Theonion (Allium cepaL.Tooltip Carl Linnaeus, fromLatincepa), also known as thebulb onion orcommon onion, is avegetable that is the most widely cultivated species of the genusAllium. Theshallot is abotanical variety of the onion which was classified as a separate species until 2011. The onion's close relatives includegarlic,scallion,leek, andchives.
The genus contains several other species variously called onions and cultivated for food, such as the Japanese bunching onionAllium fistulosum, thetree onionAllium ×proliferum, and the Canada onionAllium canadense. The namewild onion is applied to a number ofAllium species, butA. cepa is exclusively known from cultivation. Its ancestral wild original form is not known, although escapes from cultivation have become established in some regions. The onion is most frequently abiennial or aperennial plant, but is usually treated as anannual and harvested in its first growing season.
The onion plant has a fan of hollow, bluish-green leaves, and its bulb at the base of the plant begins to swell when a certain day-length is reached. The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem. In the autumn (or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become more dry, and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly theonion fly, theonion eelworm, and various fungi which can cause rotting. Some varieties ofA. cepa, such asshallots andpotato onions, produce multiple bulbs.
Onions are cultivated and used around the world. As a food item, they are often served raw as avegetable or part of a prepared savoury dish, but can be eaten cooked or used to makepickles orchutneys. They arepungent when chopped and contain certain chemical substances which may irritate the eyes.
Taxonomy and etymology
The onion plant (Allium cepa), also known as the bulb onion[2] or common onion,[3] is the most widely cultivated species of the genusAllium.[4][5] It was first officiallydescribed byCarl Linnaeus in his 1753 workSpecies Plantarum.[6] Synonyms during its taxonomic history are:[7][8]
A. cepa is known exclusively from cultivation,[9] but related wild species occur in Central Asia and Iran. The most closely related includeA. vavilovii from Turkmenistan andA. asarense fromIran.[10][11] The genusAllium contains other species variously called onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), Egyptian onion (A. × proliferum), and Canada onion (A. canadense).[3] The vast majority ofcultivars ofA. cepa belong to the common onion group (A. cepa var. cepa) and are usually referred to simply as onions. The Aggregatum Group of cultivars (A. cepa var. aggregatum) includes both shallots,[12] formerly classed as a separate species,[13][14] and potato onions.[12] Related species includegarlic,leek, andchives.[15]
Cepa is commonly accepted as Latin for "onion"; thegeneric nameAllium is the classical Latin name for garlic.[16] It has an affinity with Spanish:cebolla, Italian:cipolla, Polish:cebula, and the GermanZwiebel (this last altered byfolk etymology). The English word "chive" is from the Old Frenchchive , in turn fromcepa.[17]
Description
The onion is abiennial plant but is usually grown as an annual. Modern varieties typically grow to a height of 15 to 45 cm (6 to 18 in). The leaves are yellowish- to bluish green and grow alternately in a flattened, fan-shaped swathe. They are fleshy, hollow, and cylindrical, with one flattened side. They are at their broadest about a quarter of the way up, beyond which they taper to blunt tips. The base of each leaf is a flattened, usually white sheath that grows out of the basal plate of a bulb. From the underside of the plate, a bundle offibrous roots extends for a short way into the soil. As the onion matures, food reserves accumulate in the leaf bases, and the bulb of the onion swells.[18]
In the autumn, the leaves die back, and the outer scales of the bulb become dry and brittle, so the crop is normally harvested. If left in the soil over winter, the growing point in the middle of the bulb begins to develop in the spring. New leaves appear, and a long, stout, hollow stem expands, topped by abract protecting a developing inflorescence. Theinflorescence takes the form of a roundedumbel of white flowers with parts in sixes. The seeds are glossy black and triangular in cross-section.[18] The average pH of an onion is around 5.5.[19]
Humans have grown and selectively bred onions in cultivation for at least 7,000 years.[20] The geographic origin of the onion is uncertain; ancient records of onion use span botheastern andwestern Asia.[21][22] Domestication likely took place inWest orCentral Asia.[12][23] Onions have been variously described as having originated inIran, westernPakistan andCentral Asia.[21][23]: 1[22][24] The onion speciesAllium fistulosum (spring onion, bunching onion) andAllium tuberosum (Chinese leek) were domesticated in China around 6000 BC alongside other vegetables, grains, and fruits.[25]
Ancient Egyptians revered the onion bulb, viewing its spherical shape and concentric rings as symbols of eternal life.[23] Onions were used in Egyptian burials, as evidenced by onion traces found in the eye sockets ofRamesses IV.[28]Pliny the Elder of the first century AD wrote about the use of onions and cabbage inPompeii. He documented Roman beliefs about the onion's ability to improve ocular ailments, aid in sleep, and heal everything from oral sores and toothaches to dog bites,lumbago, and evendysentery. Archaeologists unearthing Pompeii long afterits 79 AD volcanic burial have found gardens resembling those in Pliny's detailed narratives. According to texts collected in the fifth/sixth century AD under the authorial aegis of "Apicius" (said to have been agourmet), onions were used in many Roman recipes.[23]
