Onions are members of the allium plant family, which also includes garlic, leeks, and shallots. There are scores of different varieties of onions, with new ones constantly emerging. In general, onions are divided into 2 categories: spring onions, which have a mild flavor and whose green tops and bulbs are eaten; and globe onions, which have a more pungent flavor and dry outer skins that are discarded. Shallots possess features of onions and garlic, but are milder than either.
With so many sizes, shapes, and flavors, and so many varieties of spring onion and globe onions choosing an onion can be somewhat tricky. Scallions should have crisp, dark green tops and firm white bottoms. Although they will keep for a few days in the refrigerator, scallions should be used before they begin to soften. Globe onions should be firm, with cracky, dry skin. Reject any that feel soft, have black spots (indicating mold), or have green sprouts showing at the top (these are well past their prime). They should have a mild odor—a strong, oniony smell points to decay. Globe onions should be stored in a cool, dry place away from direct light, which can give them a bitter taste. They should not be stored near potatoes, which give off moisture and a gas that causes onions to spoil more quickly.
Red onions have a mild, somewhat sweet flavor, which makes them a favorite for salads.
Cooks favor onions more for the flavor they impart to other foods than for their nutritional content. Although onions are not high on the nutritional scale, the green tops of spring onions are a good source of vitamin C and beta carotene. A cup of boiled onions provides about 225mg of potassium.
Recent research has verified some for the centuries-old beliefs about onions. Researchers have now documented that adenosine, a substance in onions, hinders clot formation, which may help prevent heart attacks. Studies also indicate that onions may protect against the artery-clogging damage of cholesterol, by raising the levels of the protective high-density lipoproteins (HDLs). Still other studies suggest that eating ample amounts of onions may help prevent high blood pressure.
Sulfur compounds in onions can cause bad breath and an unpleasant skin odor; however, they also block the cancer-causing potential of some carcinogens. In addition, onions contain substances that have a mild antibacterial effect, which may validate the old folk remedy of rubbing a raw onion on a cut to prevent infection.
cutting an onion allows its sulfur compounds to combine with enzymes to form a type of sulfuric acid, which is what brings tears to the eyes. This effect may help clear congested nasal passages during a cold. a syrup made from onions and honey is an old cough remedy. but consuming raw onions can cause bloating and gas in some people.