The onion (Allium cepa) (Latin 'cepa' = onion), also known as the bulb onion or common onion, is used as a vegetable and is the most widely cultivated species of the genus Allium. This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), the Egyptian onion (A. ×proliferum), and the Canada onion (A. canadense). The name "wild onion" is applied to a number of Allium species but A. cepa is exclusively known from cultivation and 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 the bulb at the base of the plant begins to swell when a certain day-length is reached. In the autumn the foliage dies down and the outer layers of the bulb become 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 that 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 foodstuff they are usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make pickles or chutneys. They are pungent when chopped and contain certain chemical substances which irritate the eyes. Onions contain phenolics and flavonoids that have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties.
- 1 Taxonomy and etymology
- 2 Description
- 3 Uses
- 4 Nutrients and phytochemicals
- 5 Eye irritation
- 6 Cultivation
- 7 Varieties
- 8 Production and trade
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
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 Carolus Linnaeus in his 1753 work Species Plantarum. A number of synonyms have appeared in its taxonomic history:
- 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.
Allium cepa is known exclusively from cultivation, but related wild species occur in Central Asia. The most closely related species include Allium vavilovii (Popov & Vved.) and Allium asarense (R.M. Fritsch & Matin) from Iran. However, Zohary and Hopf state that "there are doubts whether the A. vavilovii collections tested represent genuine wild material or only feral derivatives of the crop."
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 and potato onions.
The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), Egyptian onion (A. ×proliferum), and Canada onion (A. canadense).
The onion plant (Allium cepa) is unknown in the wild but has been grown and selectively bred in cultivation for at least 7,000 years. It is a biennial 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 blueish-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 towards a blunt tip. The base of each leaf is a flattened, usually white sheath that grows out of a basal disc. From the underside of the disc, a bundle of fibrous roots extends for a short way into the soil. As the onion matures, food reserves begin to accumulate in the leaf bases and the bulb of the onion swells.
In the autumn the leaves die back and the outer scales of the bulb become dry and brittle, and this is the time at which 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 a bract protecting a developing inflorescence. The inflorescence takes the form of a globular umbel of white flowers with parts in sixes. The seeds are glossy black and triangular in cross section.
Bulbs from the onion family are thought to have been used as a food source for millennia. In Bronze Age settlements, traces of onion remains were found alongside date stones and fig remains that date back to 5000 BC. However, it is not clear if these were cultivated onions. Archaeological and literary evidence such as the Book of Numbers 11:5 suggests that onions were probably being cultivated around two thousand years later in ancient Egypt, at the same time that leeks and garlic were cultivated. Workers who built the Egyptian pyramids may have been fed radishes and onions.
The onion is easily propagated, transported and stored. The ancient Egyptians worshipped it, believing its spherical shape and concentric rings symbolized eternal life. Onions were even used in Egyptian burials, as evidenced by onion traces being found in the eye sockets of Ramesses IV.
In ancient Greece, athletes ate large quantities of onion because it was believed to lighten the balance of the blood. Roman gladiators were rubbed down with onions to firm up their muscles. In the Middle Ages, onions were such an important food that people would pay their rent with onions, and even give them as gifts. Doctors were known to prescribe onions to facilitate bowel movements and erections, and to relieve headaches, coughs, snakebite and hair loss.
Onions were taken by the first settlers to North America, where the Native Americans were already using wild onions in a number of ways, eating them raw or cooked in a variety of foods. They also used them to make into syrups, to form poultices and in the preparation of dyes. According to diaries kept by the colonists, bulb onions were one of the first things planted by the Pilgrim Fathers when they cleared the land for cropping in 1648.
Onions were also prescribed by doctors in the early 16th century to help with infertility in women. They were similarly used to raise fertility levels in dogs, cats and cattle, but this was an error as recent research has shown that onions are toxic to dogs, cats, guinea pigs and many other animals.
Onions are commonly chopped and used as an ingredient in various hearty warm dishes, and may also be used as a main ingredient in their own right, for example in French onion soup or onion chutney. They are very versatile and can be baked, boiled, braised, grilled, fried, roasted, sautéed or eaten raw in salads. Their layered nature makes them easy to hollow out once cooked, facilitating stuffing them. Onions are a staple in Indian cuisine, used as a thickening agent for curries and gravies. Onions pickled in vinegar are eaten as a snack. These are often a side serving in pubs and fish and chip shops throughout the United Kingdom and The Commonwealth, usually served with cheese and/or ale in the United Kingdom. In North America, sliced onions are battered and deep fried and served as onion rings.