In theAge of Discovery, onions were taken toNorth America by the first European settlers in part of theColumbian exchange. They found close relatives of the plant such asAllium tricoccum readily available and widely used inNative American gastronomy.[21] According to diaries kept by some of the first English colonists, the bulb onion was one of the first crops planted in North America by thePilgrim fathers.[23] Between 1883 and 1939, inventors in the United States patented 97 inventions meant to make onion-growing more efficient through automation.[29]
Three colour varieties of onions offer different possibilities for the cook:
Yellow or brown onions are sweet, with many cultivars bred specifically to accentuate this sweetness, such asVidalia, Walla Walla, Cévennes, and Bermuda.[30] Yellow onions turn a rich, dark brown when caramelised and are used to add a sweet flavour to various dishes, such as French onion soup.[31]
Red or purple onions, known for their sharp pungent flavour, are commonly cooked in many cuisines, and used raw and ingrilling.[31]
White onions are mild in flavour; they have a golden colour when cooked and a particularly sweet flavour when sautéed.[31][23]
While the large, mature onion bulb is most often eaten, onions can be eaten at immature stages. Young plants may be harvested before bulbing occurs and used whole asspring onions orscallions. When an onion is harvested after bulbing has begun, but the onion is not yet mature, the plants are sometimes referred to as "summer" onions. Onions may be bred and grown to mature at smaller sizes, known as pearl, boiler, or pickler onions; these are not truepearl onions which are a different species.[32] Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient, while pickler onions are often preserved invinegar as a long-lasting relish.[33]Onions pickled in vinegar are eaten as a side serving with traditionalpub food such as aploughman's lunch.[34]
Onions are commonly chopped and used as an ingredient in various hearty warm dishes, and may be used as a main ingredient in their own right, for example inFrench onion soup, creamed onions, andonion chutney. They are versatile and can be baked, boiled, braised, grilled, fried, roasted, sautéed, or eaten raw in salads.[35] Onions are a major ingredient of somecurries; the Persian-styledopiaza's name means "double onion", and it is used both in the dish's sour curry sauce and as a garnish.[36]Onion powder is a seasoning made from finely ground, dehydrated onions; it is often included in seasoned salt and spice mixes.[37]
Onions have particularly largecells that are easy to observe under low magnification. Forming a single layer of cells, the bulb epidermis is easy to separate for educational, experimental, andbreeding purposes.[38][39] Onions are therefore commonly used inscience education to teach the use of amicroscope for observing cell structure.[40] Onion skins can be boiled to make an orange-brown dye.[41]
Most onion cultivars are about 89% water, 9%carbohydrates (including 4% sugar and 2%dietary fibre), 1%protein, and negligiblefat (table). Onions contain low amounts ofessential nutrients and have an energy value of 166 kJ (40kilocalories) in a 100 g (3.5 oz) amount. Onions contribute savoury flavour to dishes without contributing significant caloric content.[23]
Phytochemicals
Onion varieties vary widely inphytochemical content, particularly forpolyphenols, with shallots having the highest level, six times the amount found inVidalia onions. Yellow onions have the highest totalflavonoid content, an amount 11 times higher than in white onions. Red onions have considerable content ofanthocyaninpigments, with at least 25 different compounds identified representing 10% of total flavonoid content.[44] Likegarlic,[45] onions can show an additional colour – pink-red – after cutting, an effect caused by reactions ofamino acids with sulfur compounds.[46] Onion polyphenols are underbasic research to determine their possible biological properties in humans.[44][47]
Adverse effects and toxicity
Some people suffer fromallergic reactions after handling onions.[48] Symptoms can include contactdermatitis, intense itching,rhinoconjunctivitis, blurred vision, bronchialasthma, sweating, andanaphylaxis. Allergic reactions may not occur when eating cooked onions, possibly due to thedenaturing of the proteins from cooking.[49]
Eye irritation
Freshly cut onions can produce a stinging sensation in the eyes of people nearby and often uncontrollabletears. This is caused by the release of avolatile liquid,syn-propanethial-S-oxide and itsaerosol, which stimulates nerves in the eye. This gas is produced by a chain of reactions which serve as adefence mechanism: chopping an onion causes damage tocells which releasesenzymes calledalliinases. These break downamino acidsulfoxides and generatesulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, thelacrimatory factor synthase (LFS), producing thesyn-propanethial-S-oxide.[4] This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons.Lacrimal glands producetears to dilute and flush out the irritant. Eye irritation can be minimised by cutting onions under running water or submerged in a basin of water.[50] Leaving the root end intact also reduces irritation as the onion base has a higher concentration of sulphur compounds than the rest of the bulb.[51]
Cut onions emit a chemical compound which cause thelacrimal glands in the eyes to become irritated, releasing tears.
