The science behind: what actually happens when we reheat food?
When we reheat food that has already been cooked, its chemical structure changes – mainly proteins, fats, and some minerals. Proteins can start to decompose, releasing substances that are no longer suitable for our bodies. Some bacteria can develop in the food during cooling to the extent that reheating cannot destroy them anymore – and that's the problem.
In 2023, the National Institute of Public Health (NIJZ) recorded more than 430 cases of food poisoning, of which 15% were related to home-prepared leftover food that was incorrectly stored or reheated. This is not negligible.
- Spinach, chard, kale, celery – why can leafy greens become dangerous?
All the plants listed belong to the leafy greens category, which contains a lot of nitrates – natural substances that plants absorb from the soil, especially if fertilized with artificial fertilizers. Nitrates themselves are not dangerous. The problem arises when they are reheated – then they can convert to nitrites, which under certain conditions in the body can turn into nitrosamines, which have been scientifically proven to be carcinogenic (meaning they can cause cancer).
In 2018, the European Food Safety Agency (EFSA) published a report stating that the highest amounts of nitrates are found in spinach and chard, where levels can exceed 3000 mg of nitrate per kilogram.
Therefore, if you cook spinach or chard, eat it immediately or quickly chill it and do not reheat it. Instead, use it for a cold dish, like a spread or salad.
- Chicken and turkey – proteins can be deceptive
Poultry meat is full of proteins that are sensitive to heat. Once cooked, these proteins stabilize. However, if the meat is reheated, the proteins can change and become harder to digest. Additionally, bacteria such as Campylobacter, which causes digestive issues (vomiting, diarrhea, cramps), can be present. This bacterium is responsible for 30% of all meat-related poisoning cases in Europe, according to a 2022 ECDC study.
What's worse is that if the meat is overcooked and then slowly cooled at room temperature, bacteria can multiply rapidly. In a few hours, a single bacterium can become a colony of millions!
Advice? If you have any chicken leftovers, eat them cold, for example in a sandwich or salad. If you do reheat it, make sure to heat it well (above 75 {-15696}C) and never leave it on the counter for more than two hours.
- Eggs – sounds incredible, but reheating them can make them toxic
Scrambled, boiled, or fried eggs are great for breakfast, but be careful – reheating them can cause bacteria to multiply, especially Salmonella. This bacterium is present in Slovenia on average in 1 out of 1500 eggs, but the risk increases if the eggs are not stored in the fridge or left at room temperature.
Heat during reheating can also cause chemical changes in sulfur and iron, present in the egg yolk – under certain conditions, substances that irritate the stomach and intestines can form.
Therefore, the golden rule is: do not reheat eggs. If you have leftovers, store them in the fridge as soon as possible and use them cold the next day, for example as a spread or salad topping.
- Potatoes – extremely common but surprisingly sensitive
Cooked potatoes left at room temperature are an ideal breeding ground for the bacterium Clostridium botulinum, which causes botulism – a rare but very dangerous disease. This bacterium multiplies in anaerobic conditions (meaning without the presence of air), which can happen if potatoes are wrapped in aluminum foil and left standing.
Additionally, when reheating, the starch in potatoes changes and can cause digestive issues. The highest number of potato poisoning cases was recorded at picnics, where boiled potatoes were exposed to heat and sunlight all day.
Solution? Do not reheat potatoes. Use them cold for potato salad or bake them in the oven, where they will have a nice crispy crust.
- Mushrooms – sensitive to heat and time
Mushrooms contain proteins that quickly break down if not stored correctly. If left at room temperature for more than two hours, bacteria start to develop. In France, in 2016, a severe poisoning case was recorded when a family reheated mushroom soup that had been sitting on the kitchen counter for 8 hours – one family member ended up in the hospital.
If you do reheat mushrooms, make sure to cool them properly in the fridge (for a maximum of 24 hours) and heat them to at least 70 {-15696}C.
- Rice – silent and dangerous
Rice is one of the most dangerous "innocent" foods. Why? The Bacillus cereus bacterium, which is present in soil and water, can survive cooking and start multiplying if rice is kept at room temperature. Reheating these bacteria does not destroy them.
In a study published in the Journal of Food Protection, researchers found that at a temperature of around 30 {-15696}C, the number of bacteria can double every 20 minutes! So, if rice sits on the table for 2-3 hours, there could already be enough bacteria in it to cause poisoning.
Symptoms? Vomiting, diarrhea, cramps – and all this can occur within a few hours of consumption.
Therefore, the rule is: always cool rice within an hour of cooking, store it in the fridge, and consume it within one day at most. If in doubt, it's better to discard it.
The most important advice...
Always cool food within an hour of cooking. Store it in a sealed container in the fridge and consume it within 48 hours at most. Reheat only once – each additional reheating increases the risk of harmful substances formation or bacterial growth.
And remember – more than half of food poisoning cases happen at home, not in restaurants. Simple measures like rapid cooling, proper storage, and avoiding reheating certain foods are what protect us.
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