Bacteria are everywhere. Most are harmless but some are “pathogenic,” meaning they can cause disease. They can carry out their mischief in two ways. One possibility is that they can survive passage through the stomach, start multiplying in the intestine, and damage intestinal cells by invading them. This causes the body’s immune system to spring into action causing inflammation with its symptoms of pain and fever. Diarrhea is common as the body tries to eliminate the pathogens. Salmonella, Campylobacter and Listeriaare examples of bacteria that can contaminate food and then multiply in the body. Symptoms can take days to appear and usually resolve within a week.
Alternatively, there are bacteria such as Staphylococcus aureus, Bacillus cereus and Botulinum clostridium that can contaminate food, multiply inside the food and produce toxins that are then ingested. In this case the onset of symptoms, usually vomiting and nausea, is quick, within hours. Fever is rare and symptoms usually resolve within 24 hours. The toxin produced by Botulinum clostridium is particularly dangerous because it can attack nerves causing paralysis and even death. Some bacteria, Escherichia coli O157:H7 and Clostridium perfringens for example, can multiply in the intestine and produce toxins. E. coli O157:H7 was responsible for the infamous food poison outbreak in 1992-1993 that killed 4 children and infected 732 people who had eaten contaminated hamburgers at Jack in the Box restaurants.
It is impossible to eliminate all bacteria from food. Cooking can destroy most, but the survivors begin to reproduce as the food cools. This can be minimized if the food is cooled in such a way that the bacteria spend less than two hours in the “danger zone,” which is between 4oC and 60oC. The longer food spends in this zone, the greater the chance that bacteria multiply and produce toxins.
A large pot of food cools slowly, and even if it is put in the fridge within two hours, the inside will still be warm enough to allow bacteria to multiply. So, divide that large batch of stew or soup into small containers and refrigerate within two hours. There will still be some remnants of the original bacteria, plus some picked up from the air, utensils, surfaces, raw foods or hands. Remember that refrigeration does not kill bacteria, only slows their growth. This means that leftovers should be eaten within about 3-4 days even if they look and smell fine. Food spoilage and food safety are not the same. Spoilage refers to changes in smell, texture and taste, while safety refers to the absence of pathogenic bacteria or toxins. Spoiled food is not necessarily unsafe and food that does not look to be spoiled may be dangerous to consume. Food safety isn’t about how the food looks or smells, it is about how it was cooled and the length of time spent in the danger zone.
Now for the questions about reheating leftovers. As mentioned, food stored in the fridge still contains bacteria which is why leftovers should be reheated to at least 75oC. However, if the initial cooling was not properly done, and toxin producing bacteria survived, their toxin may not be destroyed by heat. Rice is a special case. It contains spores of Bacillus cereus that survive cooking and can produce live bacteria if the food stays too long in the danger zone. These bacteria produce toxins that are not destroyed by heat. This means that if cooked rice has not been properly cooled, reheating can cause illness. But the problem is not reheating the cooked rice, the problem stems from letting the rice stay too long in the danger zone after the original cooking and before it was refrigerated.
The scary videos about reheating rice do not mention that this can be safely done if the proper cooling procedure has been followed. So, they sort of get the rice risk half right. However, when it comes to other risks of reheating that they talk about, such as formation of malondialdehyde (MDA) and nitrites on reheating, they descend into unjustified fearmongering.
Let’s deal first with malondialdehyde, a breakdown product of polyunsaturated fats that can form in foods or in the body after ingestion of these fats. It is of concern because malondialdehyde reacts with DNA and can cause mutations that lead to cancer as has been shown in laboratory studies. MDA cannot be totally avoided because it forms naturally in the body from fat and is also found in fried and ultra-processed foods. For some bizarre reason, the “do not reheat videos” choose mushrooms as being dangerous when reheated because of the formation of malondialdehyde. Mushrooms actually contain very little polyunsaturated fats, so they are an insignificant source of MDA. If you want to reduce exposure to MDA, then consume antioxidant-rich foods such as fruits and vegetables that can neutralize the free radicals that trigger the formation of this compound. Mushrooms actually contain antioxidants, so it is hard to understand why they are vilified when it comes to reheating leftovers. Handle mushrooms the same way as any leftover.
Now for the nitrites. Vegetables high in nitrates such as spinach, beetroot and celery are also fingered as being dangerous when reheated. The claim is that when heated, nitrates turn into nitrites that then react with amines in the food or in the body to form carcinogenic nitrosamines. However, the conversion of nitrate to nitrite requires enzymes that are not present in vegetables! Bacteria produce such enzymes, so if spinach was not cooled properly after cooking and became contaminated, then indeed there can be nitrite production. Still, compared with the nitrates being converted to nitrites by enzymes in saliva, this is trivial. Furthermore, while nitrites in the body can form nitrosamines, they also can be converted into nitric oxide that dilates blood vessels and increases blood flow, which is why nitrate rich foods such as beets or beet juice are consumed by athletes looking to increase oxygen and nutrient delivery to muscles. There is no issue with reheating spinach.
Where does all this leave us about heating leftovers? Is it a risky business? Could be, if cooked food has been left out more than two hours before refrigerating, or if it has been in a large pot so that even after placing in the fridge the temperature in the center of the pot is in the “danger zone.” If the food has been properly cooled, there is no problem with reheating. It is best to reheat only as much as will be consumed because repeated heating and cooling increases the risk that it will not be done properly. Malondialdehyde can be a problem if foods that contain polyunsaturated fats are heated to a high temperature as in frying, but reheating only increases the presence of this chemical by a trivial amount. Nitrites are not increased by reheating foods that contain nitrates.
Bottom line: You can ignore videos that tell you to never reheat certain foods because they can cause cancer. Reheating leftover food is not a problem as long as it is thoroughly heated and the food was originally cooked properly and refrigerated within 2 hours in small containers. There is one more rule that applies to food safety. When in doubt, throw it out!