The amount of sulfenic acids and lacrimal factor released and the irritation effect differs amongAllium species. In 2008, theNew Zealand Institute for Crop and Food Research created "no tears" onions bygenetic modification to prevent the synthesis of lachrymatory factor synthase in onions.[52] One study suggests that consumers prefer the flavour of onions with lower LFS content.[53] Since the process impedes sulfur ingestion by the plant, some find LFS− onions inferior in flavour.[54]
A method for efficiently differentiating LFS− and LFS+ onions has been developed based onmass spectrometry, with potential application in high-volume production;[55]gas chromatography is also used to measure lachrymatory factor in onions.[56][57] In early 2018,Bayer released the first crop yield of commercially available LFS-silenced onions under the name "Sunions".[58] They were the product of 30 years of cross-breeding; genetic modification was not employed.[58][59]
Guinea hen weed andhoney garlic contain a similar lachrymatory factor.[60] Synthetic onion lachrymatory factor has been used in a study related to tear production,[61] and has been proposed as a nonlethal deterrent against thieves and intruders.[62][63]
Onion bulbs to be planted for seed productionLarge-scale onion cultivation
Onions are best cultivated in fertile, well-drained soils. Sandy loams are good as they are low in sulphur, whereas clay-rich soils usually have high sulphur content and produce pungent bulbs. Onions require a high level ofnutrients in the soil.Phosphorus is often present in sufficient quantities, but may be applied before planting because of its low level of availability in cold soils.Nitrogen andpotash can be applied at regular intervals during the growing season, the last application of nitrogen being at least four weeks before harvesting.[66]
Bulbing onions are day-length sensitive; their bulbs begin growing only after the number of daylight hours has surpassed some minimal quantity. Most traditional European onions are referred to as "long-day" onions, producing bulbs only after 14 hours or more of daylight occurs. Southern European and North African varieties are often known as "intermediate-day" types, requiring only 12–13 hours of daylight to stimulate bulb formation. "Short-day" onions, which have been developed in more recent times, are planted in mild-winter areas in the autumn and form bulbs in the early spring and require only 11–12 hours of daylight to stimulate bulb formation.[67] Onions are a cool-weather crop and can be grown inUSDA zones 3 to 9.[68] Hot temperatures or other stressful conditions cause them to "bolt", meaning that a flower stem begins to grow.[69]
Onions are grown from seeds or from partially grownbulbs called "sets" orstarter bulbs. Onion seeds are short-lived and fresh seeds germinate more effectively when sown in shallow rows, or "drills," with each drill 12" to 18" apart.[68][70] In suitable climates, certain cultivars can be sown in late summer and autumn to overwinter in the ground and produce early crops the following year.[18]
Onion bulbs are produced by sowing seeds in a dense pattern in early summer, then harvested in the autumn when the bulbs are still small, followed by drying and storage. These bulbs are planted the following spring and grow into mature bulbs later in the growing season.[71] Certain cultivars used for growing and storing bulbs may not have as good storage characteristics as those grown directly from seed.[18]
Routine care during the growing season involves keeping the rows free of competing weeds, especially when the plants are young. The plants are shallow-rooted and do not need much water when established. Bulbing usually takes place after 12 to 18 weeks. The bulbs can be gathered when needed to eat fresh, but if stored, they are harvested after the leaves have died back naturally. In dry weather, they may be left on the surface of the soil for a few days for drying, then are placed in nets, roped into strings, or laid in layers in shallow boxes to be stored in a cool, well-ventilated place.[18]
Pests and diseases
Onions suffer from several pests and diseases. The most serious for the home gardener are likely to be theonion fly, stem and bulb eelworm, white rot, and neck rot. Diseases affecting the foliage include rust and smut, downy mildew, and white tip disease. The bulbs may be affected by splitting, white rot, and neck rot. Shanking is a condition in which the central leaves turn yellow and the inner part of the bulb collapses into an unpleasant-smelling slime. Most of these disorders are best treated by removing and burning affected plants.[72] The larvae of the onion leaf miner or leek moth (Acrolepiopsis assectella) sometimes attack the foliage and may burrow down into the bulb.[73]
The onion fly (Delia antiqua) lays eggs on the leaves and stems and on the ground close to onion, shallot, leek, and garlic plants. The fly is attracted to the crop by the smell of damaged tissue and is liable to occur after thinning. Plants grown from sets are less prone to attack. The larvae tunnel into the bulbs and the foliage wilts and turns yellow. The bulbs are disfigured and rot, especially in wet weather. Control measures may include crop rotation, the use of seed dressings, early sowing or planting, and the removal of infested plants.[74]
