How should vegetables be eaten to minimize nitrite intake?

You may have heard that nitrite can be used to preserve sausages, canned foods and other products, but large amounts of it can cause human poisoning. The substance can also be converted into nitrosamines in the body, which is a first-class carcinogen, so even within the safe range of addition, it is not recommended to consume processed meat products for a long time or in excess.

In addition to nitrite as a food additive, we generally believe that green and healthy vegetable foods can also contain a lot of nitrite if they are not eaten and stored properly. Pickled vegetables and leftovers are two foods with extremely high nitrite content.

Various pickles (Photo source: Veer Gallery)

Why do these vegetables produce nitrite? What are the harms of eating them? Today we will talk about the nitrite problem in vegetables.

Part 1

Where does nitrite come from in vegetables?

Plants cannot grow without three macronutrients, namely nitrogen (N), phosphorus (P) and potassium (K). There is an agricultural proverb that says: "Nitrogen grows leaves, phosphorus grows fruits", which means that the nitrogen content in leaves is very high. For example, the nitrogen content in leafy vegetables such as rapeseed and spinach is very high, while the nitrogen content in melon and fruit vegetables such as eggplant and zucchini is relatively low.

Nitrogen can exist in plants in various states. In living plants, the main forms of nitrogen include ammonium (), nitrate () and free amino acid (), while nitrogen in the nitrite state () is very rare.

Figure 1 Nitrogen deficiency in plants can lead to poor leaf development.

(Image source: flickr- International Maize and Wheat Improvement Center)

The vegetables will not die immediately after being picked, but will continue to survive for a long time. However, the plants cannot obtain nutrients and water without the soil, so the picked vegetables will start a series of "self-rescue" behaviors.

During this process, the active action of various enzymes in plant cells gradually converts other types of nitrogen into nitrite. Therefore, as the storage time increases, the nitrite in vegetables will increase.

If vegetables are pickled, the nitrite in the vegetables will increase hundreds or thousands of times in the initial period of time, but this is not caused by the plants' self-rescue. In fact, the pickling ingredients (salt, etc.) can directly kill plant cells in a short period of time, leaving no time for the vegetables to save themselves.

When plant cells die and burst, the nutrients inside are exposed, and a large number of bacteria in the air will settle on the surface of the plant. These bacteria multiply in large numbers and convert a large amount of nitrogen in the plant into nitrite.

Similarly, cooking kills plant cells, so cooking vegetables will cause cells to rupture and expose their contents. Therefore, the nutrients in vegetables will also attract a large number of bacteria to "enjoy". Experiments have shown that the nitrite concentration in green vegetables increases significantly after cooking, and can reach as high as 55mg/kg when stored at room temperature for more than 48 hours. For reference, the national standard limits the nitrite content in pickled vegetables to no more than 20mg/kg.

Figure 2 The cell structure of cooked vegetables is destroyed, making it easy for bacteria to grow and produce nitrite.

(Photo source: Veer Gallery)

Part 2

How to avoid excessive intake of nitrite?

Knowing the sources of nitrite in vegetables, we can target these sources accordingly to reduce the risk of consuming too much nitrite.

First of all, in order to avoid the production of a large amount of nitrite due to the long-term storage of vegetables after purchase, we must try to eat fresh vegetables. If you buy too much at one time, you should also try to package them well and store them in the refrigerator. However, if you do not use the refrigerator to store them, you must also remove the packaging to prevent the rapid increase of nitrite caused by the growth of nitrifying bacteria under anaerobic conditions.

Figure 3: Try to add packaging when storing fresh vegetables in the refrigerator

(Image credit: flickr - Angelia Hardy)

Secondly, be cautious when eating pickled vegetables. After the 3.15 program exposed the Laotan Pickled Cabbage incident this year, many people on social networking sites said that they would make their own pickles in the future. However, everyone should still be aware that homemade pickles must be pickled for at least 15 days!

