Science Fair Project Encyclopedia
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 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. See how to prepare an onion cell slide for details.
Onions are probably one of the oldest crops grown by humans, being mentioned in the Bible 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.
Culinary and medicinal uses
Onions are available in fresh, frozen, canned, and dehydrated forms. Onions can be used, usually chopped or sliced, in almost every type of non-dessert 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
They appear to be at least somewhat effective against colds, heart disease, diabetes, and other diseases and contain antiinflammatory, anticholesterol, and anticancer components.
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). It also helps to breathe exclusively through the mouth during the preparation. 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.
Types of onion (Allium cepa)
- Bulb onions
- Multiplier onions
- Shallot (most of the types in the markets are Allium cepa)
- Potato onion
- Tree onions or Egyptian onions
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.
Eye Irritation Information
- How stuff works article on eye irritation by onions
- Library of Congress article (more details on eye irritation)
- Article at americanchemistry.com
Eclectic Herbal Information
- King's American Dispensatory @ Henriette's Herbal
- Mrs. Grieve's "A Modern Herbal" @ Botanical.com
- Potato Onion (Allium cepa, var. aggregatum) Mrs. Grieve's "A Modern Herbal" @ Botanical.com
- Tree Onion (Alliurn cepa, var. proliferum) Mrs. Grieve's "A Modern Herbal" @ Botanical.com
- Allium cepa (all-cep.) "Kent's Lectures on Homeopathic Materia Medica" by Dr Robert SÚror
- Allium cepa "A Primer of Materia Medica for practitioners of Homœopathy" by Timothy Allen
- Imani, S. et al. Plant biochemistry: an onion enzyme that makes the eyes water. Nature, v. 419, Oct. 17, 2002: 685.
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