Away from Nitrites in Vegetables
Many experts warn that nitrites in vegetables are likely a more harmful component than pesticides. Due to the excessive application of nitrogen fertilizers, the nitrate content in vegetables is often high. After converting to nitrites, they may react with protein decomposition products to form nitrosamines, becoming a hidden danger for inducing cancers such as stomach cancer.
Surveys have found that about 80% of the nitrites in our diet come from vegetables.
Recently, many people have been asking: What harmful components are in leftover vegetables? Why can't we eat them?
The danger lies in nitrites
Vegetables generally contain a high amount of nitrates, especially those grown with large amounts of nitrogen fertilizer. Under the action of nitrate reductase, nitrates are converted into nitrites. Nitrites themselves are somewhat toxic and can block the ability of red blood cells to transport oxygen, leading to tissue hypoxia. If the amount of nitrites is large, it can lead to typical nitrite poisoning symptoms such as "cyanosis." However, in daily life, the amount of nitrites in vegetables is not enough to cause acute poisoning, but it brings another problem—nitrites can react with amino acids and primary amines in food to form carcinogenic nitrosamines and nitrosamides, which may increase the risk of stomach cancer.
Among vegetables, leafy greens and root vegetables have higher nitrate content, so these vegetables must be very fresh. Once you find that leafy vegetables are wilting or losing leaves, it means their nitrite content has already risen significantly. If the leaves appear water-soaked or have even changed their flavor, the nitrite content is already dangerously high and should not be eaten. In contrast, vegetables like tomatoes, eggplants, and winter melons have lower nitrate content and therefore pose less risk.
So, under what circumstances do nitrates turn into nitrites? First, when vegetables are left at room temperature, the inside of the vegetable is sterile, but its own enzymes will reduce nitrates to produce nitrites. Second, after vegetables are cooked, their own enzymes are destroyed, but bacteria multiply rapidly. Many bacteria can produce nitrate reductase, thus creating nitrites.
The so-called harm of overnight or leftover vegetables is twofold: one is that stale vegetables produce nitrites, and the other is that if cooked vegetables are not eaten promptly, they will also produce nitrites.
How to make leftover vegetables safe?
However, if you have to throw away all the leftovers, thrifty housewives and husbands may find it hard to do. If you really can't finish the vegetables in a single meal, what should you do? This comes down to controlling the production of nitrites.
Since nitrites in leftovers come from bacterial growth, this means that as long as you can find a way to slow down bacterial growth, you can reduce nitrite production. It is well known that bacteria reproduce more slowly at low temperatures. Therefore, putting leftovers in the refrigerator is the common choice.
However, to solve the nitrite problem, the vegetables must be put into the refrigerator immediately after cooking, when the number of bacteria is still low, not after being in a serving dish and stirred for an hour or two. Stirring with chopsticks is actually inoculating the bacteria, mixing them thoroughly with the vegetables, and facilitating their large-scale reproduction. The longer the vegetables are left at room temperature and the more they are stirred, the more the bacteria will multiply.
Therefore, the leftover vegetables from a long banquet are the most dangerous. Reheating them before the next meal only kills the bacteria but does not eliminate the nitrites that have already been produced.
If you want to eat a batch of cooked vegetables over two meals, it's a good idea to divide it into two portions immediately after cooking. One portion should be covered and put into the fridge as soon as it is slightly cooled, without being stirred at all. The other portion is to be eaten in the current meal, with none left over. The next meal, take out the refrigerated portion and heat it slightly before eating. This way, the risk of nitrites is much reduced.
On the other hand, if there is a lot of bacterial growth but the vegetables themselves have little nitrate, the amount of nitrites produced will not be too high. Nitrates are particularly soluble in water. Therefore, if you blanch vegetables in water before cooking, you can remove most of the nitrates, making the vegetables safer. Of course, the cost is also a loss of some vitamin C and potassium. If the vegetable ingredients are not very fresh or may be contaminated with pesticides, blanching is still worthwhile, as most of the harmful substances will dissolve in the water, making the food much safer.
