Rice: The Silent Killer in Your Kitchen
However, reheated rice can cause food poisoning, no matter how carefully you reheat your leftovers. In fact, this leftover is so iffy that the nasty symptoms that can follow eating it have gained the name “reheated rice syndrome.” What makes rice particularly dangerous is that the bacteria is heat-resistant, so no amount of microwaving or time spent bubbling in a sauce in the oven will kill it, once it forms. B. cereus is one of the most common causes of food poisoning in the United States.
The bacteria multiplies when rice sits in the danger zone between 40-140°F, producing toxins that can cause severe symptoms including belly pain, cramping, and vomiting. Once these bacteria multiply, reheating won’t destroy their toxins. Your innocent bowl of leftover rice might be harboring an invisible villain called Bacillus cereus.
Uncooked rice and pasta can contain spores of the bacterium, Bacillus cereus, which is common and widespread in our environments. Notably, B. cereus can survive even after the food has been properly cooked. The FDA advises that food should only be reheated once before consumption. Repeated reheating increases the risk of bacterial growth, which can lead to foodborne illnesses if proper precautions are not taken.
Spinach: The Nitrate Nightmare
Spinach is chock full of nitrates, which are naturally occurring compounds that may help reduce your risk of cancer and other chronic health conditions, according to WebMD – all good stuff. But consuming too many nitrates can lead to “serious health effects,” per the Centers for Disease Control and Prevention (CDC), especially for babies. And a study from the University of Medical Sciences in Iran showed that cooking significantly increased nitrate levels in spinach, while research from Foods demonstrated that stir-frying increased nitrate content by 31%. Plus, if you cook your spinach at high heat, nitrates can convert into nitrosamines, which are carcinogenic.
When reheated, especially in dishes that are cooled slowly or kept warm for a while, these nitrates can convert into nitrites, and then potentially into nitrosamines, which can be carcinogenic. Best practice: Consume dishes with cooked spinach or leafy greens fresh.
When spinach is cooked and then left at room temperature for an extended period, these nitrates can convert into nitrites, and potentially nitrosamines. Nitrites, in large amounts, can be harmful, especially to infants and pregnant women. The formation of nitrosamines, some of which are carcinogenic, is also a worry. Nitrates on their own are harmless and even beneficial, but when you reheat spinach, especially at high temperatures, these nitrates can break down into nitrites and, in some cases, even nitrosamines—compounds that are potentially harmful when consumed in large amounts. So, while it’s safe when cooked once, reheating can lead to these chemical changes that make your leftover greens less healthy and possibly risky.
Chicken: The Protein Bomb
Poultry proves particularly problematic when improperly reheated. Salmonella and Campylobacter bacteria thrive in the protein-rich environment of cooked chicken left in the temperature danger zone (40°F-140°F). Microwaving creates hot and cold spots where bacteria can survive. Always use a food thermometer to ensure chicken reaches 165°F throughout before serving.
Chicken is another favourable breeding ground for bacteria like E-coli and Salmonella. Chicken contains a high amount of proteins. Never reheat this food as the protein composition will change, turning the meat toxic, which will affect the digestive system, causing diarrhoea and vomiting.
Chicken, particularly when reheated, poses a risk of salmonella contamination. The bacteria can survive the initial cooking process and multiply when the chicken is stored improperly. When reheating chicken, it’s crucial to ensure it reaches an internal temperature of 165°F (74°C) to eliminate any remaining bacteria. Multiple reheatings also toughen proteins, making them harder to digest.
Mushrooms: The Protein Breakdown
The delicate structure of mushrooms makes them particularly vulnerable to bacterial contamination. Even properly stored mushrooms should only be reheated once to minimize risk of foodborne illness and preserve their nutritional value. Reheating mushrooms can lead to digestive problems if they are not handled correctly. Mushrooms contain proteins that can break down when reheated multiple times, leading to adverse reactions in some individuals.
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, but reheating them causes the proteins to break down and release toxins. This can lead to digestive issues, like stomach upset, and in severe cases, heart complications. Mushrooms contain proteins that can easily degrade after cooking. Reheating them, especially if not stored properly, can lead to digestive upset like stomachaches, bloating, or even more severe reactions.
