All green leafy vegetables contain more nitrates in the green leaves, just you cook the vegetables placed in too long a period of time (more than 12 hours), in the bacterial decomposition, nitrates in the green leaves will become nitrites, medically proven that nitrites are carcinogenic.

Oxtail soup without vegetables, hot open, cold after the compound plastic wrap in the refrigerator, the next day is not a big problem, it is best not to put long.