Onion which is also known as bulb onion is a vegetable that is cultivated throughout the world. It belongs to the genus of Allium. Onions are the modified leaves of the plant that become fleshy as they store water, food materials and mucilage of the plants. Onion contains 89% water, 9% carbohydrates (including 4\% sugar and 2\% dietary fibre), 1\% protein, and negligible fat. Onion also adds a savoury flavour to dishes without adding to its calories.