Eggs: The Staphylococcus Trap
Morning scramble leftovers might scramble your stomach later! Eggs provide the perfect medium for Staphylococcus aureus bacteria, which produce heat-resistant toxins even after thorough cooking. The proteins in eggs also undergo chemical changes during storage and reheating. These changes can create compounds that trigger digestive distress in sensitive individuals.
Eggs can be risky to reheat due to the potential for bacteria growth, especially salmonella. Reheating eggs, particularly scrambled eggs, omelets, or fried eggs that have been sitting out, can lead to the proliferation of Salmonella bacteria if not handled and reheated properly. The protein structure can also become rubbery and unappetizing.
Reheating eggs, particularly hard-boiled eggs, can lead to the release of sulfurous compounds, resulting in an unpleasant odor. Additionally, reheating eggs may cause changes in texture and flavor. Eggs provide a great source of protein, but reheating them degrades the proteins, leading to digestive discomfort such as bloating and indigestion.
Potatoes: The Botulism Battleground
Spuds might seem harmless, but improperly stored baked potatoes create the perfect breeding ground for Clostridium botulinum. The aluminum foil wrapping many people use creates an oxygen-free environment where this dangerous bacteria thrives. Even a few bites of affected potato can cause botulism—a potentially fatal illness. The toxin attacks your nervous system, causing difficulty breathing, muscle paralysis, and in severe cases, the end of your life.
Potatoes, when reheated, can change their starch structure, potentially leading to the formation of toxic compounds called solanine. Solanine is bitter and can cause nausea, vomiting, and other gastrointestinal issues. Potatoes can be deceptively harmful if not reheated properly. Like rice, when potatoes are cooked, left to cool, and stored incorrectly, they can develop the bacteria Clostridium botulinum.
Potatoes should never be left at room temperature. When stored improperly, they can promote the growth of Clostridium botulinum, a bacterium that causes botulism, which can lead to paralysis or even death. This bacteria can thrive in low-oxygen environments, such as wrapped potatoes.
Seafood: The Histamine Hazard
Yesterday’s fish fillet might be today’s trip to the emergency room. Seafood contains proteins that break down rapidly after cooking, creating a breeding ground for bacteria like Vibrio and histamine-producing microbes. Once these bacteria multiply, reheating won’t destroy their toxins. Scombroid poisoning from improperly stored fish causes facial flushing, headache, and heart palpitations within minutes of consumption.
Fish and seafood carry two types of food poisoning: ciguatera poisoning, which can occur when you eat tropical reef fish that have built up a high degree of certain toxins, and scombroid poisoning, which can occur when you eat fish that contains a high level of histamine toxicity. What’s particularly terrifying is that the fish can look, smell, and taste completely fine.
Seafood, particularly fish, contains delicate proteins and fats that break down quickly. It’s highly susceptible to rapid bacterial growth and histamine production (which can cause scombrotoxin poisoning) if not kept consistently cold. When reheated, the proteins in fish and shellfish degrade, leading to a loss of flavour and texture. Additionally, histamines can form during reheating, which can cause food poisoning symptoms such as nausea and vomiting.
Pasta: The Starchy Menace
Mama mia! Your leftover spaghetti could be serving up a side of food poisoning. Like rice, pasta provides the perfect starchy environment for Bacillus cereus to multiply when left at room temperature. Similarly, pasta and other cooked grains can also be problematic if left out too long or reheated multiple times. The cooling and reheating process can promote bacterial growth, leading to potential foodborne illnesses.
Pasta and noodles are often reheated, but this practice can lead to bacterial growth if not stored correctly. Reheating can also alter the texture and reduce their nutritional value. Best practices include ensuring they are properly cooled and reheated only once to maintain safety.
Leafy Greens: The Celery Catastrophe
Celery has a high nitrate content that can become toxic if you reheat it. The nitrates transform into nitrites after it is reheated and can cause food poisoning. Leafy greens like spinach and kale can pose dangers when reheated, particularly due to their high nitrate content. When greens are reheated, especially multiple times, nitrates can convert into harmful substances called nitrosamines, which may be linked to certain types of cancer.
