People are constantly being reminded: Don't eat leftovers, it causes cancer! The statement may sound alarming, but let's examine what actually happens to food overnight. When we say "don't eat leftovers," we're not just talking about reheated leftovers, but also referring to vegetables that were bought but not cooked immediately and were then cooked the next day. So why shouldn't we eat leftovers?
Why shouldn't we eat leftovers?
The main reason why leftovers are considered taboo is because they contain a chemical compound called nitrite. When consumed in large amounts, nitrite can be toxic to the body. Ingesting 0.2-0.5 grams of nitrite can cause poisoning, and approximately 3 grams can be lethal. What's even more concerning is that under certain conditions, nitrite can be converted into nitrosamine, a potent carcinogen that can induce liver cancer, nasopharyngeal cancer, esophageal cancer, stomach cancer, and so on.
How does nitrite form in leftovers?
Where does nitrite come from? To understand nitrite, we need to talk about its precursor – nitrate.
Nitrate can be found in almost all plant-based foods. Vegetables such as spinach, cabbage, kale, leeks, radishes, celery, and beets contain high levels of nitrate. Nitrate itself is not toxic to the body, but once converted into nitrite, its properties change significantly.
After vegetables are harvested, the enzymes that reduce nitrate to nitrite are released. Additionally, the ubiquitous bacteria in the environment can also facilitate this conversion. In other words, if vegetables are not consumed immediately after harvesting and are instead stored for a period of time, nitrite will be produced.
Small amounts of nitrite are not harmful to the body. The standard for determining the nitrite content in food is "not exceeding 4 milligrams per kilogram of vegetables and 3 milligrams per kilogram of meat." If the content exceeds these limits, it indicates contamination.
Beware of excessive nitrite in raw vegetables
The rate of conversion from nitrate to nitrite depends on the type of vegetable and storage conditions. Green leafy vegetables such as spinach, which already have high nitrate content, often exceed the national standard of "4 milligrams per kilogram" during transportation and distribution. There have been reports stating that cabbage purchased from the market already had excessive nitrite levels, meaning that if vegetables are not consumed immediately after harvesting and are instead cooked the next day, nitrite levels will increase.
Bacteria contribute to nitrite formation
When vegetables are cooked and heated, the enzymes that convert nitrate to nitrite lose their activity, effectively blocking this pathway. Additionally, bacteria are mostly killed during the cooking process. Therefore, cooking helps reduce nitrite production.
However, during storage, bacteria present in the air can enter and grow in the cooked vegetables, leading to an increase in nitrite. Whether cooked or raw vegetables are more suitable for bacterial growth and the production of nitrite depends on the "cooking – packaging – refrigeration" conditions.
Canned vegetables, for example, may not undergo significant changes even after being stored for days or weeks. However, when we pack homemade food for storage, it is difficult to completely isolate it from the air, resulting in the inevitable production of nitrite.
How to reduce the risk of nitrite intake
"Leftovers" are not the key factor in nitrite production, and heating does not increase the carcinogen content. If cooked vegetables are packaged while still hot and quickly refrigerated, the risk of "leftovers" can be ignored.
Firstly, reduce the storage time for vegetables, especially leafy greens, and increase the frequency of grocery shopping.
Secondly, wash and package vegetables that need to be stored to reduce the bacteria they carry. Cooked vegetables that haven't been finished can also be properly packaged and stored in the refrigerator.
Thirdly, refrigeration significantly reduces nitrite production. If frequent grocery shopping is difficult to achieve, frozen vegetables are a good alternative, although this may affect the taste and texture of the vegetables.
No need to worry about nitrite in meat
Whether it is meat or fish, they naturally contain low levels of nitrite, so there is usually no concern about nitrite formation during storage. The key issue is whether there is external nitrite added.
For the average person, as long as they don't use condiments that contain nitrate (and don't mistake nitrite for table salt), there is no need to worry about increased nitrite levels in "leftover meat."