Onion types and products
Common onions are normally available in three colour varieties. Yellow or brown onions (called red in some European countries), are full-flavoured and are the onions of choice for everyday use. Yellow onions turn a rich, dark brown when caramelized and give French onion soup its sweet flavour. The red onion (called purple in some European countries) is a good choice for fresh use when its colour livens up the dish. It is also used in grilling and char-broiling. White onions are the traditional onions that are used in classic Mexican cuisine. They have a golden colour when cooked and a particularly sweet flavour when sautéed.
While the large mature onion bulb is the onion 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.
Additionally, onions may be bred and grown to mature at smaller sizes. Depending on the mature size and the purpose for which the onion is used, these may be referred to as pearl, boiler, or pickler onions, but differ from true pearl onions which are a different species. Pearl and boiler onions may be cooked as a vegetable rather than as an ingredient and pickler onions are often preserved in vinegar as a long-lasting relish.
Onions are available in fresh, frozen, canned, caramelized, pickled and chopped forms. The dehydrated product is available as kibbled, sliced, rings, minced, chopped, granulated and powder forms. Onion powder is a spice widely used when the fresh ingredient is not available. It is made from finely ground, dehydrated onions, mainly the pungent varieties of bulb onions, and has a strong odour. Being dehydrated, it has a long shelf life and comes in several varieties: yellow, red and white.
Onions have particularly large cells that are readily observed 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 employed in science education to teach the use of a microscope for observing cell structure.
The pungent juice of onions has been used as a moth repellent and can be rubbed on the skin to prevent insect bites. When applied to the scalp it is said to promote growth of hair and on the face to reduce freckling. It has been used to polish glass and copperware and to prevent rust on iron. If boiling water is poured onto chopped onions and left to cool, the resulting liquor can be sprayed onto plants to increase their resistance to pests, and the onion plants when growing are reputed to keep away moles and insects. Onion skins have been used to produce a yellow-brown dye.
Nutrients and phytochemicals
|Nutritional value per 100 g (3.5 oz)|
|Energy||166 kJ (40 kcal)|
|Dietary fiber||1.7 g|
|Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
Most onion cultivars are about 89% water, 4% sugar, 1% protein, 2% fibre and 0.1% fat. They contain vitamin C, vitamin B6, folic acid and numerous other nutrients in small amounts. They are low in fats and in sodium, and with an energy value of 166kJ (40 kcal) per 100 g (3.5 oz) serving, they can contribute their flavour to savoury dishes without raising caloric content appreciably.
Onions contain phytochemical compounds such as phenolics and flavonoids that basic research shows to have potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant properties. These include quercetin and its glycosides quercetin 3,4'-diglucoside and quercetin-4'-glucoside.
There are considerable differences between onion varieties in polyphenol content, with shallots having the highest level, six times the amount found in Vidalia onions, the variety with the smallest amount. Yellow onions have the highest total flavonoid content, an amount 11 times higher than in white onions. Red onions have considerable content of anthocyanin pigments, with at least 25 different compounds identified representing 10% of total flavonoid content.
Some people suffer from allergic reactions after handling onions. Symptoms can include contact dermatitis, intense itching, rhinoconjunctivitis, blurred vision, bronchial asthma, sweating and anaphylaxis. There may be no allergic reaction in these individuals to the consumption of onions, perhaps because of the denaturing of the proteins involved during the cooking process.
While onions and other members of the genus Allium are commonly consumed by humans, they can be deadly for dogs, cats, guinea pigs, monkeys and other animals. The toxicity is caused by the sulfoxides present in raw and cooked onions which many animals are unable to digest. Ingestion results in anaemia caused by the distortion and rupture of red blood cells. Sick pets are sometimes fed with tinned baby foods and any that contain onion should be avoided. The typical toxic doses are 5 g (0.2 oz) per kg (2.2 lb) bodyweight for cats and 15 to 30 g (0.5 to 1.1 oz) per kg for dogs.
Chopping an onion causes damage to cells which allows enzymes called alliinases to 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 lachrymatory factor synthase (LFS), giving syn-propanethial-S-oxide, a volatile gas known as the onion lachrymatory factor or LF. This gas diffuses through the air and soon reaches the eye, where it activates sensory neurons, creating a stinging sensation. Tear glands produce tears in order to dilute and flush out the irritant.
Eye irritation can be avoided by cutting onions under running water or submerged in a basin of water. 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. Refrigerating the onions before use reduces the enzyme reaction rate and using a fan can blow the gas away from the eyes. The more often one chops onions, the less one experiences eye irritation.
