15 Leftovers That Can Be Dangerous To Eat

Leftovers make a simple lunch when you're in between meetings or don't have the energy to prep an entirely new meal for yourself. But by the time you get to your leftovers, it's important to note that hazardous pathogens may have gotten to your day-old pizza first. Reheating and eating some leftover foods can lead to foodborne illness and endanger your health and well-being (via the CDC). Other foods are simply dangerous to reheat because they tend to explode in the microwave (via Kitchen Appliance Answer).

When it comes to leftovers and basic food safety, it's first important to note the expiration date on the package. Foods with perishable ingredients, such as eggs or dairy, can quickly spoil even after cooking (via USDA). When examining food, think with your nose and eyes; the USDA notes that spoiled food will typically have an off-color or strange smell. It is equally as important to do your research; there are also some foods you should never refrigerate —including potatoes, according to the FDA — because the cool conditions of the fridge can promote spoilage and staleness. To avoid these possible hazards, here are some leftovers that can be dangerous to eat.

Mushrooms are common ingredients in pasta and soups and can be especially tasty if eaten the next day. But can eating leftover mushrooms actually make you sick?

As a rule of thumb, cooked mushrooms should not be left out for more than two hours after cooking to prevent contact with foodborne illnesses, per Chew the World. Dried, frozen, or canned mushrooms are not as dangerous as eating raw mushrooms, according to Lauren Groverman. This is because drying removes the moisture bacteria need to thrive, while freezing makes an inhospitable environment for the bacteria to grow.

If you're looking for the maximum storage life on your sautéed mushrooms, we recommend preparing, freezing, and thawing air-tight portions as needed. Cooked mushrooms need to be consumed within a day and should be reheated to a temperature of at least 165 F to kill off any potential lingering pathogens (via Valuable Kitchen). If you run past the 24-hour deadline, prioritize your safety and toss the mushrooms.

Fresh or raw seafood can be dangerous if left at room temperature for too long, so reheating leftover grilled salmon or tilapia may not always be the best call. Reheating might not kill off any remaining pathogens that can cause foodborne illness (via the FDA). Plus, you are likely unaware of how the fish was actually stored prior to purchase and if it came into contact with pathogens invisible to the human eye.

The FDA also notes that fish and seafood should never be left unrefrigerated for longer than two hours, and the fridge needs to be 40 F or colder. Always wash your hands before and after handling raw fish to prevent the cross-contamination of pathogens to other kitchen surfaces and the human body. Throw fish away if it has been at room temperature for over two hours or at temperatures greater than 90 F for over one hour. If you plan to have leftovers, we recommend storing the fish in the fridge and reheating smaller portions.

Does thinking about buffet food make your stomach queasy? Buffets can be dangerous because of the contact with other patrons (and their sneezes, coughs, and saliva), as well as the risk of leaving food a room temperature for several hours (via CDC). Keep yourself safe by skipping the buffet line altogether, or make sure to use clean tongs and plates and wash your hands before and after taking food from the buffet, per Washington State University Extension.

Food poisoning is always a risk if you reheat food from a buffet. Potatoes, spinach, mushrooms, and rice are particularly dangerous offenders lurking in those silver trays. Although you might think that reheating buffet leftovers in the microwave is enough to kill off any lurking bacteria, a study by Michigan State University showed that it might not get hot enough to do so (via Food Safety News). If you're considering taking home a doggy bag for later, we recommend leaving your leftovers on your plate.

Reheated potatoes can impact your health, but not for the same reason that french fries may make you sick. Cooked potatoes that have sat for too long may be a carrier for the Clostridium botulinum bacteria — better known as botulism (via BC Centre for Disease Control). Botulism spreads especially well in aluminum foil or plastic wrap, which doesn't bode well for the gratin or scalloped potatoes you took home from the family get-together last night.

According to the CDC, botulism is a particularly dangerous foodborne illness but is considered relatively rare in the United States. Botulism thrives on potatoes because the bacteria prefer low-oxygen, low-acid, and low-sugar surfaces. Reheating potatoes does not necessarily kill off the bacteria, but roasting or cooking the potatoes thoroughly before refrigerating them can reduce the risk of other foodborne illnesses. Cleaning the potatoes well with a vegetable brush and removing the eyes before preparing them is also important because the bacteria do not have anywhere else to hide, per Food Safety News. 

If you have ever frozen a batch of pasta sauce, you know the secret is just popping a container in the microwave when you need some sauce for pasta or meatball sandwiches. But microwaving your red sauce can actually cause a dangerous risk for home cooks that isn't biological. 

The volatility of red sauce is due to its viscous consistency, according to AZ Central. When the sauce heats up in the microwave, its heavy components — the crushed tomatoes, spices, onion, and garlic — heat up slower than the surrounding water. The water molecules start moving around, producing steam, and searching for a way to release from the container. This movement can cause the sauce to splatter and burn.

One way to prevent explosions in your microwave is to opt for the stovetop instead. The constant stirring motion of the sauce on the stove allows steam to escape and prevents the build-up of pressure. Not only will you avoid a kitchen mess, but your sauce will also be evenly heated and ready for a quick meal.

Reheating shell-on, hard-boiled eggs will cause a dangerous explosion in your microwave. Although science hasn't definitively given humanity a reason for why whole hard-boiled eggs explode in the microwave, the assumption is that it's due to the build-up of pressure in the yolk of the egg. The yolks contain pockets of water that become unstable with heat application. When the water heats, it expands, breaking through the whites and the shell of your egg (via JASA). Not only can the rogue eggshell damage your microwave, but the heat from the egg may also cause scalding burns.

If you need to reheat hard-boiled eggs at home, we recommend placing the eggs in a tempered (heat-safe) container and pouring hot water over the eggs. Let the eggs bathe in the warm water for 10 to 20 minutes until the eggs are warm. Crack your eggs open and enjoy a scald-free eating experience.

If you took out a larger portion of leftovers than you can eat, you can probably relate to the internal dilemma of whether or not you should place the remainder back in your fridge. The answer, from a food safety perspective, is a definite no. If you reheat leftovers more than once, you can increase your exposure to spoiled food and pathogens. Repeatedly changing temperatures in food creates a hospitable environment for bacteria, mold, and fungus (via BBC Good Food).

To prevent contamination, the USDA recommends heating the food to 165 F (check it with a thermometer to be as exact as possible) for at least two minutes. You should also ensure your leftovers are equally heated throughout; it will result in a better (and safer) leftover experience. If you plan to eat some leftovers (but not all), take part of the food out of the container and heat it up on a separate plate. This will reduce the amount of time the food is at room temperature collecting bacteria.