Because after about 10 days of pickling, the bacteria in the vegetables gradually die, and the nitrite produced is also decomposed by beneficial bacteria. Therefore, pickling for a few more days can effectively avoid nitrite poisoning. When pickling, pay attention to the need to add enough salt, otherwise it will not be able to effectively kill the bacteria.

Figure 4 Effect of fermentation time on nitrite content in kimchi, I-IV are different fermentation methods, V is the control. It can be seen that it takes at least 12 days to reduce the nitrite content to the level before fermentation.

(Image source: Reference 1)

Finally, be sure to minimize leftovers.

According to the data in a series of scientific research documents I consulted, the nitrite content of leftovers stored for 24 hours has not increased significantly; but after more than 24 hours, the nitrite content will quickly rise to exceed the standard. Therefore, it is still okay to pack up the leftovers for the next day, but never leave them for the third day.

Figure 5 Changes in nitrite content in green vegetables after stir-frying and storage.

(Image source: Reference 2)

Part 3

Is it safe to store vegetables in the refrigerator?

Since low temperature can significantly inhibit bacterial growth, refrigeration can effectively inhibit the increase of nitrite in leftovers. Leftovers do not need to be packaged, but fresh vegetables need to be packaged (boxes, plastic wrap, etc.) because the cell structure of fresh vegetables is intact and can resist bacteria, and the oxygen-deficient environment created by packaging can inhibit the respiration and transpiration of plant cells.

In a low temperature environment, most bacteria will be inhibited, but there is a kind of "strange" bacteria that likes low temperature environment very much, that is Listeria.

In a 4°C environment, Listeria can still grow and reproduce normally, thus contaminating refrigerated food. Although this bacterium is not a common pathogen and has a weak ability to infect, it still has a strong pathogenicity for people with low immunity (pregnant women, young children, the elderly, and patients with underlying chronic diseases).

Figure 6 Listeria under a scanning electron microscope.

(Photo credit: PHIL-Elizabeth White)

Not long ago, the media just reported a case of a pregnant woman infected with Listeria, which led to a super-premature birth of her baby. Thanks to the efforts of doctors, the baby, who was born at 24 weeks and weighed only 600 grams, survived safely. But this case once again sounded the alarm for us: leftovers in the refrigerator must not be stored for too long, try not to leave them for more than 48 hours, and they must be fully heated before eating!

Here is a special reminder for office workers who bring their own lunch boxes. When heating in a microwave oven, it is easy for the surface of the food to be hot but the inside to be cold. Therefore, the heating time must be long enough to ensure that the inside of the food is also heated thoroughly.

Image source: Veer Gallery

To sum up, we should try to eat fresh vegetables and avoid storing them for too long; fresh vegetables should be packaged when stored in the refrigerator; the amount of food should be arranged scientifically according to the number of people eating, and try not to have leftovers; leftovers should not be eaten for more than one day and should be fully heated before eating; leftovers that are more than one day old should be avoided.

The older generation is often reluctant to throw away leftovers for the sake of saving money, but at the same time, the elderly have weaker immunity and are more likely to be infected by Listeria. Children should use reasoning and examples to explain the harm of eating leftovers and take practical actions to protect their health.

References:

1. Li Hanchen, Tian Lixia, Zhao Xiyan. Effects of different treatment methods on the acidity and nitrite content of cabbage fermentation[J]. Journal of Hebei Science & Technology Normal University, 2004, 18(3):4.

2. Ye Hongyang. Preliminary study on the variation pattern of nitrate in vegetables and dietary exposure assessment in Hangzhou[D]. Zhejiang A&F University, 2018.

3. GB2762-2012 National Food Safety Standard Limits of Contaminants in Food

4. GB5009.33-2016 "National Food Safety Standard Determination of Nitrite and Nitrate in Food"

5. "Just because of negligence, a 27-year-old pregnant mother was infected with Listeria and gave birth to a premature baby", Peninsula Morning Post, 2022-3-25

Author: Mu Yunshen

Produced by: Science Popularization China

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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