Onion

For the parody newspaper, see The Onion. For the computer networking technique, see Onion Routing.

Onion
Scientific Classification
Kingdom: Plantae Division: Magnoliophyta Class: Liliopsida Order: Liliales Family: Liliaceae Genus: Allium Species: cepa
Binomial name
Allium cepa L.

Onion in the general sense can be used for any plant in the genus 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.

Table of contents

1 Why do onions make you cry? 2 Related species 3 External links 4 References

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. When the enzymes mix with the sulfides (amino acid sulfoxides), sulfenic acids are formed. Sulfenic acid is unstable and decomposes into a volatile gas called syn-ropanethial-S-oxide. The gas then dissapates through the air and eventually reaches one's eye, where it will react with the water to form a mild sulfuric acid. The sulfuric acid irritates the nerve endings in the eyes, making them sting.

The release of gas can thus best be prevented by cutting the onions under running tap water or completely under water. This is not very practical of course. 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 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

External links

• How stuff works article on eye irritation by onions
• Library of Congress article (more details on eye irritation)
• Article at americanchemistry.com

References

• Imani, S. et al. Plant biochemistry: an onion enzyme that makes the eyes water. Nature, v. 419, Oct. 17, 2002: 685.