onions

Onions (Allium cepa) are a bulbous vegetable in the Amaryllidaceae family and one of the most widely cultivated vegetables. The edible bulb forms underground, with thin papery skins that range from yellow-brown to red or white. Leaves are long and hollow. Major cultivated varieties include yellow, red, and white onions; green onions and leeks are closely related but distinct.

Biology and cultivation: Onions are typically grown as annuals; the bulb enlarges in response to day length (photoperiod). They prefer well-drained soil, full sun, and regular moisture. Spacing about 8-15 cm apart depending on size. They are susceptible to pests such as onion maggot and diseases such as basal rot. Harvest when the tops fall over and dry.

Chemistry and flavor: Cutting onions releases sulfur compounds from alliinase converting amino acid sulfoxides into volatile thiosulfinates, notably syn-propanethial-S-oxide, which causes tearing and a characteristic pungent aroma. Flavor is mild when raw and sweet when cooked; caramelization enhances sweetness.

Uses: Onions are a culinary staple used fresh in salads, pickled, sautéed, roasted, or used as a flavor base in soups, stews, and sauces. They pair with many cuisines and can be stored long term in a cool, dry, ventilated place. Once cut, onions keep best in the refrigerator in a sealed container.

Nutrition and health: Onions are low in calories and provide vitamin C, B6, potassium, and fiber. They contain various phytochemicals with potential antioxidant properties. Allergic reactions are uncommon but can occur.

Global production: The crop is grown worldwide, with major producers including China, India, and the United States. See also related Allium vegetables such as garlic, shallots, chives, leeks.