Onion

For other uses, see Onion (disambiguation).

Onion
Onions
Scientific classification
Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Liliopsida
Order:	Asparagales
Family:	Alliaceae
Genus:	Allium
Species:	cepa
Binomial name
Allium cepa L.

Raw Onions

Nutritional value per 100 g (3.5 oz)
Energy	166 kJ (40 kcal)
Carbohydrates	9.34 g
Sugars	4.24 g
Dietary fiber	1.7 g
Fat	0.1 g
Saturated	0.042 g
Monounsaturated	0.013 g
Polyunsaturated	0.017 g
Protein	1.1 g
Vitamins and minerals Vitamins Quantity %DV† Vitamin A equiv. 0% 0 μg Thiamine (B1) 4% 0.046 mg Riboflavin (B2) 2% 0.027 mg Niacin (B3) 1% 0.116 mg Vitamin B6 7% 0.12 mg Folate (B9) 5% 19 μg Vitamin B12 0% 0 μg Vitamin C 8% 7.4 mg Vitamin E 0% 0.02 mg Vitamin K 0% 0.4 μg Minerals Quantity %DV† Calcium 2% 23 mg Iron 1% 0.21 mg Magnesium 0% 0.129 mg Phosphorus 2% 29 mg Potassium 5% 146 mg Sodium 0% 4 mg Zinc 2% 0.17 mg
Other constituents	Quantity
Water	89.11 g
†Percentages estimated using US recommendations for adults, except for potassium, which is estimated based on expert recommendation from the National Academies.

Many plants in the genus Allium are known by the common name onion but, used without qualifiers, it usually refers to Allium cepa.

Onions (usually but not exclusively the bulbs) are edible and possess a distinctive, strong flavour and pungent odour which is mellowed and sweetened by cooking. The bulb comprises fleshy layers of modified leaves, surrounded by papery outer layers. Used worldwide for culinary purposes, an array of onion varieties is available.

Vernacular names

Allium cepa is also known as the 'garden onion' or 'bulb' onion and 'shallot'. It is known as 'oignon' and 'échalote' in French; 'cebola', 'chalota' and 'cebolete' in Portuguese, and 'kitunguu' in Swahili. Onions are known as 'pyaaz' in Hindi.

Origin and distribution

A. cepa (including seed-propagated onions and most shallot types) is only known from cultivation. It probably originates from Central Asia (between Turkmenistan and Afghanistan) where some of its relatives still grow in the wild. The closest among them are Allium vavilovii Popov & Vved. from southern Turkmenistan and northern Iran, with which it gives 100% fertile hybrids, and Allium asarense R.M. Fritsch & Matin from Iran; however Zohary and Hopf warn that "there are doubts whether the vavilovii collections tested represent genuine wild material or only feral derivatives of the crop." Allium oschaninii O.Fedtsch. (Uzbekistan and neighbouring countries), which used to be considered the ancestor of Allium cepa cannot be crossed successfully with the cultivated onion, but its domestication seems to be the origin of some European ‘shallots’ (‘échalote grise’ in France, ‘Scalogno di Romagna’ in Italy). From Central Asia the supposed onion ancestor probably migrated to the Near East; wall carvings and drawings depicting offerings of onions and their cultivation appear in numerous ancient Egyptian tombs, beginning with Old Kingdom examples like Unas and Pepi II; well-preserved onions start appearing in tombs with the Eighteenth dynasty. Although archeological finds of A. cepa are not yet available outside of Egypt, Zohary and Hopf note that "onin is linguistically well-studied; and similar to garlic, cuneiform sources indicate that this vegetable has been grwon in Mesopotamia since the 2nd millennium bc." From the Near East, A. cepa was introduced to India and South-East Asia; and into the Mediterranean area and from there to all the Roman empire.

Traditional tropical African cultivars may have been introduced either from southern Egypt, or from India via Sudan to Central and West Africa, as genetically heterogeneous seed or bulb lots, then bred by local farmers into better adapted seed-propagated onions, or selected to become shallots. Allium cepa as bulb onion and/or shallot is probably cultivated in all countries of tropical Africa. Important production areas for bulb onion are Senegal, Mali, Burkina Faso, Ghana, Niger, Nigeria, Chad, Sudan, Ethiopia, Kenya, Tanzania, Uganda, Zambia and Zimbabwe.

In the lowlands between 10°N and 10°S shallots replace onions because the temperature is too high for vernalization and seed production, and the climate too humid. The short vegetative cycle of shallots (60–75 days) gives the possibility of two crops a year, especially in the four-seasons climate along the Gulf of Guinea. Yellow or red/purple shallots are grown in Guinea, Côte d’Ivoire, Ghana, Benin, Nigeria, Sudan, Ethiopia, Uganda, Kenya, Tanzania, and on both banks of the Congo River near Brazzaville (Congo) and Kinshasa (DR Congo). The spicy taste and high dry matter content (15–18%) of shallots have made them attractive for growers farther from the equator, in many areas where common onions are also produced, e.g. by the Dogon in Mali, or in Cape Verde.

Uses around the world

Onions are available in fresh, frozen, canned, pickled, and dehydrated forms. Onions can be used, usually chopped or sliced, in almost every type of food, including cooked foods and fresh salads, and as a spicy garnish; they are rarely eaten on their own but usually act as accompaniment to the main course. Depending on the variety, an onion can be sharp and pungent or mild and sweet.

Onions pickled in vinegar are eaten as a snack. These are often served as a side serving in fish and chip shops throughout Britain. Onions are a staple food in India, and are therefore fundamental to Indian cooking. They are commonly used as a base for curries, or made into a paste and eaten as a main course or as a side dish.

