Onion
- For the parody newspaper, see The Onion. For the computer networking technique, see Onion Routing.
Allium but used without qualifiers usually means Allium cepa, 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 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 through an optical microscope. See how to prepare an onion cell slide for details.
Why do onions make you cry?
As onions are sliced, cells are broken open. Onion cells have two sections, one with enzymes called allinases, 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-ropanethial-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). For more tips and information, please check links in External links section.
Different species of onions will release different amounts of sulfenic acids, thus some will cause more tear formation and irritation than others.
Related species
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 which are grown for ornament.
Several species of wild onion, including A. canadense and A. diabolense, can be collected in the wild and their leaves and bulbs used as food.
allium cepa
