Most people don’t think anything of reheating leftover meals the day after they were cooked. Whether it's safe to reheat food is simply not something the vast majority of diners bother to ask. After all, not letting a meal go to waste seems like a great way to cut down on your trash and save a bit of money. While you may not have considered the dangers of sticking that cold takeout in the microwave, there are some truly gross facts about reheating food that might make you think twice in the future.
When it comes to food safety and leftovers, reheating can be surprisingly risky. The fact that any remaining food is often left sitting at room temperature rather than being stored away appropriately means that harmful bacteria can easily grow. Meanwhile, microwaves might not completely destroy those bacteria before you dig in.
Is it safe to reheat food? Sometimes. But improper food handling can leave you with more than an upset stomach.
The most common danger posed by reheated leftovers is food poisoning. This condition can be caused by bacteria such as salmonella and E. coli, or viruses such as the norovirus. The symptoms are wide-ranging, but can include vomiting, diarrhea, and stomach pains. However, in severe cases, victims can also suffer from exhaustion, aching muscles, fever, and even death.
While there is little data to determine exactly how many people get sick from eating reheated food every year, the Centers for Disease Control and Prevention estimate that some 48 million people get sick from foodborne diseases every year in the U.S. Of those, around 128,000 are hospitalized and 3,000 die. Although many of the diseases come from mishandling of food or inappropriate storage methods, a significant proportion arise from the improper reheating of leftover food.
Reheated rice is a big culprit for food poisoning, though not due to the reheating itself. Uncooked rice often contains spores of dangerous bacteria that can survive the cooking process. If the cooked rice is then left for an extended period of time at room temperature, this bacteria can multiply. Reheating the rice won’t get rid of the spores and they can be powerful enough to cause severe vomiting and diarrhea.
Food poisoning from rice is so common that illness from this particular bacteria used to be called "fried rice syndrome." This nickname arose from the fact that people used to get sick after eating rice that had sat out for extended periods of time at Chinese food buffets. Fortunately, restaurants' health standards are much higher now.
Several green vegetables, such as celery and spinach, contain nitrates. When cooked, bacteria can lead to these nitrates being converted into nitrites. While this will happen naturally in the cooking process, reheating the vegetables can cause more nitrites than usual to form. They can lead to methemoglobinemia, where the body cannot properly transfer oxygen around the body in blood.
Potatoes are especially risky to reheat, because they can contain the bacterium Clostridium botulinum, which can lead to botulism. The problem is not in the reheating itself, but the way that people often store potatoes after cooking. If stored at room temperatures in tin foil, where little or no oxygen can get to the potato, then the bacteria can grow. The food would then have to be heated to a high temperature for several minutes to make it safe to eat once again.