"Leftover food" has nothing to do with "night"

The saying that "leftover vegetables cause cancer" was once widely circulated. So, how harmful are "leftover" vegetables? How should vegetables be stored in the safest way?

I would like to share with you the article "Does "Leftover Food" Really Cause Cancer" written by Yun Wuxin, the scientific consultant of "A Bite of China", to help my friends clear up their doubts.

Anyone who pays attention to food health must have heard the saying that "leftover food causes cancer." There were even reports on the Internet and in newspapers of people being sent to the emergency room after eating leftover food. How many carcinogens are there in vegetables? What happens during the "overnight" process?

Carcinogens, the inevitable existence

All our diets, including water, meat, vegetables, fruits, etc., inevitably contain nitrates and nitrites. According to statistics from Europe, the United States and other countries, in a normal diet, vegetables are the main source of nitrates, and nitrites are often related to the conversion of nitrates.

Current scientific research results generally believe that nitrates themselves are non-toxic. If nitrite enters the human body in large quantities, it may cause "methemoglobinemia", in which the blood loses its ability to carry oxygen, resulting in symptoms of hypoxia, which may be life-threatening in severe cases. A broader concern about nitrite is that it may be converted into nitrosamines in the human body, which are carcinogens.

However, under normal circumstances, the content of nitrate and nitrite in vegetables is still quite far from the dose that is harmful to the human body. Moreover, vegetables have many clear benefits for human health. Therefore, the scientific community and food hygiene agencies still recommend that people eat more vegetables.

So the question becomes: How can we obtain the benefits of vegetables while minimizing possible harms?

"Leftover food" has nothing to do with "night"

If you cook a dish in the evening and don’t finish it, and eat it the next day, of course it is called “leftover food”. However, as someone asked: What if I eat it at midnight? What if I fry it in the morning and eat it at night?

From a food science perspective, whether or not to store it overnight is not the issue. The essence of the problem is what happens to the prepared dishes during the storage process. What we are worried about is the conversion of nitrates in vegetables into nitrites. This conversion process can be achieved by the reductase naturally found in vegetables, but when the vegetables are heated and cooked, these enzymes lose their activity and this pathway is cut off.

Another pathway is the action of bacteria. The vegetables have been cooked and the bacteria in them have been almost killed. However, during the eating process, some bacteria on the chopsticks will enter the leftovers; during the storage process, some bacteria in the air may also enter.

This process has nothing to do with whether it is kept overnight or not, it only has to do with the storage conditions. How much nitrite will be produced in the dish in the end depends first on the vegetables themselves; secondly on the conditions under which the cooked vegetables are stored; and thirdly on how long they are stored.

How to preserve and eat vegetables?

Vegetables have clear health benefits, and we cannot stop eating them just because they “may” contain nitrates and nitrites. The lifestyle of modern society makes it impossible for many people to eat fresh vegetables picked from the ground for every meal. For many people, buying food and eating it for several days is a very common thing. Therefore, preserving vegetables becomes a very important issue in food health.

The production of nitrite in vegetables uses the nitrate in the vegetables as the raw material. The conversion condition is mainly bacterial growth, and “overnight” is just a matter of time. There are many ways to reduce the production of nitrite.

First, reduce the storage time of vegetables, especially green leafy vegetables, and increase the frequency of shopping for vegetables. Secondly, vegetables that need to be preserved should be washed and packaged to reduce the bacteria they carry. If you don’t finish eating the prepared vegetables, you can also seal them and store them in the refrigerator.

"Overnight" is not the key to the production of nitrite, and heating will not increase the content of carcinogens. Of course, many vitamins in vegetables will be destroyed when heated, and vegetables that are heated multiple times will also taste unpalatable. From the perspective of "taste", "overnight food" is indeed inferior; from a nutritional perspective, multiple heating does have a certain impact; from a safety perspective, there is no problem with heating.

Leftover food does not have the "carcinogenic" ability as it is said to be.