Vegetable broth also has a nitrite problem
In the past, people knew that stir-fried broth dissolves vitamin C and should be drunk. But today, people pay more attention to food safety. If the ingredients are not fresh, the nitrite content in the vegetable broth can be high, especially in leftover broth, so it's best not to drink it.
When eating hot pot, although the broth, which has absorbed the umami of meat and vegetables, is very tempting, it may contain a considerable amount of nitrites, which can have adverse effects on the body. If the color of the cooked meat is pink, do not drink the broth. This is because coloring agents contain nitrites. If the meat is brown and has not been colored, the broth can be drunk.
If you have boiled a large amount of vegetables, do not drink the broth, as stale vegetables also contain a fair amount of nitrites. If you pickled vegetables like sauerkraut are boiled, their nitrites can also leach into the broth. The boiled vegetables and pickles themselves have a reduced nitrite content, while the broth's concentration will rise. Similarly, blanching water contains a relatively high amount of nitrites, so it should not be consumed. Various vegetable broths should not be left at room temperature for too long; it is best to finish them in one meal, or put them in the refrigerator as soon as possible.
A healthy diet can reduce risk
Although nitrosamines are toxic, they are usually not potent enough to cause cancer quickly, so those who need to pack meals for work need not panic. Just try to put the freshly cooked meals into a lunchbox and store them at low temperatures.
Many components in food can prevent the formation of nitrosamines, such as vitamins C and E, and other reducing substances like flavonoids are also helpful. Obtaining balanced nutrition from natural foods can improve the body's detoxification and immune surveillance capabilities, reducing the chance of cancerous cell changes. As long as you have a reasonable diet and eat more fresh, natural foods, you can significantly reduce the risk of stomach cancer from leftovers.
At the same time, one should not forget that there are many other cancer risk factors in the diet. Leftovers and pickled vegetables are not the only sources. For example, eating too much salt also increases the risk of stomach cancer, and few families control the salt in their food. Another example is that eating fried and smoked foods contains carcinogens, and the carbon left in a wok after stir-frying may also bring carcinogens. Imbalances in nutrients themselves can lead to an increase in the incidence of various cancers, and these problems are often overlooked.
But how can we stay away from nitrites in vegetables? Let's look at the findings from the research below.
Many people, after buying vegetables, worry about pesticides on the surface and like to soak them in water or salt water for about 20 to 30 minutes. Is this method really beneficial for food safety?
A recent domestic study gives us the answer: compared to rinsing vegetables, soaking them increases the nitrite content, which is not good for food safety. A study by a graduate student from my university found that soaking in salt water is no more effective at removing pesticides than washing with detergent and then rinsing with tap water.
Research has shown that vegetables washed with detergent and then rinsed have a lower nitrite content than samples soaked in clean water for 20 minutes. The study suggests that this might be because soaking is an anaerobic state, which increases the activity of nitrate reductase and decreases the activity of nitrite reductase, thereby increasing the nitrite content in the vegetables. Long soaking may also cause damage to the leaves, increasing the loss of nutrients.
In freshly harvested vegetables, the nitrite content is minimal. When vegetables are stored at room temperature for 1-3 days, their nitrite content peaks; under refrigerated conditions, it peaks in 3-5 days. For leafy vegetables like spinach and bok choy, the amount of nitrite produced is particularly large, and refrigerated storage is far better than room temperature storage. For cucumbers and potatoes, the difference is not so obvious. Frozen storage has little change, and the differences among various vegetables are not significant.
Therefore, if you buy leafy vegetables and don't eat them right away but keep them for two or three days, their nitrite content is likely to rise, especially for leafy greens. However, for long-stored Chinese cabbage, the nitrate and nitrite content actually decreases after being stored for many days, possibly due to nutrient loss and conversion into other nitrogen-containing substances during storage. Therefore, there is no need to worry about the nitrite problem in winter-stored Chinese cabbage.
When making cold salads, adding minced garlic and lemon juice can help improve safety because garlic can reduce nitrite content, and the organic sulfides in garlic juice, vitamin C in lemon juice, and other reducing substances can block nitrites from forming carcinogenic nitrosamines. Similarly, when pickling vegetables, adding scallions, ginger, garlic, and chili sauce can help reduce the nitrite content.