In vegetables like spinach and celery, nitrates are naturally occurring. When these vegetables are cooked and then allowed to sit or are repeatedly reheated, bacteria can convert nitrates into nitrites. Nitrites can be harmful, especially for infants, and can potentially form carcinogenic nitrosamines when exposed to high heat.
Other leafy green vegetables, such as lettuce, kale, beetroot, celery, and rocket (arugula), also naturally contain nitrates. The same principles apply to these vegetables: proper storage after cooking is crucial to prevent the conversion of nitrates to nitrites.
Beets: The Root Trouble
Beetroot is high in nitrates, and when it is re-exposed to heat, it can turn toxic. Reheating any dish which has beetroot will release carcinogenic properties, which may cause infertility and cancer. Like spinach, beets are one of the handful of foods that are high in nitrates. Because of this, you should limit how much heat you expose your beets to, as that heat can lead to the creation of nitrosamines, which increase your risk of cancer.
Beets are another vegetable that should not be reheated. Like spinach and celery, they are high in nitrates, which convert into harmful nitrites when reheated. Reheating leftover cooked beets may just be a risky endeavor that you want to skip.
Carrots: The Orange Offender
Carrots, like beets, contain nitrates that convert into nitrites when reheated. While fresh carrots are nutritious, reheating them can result in a loss of nutrients and potential toxicity. Other nitrate-rich vegetables, including turnips, carrots, and beetroot, pose similar risks.
Like spinach, these vegetables can be safely reheated if they are cooled quickly and refrigerated promptly after cooking. Yet many families don’t follow proper cooling procedures, making reheating dangerous. However, due to the abuse of chemical fertilizers and unreasonable planting methods, the nitrate content of intensively planted vegetables tends to reach excessively high levels. Approximately 80% of total nitrate ingestion has been reported to come from vegetables, and leafy vegetables easily accumulate nitrate.
Oils: The Toxic Transformation
Certain oils, especially those with a low smoke point like avocado and walnut oils, can turn toxic when reheated. When oils are heated beyond their smoke point (around 375°F), they produce harmful toxins that can increase LDL (bad cholesterol) and the risk of heart disease. A study found that frequent use of deep-fried oil, particularly reused deep-fried oil, has been linked with neurodegeneration, higher levels of oxidative stress, liver inflammation, high cholesterol, and colon damage in rats, as noted in the Journal of Biological Chemistry. This is because reheated oils become more and more unstable with each use and, as they become more unstable, they release more toxins that result in negative health effects.
Rather than risk these severe health consequences, families should consider discarding cooking oil after one use. Rather than risk this bevy of bad effects, consider just tossing your cooking oil and opting for fresh oil, each time you want to fry something.
Gravy and Sauces: The Dairy Disaster
Gravy and sauces, especially those made with dairy or meat, can spoil quickly. When left at room temperature or reheated, they can become breeding grounds for bacteria and harmful compounds. When reheating leftovers, be sure they reach 165° F as measured with a food thermometer. Reheat sauces, soups and gravies by bringing them to a rolling boil.
The challenge is that most families don’t properly bring gravies to a rolling boil, creating dangerous bacterial pockets. Microwaves cook food from the outside in. This leaves cold spots where bacteria can grow. One study found that simply zapping a dish for 5 minutes didn’t kill salmonella.
Buffet Foods: The Multi-Exposure Risk
Without the right precautions, that’s a recipe for disaster, with food entering the “danger zone” of bacteria-friendly temperatures and conditions, allowing bacteria to produce toxins and poisons that can’t be killed via heat (like the toxins produced by staphylococcus). If you need those buffet leftovers, it’s important to be sure the food was cooked safely in the first place, and that the buffet food was then kept at 140 F or higher the entire time it was served, and that it was only served for two hours or less.
The bacteria that cause food poisoning grow quickly at room temperature. To make sure it’s safe, put food in the fridge or freezer in the 2 hours after it’s cooked or taken off a heat source, such as a warming tray. If it’s hot outside (above 90 F), do this within 1 hour.
One in 3 people admit to eating pizza left out overnight. If those slices have been sitting out for hours, throw them away.
