Onion

Author: Dr Marius Rademaker, Dermatologist, Hamilton, New Zealand, 1999.


Common name: Onion
Botanical name: Allium cepa
Family: Liliaceae (lily family). Some authorities regard the onions, garlics, leeks and their relatives in a family of their own, the Alliaceae, but others put them in the lily or Liliaceae family. There are about 400 species in the genus Allium.
Origin: The onion was probably developed from a wild ancestor in Southwest Asia. Onion seeds have been found in Egyptian tombs dated to 3200 BC, and some authorities believe the onion may have been one of the first vegetables domesticated by humans.
Description: The onion species, Allium cepa, can be divided into three groups: i) the Cepa Group includes the typical bulbing onions which have a single enlarged bulb, and are propagated from seeds; ii) the Aggregatum Group which includes shallots, multiplier onions, and potato onions, which do not produce seeds but are propagated from lateral bulbs off the main bulb; iii) the Proliferum Group which includes top-setting onions, walking onions, Egyptian onions and tree onions, which are propagated from bulbils (little ‘bulblets’).
Bulbing onions have cylindrical, hollow leaves and an enlarged bulb that develops at ground level. The roots come off the bottom of the bulb. The flowers are produced in the second growing season (following a required ‘rest’ period) in a rounded umbel (cluster with all flower stems originating from the same point) on a stalk 0.6-1.2 m) tall. The umbels, about 5 cm in diameter and consisting of many small purplish flowers, are quite showy. There are hundreds of onion cultivars, differing in day-length requirement, skin colour (white, brown, yellow, red, or purple), size (2.5-15 cm in diameter), shape (globe-shaped, flattened or spindle-shaped), pungency and sweetness.
Uses: Food source.
Allergens: Allergen not identified
Allergy: Allergic contact dermatitis is rare. A case of severe systemic reactions (intense itching, urticaria, confusion, blurred vision, transient loss of consciousness, sweating, tachycardia) after ingestion of raw or lightly-cooked onion is described. Irritancy is very common.
Cross reactions: Garlic, shallots, and certain pollens.
Other information: There are two main kinds of onions, based on the day-length required for bulb formation. Short-day varieties start forming an enlarged bulb when days are 12 or 13 hours long; long-day varieties don't form a bulb until days are 14-16 hours long. For both types, bulb enlargement is arrested during hot, freezing or dry weather. Near the equator, where days are 11-13 hours long throughout the year, long-day onions will never form a bulb; and in New Zealand, where days can be 14-20 hours long during the growing season, short-day onions will never form a bulb.
Onions and their relatives have almost no odour until cut into. When the cell walls are damaged, odourless compounds come into contact with each other and react to form ammonia, pyruvic acid and various disulfides, the last of which cause the distinctive smell of onions. These compounds can form sulfuric acid in the eyes. Cooking reduces the odoriferous compounds and converts some of them to sugars.
If you have to peel a lot of onions, drop them in boiling water for a few seconds, then cool; the skins should slip right off. If you peel onions under running water, or if you chill them in the refrigerator first, there will be less crying. Rub your hands with salt or vinegar to remove the odour.
Patch test: Open application test with onion ‘as is’, immediate or delayed scratch-chamber tests.

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