Storing and reheating leftovers has become a common kitchen practice, especially with the rising cost of food and growing concerns about waste. Most of us barely think twice before popping yesterday’s meal into the microwave or stovetop for a quick reheat. Yet, the CDC reports that an estimated 1 in 6 Americans get sick, 128,000 are hospitalized and 3,000 die every year from foodborne illnesses. Though some foods are perfectly safe to reheat when handled properly, certain seemingly innocent leftovers can transform into dangerous toxin factories that no amount of heat can neutralize.
The problem isn’t just about bacteria growing while food sits out too long. Some foods undergo chemical changes that create heat-resistant toxins, making even thorough reheating ineffective at eliminating the danger. Ready to discover which innocent-looking leftovers could be lurking in your fridge like ticking time bombs?
Rice: The Bacillus Cereus Time Bomb
Uncooked rice can contain spores of a bacterium called Bacillus cereus. What makes rice particularly treacherous is that the spores of Bacillus cereus can survive being cooked. While the parent bacterium is killed when rice is cooked, its spores can survive the temperature of boiling water. The relevant bacteria in this case is Bacillus cereus (B. cereus), a type of fast growing, “spore-forming” bacteria that spreads on starches. When cooked rice is left at room temperature, these microscopic survivors begin multiplying rapidly.
If rice is not refrigerated within two to three hours of cooking, the spores can grow into bacteria which in turn release the rice toxins which give rise to food poisoning symptoms such as diarrhoea, abdominal pain and vomiting. The longer contaminated cooked rice sits out, the more dangerous it becomes. If the bacteria has multiplied on a serving of rice, its toxins will be equally as dangerous after reheating it in the microwave. These toxins are heat stable and will survive getting zapped in the microwave or cooked in another dish. The symptoms hit fast too, with symptoms from Bacillus cereus forming within 6 to 12 hours, which is faster than most foodborne illnesses.
Cooked Potatoes: A Botulism Breeding Ground
Potatoes might seem harmless, but they can harbor one of the most deadly bacteria known to humans. Cooked potatoes, especially if wrapped in foil and left to cool slowly at room temperature, can become a breeding ground for Clostridium botulinum, the bacteria that causes botulism. According to Food Safety News, the source of the botulism was “potatoes stored at room temperature for two weeks.” As the Food Poisoning Bulletin explains, the greatest risk for potato-based-botulism comes from baked potatoes stored in the fridge in aluminum foil. The foil wrapping creates that perfect oxygen-free environment where this deadly bacteria loves to multiply.
When stored improperly, they can promote the growth of Clostridium botulinum, a bacterium that causes botulism, which can lead to paralysis or even death. Potatoes have even been linked to cases of botulism, which the Centers for Disease Control and Prevention describes as “a rare but serious illness caused by a toxin that attacks the body’s nerves.” Per the USDA, a botulism outbreak that occurred in 1997, affecting 17 people, was traced back to potatoes. The bacteria thrives in low-oxygen environments, making wrapped or improperly stored potatoes particularly dangerous.
Chicken: A Salmonella Playground
Chicken tends to dry out, toughen up, and lose its juicy flavour when it’s reheated, but that’s the least of your worries. Keeping cooked chicken at room temperature uncovered promotes the growth of bacteria, which reproduces most effectively between 5ºC to 60ºC. This could potentially lead to food going foul and causing food poisoning. Chicken is another favourable breeding ground for bacteria like E-coli and Salmonella. Chicken contains a high amount of proteins, which can break down during improper storage and reheating.
The protein structure in chicken makes it particularly susceptible to bacterial contamination. Before eating chicken, you have to cook it thoroughly to eliminate all present bacteria. Since microwaves don’t fully or evenly cook all parts of the meat, you’re more likely to be left with surviving bacteria, such as salmonella. Studies suggest that microwaves don’t fully or evenly cook all parts of the meat, potentially leaving surviving bacteria such as salmonella. Even when properly cooked initially, reheated chicken creates uneven temperature distribution, leaving cold spots where dangerous bacteria can survive.
Spinach and Leafy Greens: The Nitrate Nightmare
Spinach and other leafy greens contain naturally occurring compounds called nitrates, which are normally harmless and even beneficial. However, the reheating process can transform these innocent compounds into something far more sinister. According to the EUFIC, nitrates in these vegetables could be converted into nitrites and then into nitrosamines, which are known to have carcinogenic properties, when reheated. When you reheat cooked spinach, the nitrates turn to nitrites which act like carcinogens, adversely affecting your body.
Spinach does contain a high amount of nitrates, and this is converted to nitrites by enzymes within the plant cells, and by bacterial action on the vegetables before cooking, after cooking, and after ingestion. It is probably not the action of reheating the spinach that is the issue, but that the cooked spinach has been left at room temperature before being refrigerated, since this period of time allows for an accumulation of nitrites. A study published in the scholarly journal Foods recommends not storing boiled spinach for more than 12 hours at room temperature, if you want to avoid a “direct nitrate safety risk” and “adverse health effects.” If you want to save your celery, kale or spinach to eat later as leftovers, plan to reheat them in a conventional oven rather than a microwave. When blasted in the microwave, naturally occurring nitrates (which are very good for you on their own) may convert to nitrosamines, which can be carcinogenic, studies show.
Mushrooms: Protein Breakdown Disaster
Mushrooms are delicate organisms with complex protein structures that don’t respond well to reheating. Mushrooms, particularly after being cooked, can change their chemical structure upon reheating. This can lead to digestive discomfort and potential toxin build-up. The delicate texture and nutritional content of mushrooms are also negatively affected by reheating. Mushroom leftovers should not be reheated, as the protein and the nutrient levels in the mushrooms, can turn toxic and cause digestive problems. In some cases, heating mushrooms more than once can affect the heart as well.
Mushrooms are rich in proteins and minerals but are highly perishable. Reheating mushrooms can lead to the breakdown of their proteins, producing compounds that may upset your stomach or cause food poisoning. Improper storage increases the risk of bacterial growth. Reheating these ingredients will actually change the composition of the protein. This has a number of effects such as to change the flavor of the ingredient, to remove the health benefits and to cause severe digestive problems. The high moisture content and delicate cellular structure of mushrooms makes them particularly vulnerable to bacterial contamination and protein degradation.
Hard-Boiled Eggs: Sulfur Compound Formation
Eggs might seem like a safe bet for leftovers, but reheating them can create unexpected dangers. Eggs are highly nutritious but can become toxic when reheated. Reheating scrambled or boiled eggs can cause them to release sulfur compounds, which may lead to stomach upset or worse. Eggs are also prone to contamination by bacteria like Salmonella if left out too long. Bacteria can thrive at temperatures between 4 °C and 60°C and can spoil the food or make you ill. High-protein foods like eggs contain a lot of nitrogen, and this may get oxidised due to reheating.
It boils down to how cooked eggs and egg-containing dishes are stored. According to the US Food and Drug Administration, cooked eggs and egg dishes should not be left out of the refrigerator for more than 2 hours (1 hour in hot weather). The protein structure in eggs is particularly sensitive to temperature changes, and reheating can cause the formation of toxic sulfur compounds that weren’t present in the original cooked egg. These compounds can cause significant digestive distress and may be especially harmful to vulnerable populations like children and elderly individuals.