The amount of sulfenic acids and LF released and the irritation effect differs among Allium species. In 2008, the New Zealand Crop and Food institute created a strain of "no tears" onions by using gene-silencing biotechnology to prevent synthesis by the onions of the LFS enzyme.
Onions are best cultivated in fertile soils that are well-drained. Sandy loams are good as they are low in sulphur, while clayey soils usually have a 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 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 what is referred to as "long-day" onions, producing bulbs only after 14+ hours 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. Finally, "short-day" onions, which have been developed in more recent times, are planted in mild-winter areas in the fall 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. Hot temperatures or other stressful conditions cause them to "bolt", meaning that a flower stem begins to grow.
Onions may be grown from seed or from sets. Onion seeds are short lived and fresh seed germinates better. The seeds are sown thinly in shallow drills, thinning the plants in stages. 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. Onion sets are produced by sowing seed thickly in early summer in poor soil and the small bulbs produced are harvested in the autumn. These bulbs are planted the following spring and grow into mature bulbs later in the year. Certain cultivars are used for this purpose and these may not have such 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 a great deal of water when established. Bulbing usually takes place after twelve to eighteen weeks. The bulbs can be gathered when needed to eat fresh but if required to be kept in store, should be harvested after the leaves have died back naturally. In dry weather they can be left on the surface of the soil for a few days to dry out properly, then they can be placed in nets, roped into strings or laid in layers in shallow boxes. They should be stored in a well-ventilated, cool place such as a shed.
Pests and diseases
Onions suffer from a number of plant disorders. The most serious for the home gardener are likely to be the onion 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 when 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. The larvae of the onion leaf miner or leek moth (Acrolepiopsis assectella) sometimes attacks 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. There is no cure and affected plants should be uprooted and burnt. 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 base of the bulbs is attacked and becomes 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. There is no cure for this fungal disease 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 downwards in 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 manifest itself when the bulb is in store. Anti-fungal 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.
Storage in the home
Cooking onions and sweet onions are better stored at room temperature, optimally in a single layer, in mesh bag 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. Also, they draw moisture from vegetables with which they are stored which may cause them to decay.
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 approximately 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.
Common onion group (var. cepa)
Most of the diversity within A. cepa occurs within this group, the most economically important Allium crop. Plants within this group form large single bulbs, and are grown from seed or seed-grown sets. The majority of cultivars grown for dry bulbs, salad onions, and pickling onions belong to this group. The range of diversity found among these cultivars includes variation in photoperiod (length of day that triggers bulbing), storage life, flavour, and skin colour. Common onions range from the pungent varieties used for dried soups and onion powder to the mild and hearty sweet onions, such as the Vidalia from Georgia, USA, or Walla Walla from Washington that can be sliced and eaten on a sandwich instead of meat.
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. 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. However, intermediate forms exist.
I'itoi onion (Allium cepa) 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 1 inch below surface and 12 inches apart. Bulbs will multiply into clumps and can be harvested throughout the cooler months. Tops will 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
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 × 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.
The Wakegi onion is also known to be a hybrid between A. cepa and A. fistulosum, with the A. cepa parent believed to be from the Aggregatum Group of cultivars. It has been grown for centuries in Japan and China for use as a salad onion.
Under the rules of botanical nomenclature, both the Egyptian onion and Wakegi onion should be combined into one hybrid species, having the same parent species. Where this is followed, the Egyptian onion is named A. ×proliferum Eurasian Group and the Wakegi onion is named A. ×proliferum East Asian Group.
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.
Some authors have used the name A. cepa var. viviparum (Metzg.) Alef. for the triploid onion, but this name has also been applied to the Egyptian onion. The only name unambiguously connected with the triploid onion is A. ×cornutum.
Scallions 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.
Production and trade
It is estimated that around the World, over 9,000,000 acres (3,642,000 ha) of onions are grown annually. About 170 countries cultivate onions for domestic use and about eight percent of the global production is traded internationally.
|Top Ten Onions (dry) Producers — 2012 (metric tons)
|Republic of Korea||1,411,650|
|Source: UN Food & Agriculture Organisation (FAO)|
The Onion Futures Act, passed in 1958, bans the trading of futures contracts on onions in the United States. This prohibition came into force after farmers complained about alleged market manipulation by Sam Seigel and Vincent Kosuga at the Chicago Mercantile Exchange two years earlier. The subsequent investigation provided economists with a unique case study into the effects of futures trading on agricultural prices. The act remains in effect as of 2013[update].
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