Reheating leftovers saves time and effort, offering a second chance to enjoy a favorite meal. However, some foods pose health risks when reheated due to two main reasons: the potential for food poisoning and the chemical changes that occur in certain foods.
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The foods most susceptible to foodborne illnesses are those high in protein and moisture—conditions that favor the growth of certain microbes.
According to Healthline, safe reheating can effectively kill harmful microbes in some foods, especially if the food is heated thoroughly to the proper temperature (70°C or 158°F for most foods for at least two minutes). Steam rising from the food is usually a good sign that it’s been heated properly.
It’s also crucial to store leftovers properly before reheating, a point that will be elaborated on later.
High-Risk Foods
Reheating seafood, boiled or scrambled eggs, and rice carries a risk of activating harmful bacteria, which can cause health issues.
Chicken is also on the list and should be reheated thoroughly, ensuring the heat reaches all parts to avoid activating harmful bacteria such as Salmonella.
Shellfish and fish are particularly sensitive, as they may contain pathogens or toxins like histamine, which can cause illness. Experts recommend consuming seafood-based leftovers within 3 days of refrigeration, and cooling them within an hour of cooking.
Fried Rice Syndrome
This term became associated with Chinese restaurants, where fried rice would often be left at room temperature for too long. According to the BBC, this increases the risk of a bacterium known as Bacillus cereus.
While heating can kill the bacteria, the toxins they produce during this process can still lead to foodborne illness.
To avoid this, rice should be stored in a sealed container, cooled within one hour after cooking, and consumed within three days—after thorough reheating.
Pasta is also considered a potential risk when reheated, as are sauces containing cream or milk. These should be cooled quickly and reheated thoroughly if consumed again. They may also be eaten cold if stored properly.
The Risk of Reheating Spinach
Nutritionist Harini Bala, in a viral TikTok video, warns against reheating three specific foods. Unsurprisingly, rice is one of them—but surprisingly, so is spinach.
She explains: "These leafy greens contain oxalic acid, which is harmless when spinach is eaten raw."
However, once reheated, the oxalic acid "crystallizes" and becomes "toxic" in high doses, potentially increasing the risk of kidney stones.
Bala adds: "Spinach also contains nitrates, which turn into nitrites when reheated. Nitrites can interact with chemicals in the gut to form compounds believed to raise the risk of intestinal tumors."
Other studies have found spinach leaves to be rich in Listeria bacteria, a common cause of foodborne illnesses.
The third food she warns against reheating is tea leaves.
What About Molokhia Leaves?
According to Daraj.com, based on available research and animal studies, molokhia appears to have anti-toxic properties—even when dried. Rehydrating it with water after drying does not seem to reactivate harmful bacteria.
However, it's believed that letting it sit after cooking may increase its toxicity risks, though there are no sufficient reports of food poisoning linked to molokhia.
Some sources claim reheating molokhia is harmful because it’s a leafy green containing nitrates, although this has yet to be thoroughly studied.
Reheat with Caution
Mushrooms should generally not be reheated. If necessary, they must reach 70°C (158°F) during reheating. The same applies to potatoes, which should be heated evenly and thoroughly.
Mushrooms contain proteins that can be degraded by enzymes and bacteria if not stored properly, leading to digestive discomfort if reheated inadequately.
Buffet-style foods should also not be reheated, as they often sit out at room temperature for extended periods.
General Guidelines
According to Healthline, safe handling of leftovers is crucial for health and taste:
Cool leftovers quickly (within 2 hours, or 1 hour in hot weather), then refrigerate and eat within 3–4 days.
Alternatively, freeze leftovers for up to 3 months. After that, they’re still safe to eat, but texture and flavor may decline.
Thaw frozen leftovers completely before reheating by transferring them to the fridge or using the microwave defrost setting.
Partially thawed leftovers can be reheated on the stove, in the oven, or in the microwave.
Reheat leftovers until they are steaming hot—all parts should reach 70°C (158°F) for 2 minutes. Stir the food during reheating to ensure even heating, especially in the microwave.
Do not reheat leftovers more than once.
Do not refreeze thawed leftovers.
Serve reheated leftovers immediately.
