Can you still eat the leftover food from the Chinese New Year? Does it really cause cancer? Tell your parents right away

Expert of this article: Fu Shufang, Master of Food Science and Nutrition Engineering, China Agricultural University, technician of the National Key Laboratory of Cardiovascular Diseases

The Spring Festival is a festival for families to get together and share delicious food. At the New Year’s Eve dinner, we emphasize “abundance every year”.

But people often say that leftover food contains nitrite, which can cause cancer.

Nitrite itself is not carcinogenic, but in the gastric acid environment, nitrite can react with amines, the decomposition product of protein in the stomach, to produce nitrosamines, which are extremely teratogenic and carcinogenic, and are classified as a Class 1 carcinogen by the World Health Organization.

So from a scientific point of view, are leftovers healthy? Are there any harms in eating overnight food?

Do uncooked vegetables also contain nitrites?

Plant foods contain nitrates and nitrites. Plants absorb nitrogen from soil and fertilizers during their growth and synthesize amino acids through a series of biochemical reactions. In this process, nitrates are inevitably produced, and nitrate reductase also exists in plants, which can reduce part of nitrates to nitrites, but the content of nitrite is generally very low.

After the plants are harvested, more nitrate reductase is released. At the same time, during the storage process, the bacteria in the environment also contain nitrate reductase, which can greatly increase the nitrite content.

Green leafy vegetables have a relatively high nitrate content, so if the purchased vegetables are stored for too long, the nitrite content will also increase. Generally, if refrigerated, the nitrite will reach a peak after 3-5 days. Therefore, it is better to buy green leafy vegetables in small quantities and multiple times, and try to eat fresh vegetables.

What are the health risks of leftover food?

The biggest health risk of leftover food comes from nitrite.

During the high-temperature cooking process, the nitrate reductase activity of food is lost. However, heated food is also more conducive to the growth of bacteria. If left at room temperature for a long time, the nitrate reductase in the bacteria will cause the nitrite content to increase, thus posing a health risk.

Ningbo Quality Inspection Institute once selected 32 kinds of raw materials such as vegetables, meat, eggs, etc. commonly found in the market, cooked more than 30 kinds of home-cooked dishes, and then placed them at room temperature (25°C) and refrigerated conditions (4°C) to observe the changes in nitrite and total colony count after different periods of time.

Leafy vegetables

The experiment found that after being placed at room temperature for 12 hours, the nitrite content of leafy vegetables increased significantly, and some exceeded the upper limit of 20 mg/kg of nitrite (calculated as sodium nitrite) in pickled vegetables specified in "GB2762-2017-National Food Safety Standard Limits of Contaminants in Food". After being placed for 24 hours, some even reached more than 100 mg/kg.

After 24 hours of refrigeration, the nitrite content of all vegetables is still within the safe range. However, after repeated heating, most of the vitamins in vegetables will lose their activity, and the nutritional value will be greatly reduced. Therefore, it is better to develop a good habit of cooking as much as you eat, and try to eat fresh food, even if it is refrigerated for no more than 24 hours.

Fish, meat, soy products, etc.

Their own nitrite content is very low, and some processed cooked meats use a small amount of nitrite as a food additive to enhance color and inhibit bacteria.

Therefore, there is generally no risk of elevated nitrite levels in home-cooked meat dishes left overnight. However, fish, meat, and soy products are all protein-rich foods. When left at room temperature, they become a natural culture medium that is very suitable for the growth of bacteria, including many harmful bacteria. If consumed, they can easily cause gastroenteritis and food poisoning.

Rice, pasta and other starchy foods

If left at room temperature, it is easy to be contaminated by Bacillus cereus, which can cause abdominal pain, diarrhea, vomiting and other symptoms after consumption. For cold dishes, since they have not been cooked and sterilized at high temperatures, the bacterial content is relatively high. Long-term storage is more likely to lead to food safety risks. It is recommended to eat them as a meal after cooking, and not to leave them overnight.

It is worth noting that overnight food does not only refer to food that has been left overnight. If it is left for more than 8-10 hours, it can be considered "left overnight". Therefore, if there are leftovers that need to be left overnight, remember to put them in the refrigerator. In addition, the elderly and children generally have weaker immunity, so it is not recommended to eat leftovers often.

Can we eat leftover food?

The nitrite content in vegetables stored in the refrigerator overnight is much lower than the national standard for processed foods. Therefore, overnight vegetables are actually edible if they are properly stored. It is an alarmist statement to simply and crudely say that overnight vegetables cause cancer.

The food safety risk of long-term storage of meat dishes comes from harmful microorganisms, and the risk of cancer is very low.

How to store overnight food?

There are many ways we can avoid these food risks in our lives.

1. Try to reduce the storage time of vegetables, especially green leafy vegetables, and buy them right before eating.

2. Wash the leftover vegetables in advance and wrap them in plastic wrap to reduce the bacteria they carry.

3. Before putting meat, seafood, etc. into the freezer, it is best to cut them into small pieces and take out as much as you want to eat. Avoid repeated freezing and thawing, which will accelerate spoilage or cause destruction and loss of nutrients.

4. For prepared meals, if you realize in advance that you cannot finish them, you can package them and refrigerate them before eating to reduce bacterial contamination.

5. Overnight refrigerated food must be heated thoroughly at high temperature, that is, heated to above 100°C for a few minutes, because most harmful microorganisms and toxins are not heat-resistant.

The World Health Organization once proposed the "Five Key Points for Safety", which mentioned that cooked food should not be left at room temperature for more than 2 hours, leftovers should not be reheated more than once, and the refrigerator should be cleaned regularly. The refrigerator is not a safe, and food should not be stored for too long, whether it is refrigerated or frozen.

Everyone must keep this in mind!

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