Why does food spoil? There are more ways to keep food fresh than just freezing it

Mammoths are animals that can adapt to cold climates and became extinct a long time ago. Unlike dinosaur fossils, we have found that many mammoths have preserved almost complete skin, flesh and hair. Why is this? In fact, it is due to low temperatures.

Our clever ancestors knew how to use low temperatures to preserve food very early on. For example, the royal families of all dynasties would build ice storages, which were used for cooling in the hot summer. They were also used to preserve fruits and other foods. However, ordinary people hardly used them at home at that time.

It is only in modern times that this technology has been used on a large scale for food preservation, or commercialized. In 1912, American Clarence Birdseye saw that the fish caught by Indians in the severe cold quickly froze hard after being taken out of the water, and when they were thawed several months later, they were still as fresh as when they were just out of the water. He thought this was very interesting, and wondered if this method could be used on other foods, would it have the same effect? ​​So in 1924, he founded a seafood company specializing in freezing technology. Finally, in 1930, the world's first batch of frozen foods were produced, including not only fish and meat, but also green beans, spinach, cherries, etc.

We should know not only the facts but also the reasons behind them. How does low temperature storage keep food fresh?

In fact, the principle is very simple. Food spoilage mainly comes from two aspects. One is that the ingredients of the food itself are degraded under the action of enzymes, and the other is that the reproduction of microorganisms will also accelerate the spoilage of food. Both of these points are closely related to temperature. For example, the suitable temperature for the activity of most enzymes in food is 30-40℃. At this temperature, the rate of enzyme-mediated biochemical reactions will increase rapidly, and food spoilage will naturally be faster. At this time, it is also more susceptible to invasion by certain microorganisms. With both methods, food will quickly go bad.

If we lower the temperature at this time, it will be a different picture. As the temperature drops, not only will the activity of enzymes decrease, but the reproduction of microorganisms will also be inhibited. For example, at -18℃, the respiration of fruits and vegetables basically stops, and the enzymes and microorganisms are almost in a static state. The chemical processes such as lipid oxidation degradation and protein hydrolysis and denaturation also stop, and the process of food corruption is like pressing the pause button. Imagine that the nutrients are no longer decomposed, and such food will naturally remain fresh no matter how long it is stored.

How fresh will it be? Let's take vegetables as an example. If they are kept at room temperature of 20℃, the loss of vitamin C can reach 40% in just 2-3 days. But if they are quick-frozen vegetables, the loss rate of vitamin C is only 7.9% after 100 days. Even if the loss during the freezing process is taken into account, it is only 27.9%, which is much less in comparison. In fact, many vegetables and fruits we eat in winter and spring are also stored at low temperatures, such as apples, bananas, garlic, cabbage, etc.

We occasionally use refrigerators at home to freeze meat, vegetables, dumplings and other foods, but the taste is much worse than those on the market. What's the problem?

Figure: The picture comes from Tuchong.com

Actually, the water in food will turn into ice crystals when it is frozen, which will damage the cells of vegetables and meat like a saw. When thawed, it will soften and ooze juice, and the loss of liquid will take away minerals, vitamins and other nutrients, which will cause the loss of flavor and nutrition, leading to the decline of food quality. Therefore, in order to maintain the flavor and nutrition of food, the key is to reduce the damage caused by this change, and the speed of freezing plays an important role in this.

If the freezing speed is fast, the number of ice crystal nuclei in the cells will be greater and smaller than that in the intercellular spaces, and their distribution will be close to the distribution of liquid water in natural foods. Mechanical damage will naturally be less, and it will be easier to recover after thawing, so the amount of juice flowing will be less.

Although we can't do quick freezing at home, there are some small tricks that you may want to try. First, shorten the freezing time as much as possible to freeze the food quickly. For example, directly adjust the freezer of the refrigerator to the lowest temperature. At the same time, divide the food into small pieces as much as possible and spread them flat in the freezer to reduce the thickness. Second, do not thaw repeatedly. During this process, water will crystallize repeatedly, the ice crystals will become larger, and the quality of the food will decrease significantly. Third, frozen food should be packaged in small packages, and a little water should be retained inside to prevent the surface of the food from drying out. Fourth, frozen food also has a shelf life. Try not to store it for too long. For example, meat is best eaten within 6 months, otherwise the taste will deteriorate.

In fact, in addition to low-temperature storage, there are many other food preservation methods that are also very useful. Do you know them all?

For example, dehydration is to use high temperature or air drying to dehydrate food, thereby extending the shelf life. The dried fruits, dried vegetables, dried meat and dried bacteria we eat every day use this method. The food pickling method uses salt or sugar under high osmotic pressure to quickly dehydrate food to inhibit bacteria and extend the shelf life. The pickles and preserved fruits we eat every day use this method. There is also the vacuum preservation method, which reduces the rate of food spoilage by isolating food from oxygen. There is also the irradiation method, which uses low-dose irradiation to kill pathogenic bacteria and spoilage bacteria in meat. In addition, the use of microbial fermentation can also achieve the effect of extending the shelf life, such as stinky tofu, bacon, ham, and fermented black beans.

In comparison, low-temperature preservation has a wider range of applications, and is applicable to meat, vegetables, and milk. It can better preserve the original flavor and nutrition of food because it hardly destroys the seven major nutrients, and the loss of protein, carbohydrates, fat, and minerals is also very small. Therefore, it can significantly extend the shelf life of food. The only drawback is that there are certain requirements for equipment to maintain low temperatures. If the temperature changes and the storage is improper, the food will deteriorate.

So, what new technologies have emerged in the field of cryogenic preservation in recent years? In fact, there are quite a lot of them, and you may not have heard of them yet. For example, the membrane wrapping freezing method can inhibit the expansion and deformation of food, and the food texture tastes good; ultrasonic freezing technology helps to improve the quality of frozen food; high-pressure freezing technology can preserve the quality of the original food; and food decompression freezing technology can quickly relieve the internal pressure of food, and the ice crystals are small and uniform, which causes less damage to the internal food tissue.

It is said that low temperature can inhibit the growth of fungi, but you would never guess that fungi can also be used for freezing. This is the ice nucleation active bacteria freezing technology. Ice nucleation active bacteria is a kind of bacteria that can freeze water droplets at around -2℃. The advantage of this technology is that it can increase the freezing point of water in food materials, shorten the freezing time, save energy, and promote the growth of ice crystals, making it easier to separate the subsequent ice products from the concentrated materials.

Whether it is low temperature storage or other food preservation methods, they are all good choices to extend the shelf life of food, and you can choose according to your needs. According to the recommendations of the Chinese Residents' Dietary Guidelines, food should be eaten fresh as much as possible.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Zhang Yu (Doctor of Nutrition)

Reviewer: Chen Ran (Associate Researcher, COFCO Nutrition and Health Research Institute)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.