Don’t leave a cut lemon overnight! What does “leaving overnight” really mean?

On March 17, topics related to a certain beverage chain brand became a hot topic on Weibo, sparking heated discussions among netizens.

According to a report by Hubei Jingshi Live on 14th, at the Ninghe Xintiandi store of the beverage chain brand in Yichang, Hubei, the reporter found that a store clerk hid the remaining lemon and orange slices of the day in the locker under the operating table, ready for use the next day. The headquarters of the beverage chain brand requires that the fresh orange and fresh lemon ingredients must be cut and used immediately, and if not used up, they must be discarded at closing time.

After the beverage chain was exposed by the media, the Hubei Yichang Market Supervision and Administration Bureau rushed to the store involved. Law enforcement officers found that there were surplus raw materials in the second-floor warehouse, which were suspected of irregular labels. They seized them and filed a case against the store.

On March 18, a staff member of a store of the beverage chain in Hangzhou responded that they had received regulations from the headquarters: "Lemons cannot be kept overnight, and photos must be taken and registered when the store closes every day, and records of scrapping must be kept."

What are the hidden dangers of leaving things overnight?

In fact, there has always been controversy about things that are left overnight.

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So can we eat leftover food? Do you have all kinds of arguments with your mother, father, and teammates at home about leftover food and fruit?

"It is best not to eat overnight food. It contains high nitrites and eating too much can cause cancer."

"Just heat up the leftover food thoroughly. If the temperature is high, no germs will come out!"

"How can this be considered an overnight dish? I just cooked it at noon and it's just right for a midnight snack at 9pm. It didn't stay overnight, so how can it be considered an overnight dish?"

Today, the editor will take you to find out.

Eat cut fruit within 2 hours

It is best to eat cut fruit as soon as possible, because once the fruit is cut, it loses the protection of the skin, the nutrients inside will be lost, and it is also easily contaminated by bacteria such as Salmonella. This is especially true for fruits that are easily oxidized, such as apples and bananas, whose flesh will turn black when cut and exposed to the air. At this time, soaking these fruits in salt water or lemon water can slow down the oxidation and blackening rate.

In an interview with the media, Fan Zhihong, an associate professor at China Agricultural University, said that watermelons cut at room temperature in summer can only be kept for less than 2 hours . Watermelons that cannot be eaten immediately should not be cut into small pieces but should be immediately wrapped in plastic wrap and refrigerated. In addition, knives and cutting boards for cutting watermelons should be kept as clean as possible and should not be used together with those for cutting raw meat. For example, pre-opened coconuts, which have been popular in the past two years, can cause bacteria to multiply if stored at room temperature for a long time. Eating them may cause bacterial food poisoning, resulting in symptoms such as nausea and vomiting, abdominal pain and diarrhea, physical weakness, dizziness and headache, and even chills and fever.

Regarding the safe storage time of cut fruits, in fact, at room temperature, try to eat them within two to three hours. In winter, when the indoor temperature is low, they can only be kept for half a day longer. If they are refrigerated, two points need to be noted: first, separate raw and cooked fruits, and second, keep them sealed . In this way, it is relatively safe to store them overnight for 12 hours.

Of course, this is not absolutely safe, especially for large fruits such as watermelon, cantaloupe, peaches, etc. that are high in sugar and easily contaminated by bacteria in the summer. People with weak stomachs, especially the elderly and children, are recommended to buy and eat them immediately.

If it is left from morning to night, does it count as “overnight”?

Before discussing whether overnight water, overnight tea, and overnight vegetables can be eaten, I would like to ask you a question first.

We usually assume that water stored between 7am and 6pm is not considered overnight water and can be drunk. However, water stored between 7pm and 6am is considered overnight water and cannot be drunk. The water is stored for the same period of time, so why can't the water stored overnight be drunk?

In fact, the real meaning of overnight is that it is left for a period of time. Generally, it can be considered overnight if it exceeds 5-8 hours . It is not distinguished by whether it is left overnight or not.

The reason why we dare not drink overnight water or eat overnight food is because we are worried that the nitrite produced overnight will form nitrosamines (strong carcinogens) in the body.

In fact, the food and water we eat every day actually contain nitrites. But they are basically within the safe range, and most of the nitrites will be excreted in urine in their original form. Only under certain conditions, such as pH, microorganisms and temperature, can they be converted into nitrosamines.

Leftover food is related to cancer, but not significantly

Some people believe that overnight food contains high levels of nitrite and eating too much can cause cancer, but in fact the relationship between the two is not as close as you think.

First of all, nitrites are not directly carcinogenic. Nitrite itself is not carcinogenic. It is only when it is taken in large quantities and repeatedly that it combines with amines under the action of gastric acid to form nitrosamines, which are confirmed as Class 2A carcinogens by the International Agency for Research on Cancer that there is a risk. Did you see that? The two words "large quantities" and "repeatedly" are emphasized!

Secondly, the daily diet is far from reaching the amount that causes cancer. The dose of nitrite poisoning for adults is about 200 mg or more. What does this mean? Generally, overnight food stored in the refrigerator for 24 hours (excluding pickled vegetables and processed meat) will not contain more than 10 mg/kg of nitrite.

