Is it better to put food in the refrigerator when it is cold, or to put it in the refrigerator while it is hot? Here is the answer

Expert of this article: Dou Pan, deputy director of the Clinical Nutrition Department, Peking University First Hospital, nutritionist

What do you do with leftovers?

In order to avoid waste, many people choose to store leftovers in the refrigerator and then heat them up for consumption later.

Of course, people still worry about the safety of leftovers, and have often heard that they are harmful to health.

So, these doubts arise at this time: Is it better to put the food in the refrigerator when it is cold, or while it is still hot? What should I pay attention to when storing leftovers? Which foods are suitable for the refrigerator and which are not?

Next, I will explain it to you in detail.

Is it better to put the food in the refrigerator when it is cold, or to store it while it is still hot?

The truly scientific approach is: put leftover food into the refrigerator while it is still hot; the sooner the safer.

In fact, there are bacteria on meals and food, such as "Escherichia coli", which is a "conditional pathogen" that does not cause much harm to the human body and can only make people sick when it reaches a certain number, and "Staphylococcus aureus" and "Salmonella", which are highly toxic pathogens that can cause great harm to the human body when the number is small. Once these bacteria multiply in large numbers and are eaten by people, they will cause abdominal pain, diarrhea, nausea, vomiting and other symptoms, and even endanger life.

Temperature has a great impact on the growth rate of bacteria in food: when the temperature exceeds 60℃, most bacteria cannot survive; when the temperature is below 4℃, the growth of most bacteria slows down significantly. 4℃-60℃ is called the "dangerous temperature zone" of food, during which most bacteria can grow and reproduce relatively quickly.

Therefore, the longer food is left at room temperature, the more bacteria will multiply. The purpose of putting food in the refrigerator while it is still hot is to reduce the temperature to below 4°C as soon as possible before bacteria multiply in large quantities, thus reducing the time that bacteria can multiply rapidly. Therefore, unfinished food should be taken out in advance, placed in a fresh-keeping box, and stored in the refrigerator as soon as possible.

In addition, some people may have such questions: "Will the refrigerator be damaged if hot food is placed in the refrigerator?"

The answer is: No, the refrigerator is not that fragile. Although putting food in the refrigerator while it is still hot will temporarily raise the temperature of the refrigerator compartment, the refrigerator's temperature sensor will start cooling down when it senses the temperature change, allowing the refrigerator compartment to return to a low temperature state.

What should we pay attention to when storing leftovers?

1. Try not to eat "saliva dishes". The "leftovers" to be eaten the next day are the food that was stored with clean utensils before eating the day before. In this way, the "leftovers" are less contaminated by bacteria and can be refrigerated for consumption the next day; however, if all that is left are "saliva dishes", the bacteria in the overnight leftovers will definitely increase, so it is not recommended to eat them.

2. Categorize and store. Categorize refrigerated leftovers to avoid cross-infection of bacteria, and store them in clean and airtight containers, such as fresh-keeping boxes, fresh-keeping bags, or cover bowls and plates with a layer of plastic wrap.

3. The storage time should not be too long, and it should be fully heated before eating. Although most pathogens can be killed after the leftovers from the previous day are fully heated at high temperature or in a microwave, it is not recommended to store leftovers in the refrigerator for too long.

Which foods are suitable for the refrigerator and which are not?

Suitable for refrigerator:

1. Stem and leafy vegetables such as rapeseed and cabbage

These vegetables are difficult to store for a long time, and their moisture and vitamins are easily lost. Therefore, you can put these vegetables in a plastic bag and put them in the refrigerator. Control the temperature at 0~5 degrees Celsius and the humidity at around 85%. Water them regularly, and they can be stored for up to 3~4 days.

2. Watermelon

The cut watermelon needs to be wrapped in plastic wrap and placed in the refrigerator to prevent bacterial growth, and should be eaten as soon as possible within 24 hours.

Not suitable for refrigerators:

1. Root vegetables such as sweet potatoes and potatoes

These vegetables don't lose moisture easily, so just put them in a cool, dry place. The high humidity in the refrigerator can easily make sweet potatoes, onions, potatoes, etc. soften and moldy, thus losing their fresh taste.

2. Tomatoes, cucumbers and other melon and eggplant vegetables

Unripe tomatoes can be stored at room temperature and do not need to be placed in the refrigerator. This is because the ripening process of tomatoes will be terminated in the low temperature of the refrigerator, resulting in the loss of freshness. If the tomatoes are already ripe, it is best to eat them quickly, or put them in the refrigerator with plastic wrap and store them separately to prevent the natural ripening hormone (ethylene) released after they ripen from affecting other vegetables and fruits.

Other melon and eggplant vegetables, such as cucumbers, are not suitable for storage in the refrigerator at too low a temperature, otherwise they are prone to frostbite, so they should be bought and eaten immediately.

3. Bananas, lychees and other tropical fruits

These tropical fruits are best stored in an environment above 8°C. The average temperature of refrigerators and freezers is generally around 4°C, which not only makes these fruits difficult to mature, but also easily freezes, and the skin is prone to black spots and depressions. It is best to store these tropical fruits in a dark, cool and ventilated place and eat them as soon as possible.

In short, it is best to buy and eat vegetables and fruits immediately to ensure their freshness.

Be especially vigilant about Listeria!

In addition to the aforementioned bacteria such as E. coli and Staphylococcus aureus, there are also Listeria monocytogenes, which is easily overlooked but deadly. It can survive for a long time in a freezer below 0°C, and refrigerating food cannot inhibit its reproduction at all. Listeria monocytogenes is very lethal to people with low immune function, pregnant women, fetuses, infants, and the elderly, and may cause life-threatening diseases such as meningitis, sepsis, miscarriage, and neonatal infection.

Foods that are easily infected with Listeria include: raw and cold seafood; cooked food and meat that have been stored for too long; ice cream; ready-to-eat fruit and vegetable salads; unpasteurized freshly squeezed cow's and goat's milk; raw mushrooms and incompletely cooked bean sprouts; and cheese made from raw milk that has not been pasteurized at high temperatures.

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