Bulb Onion

An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), 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 2010. Its close relatives include garlic, scallion, leek, and chive.

This genus also contains several other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (''Allium fistulosum''), the tree onion (''A.'' × ''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 usually served cooked, as a vegetable or part of a prepared savoury dish, but can also be eaten raw or used to make Pickled onion, 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.''Eric Block, "Garlic and Other ''Alliums'': The Lore and the Science" (Cambridge: Royal Society of Chemistry, 2010) It was first officially described by Carl 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.

''A. cepa'' is known exclusively from cultivation, but related wild species occur in Central Asia and Iran. The most closely related species include '' A. vavilovii'' from Turkmenistan and '' A. asarense'' from Iran.Grubben, G.J.H.; Denton, O.A. (2004) Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen; Backhuys, Leiden; CTA, Wageningen.

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'').

''Cepa'' is commonly accepted as Latin for "onion" and has an affinity with Ancient Greek: κάπια (''kápia'') and Albanian: ''qepë'' and is ancestral to Aromanian: ''tseapã'', Catalan: ''ceba'', Occitan: ''ceba'', Portuguese: ''cebola'', Spanish: ''cebolla'', Italian: ''cipolla'', and Romanian: ''ceapă''. The English word "chive" is also derived from the Old French ''cive'', which derived from ''cepa''.

Description

The onion 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 . 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.

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 a bract protecting a developing inflorescence. The inflorescence takes the form of a rounded umbel of white flowers with parts in sixes. The seeds are glossy black and triangular in cross-section. The average pH of an onion is around 5.5.

History

The history of ancestral onion species is not well documented. Ancient records of onion use spans western and eastern Asia, so the geographic origin of the onion is uncertain. Yet, domestication likely took place in West or Central Asia. Onions have been variously described as having originated in Iran, western Pakistan and Central Asia.

Traces of onions recovered from Bronze Age settlements in China suggest that onions were used as far back as 5000 BC, not only for their flavour, but also for the bulb's durability in storage and transport. Ancient Egyptians revered the onion bulb, viewing its spherical shape and concentric rings as symbols of eternal life. Onions were used in Egyptian burials, as evidenced by onion traces found in the eye sockets of Ramesses IV.

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.

In the Age of Discovery, onions were taken to North America by the first European settlers, who found close relatives of the plant such as ''Allium tricoccum'' readily available and widely used in Native American gastronomy. According to diaries kept by certain first English colonists, the bulb onion was one of the first crops planted by the Pilgrim fathers.

Uses

Onion types and products

Common onions are normally available in three colour varieties:
* Yellow or brown onions are sweeter and are the onions of choice for everyday use in European cuisine