Thirdly, the nitrite content is also related to the storage and ingredients of overnight dishes. The Ningbo Food Inspection Institute of Zhejiang Province conducted an experiment on overnight dishes. The prepared dishes were stored in an environment of 4°C (simulated refrigerator) and 25°C (simulated room temperature), and the nitrite content was tested after 0 hours, 6 hours, 12 hours, and 24 hours. The results showed that in an environment of 4°C, the nitrite content in the dishes at each time period was basically the same as when they were just out of the pot, with little change. However, in an environment of 25°C, the nitrite content of overnight dishes such as "leafy vegetables" increased significantly, while the nitrite content of meat did not change much.

So to put it simply, the nitrite content in ordinary overnight meals generally does not exceed the standard, and it requires meeting multiple conditions to reach carcinogenicity.

Eat overnight food to do these two things

Throwing away leftovers is also a waste. If you want to eat leftovers safely, the most important thing is to do these two things:

Reasonable storage

Leftovers should not be stored at room temperature for more than 2 hours. They should be put into the refrigerator and sealed in time. Some foods may need to be frozen. But please note that the refrigerator is not a safe. Raw and cooked food should be stored separately, and food cannot be stored without time limit. Frozen food should not be thawed at room temperature for a long time. It can be divided into small packages according to the amount needed by the family and stored in the refrigerator.

Question: “Isn’t there a saying that food should be completely cold before it can be put into the refrigerator?”

A: Actually not. If food is left at room temperature (4℃-60℃ danger zone) for more than 2 hours, pathogenic bacteria will multiply faster. Especially high-protein foods, such as meat and dairy products, may produce high-temperature resistant Bacillus during the cooling process, which is difficult to eliminate even with subsequent refrigeration. The refrigeration system of modern refrigerators can fully withstand the load of warm food. It is recommended to divide the food into small portions and put it in the refrigerator within 1 hour after cooking. A more scientific approach is to use shallow containers to hold food, or to quickly cool it to below 60℃ in an ice bath and then refrigerate it immediately. This can not only avoid excessive energy consumption of the refrigerator, but also minimize the proliferation of bacteria and prevent foodborne diseases caused by Salmonella and Staphylococcus aureus.

Heating should be thorough

Thorough heating and cooking are the most important measures we take at home to control microbial foodborne diseases, especially leftovers in the refrigerator, which must be heated thoroughly - that is, heat the entire thing to 100°C and keep boiling for more than 3 minutes.

Specifically, there are these operations: It is recommended to chop large pieces of meat or extend the heating time, and steam for more than 10 minutes after the water boils; stir-fry dishes can be returned to the pan and stir-fried at high temperature for 3-5 minutes; if using a microwave oven, the heating time needs to be appropriately extended, and the food needs to be turned 2-3 times in the middle to avoid partial underheating. But be careful, repeated heating is also unhealthy, because during repeated heating, if the operation is improper (such as not washing the dishes or hands thoroughly), new bacteria may be introduced, and the nutritional content of the food is also seriously lost.

These 4 overnight dishes are too dangerous. It’s best to throw them away if you can’t finish them!

Overnight vegetables

Eating vegetables regularly has many benefits. According to the "Dietary Guidelines for Chinese Residents", adults should eat 300-500 grams of vegetables every day. However, there is an old saying: "It is better to have leftover meat than vegetables."

If vegetables are left for too long after cooking or are improperly stored, a large amount of nitrite will be produced due to the decomposition of nitrifying bacteria. The Ningbo Food Inspection Institute of Zhejiang Province once conducted an experiment and found that after celery, lettuce, water spinach and other green leafy vegetables were fried and stored at room temperature of 25°C, the nitrite content reached an astonishing 100 mg/kg after 24 hours.

It is strongly recommended not to eat green leafy vegetables overnight.

Overnight seafood

It is difficult to completely eliminate the various bacteria that may exist in seafood products such as shrimps and crabs using general cooking and eating methods. If they are not cooked while fresh and eaten in time, they may become corrupt from the inside out!

If you don't finish eating it on the same day and eat it overnight, it may cause bacteria to multiply and produce bacterial toxins, causing damage to liver and kidney function.

Overnight Tremella, Mushrooms

Many people will stew a pot of Tremella soup and put it in the refrigerator to eat it several times, but this practice is actually very dangerous!

Generally speaking, dried Tremella and mushrooms, whether grown indoors or collected outdoors, have lower nitrite content than vegetables. However, if they are soaked at room temperature for too long, or left for a long time after being cooked, not only will the production of nitrosamines increase after consumption, but they will also be more likely to breed bacteria, causing abdominal pain, diarrhea and other symptoms after consumption.

Overnight soft-boiled eggs

Many people prefer to eat half-cooked "soft-boiled eggs" rather than fully cooked white-boiled eggs. However, these half-cooked eggs are not sterilized thoroughly and may contain a variety of pathogenic bacteria such as salmonella; if left for a long time, it is easier for bacteria to grow, which will harm the intestines after consumption.

If the eggs are cooked the first time and stored at a low temperature and in a sealed environment, it is generally okay to eat them overnight.

Source: Beijing Youth Daily, Science Popularization China, I am a Great Doctor official microblog, West China Hospital of Sichuan University, Modern Express, China Blue News