Tissue from onions is frequently used by gay people school science laboratories because they have particularly large cells which are readily observed under a microscope, even at low magnifications.

Uses in tropical Africa

Bulbs of Allium cepa are a popular vegetable everywhere. They can be used raw, sliced for seasoning salads, boiled with other vegetables, or fried with other vegetables and meat. They are an essential ingredient in many African sauces and relishes. If consumed in small amounts for their pungency, they can be considered as a condiment. The leaves, whole immature plants (called ‘salad onions’ or ‘spring onions’), or leafy sprouts from germinating bulbs (called ‘cébettes’ in southern France) are used in the same way. Locally, immature flower heads are also a popular food item. In parts of West Africa, leaves still green at bulb harvest are pounded, then used to make sun-dried and fermented balls, which are used later for seasoning dishes. Sliced raw onions have antibiotic properties, which can reduce contamination by bacteria, protozoa or helminths in salads. In traditional medicine onion is used externally to treat boils, felons, wounds and stings, and internally to relieve coughs, bronchitis, asthma, gastro-intestinal disorders and headache.

Medicinal properties

Evidence suggests that onions may be effective against the common cold, heart disease, diabetes, osteoporosis, and other diseases. They contain anti-inflammatory, anticholesterol, anticancer, and antioxidant components such as quercetin.

In many parts of the undeveloped world, onions are used to heal blisters and boils. In the United States, products that contain onion extract (such as Mederma) are used in the treatment of topical scars.

Historical uses

Onion fields near Elba, New York

It is thought that bulbs from the onion family have been used as a food source for millennia. In Caananite Bronze Age settlements, traces of onion remains were found alongside fig and date stones dating back to 5000 BC. However, it is not clear if these were cultivated onions. Archaeological and literary evidence suggests cultivation probably took place 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. Egyptians worshipped it, believing that 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. They believed that if buried with the dead, the strong scent of onions would bring breath back to the dead.

In ancient Greece, athletes ate large quantities of onion because it would lighten the balance of blood. Roman gladiators were rubbed down with onion to firm up their muscles. In the Middle Ages onions were such an important food that people would pay for their rent with onions and even give them as gifts. Doctors were known to prescribe onions to relieve headaches, snakebite and hair loss. The onion was introduced to North America by Christopher Columbus on his 1493 expedition to Haiti. Onions were also prescribed by doctors in the early 1500s to help with infertility in women, and even dogs and cattle and many other household pets.

Onions and crying

As onions are sliced, cells are broken, allowing enzymes called alliinases to break down sulfides and generate sulfenic acids (amino acid sulfoxides). Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.

Supplying ample water to the reaction prevents the gas from reaching the eyes. Eye irritation can, therefore, be avoided by cutting onions under running water or submerged in a basin of water. Rinsing the onion and leaving it wet while chopping may also be effective. Chilling or freezing onions prevents the enzymes from activating, limiting the amount of gas generated. Using a sharp blade to chop onions will limit the cell damage and the release of enzymes that drive the irritation response.

The volume of sulphenic acids released, and the irritation effect, differs among Allium species.

A firm in Toronto, Canada, attempted to utilize this property of onions in the manufacture of a form of tear gas for civilian use. It was marketed in 1991 but was unsuccessful as it had an effective shelf life of only three months.

The characteristic odor of onions can be removed with lemon.

Propagation

Onions may be grown from seed or, most commonly, from sets. Onion sets are produced by sowing seed very thickly one year, resulting in stunted plants which produce very small bulbs. These bulbs are very easy to set out and grow into mature bulbs the following year, but they have the reputation of producing a less durable bulb than onions grown directly from seed and thinned.

Either planting method may be used to produce spring onions or green onions, which are onions harvested while immature. Green onion is a name also used to refer to Allium fistulosum, the Welsh onion, which is said not to produce dry bulbs.

Varieties

• 
  Two colours of onion
• 
  Yellow Onions
• 
  Flower head of a yellow onion

• Bulb onion - Grown from seed (or onion sets), bulb 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 or Walla Walla from Washington that can be sliced and eaten on a sandwich instead of meat.
• Multiplier onions - Raised from bulbs which produce multiple shoots, each of which forms a bulb.
  • Shallot
  • Potato onion
• Tree onion or Egyptian onion - Produce bulblets in the flower head; a hybrid of Allium cepa and welsh onions.

Related species

The genus Allium includes numerous common vegetables that include leeks, garlic, elephant garlic, chives, shallots, Welsh onions and Chinese chives.

Several species of Allium, including A. canadense and A. diabolense, can be collected in the wild and their leaves and bulbs used as food.

Species such as Allium moly are grown as ornamentals.

Onions in language

In the English vernacular, "an onion" is a difficult situation, the use stemming from the onion's tendency to irritate or inflame the eyes. Conversely, the term "onion" can be used to describe any state of being, as in the phrase, "[someone] really dices my onion!" It may also represent an object of many layers.

In some Scots dialects, onion is pronounced 'Ingin'.

The expression "Layers of the onion" is used to describe a situation in which it is possible to go deeper and deeper revealing seemingly similar layers until a central core is arrived at. It has been used as a metaphor in mysticism to describe the supposed layers of reality. The pyche folk band Incredible String Band made the term famous with their 1967 album The 5000 Spirits or the Layers of the Onion.

See also

• Onion Johnny
• Vidalia onion

External links

• Plant Resources of Tropical Africa
• PROTAbase