The term Onion (pyaz) refers to any one of a number of plants of the Allium Family, we are usually talking about the garden variety. In the past it was regarded as part of the lily family of plants. Other members of the family are spring onions (scallions), garlic, shallots and leeks; there are also a number of varieties such as spanish, red, white and so on. They all have slightly different characteristics which make them more or less suitable for different types of cooking.
A word about the taste. It is said that onions and apples have exactly the same taste and the apparent distinction is caused by differences in smell and texture. Smell actually accounts for about 70 percent of what we perceive as taste (which is why you can't taste your food when you have a cold). As a vegetable they have quite a sweet taste and moreso when cooked.
The bulb which grows underground is the plants foodstore and so is is full of nutrients - more of that later.
It is unclear where this vegetable originated; some say central Asia, others, Pakistan or Iran. What is certain is that they have been used as food for at least 7000 years, although it is unclear at what stage they were cultivated. Since they are mentioned in the Bible (Number 11:5) it seems likely that they were cultivated in Egypt in about 3000 BC. They were certainly worshipped by the Egyptians who believed the concentric layers symbolized eternal life. They were used in burial rituals; Ramases IV eye sockets were found to have traces of onion in them and paintings of them have been found in pyramids and other tombs.
Ancient Greek athletes believed (with uncanny accuracy) that they improved the blood and so ate them in large quantities. Roman soldiers used them as a body rub to tighten the muscles.
In Europe they were one of the three commonest vegetables along with beans and cabbage, and were valuable enough to be used as barter for rent and other goods.
India is a major producer - by far the world's largest. A strange fact though - people of the Jain religion and certain sects of Hindus and some Buddhists will not eat this vegetable, or garlic, ginger and a number of other root vegetables. This is to do with complex religious beliefs about destroying the plants life source and denying oneself something that may be an aphrodisiac (apologies for the simplification if you're an expert).
Firstly the obligatory word about them making you cry and how to avoid it. The cause of the tears is something called sulphenic acid which is a gas created by enzyme driven reactions when the onion is damaged (like by your knife). The commonly touted ways of avoiding this are to slice them under water (which pretty effectively stops the gas reaching your eyes) and not cutting the root off until last (the root contains a higher concentrate of the chemicals involved). I keep my onions in the fridge and they never affect my eyes - maybe my eyes are not as sensitive as some but the lower temperature does slows down the chemical reactions.
Prolonged cooking, such as in a curry will make them dissolve into the sauce and act as a thickening agent whilst adding sweetness. Slicing very thinly will exaggerate this effect so if you wish to still have chunks of the vegetable in the end result then you need to chop then quite big.
Most recipes require that they be fried in oil at the beginning of cooking; this brings out the sweetness especially if they are fried on a low heat for a long time (caramelizing). Cooking on a higher heat (sautéing) will crisp the outside slightly whilst leaving the centre soft.
Onions are absolutely loaded with Vitamins (notably C, B6, Foliate and Thiamin plus others) and minerals (manganese, potassium, magnesium and phosphorus). They have virtually no fat, cholesterol or sodium, but do contain carbohydrates, protein and dietary fibre.
Medically they are a strong anti-inflammatory and anti-oxidant; they are variously used to treat things such as bee strings, blisters and even sea-urchin wounds. Research has suggested that large consumption reduces the risk of head and neck cancers (strangely it is the very chemicals that make you try that could be resonsible for this). Onion sweetened with jaggery is a traditional sore throat remedy in India and chemist believe they have identified the chemicals responsible for this.
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