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Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Liliopsida
Order: Asparagales
Family: Alliaceae
Genus: Allium
Allium cepa
Onion in the general sense can be used for any plant in the Genus Allium but used without qualifiers usually means Allium cepa L., also called the garden onion. Onions (usually but not exclusively the bulbs) are edible with a distinctive strong flavour and pungent odour which is mellowed and sweetened by cooking. They generally have a papery outer skin over a fleshy, layered inner core. Used worldwide for culinary purposes, they come in a wide variety of forms and colors.Onions may be grown from seed or very 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 just onions harvested while immature, although "green onion" is also a common name for the Welsh onion, Allium fistulosum which never produces dry bulbs.Onions are frequently used in school science laboratories because they have particularly large cells which are easily visible even through rather low-end optical microscopes.

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Culinary and medicinal uses
Why do onions make you cry?
Types of onion (Allium cepa)
Related species

History - Contents

Onion fields near Elba, New York
Onion fields near Elba, New York
It is thought that bulbs from the onion family have been utilised as a food source for millennia. In Palestinian Bronze Age settlements, traces of onion remains were found alongside fig and date stones dating back to 5000 BC. It would be pure conjecture to suggest these were cultivated onions. The archaeological and literary evidence suggests cultivation probably took place around two thousand years later in ancient Egypt. This happened alongside the cultivation of leeks and garlic and it is thought that workers who built the pyramids were fed radishes and onions. [1]The onion is easily grown, transportable, and has good storage qualities. 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 King Ramses IV. They believed that if buried with the dead, the strong scent of onions would bring breath back to the dead. The onion then made its way to Greece where athletes ate large quantities of onion because it would lighten the balance of blood. Roman gladiators were also 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 onions as gifts. Doctors were also known to prescribe onions to end headaches, snakebites and hair loss. The onion was introduced to North America by Christopher Columbus on his 1493 expedition to Haiti.Onions are one of the earliest crops mentioned in written text, in the Bible's Book of Numbers (11:5) as part of the Egyptian diet of that time. Six types of onions were known at the time of Pliny the Elder's Natural History.

Culinary and medicinal uses - Contents

Onion set
Onion set
Onion cell slide microphotography
Onion cell slide microphotography
Onions are available in fresh, frozen, canned, 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 even sweet.
  • Chopped, it is one of the three vegetables considered the holy trinity of Louisiana Creole and Cajun cuisine.
  • Cocktail onions, or pickled pearl onions, are used to garnish drinks such as Gibsons.
They appear to be at least somewhat effective against colds, heart disease, diabetes, and other diseases and contain anti-inflammatory, anticholesterol, anticancer, and antioxidant components such as quercetin.In many parts of the underdeveloped 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.

Nutrition - Contents

(All average size)
Onion Raw Onion Fried in Oil Pickled Onion
Energy(kJ) 225 275 17
Energy(kcal) 54 66 4
Protein (g) 1.8 0.9 0.1
Carbohydrate (g) 11.9 5.9 0.7
Sodium (mg) 5.0 2.0 68.0
Calcium(mg) 38.0 19.0 3.0

Why do onions make you cry? - Contents

As onions are sliced, cells are broken open. Onion cells have two sections, one with enzymes called alliinases, the other with sulfides ( amino acid sulfoxides). The enzymes break down the sulfides and generate sulfenic acids. Sulfenic acid is unstable and decomposes into a volatile gas called syn-propanethial-S-oxide. The gas then dissipates through the air and eventually reaches one's eye, where it will react with the water to form a mild solution of sulfuric acid. The sulfuric acid irritates the nerve endings in the eyes, making them sting. The tear glands then produce tears in response to this irritation, to dilute and flush out the irritant.The release of gas can thus best be prevented by cutting the onions under running tap water or completely under water, though this may not be very practical. Wetting the onions and your hands before slicing will lessen the effect, as some of the gas will react with the moisture on the onions and on your skin (instead of the moisture on your eyes). This reaction may result as an odour which may be removed with lemon. It also helps to breathe exclusively through the mouth during the preparation. Using a sharp knife will rupture fewer cells and cause less eye irritation. For more tips and information, please check links in External links section. Chilled onions (onions kept in the fridge for a while) will make you 'cry' less than onions kept at room temperature because lower temperature inhibits the enzymes and gas diffusion. Also, some people 'freeze' the knife (leave it in the freezer for around 2 minutes) before cutting the onions to prevent the tears.Different species of onions will release different amounts of sulfenic acids, thus some will cause more tear formation and irritation than others.

Types of onion (Allium cepa) - Contents

  • Bulb onions
  • Multiplier onions
    • Shallot (most of the types in the markets are Allium cepa)
    • Potato onion
  • Tree onions or Egyptian onions
  • Vidalia onion (sweet onion grown near Vidalia, Georgia, a region with low amounts of sulfur in the soil)

Related species - Contents

The genus Allium is a large one, and most of the species are considered to be "onions" in the looser sense. Commonly raised vegetable alliums include the leeks, garlic, elephant garlic, chives, shallots, Welsh onions and garlic chives. There are also species, such as Allium moly, grown for ornament.Several species of Allium, including A. canadense and A. diabolense, can be collected in the wild and their leaves and bulbs used as food.
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