Listing description
The onion (Allium cepa L., from Latin cepa "onion"), also known as the bulb onion or common
onion, is a vegetable and is the most widely cultivated species of
the genus Allium.
Detailed description
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.[2] 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. In the autumn (or in spring, in the case of overwintering onions), 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 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 pickles or chutneys. They
are pungent when chopped and contain certain chemical substances which irritate
the eyes.
Description
The onion plant 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
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
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.[14]
In the autumn, the leaves die back and the outer scales of the
bulb become dry and brittle, so the crop is then 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.[14]
Uses
Historical use
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.[15] However,
whether these were cultivated onions is not clear. Archaeological and literary
evidence such as the Book of
Numbers 11:5 suggests that onions were probably
being cultivated around 2000 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.[15]
The onion is easily propagated, transported, and stored. The
ancient Egyptians revered the onion bulb, viewing its spherical shape and
concentric rings as symbols of eternal life.[16] Onions
were even used in Egyptian burials, as evidenced by onion traces being found in
the eye sockets of Ramesses IV.[17]
In ancient
Greece, athletes ate large quantities of onion because it was believed
to lighten the balance of the blood.[16] Roman gladiators were
rubbed down with onions to firm up their muscles.[16] In the Middle
Ages, onions were such an important food that people paid their rent
with onions, and even gave them as gifts.[16] Doctors
were known to prescribe onions to facilitate bowel movements and erections, and
to relieve headaches, coughs, snakebite, and
hair loss.[16]
Onions were taken by the first European 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.[16]
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.
Culinary uses
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.[21] Their
layered nature makes them easy to hollow out once cooked, facilitating stuffing them,
as in sogan-dolma. 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 traditionally a side serving in pubs and fish and chip shops throughout the United
Kingdom and the
Commonwealth. Pickled onions form part of a British pub ploughman's lunch, usually served with cheese and ale. In
North America, sliced onions are battered, deep-fried, and
served as onion
rings.[22]
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. White
onions are the traditional onions used in classic Mexican
cuisine; they have a golden colour when cooked and a particularly sweet
flavour when sautéed.[23][24]
While the large, mature onion bulb 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.[25] 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.[26]
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.[26] 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.[27]
Onions are available in fresh, frozen, canned, caramelized,
pickled, and chopped forms. The dehydrated product is available as kibbled,
sliced, ring, minced, chopped, granulated, and powder forms.[28]
Onion powder is a seasoning 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 is available in several varieties:
yellow, red, and white.[29]
Non-culinary uses
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.[30][31][32] Onions
are, therefore, commonly employed in science
education to teach the use of a microscope for
observing cell structure.[33]
The pungent juice of onions has been used as a moth repellent
and can be rubbed on the skin to prevent insect bites. It has been used to
polish glass and copperware and to prevent rust on iron. Onion skins have been
used to produce a yellow-brown dye.[34]
Historically, onions were often used for cromniomancy across
Europe, Africa, and northern Asia, and they continue to be used for this
practice in some rural areas.
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