Onion

The onion (Allium cepa , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus Allium. The shallot is a botanical variety of the onion which was classified as a separate species until 2011. The onion's close relatives include garlic, scallion, leek, and chives.

The genus contains several other species

cultivated for food, such as the Japanese bunching onion Allium fistulosum, the tree onion Allium × proliferum, and the Canada onion Allium canadense. The name wild onion is applied to a number of Allium species, but A. 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 a biennial or a perennial plant, but is usually treated as an annual 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 the onion fly, the onion eelworm, and various fungi which can cause rotting. Some varieties of A. cepa, such as shallots and potato onions, produce multiple bulbs.

Onions are cultivated and used around the world. As a food item, they are often served raw as a vegetable or part of a prepared savoury dish, but can be eaten cooked or used to make pickles or chutneys. They are pungent 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 or common onion, is the most widely cultivated species of the genus Allium. It was first officially described by Carl Linnaeus in his 1753 work Species Plantarum. Synonyms during its taxonomic history are:

Allium cepa var. aggregatum – G. Don
Allium cepa var. bulbiferum – Regel
Allium cepa var. cepa – Linnaeus
Allium cepa var. multiplicans – L.H. Bailey
Allium cepa var. proliferum – (Moench) Regel
Allium cepa var. solaninum – Alef
Allium cepa var. viviparum – (Metz) Mansf. but related wild species occur in Central Asia and Iran. The most closely related include A. vavilovii from Turkmenistan and A. asarense from Iran. The genus Allium 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). The vast majority of cultivars of A. 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, formerly classed as a separate species, and potato onions. Related species include garlic, leek, and chives.

Cepa is commonly accepted as Latin for "onion"; the generic name Allium is the classical Latin name for garlic. It has an affinity with Spanish: cebolla, Italian: cipolla, Polish: cebula, and the German Zwiebel (this last altered by folk etymology). The English word "chive" is from the Old French chive, in turn from cepa.

Description

The onion is a biennial plant but is usually grown as an annual. Modern varieties typically grow to a height of . 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 of fibrous 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.

File:ARS red onion.jpg|Roots, leaves and developing bulb

File:Onion Flower Bangladesh.jpg|Inflorescence with flower buds

File:Onion seed.jpg|Seeds

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History

thumb|upright|left|Woodcut print of an onion, from [[Hortus Sanitatis, 1547]]

Onions have been variously described as having originated in Iran, western Pakistan and Central Asia. Humans have grown and selectively bred onions in cultivation for at least 7,000 years. The geographic origin of the onion is uncertain; ancient records of onion use span both eastern and western Asia. Domestication likely took place in West or Central Asia. The onion species Allium fistulosum (spring onion, bunching onion) and Allium tuberosum (Chinese leek) were domesticated in China around 6000 BC alongside other vegetables, grains, and fruits.

Ancient Mesopotamian recipes that used onions and other Allium species were recorded in cuneiform script on the Yale culinary tablets. The tablets, dating from around 2000 BC, are the oldest known recipes in the world. The onion is mentioned in the Hebrew Bible; evidence of onions in ancient Israel comes from Chalcolithic Nahal Mishmar and from Bronze Age Jericho.

Ancient Egyptians revered the onion bulb, viewing its spherical shape and concentric rings as symbols of eternal life. Pliny the Elder of the first century AD wrote about the use of onions and cabbage in Pompeii. 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 even dysentery. Archaeologists unearthing Pompeii long after its 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 a gourmet), onions were used in many Roman recipes.

Uses

Culinary

Onions come in three colour varieties:

Yellow or brown onions are sweet, with many cultivars bred specifically to accentuate this sweetness, such as Vidalia, Walla Walla, Cévennes, and Bermuda. 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.
Red or purple onions, known for their sharp pungent flavour, are commonly cooked in many cuisines, and used raw and in grilling. Onions are a major ingredient of some curries; the Persian-style dopiaza's name means "double onion", and it is used both in the dish's sour curry sauce and as a garnish. Onion powder is a seasoning made from finely ground, dehydrated onions; it is often included in seasoned salt and spice mixes.

While the large, mature onion bulb is what is most often eaten, onions can be eaten at immature stages. Young plants may be harvested before bulbing occurs and used whole as spring onions or scallions. 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 true pearl onions which are a different species. Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient, while pickler onions are often preserved in vinegar as a long-lasting relish. Onions pickled in vinegar are eaten as a side serving with traditional pub food such as a ploughman's lunch.

