“Eggs almost always contain salmonella,” says food scientist, Dr Kantha Shelke. Some methods used to cook eggs require gentle heat for a short duration of time, which may not kill the bacteria. (Any method that results in a runny yolk.) Leaving them at room temperature for any length of time is a recipe for those bacteria to multiply to harmful levels. Plus, eggs always taste better fresh and don’t take too long to scramble, so they’re probably not a food you want to save for later.
Research, including a study published in 2012 in Sports Medicine, shows that the nitric oxide in beetroot can give your workout a boost and may help blood pressure. But those same compounds react with heat badly. When nitrate-rich foods are cooked, “not cooled properly, and further reheated, the nitrates can get converted to nitrites, and then to nitrosamines, some of which are known to be carcinogenic,” Shelke says. So regularly eating reheated beets or beet products may not be a good idea. The same may be true of turnips, another nitrate-rich root veggie.
Spuds seem so sturdy, but even though they’re cooked hotter and longer than eggs, they can pose a risk when left to cool and stored at room temperature too long. Doing so can potentially foster the growth of Clostridium botulinum, the bacteria that causes botulism, says Shelke. Large, foil-wrapped baked potatoes are particularly at risk, she says, because they offer bacteria the ideal low-oxygen environment to thrive in. That’s not all: potatoes are also among the foods that you shouldn’t reheat in a microwave. Zapping them (sans foil of course) for 30 to 60 seconds doesn’t kill the stuff that wreaks havoc on your GI system. Cooking a raw potato in the microwave, however, requires only a few minutes more. Go that route, instead.