The onion eelworm (Ditylenchus dipsaci), a tiny parasitic soil-livingnematode, causes swollen, distorted foliage. Young plants are killed and older ones produce soft bulbs. No cure is known and affected plants should be uprooted and burned. The site should not be used for growing onions again for several years and should also be avoided for growingcarrots,parsnips, andbeans, which are also susceptible to the eelworm.[75]
White rot of onions, leeks, and garlic is caused by the soil-borne fungusSclerotium cepivorum. As the roots rot, the foliage turns yellow and wilts. The bases of the bulbs are attacked and become covered by a fluffy white mass ofmycelia, which later produces small, globular black structures calledsclerotia. These resting structures remain in the soil to reinfect a future crop. No cure for this fungal disease exists, so affected plants should be removed and destroyed and the ground used for unrelated crops in subsequent years.[76]
Neck rot is a fungal disease affecting onions in storage. It is caused byBotrytis allii, which attacks the neck and upper parts of the bulb, causing a grey mould to develop. The symptoms often first occur where the bulb has been damaged and spread down the affected scales. Large quantities ofspores are produced and crust-like sclerotia may also develop. In time, a dry rot sets in and the bulb becomes a dry, mummified structure. This disease may be present throughout the growing period, but only manifests itself when the bulb is in storage. Antifungal seed dressings are available and the disease can be minimised by preventing physical damage to the bulbs at harvesting, careful drying and curing of the mature onions, and correct storage in a cool, dry place with plenty of circulating air.[77]
Onion oil is authorised for use in the European Union for use as a pesticide againstcarrot fly inumbelliferous crops (carrots, parsnips, parsley, celery, celeriac).[78]
Onions are a widely cultivated vegetable crop, produced in the second largest quantity aftertomatoes.[80] In 2021, the top global producers of onions were China, India, the United States, and Turkey.[81] In 2022, world production of onions and shallots (as green produce) was 5.0 milliontonnes, led byChina with 17% of the total, andMali,Angola, andJapan as secondary producers.[79]
Storage
In the home, cooking onions and sweet onions are best stored atroom temperature, optimally in a single layer, in large mesh bags in a dry, cool, dark, well-ventilated location. In this environment, cooking onions have a shelf life of three to four weeks and sweet onions one to two weeks. Cooking onions will absorb odours from apples and pears. Additionally, they draw moisture fromvegetables with which they are stored which may cause them todecay.[68][82]
Sweet onions have a greater water and sugar content than cooking onions. This makes them sweeter and milder tasting, but reduces their shelf life. Sweet onions can be stored refrigerated; they have a shelf life of around one month. Irrespective of type, any cut pieces of onion are best tightly wrapped, stored away from other produce, and used within two to three days.[51]
Most of the diversity withinA. cepa occurs within this group, the most economically importantAllium crop. Plants within this group form large single bulbs, and are grown from seed or seed-grown sets. The majority of cultivated varieties grown for dry bulbs,salad onions, and pickling onions belong to this group.[12] The range of diversity found among these cultivars includes variation in photoperiod (length of day that triggers bulbing), storage life, flavour, and skin colour.[83]
Aggregatum group (var.aggregatum)
This group contains shallots and potato onions, also referred to as multiplier onions. The bulbs are smaller than those of common onions, and a single plant forms an aggregate cluster of several bulbs from a master. They are propagated almost exclusively from daughter bulbs, although reproduction from seed is possible. Shallots are the most important subgroup within this group and comprise the only cultivars cultivated commercially. They form aggregate clusters of small, narrowly ovoid to pear-shaped bulbs. Potato onions differ from shallots in forming larger bulbs with fewer bulbs per cluster, and having a flattened (onion-like) shape. Intermediate forms exist.[12]
I'itoi onion is a prolific multiplier onion cultivated in theBaboquivari Peak Wilderness, Arizona, area. This small-bulb type has a shallot-like flavour and is easy to grow and ideal for hot, dry climates. Bulbs are separated, and planted in the fall 25 mm (1 in) below the surface and 300 mm (12 in) apart. Bulbs will multiply into clumps and can be harvested throughout the cooler months. Tops die back in the heat of summer and may return with heavy rains; bulbs can remain in the ground or be harvested and stored in a cool dry place for planting in the fall. The plants rarely flower; propagation is by division.[84]
Hybrids withA. cepa parentage
The tree or Egyptian onion is a hybrid ofA. cepa andA. fistulosum.
