Chicken and other poultry are highly likely to be contaminated with salmonella, as are eggs. This can be a problem to reheat particularly in microwaves, because heat waves do not penetrate equally into all areas of the food. Which means that some parts of the food cook faster than others. It is important especially with chicken and the meat to turn frequently to ensure that it is well warmed both outside and inside. Reheating chicken is not recommended more generally as it has a higher protein density than red meat - when reheated, proteins break down differently and can upset the stomach.
RICE
How cooked rice is stored is more important than reheating itself, according to the Food Standards Agency. If rice is allowed to sit at room temperature, the spores multiply and can produce toxins that cause vomiting or diarrhea - and reheating the rice will not rid it of these toxins.
POTATO
The problem with these is not the reheating itself, but how are the potatoes stored after they are cooked? If allowed to stand at room temperature (and then stored without refrigeration), these conditions may be suitable for the growth of Clostridium botulinum (botulism). Reheating the potato will not always kill these toxins, so storage of these must be processed with the utmost safety. They are recommended to be refrigerated after being cooked.
MUSHROOMS
Mushrooms have proteins that can be easily destroyed by enzymes and microorganisms, according to the European Food Information Council. If these are not stored properly, they can deteriorate rapidly and cause an upset stomach after reheating. However, says the Council: “If they are stored in a refrigerator for no more than 24 hours, which is the norm, there is no problem in reheating them.
SPINACH AND OTHER GREEN VEGETABLES
Spinach and other green leafy vegetables can contain high concentrations of nitrate, depending on where they are grown. Nitrate itself is totally harmless, but it can turn into nitrites and then nitrosamines - some of which are known to be carcinogens, according to the European Food Information Council.
These can affect the blood's ability to carry oxygen, and in young children it can cause a condition known as "blue baby syndrome." For this reason, the Council recommends avoiding overheating the spinach.