Other uses

thumb|Onion [[epidermis (botany)|epidermis cells are visible in true color with minimal magnification.]]

Onions have particularly large cells that are easy to observe under low magnification. Forming a single layer of cells, the bulb epidermis is easy to separate for educational, experimental, and breeding purposes. Onions are therefore commonly used in science education to teach the use of a microscope for observing cell structure. Onion skins can be boiled to make an orange-brown dye.

Composition

Nutrients

Most onion cultivars are about 89% water, 9% carbohydrates (including 4% sugar and 2% dietary fibre), 1% protein, and negligible fat (table). Onions contain low amounts of essential nutrients and have an energy value of 166 kJ (40 kilocalories) in a 100 g (3.5 oz) amount. Onions contribute savoury flavour to dishes without contributing significant caloric content. onions can show an additional colour – pink-red – after cutting, an effect caused by reactions of amino acids with sulfur compounds. Onion polyphenols are under basic research to determine their possible biological properties in humans.

Adverse effects and toxicity

Some people suffer from allergic reactions after handling onions. Symptoms can include contact dermatitis, intense itching, rhinoconjunctivitis, blurred vision, bronchial asthma, sweating, and anaphylaxis. Allergic reactions may not occur when eating cooked onions, possibly due to the denaturing of the proteins from cooking.

Eye irritation

Freshly cut onions can produce a stinging sensation in the eyes of people nearby and often uncontrollable tears. This is caused by the release of a volatile liquid, syn-propanethial-S-oxide and its aerosol, which stimulates nerves in the eye. This gas is produced by a chain of reactions which serve as a defence mechanism: chopping an onion causes damage to cells which releases enzymes called alliinases. These break down amino acid sulfoxides and generate sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the lacrimatory factor synthase (LFS), producing the syn-propanethial-S-oxide. 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.

File:Cortando cebolla.jpg|Cut onions emit a chemical compound which cause the lacrimal glands in the eyes to become irritated, releasing tears.

File:Syn-propanethial-S-oxide-3D-balls.png|Model of syn-propanethial-S-oxide, the volatile molecule that causes eye irritation

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The amount of sulfenic acids and lacrimal factor released and the irritation effect differs among Allium species. In 2008, the New Zealand Institute for Crop and Food Research created "no tears" onions by genetic modification to prevent the synthesis of lachrymatory factor synthase in onions. One study suggests that consumers prefer the flavour of onions with lower LFS content. Since the process impedes sulfur ingestion by the plant, some find LFS− onions inferior in flavour.

A method for efficiently differentiating LFS− and LFS+ onions has been developed based on mass spectrometry, with potential application in high-volume production; gas chromatography is also used to measure lachrymatory factor in onions. In early 2018, Bayer released the first crop yield of commercially available LFS-silenced onions under the name "Sunions". They were the product of 30 years of cross-breeding; genetic modification was not employed.

Guinea hen weed and honey garlic contain a similar lachrymatory factor. Synthetic onion lachrymatory factor has been used in a study related to tear production, and has been proposed as a nonlethal deterrent against thieves and intruders.

Other effects

Onions are toxic to animals including dogs, cats, and guinea pigs.

Onions can accumulate heavy metals such as cadmium and lead, particularly when cultivated in contaminated soils. The European Union regulates the presence of these contaminants. Several studies monitoring onion crops have reported cadmium or lead concentrations that exceed regulatory limits.

Producing onions

Cultivation

thumb|Onion bulbs to be planted for seed production

thumb|Large-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 of nutrients 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 and potash can be applied at regular intervals during the growing season, the last application of nitrogen being at least four weeks before harvesting.

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. Onions are a cool-weather crop and can be grown in USDA zones 3 to 9.

Onions are grown from seeds or from partially grown bulbs called "sets" or starter 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. 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. Certain cultivars used for growing and storing bulbs may not have as good storage characteristics as those grown directly from seed.

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. The larvae of the onion leaf miner or leek moth (Acrolepiopsis assectella) sometimes attack the foliage and may burrow down into the bulb.

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.

The onion eelworm (Ditylenchus dipsaci), a tiny parasitic soil-living nematode, 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 growing carrots, parsnips, and beans, which are also susceptible to the eelworm.

White rot of onions, leeks, and garlic is caused by the soil-borne fungus Sclerotium 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 of mycelia, which later produces small, globular black structures called sclerotia. 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.

Neck rot is a fungal disease affecting onions in storage. It is caused by Botrytis 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 of spores 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.