Some hybrids are cultivated that haveA. cepa parentage, such as thediploidtree onion or Egyptian onion (A. ×proliferum), and thetriploid onion (A. ×cornutum).[85]
The tree onion or Egyptian onion produces bulblets in the umbel instead of flowers, and is now known to be ahybrid ofA. cepa andA. fistulosum. It has previously been treated as a variety ofA. cepa, for exampleA. cepa var.proliferum,A. cepa var.bulbiferum, andA. cepa var.viviparum.[86][87] It has been grown for centuries in Japan and China for use as a salad onion.[88][3]
The triploid onion is a hybrid species with three sets of chromosomes, two sets fromA. cepa and the third set from an unknown parent.[87] Various clones of the triploid onion are grown locally in different regions, such as 'Ljutika' in Croatia, and 'Pran', 'Poonch', and 'Srinagar' in the India-Kashmir region. 'Pran' is grown extensively in the northern Indian provinces of Jammu and Kashmir. There are very small genetic differences between 'Pran' and the Croatian clone 'Ljutika', implying amonophyletic origin for this species.[89]
Spring onions or salad onions may be grown from theWelsh onion (A. fistulosum), as well as fromA. cepa. Young plants ofA. fistulosum andA. cepa look very similar, but may be distinguished by their leaves, which are circular in cross-section inA. fistulosum rather than flattened on one side.[90]
In popular culture
AnOnion Johnny, his bicycle laden with onions and garlic, in London, 2008The onion featuring in the beak of acrane on the coat of arms ofTervola, Finland
The name 'the Big Onion' was formerly used ofNew York City, before it became 'the Big Apple', andChicago became 'the Big Onion'.[26][91] The Chicago public radio stationWBEZ named a series about the city's geographyThe Big Onion; it was explained that the nickname derived from thenative American name for the city, which means a type of wild onion.[92][a]
The 10th centuryExeter Book, written inOld English, containsa riddle which seems to be about an onion, with sexual overtones. The "wondrous creature, a joy to women" stands "in a bed"; "My column is erect and tall"; a woman "rubs me to redness" but at once "she feels my meeting"; the riddle ends "Wet will be that eye."[94]
In theOdyssey,Homer included the lines "I saw the shining tunic about his skin, like the skin of a dried onion, so soft was it, and it shone in the sun".[95] R. Drew Griffith comments that the double comparison of the tunic thatPenelope gave to the disguisedOdysseus to onion and sun "risks being funny", and notes that Theopompus indeed found it "ridiculous".[96] Griffith suggests that Homer included the onion because of its capacity to produce tears, hinting at Penelope's sorrow at Odysseus's long absence.[96]
Onion Johnnies wereBreton farmers and agricultural labourers who travelled fromRoscoff in Brittany, originally on foot and later on bicycles, selling strings of distinctive pinkonions door to door in Britain.[97]
In India, when the price of onions became very high in 2015, theHindustan Times recorded that people shared many onion jokes, such as thepunningप्या (र)ज़ लो, प्याज़ दो (pya (r)z lo, pyaz do, "take love, give me onions").[98]
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