Although it is a fine tradition of our Chinese nation to cherish food and not waste it, it is recommended not to keep leftover vegetables. First of all, the leftover vegetable nutrients, such as vitamin C, chlorophyll, carotene, etc., are not easy to keep for a long time. After overnight storage, they have been lost, leaving only a little dietary fiber. Secondly, the leftover vegetables, especially green leafy vegetables, are easy to produce nitrite when stored for a long time, and nitrite combines with amines produced by protein decomposition to produce carcinogen nitrosamines.