File:Uienvlieg maden.jpg|Larvae of the onion fly

File:Urocystis colchici var. cepulae on an onion seedling.jpg|Onion smut, Urocystis colchici var. cepulae, on a seedling

File:Zwiebel Blattfleckenkrankheit (Cladosporium allii-cepae) Anamorph - Noé López G.LLG-Bernburg.jpg|Onion leaf spot caused by Cladosporium allii-cepae

File:Onion (Allium cepa) Bacterial soft rot (42052268015).jpg|Bacterial soft rot caused by Erwinia carotovora subsp. carotovora

File:Zwiebel Schmutzfleckenkrankheit (Colletotrichum dematium f. sp. circinans) - G-Bedlan, Wien.jpg|Anthracnose caused by Colletotrichum dematium subsp. circinans

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Onion oil is authorised for use in the European Union for use as a pesticide against carrot fly in umbelliferous crops (carrots, parsnips, parsley, celery, celeriac).

Production

{|class="wikitable floatright col2right"

! colspan=2|Production of onions<br> and shallots (green) in 2022

|||864,488

|||610,576

|||558,480

|||510,462

|World ||4,970,615

|colspan=2|<small>Source: UN FAO</small>

thumb|Onion grading at the [[Centre for Agroecology, Water and Resilience, England, using a manually operated sorter]]

Onions are a widely cultivated vegetable crop, produced in the second largest quantity after tomatoes. In 2021, the top global producers of onions were China, India, the United States, and Turkey. In 2022, world production of onions and shallots (as green produce) was 5.0 million tonnes, led by China with 17% of the total, and Mali, Angola, and Japan as secondary producers.

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.

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.

I'itoi onion is a prolific multiplier onion cultivated in the Baboquivari 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 below the surface and 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.

Hybrids with A. cepa parentage

thumb|The tree or Egyptian onion is a hybrid of A. cepa and A. fistulosum.

Some hybrids are cultivated that have A. cepa parentage, such as the diploid tree onion or Egyptian onion (A. ×proliferum), and the triploid onion (A. ×cornutum).

The tree onion or Egyptian onion produces bulblets in the umbel instead of flowers, and is now known to be a hybrid of A. cepa and A. fistulosum. It has previously been treated as a variety of A. cepa, for example A. cepa var. proliferum, A. cepa var. bulbiferum, and A. cepa var. viviparum. It has been grown for centuries in Japan and China for use as a salad onion.

The triploid onion is a hybrid species with three sets of chromosomes, two sets from A. cepa and the third set from an unknown parent. 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 a monophyletic origin for this species.

Spring onions or salad onions may be grown from the Welsh onion (A. fistulosum), as well as from A. cepa. Young plants of A. fistulosum and A. cepa look very similar, but may be distinguished by their leaves, which are circular in cross-section in A. fistulosum rather than flattened on one side.

In popular culture

thumb|An [[Onion Johnny, his bicycle laden with onions and garlic, in London, 2008]]

thumb|upright|An onion in the beak of a [[Common crane|crane on the coat of arms of Tervola, Finland ]]

Onion Johnnies were Breton farmers and agricultural labourers who travelled from Roscoff in Brittany, originally on foot and later on bicycles, selling strings of distinctive pink onions door to door in Britain.

The name "the Big Onion" was formerly used of New York City, before it became "the Big Apple", and Chicago became "the Big Onion". The Chicago public radio station WBEZ named a series about the city's geography The Big Onion; it was explained that the nickname derived from the native American name for the city, which means a type of wild onion.

The 10th century Exeter Book, written in Old English, contains a riddle which seems to be about an onion, with sexual overtones. The "wondrous creature, a joy to women" stands "in a bed"; "My column<!--emending staþole to stapole, as suggested by Cavell

In the Odyssey, 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". R. Drew Griffith comments that the double comparison of the tunic that Penelope gave to the disguised Odysseus to onion and sun "risks being funny", and notes that Theopompus indeed found it "ridiculous".

In India, when the price of onions became very high in 2015, the Hindustan Times recorded that people shared many onion jokes, such as the punning (, "take love, give me onions").

Taxonomy and etymology

Description

History

Uses

Culinary

Other uses

Composition

Nutrients

Adverse effects and toxicity

Eye irritation

Other effects

Producing onions

Cultivation

Production

Aggregatum group (var. aggregatum)

Hybrids with A. cepa parentage

In popular culture

See also

Notes

References

Sources

Further reading

External links