Leftovers overnight will produce nitrites and bacteria. Generally leftover vegetables at room temperature storage 1 to 3 days nitrite content reaches its peak, refrigerated is 3 to 5 days to reach its peak, it is recommended that leftover vegetables are best eaten within 5 to 6 hours. Nitrate in vegetables will be reduced to nitrite under the action of certain bacteria, nitrite in the human body and protein-like substances combined, can generate carcinogenic amines, thus endangering human health. A variety of vegetables are heated and placed in the process, all contain nitrite to varying degrees, and the longer they are placed, the higher the nitrite content. The correct storage of leftover vegetables can effectively reduce the production of nitrite. Therefore, the leftovers need to be cooled, put into a fresh box or plastic bag and other clean containers wrapped, placed in the refrigerator at about 4 ℃ storage, and different leftovers, need to be stored separately, storage time should not exceed 6 hours.