Anyone who is concerned about food health must have heard the saying that “overnight vegetables cause cancer”. On the Internet and in the press, there are even reports of people being sent to the emergency room after eating overnight vegetables. Many experts have also explained that overnight vegetables produce nitrite, which is a carcinogenic substance; some even pointed out that “every time vegetables are heated, carcinogens increase by tens of times”. So, how many carcinogens are in vegetables? And where do they come from? What happens during the “overnight” process? How should vegetables be preserved and eaten? Nitrogen is a widespread element in nature, and plants must be fertilized with nitrogen to grow. Plants absorb nitrogen from the environment and eventually synthesize amino acids through complex biochemical reactions. In this process, nitrate is an unavoidable step. There are reductases in plants that reduce some of the nitrates to nitrites. Therefore, all plants contain nitrates and nitrites. The results of scientific research nowadays generally consider nitrate itself to be non-toxic. However, if nitrite enters the body in large amounts, it may lead to “methemoglobinemia”, where the blood loses its ability to carry oxygen, resulting in hypoxic symptoms that may be life-threatening in severe cases. A broader concern about nitrite is that it can be converted into nitrosamines in the body, which are carcinogens. All of our diets, water, meat, vegetables, fruits, etc., inevitably contain nitrates and nitrites. According to statistics from Europe and the United States and other countries, vegetables are the most important source of nitrates in a normal diet, while nitrites are often associated with the conversion of nitrates. Among plant foods, the highest content is found in green leafy vegetables. In addition to the type of vegetable itself, the amount of nitrate is also related to the growing method, harvesting period and other factors. The amount of nitrate can vary greatly between different vegetables and between different seasons of different origins of the same vegetable. However, under normal circumstances, the content of these nitrates and nitrites in vegetables is still quite far from the dose that is harmful to human body. Moreover, vegetables have many clear benefits for human health. Therefore, the scientific community and food hygiene agencies still recommend people to eat more vegetables. Thus, our concern becomes: how to get the benefits of vegetables while minimizing the possible hazards? “Overnight vegetables” have nothing to do with “night”. If you stir-fry a dish of vegetables in the evening and don’t finish it, and then eat it the next day, of course, it is called “overnight vegetables”. However, as someone asked: What if I eat it in the middle of the night? What if I stir-fry it in the morning and eat it at night? From the point of view of food science, overnight or not is not the issue. The essence of the issue is what happens to the prepared vegetables during the preservation process. What we are worried about is the conversion of nitrates to nitrites in the vegetables. This conversion process can be achieved by the original reduction enzymes in the vegetables, but in the process of the vegetables being heated for cooking, these enzymes lose their activity, and this path is cut off. Another pathway is the role of bacteria. Originally, the vegetables were cooked and the bacteria in them were almost killed. But in the process of eating, chopsticks will have some bacteria into the leftover vegetables; during the preservation process, there may also be some airborne bacteria into it. Cooked vegetables are more suitable for bacteria, and under the right conditions they will grow in large numbers, and during the growth process nitrates may be converted into nitrites. Such a process has nothing to do with overnight or not, but only with preservation conditions. How much nitrite will be produced in the final dish depends first of all on the vegetables themselves; secondly, the cooked vegetables are preserved under what conditions; thirdly, how long they have been preserved. Do not eat “overnight vegetables”, what to eat? According to the previous analysis, “overnight vegetables” may produce carcinogens nitrite. If we do not eat “overnight food”, is that the problem? It depends on what we eat: If we can buy fresh vegetables every time and eat as much as we can, then it makes sense not to eat “overnight vegetables”. However, if we just buy vegetables into the “overnight” and then do, compared with cooked and put “overnight”, what is the difference? First of all, the reductase enzymes in the vegetables still remain active, they may continue to convert nitrates into nitrites. On the other hand, the bacteria on the vegetables are still there, and external bacteria can still get into the vegetables. However, because the vegetables are intact, their natural protection mechanisms for bacteria may continue to work, so the bacteria may not grow as well as they do in “cooked” vegetables. There is no doubt that nitrites can be produced in vegetables whether they are cooked or left raw for the next day. Once produced, it is impossible to remove. As for which way to produce more, the impact of too many factors, unless for each kind of vegetable each kind of preservation conditions to do experimental testing, otherwise it is difficult to draw simple conclusions. Beyond the topic, what about “overnight meat”? Before we move on to vegetables, let’s talk about the problem of “overnight meat” in passing. Meat naturally contains very little nitrate, and usually the safety of meat is more a matter of bacterial growth. Unlike vegetables, raw meat is also very suitable for bacterial growth, and raw meat itself may carry even more bacteria. Even at the “fresh” temperature of the refrigerator (usually around 4 C), raw meat will grow a lot of bacteria within a few days. If the meat is cooked to kill the “strains” it was carrying, it will be better. However, the preserved raw meat will be heated at a high temperature for a long time when it is cooked, and the bacteria that grow will be killed. Cooked meat is usually heated much more gently (“hot dish”, as the name implies, get hot on it), has produced bacteria will not be killed, but will be more dangerous. So, for meat, the most effective way is to buy less each time, to minimize the storage time. If you want to preserve it, try to put it in the freezer, basically to prevent the growth of bacteria. “Fresh” stored meat, washed and wrapped can reduce the chance of bacterial invasion. Cooked meat should also be sealed and fully heated the next time you eat it. For meat, the usual heating does not produce any harmful ingredients, but at most only affects the taste. There are not many nitrates and nitrites in the meat itself. Industrially processed meat cooked food will generally contain some preservatives. The most commonly used preservative is none other than sodium nitrite. The safety of sodium nitrite has a large amount of test data. Simply put, it is not a hazard that can be detected in humans under legal usage. But if cooked meat products of unknown origin are used in excess, it is more dangerous. In conclusion, how to preserve and eat vegetables? Back to vegetables. Because of the clear health benefits of vegetables, it is impossible not to eat them because of the “possible” presence of nitrates and nitrites. The lifestyle of modern society makes it impossible for many people to eat vegetables fresh from the ground every meal like farmers do. For many people, buy a vegetable to eat a few days is also very common and common thing. Therefore, the preservation of vegetables has become a very important issue in food health. The production of nitrite in vegetables, the raw material is nitrate in vegetables, the transformation conditions are mainly bacterial growth, “overnight” is only a matter of time. Reduce the production of nitrite, you can multi-pronged approach. First, reduce the preservation time of vegetables, especially green leafy vegetables, and increase the frequency of buying vegetables. Second, the need to preserve vegetables, wash and wrap can reduce the bacteria carried. The vegetables that are not finished can also be sealed and kept in the refrigerator. “Overnight” is not the key to nitrite production, and heating does not increase the level of carcinogens. Of course, many of the vitamins in vegetables are destroyed when they are heated, and vegetables that are heated several times are harder to eat. From the point of view of “delicious”, “overnight vegetables” is indeed worse; from the point of view of nutrition, multiple heating does have some impact; from the point of view of safety, heating does not have any problems. Overnight vegetables, there is no legendary “carcinogenic” ability.