown to have
antimicrobial and anti-
biofilm activities, raising the possibility this waste product of the brewing industry could be developed for medical applications.[17]
Extracts of the bitter alpha-acids present in H. lupulus have been shown to decrease nocturnal activity, acting as a sleep aide, in certain concentrations.[18]
Because of the growing understanding regarding the hop's overlap in gene structures with
cannabidiolic acid synthase, the precursor structure to
cannabidiol, there is a gap in general understanding about potential unknown compounds and benefits in hops. As the understanding of the health benefits available in cannabidiol increases, there is a growing demand to further investigate the overlap between cannabidiolic acid synthase and H. lupulus.[19]
Limitations
The
genome of H. lupulus is relatively large and has been shown to be a similar size to the
human genome. The complexity of the hop genome has made it difficult to understand and identify unknown
genetic properties, however with the growing availability of accessible
sequencing, there is room for more advanced understanding of the plant.[19] Because of the growing concern of
climate change, and the assumption that there will be an increase of
heat waves, it is likely that growing large yields of hops could become more difficult. This could result in changes to the
transcriptome of the hop, or result in a decrease of certain varieties, leaving less room for further research.[10]
Taxonomy
Relation to Cannabis sativa
The hop is within the same family of plants such as
hemp and
marijuana, called
Cannabaceae.[13] The hop plant diverged from Cannabis sativa over 20 million years ago and has evolved to be three times the physical size.[20][21][12][22] The hop and C. sativa are estimated to have approximately a 73% overlap in genomic content.[23] The overlap between
enzymes includes
polyketide synthases and
prenyltransferases.[24] The hop and C. sativa also have significant overlap in the
cannabidiolic acid synthase gene, which is expressed in the tissues of the leaves in both plants.[13]
Varieties
The five varieties of this species (Humulus lupulus) are:
H. l. var. lupulus – Europe, western Asia
H. l. var. cordifolius – eastern Asia
H. l. var. lupuloides (syn. H. americanus) – eastern North America
H. l. var. neomexicanus – western North American.[25]
H. l. var. pubescens – midwestern and eastern North America[26][27]
The genus name Humulus is a medieval name that was at some point Latinized after being borrowed from a Germanic source exhibiting the h•m•l consonant cluster, as in Middle Low German homele.
According to Soviet Iranist
V. Abaev this could be a word of
Sarmatian origin which is present in the modern Ossetian language (
Ossetian: Хуымæллæг) and derives from proto-Iranian hauma-arayka, an Aryanhaoma.[30]
From Sarmatian dialects this word spread across Eurasia, thus creating a group of related words in Turkic, Finno-Ugric, Slavic and Germanic languages (see
Russian: хмель,
Chuvashхăмла,
Finnishhumala,
Hungariankomló,
Mordovianкомла,
Avarхомеллег).
The specific epithet lupulus is Latin for "small wolf".[31] The name refers to the plant's tendency to strangle other plants, mainly
osiers or basket willows (Salix viminalis), like a wolf does a sheep.[3] Hops could be seen growing over these willows so often that it was named the willow-wolf.[31]
The English word hop is derived from the
Middle Dutch word hoppe, also meaning Humulus lupulus.[32]
Distribution and habitat
The plant is
native to Europe, western Asia and North America.[31]
It grows best in the latitude range of 38°–51° in full sun with moderate amounts of rainfall.[2]
H. lupulus is first mentioned in 768 CE when
King Pepin donated hops to a monastery in Paris. Cultivation was first recorded in 859 CE, in documents from a monastery in
Freising, Germany.[33]
The chemical compounds found in H. lupulus are the main components in flavoring and bittering beer. The fragrant flower cones, known as hops, impart a bitter flavor and also have aromatic and preservative qualities.[34] Some other compounds help with creating foam in beer. Chemicals such as
linalool and
aldehydes contribute to the flavor of beer. The main components of bitterness in beer are
iso-alpha acids, with many other compounds contributing to beer's overall bitterness.[35] Until the Middle Ages, many varieties of plant were used to flavor beer, including most commonly Myrica gale.[33]H. lupulus became favored because it contains preserving agents which prolong the viability of a brew.
Culture
H. lupulus was voted the
county flower of
Kent in 2002 following a poll by the wild flora conservation charity
Plantlife.[36]
^
abcdConway, Sean; Snyder, Reid (2008).
"Humulus lupulus - Hops"(PDF). College Seminar 235 Food for Thought: The Science, Culture, & Politics of Food. Hamilton College. Retrieved July 31, 2008.
^Burnham, Robyn J. (2014).
"Vitis riparia". CLIMBERS: Censusing Lianas in Mesic Biomes of Eastern Regions. University of Michigan College of Literature, Sciences and the Arts. Retrieved 2017-04-02.
^
ab"Humulus lupulus". NewCROP, the New Crop Resource Online Program. Retrieved 2017-04-02.
^Benkherouf, Ali Y.; Soini, Sanna L.; Stompor, Monika; Uusi-Oukari, Mikko (February 2019). "Positive allosteric modulation of native and recombinant GABAA receptors by hops prenylflavonoids". European Journal of Pharmacology. 852: 34–41.
doi:
10.1016/j.ejphar.2019.02.034.
ISSN0014-2999.
PMID30797788.
S2CID73456325.
^Langezaal CR, Chandra A, Scheffer JJ (1992). "Antimicrobial screening of essential oils and extracts of some Humulus lupulus L. cultivars". Pharm Weekbl Sci. 14 (6): 353–356.
doi:
10.1007/bf01970171.
PMID1475174.
S2CID12561634.
"Czech hop varieties". Žatec: Chmelařský institut, 2012.
on-line
Barchet, Ron. "German hops in craft brewing". Wolnzach: Hopfen-Rundschau, 2014.
Biendl, Martin. "Hops and health". Wolnzach: German Hop Museum, 2013.
Hieronymus, Stan. "For the love of hops: The practical guide to aroma, bitterness and the culture of hops". Boulder, CO: Brewers Publications, a division of the Brewers Association, 2012.
Melnychuk, M. D.; Boyko, A. L.; Hryhoryuk, I. P.; Likhanov, A. F.; Klyuvadenko, A. A.; Martyn, H. H.; Kukovenko, V. I. "Hop (Humulus lupulus L.): From cell to plant". Kyiv: Printeko, 2013.
Nesvadba, Vladimír. "Development and tradition of Czech hop varietes". Žatec: Hop Research Institute, 2013.
Neve, Ray A. "Hops". London: Chapman and Hall, 1991.
Skomra, Urszula. "Polish hop cultivars". Puławy: Institute of Soil Science and Plant Cultivation, 2010.