Onions have pipe-shaped green leaves that sprout from a bulb buried beneath the surface. The bulb functions as a storehouse and consists of several fleshy layers that are arranged one on top of the other, each covered by a thin skin. Depending upon the variety, either the bulb alone may be eaten or the leaves as well. Onions are available year-round in a range of varieties. Worldwide, the largest number of onions are harvested in China, from where they are also exported to Europe. Onions are very low in calories and contain large amounts of vitamin C, potassium and zinc. Their typical aromatic to hot spicy flavour is due to sulphur-containing secondary plant compounds (sulphides), which are also what make the onion so beneficial to the health.
