Microwaves do not cook evenly, but leave hot and cold spots in the food. Microbes not killed by the cooking remain to multiply in the food if the dish is not refrigerated.

Other research in Britain and the US unearthed another possible hazard. In 1990, scientists in the Department of Medical Microbiology at the University of Leeds found that,[1] the higher the salt content of a portion of mashed potatoes, the greater the difference between its surface and internal temperatures after being microwaved (a higher salt content leading to lower internal temperatures).