Rumor: Zhejiang University College of Biosystems Engineering and Food Science conducted a test. Stir-fried green vegetables, scrambled eggs with leeks, braised pork and braised carp in the refrigerator for 24 hours after heating in a microwave oven, the results of nitrite content all exceeded the limits of the “Limits of Contaminants in Food”, which exceeded the limit of meat dishes more powerful. Nitrite is harmful to human body. Do you still dare to eat overnight vegetables? The truth: vegetables and meat dishes in the nitrite why will increase? “Overnight” after the dish in the nitrite content is really “over the limit”? How harmful is such a content of nitrosohydrochloric acid? The results of the experiment in the news are: after 24 hours of refrigeration, the nitrite in four kinds of vegetables “all exceeded the limit of the standard for contaminants in food”, including 34% of bok choy, 41% of scrambled eggs with leeks, 84% of braised pork, and 141% of braised carp. The standard for nitrite quoted in the news is “no more than 4 mg per kg for vegetables and no more than 3 mg per kg for meat”. In fact, this standard refers to the nitrite content in fresh vegetables and meat. The basis for this limit is that the original nitrite content in vegetables and meat generally does not exceed this amount. If it is exceeded, it means that it is contaminated. It is not the same thing as whether the finished product is harmful to the final direct consumption. Because the national standard does not regulate the content of the catering industry, so there is no such thing as “standard” in fried vegetables, “exceed the standard” is also not to talk about. If you want to find a relevant “national standard” for reference, it should be the amount of nitrite residues in processed foods. Because food in the restaurant industry and processed food are directly consumed, the two are more comparable. In the national standard, the residual amount of sodium nitrite in cooked meat products is not more than 30 mg per kg, while the residual standard in pickled vegetables is not more than 20 mg per kg. Leaving aside the accuracy of the experimental data reported in the news, the nitrite levels in the bok choy, scrambled eggs with leeks, braised pork and braised carp, which were claimed to be “seriously over the limit”, were 5.36, 5.64, 5.52 and 7.23 milligrams per kilogram respectively. Compared to the national standard in processed foods, it is far lower. In other words, even if these figures are accurate and reliable, it is not possible to talk about not eating them – since cooked meat products, sauerkraut, kimchi and pickles are safe to eat, why can’t you eat braised meat, braised fish and fried vegetables with much lower nitrite content? Where do nitrites in vegetables come from? Nitrogen is a widely available element in nature, and nitrogen fertilizer is necessary for plant growth. Plants absorb nitrogen from the environment and eventually synthesize amino acids through complex biochemical reactions. In this process, the production of nitrates is an inevitable step. There are also reductases in plants that reduce some of the nitrates to nitrites. Therefore, all plants contain nitrates and nitrites. 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 nitrate content can vary greatly between different vegetables and between seasons of different origins of the same vegetable. After the plants are harvested, the balance of nitrate and nitrite is broken. Reducing enzymes are released and more nitrates are converted to nitrites. In addition, bacteria, which are ubiquitous in the natural environment, can also achieve this conversion. In other words, it is difficult to avoid this conversion whenever vegetables need to be preserved for a period of time after they are harvested. In fact, there are many reports on the variation of nitrite content in various vegetables under different storage conditions. The specific rate of transformation is closely related to the type of vegetable and storage conditions. Green vegetables that are already high in nitrate, such as cabbage, can easily exceed the national standard of “4 mg/kg” during transportation and distribution. There has been more than one literature reported vegetable market back to the cabbage nitrite has exceeded the standard. In other words, as long as the vegetables are not eaten immediately after harvesting, the nitrites will also increase if they are fried “overnight”. In the process of vegetables being heated for cooking, the reductase enzyme loses its activity, the pathway to produce nitrite is cut off, and bacteria in the cooking process is also killed almost, so it seems that the cooking process can help reduce the production of nitrite. However, in the preservation process, there may still be some airborne bacteria enter. And cooked vegetables are more suitable for bacterial growth, and again conducive to the increase in nitrite. So, whether cooked or raw vegetables are more suitable for bacterial growth and thus produce more nitrites depends on the operating conditions of “stir-fry – packaging – refrigeration”. In fact, if it is canned vegetables, not to mention “overnight”, “every other week”, “every other month” is not likely to have much change. Where does the nitrite in meat come from? Whether it is meat or fish, its original nitrite content is very low, so there is a national standard of “3 mg per kg”. In addition, the nitrate content is also very low, and there is usually no concern that nitrite is produced during the preservation of meat. However, the news reports showed that the nitrite content in braised meat and braised carp increased with time. If the data is measured without problems, then it means: a considerable amount of nitrates were added to the cooking. Although nitrates themselves are very safe food additives, they are basically absent from the usual seasonings. What seasoning this restaurant used is, in turn, well worth pursuing. If nitrates were added during the process, then the results in the news are not difficult to understand. However, for the average person, their own cooking will not use the seasonings containing nitrates. “Overnight meat”, there is no concern about the increase in nitrite. Overnight vegetables, in the end can not eat? Nitrite in vegetables may increase during storage, whether cooked or not. However, the lifestyle of modern society makes it impossible for many people to pluck vegetables from the ground to eat every meal like farmers do. For many people, it is common to buy vegetables once and eat them for several days. Therefore, not preserving vegetables to reduce the production of nitrite is a real concern. 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 its production, you can multi-pronged approach. First, reduce the preservation time of vegetables, especially green leafy vegetables, 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. There are many studies in the literature on how the storage time and conditions of vegetables affect the nitrite content, and the conclusion is that refrigeration can greatly reduce the production of nitrites. If it is really difficult to achieve frequent vegetable shopping, frozen vegetables are actually a good alternative. Conclusion: Rumor-busting. Although there will be some increase in nitrite content in vegetables during the preservation process, the total nitrite content under refrigerated conditions is still significantly lower than the amount allowed by national standards in processed foods. The increase in nitrite content in meat should be due to the use of seasonings containing nitrates in the cooking process. This does not occur in regular cooking. Even if the experimental data in the news is accurate and reliable, the statement “seriously exceeded” is a misinterpretation of the national standard, and “never eat” is even more sensational. If you encapsulate the vegetables while they are hot and refrigerate them quickly, the risk of “overnight vegetables” can be ignored.