Beware of the nutritional value of quick-frozen food. For example, the shelf life of a quick-frozen food is 3 months at-18℃, but it does not mean that it can be stored for 3 months at -8℃. If it is stored at-18℃ after leaving the factory and there is no obvious quality problem within 3 months, it can be safely eaten. If it is not kept at-18℃ all the time, there is no guarantee that it will change qualitatively within 3 months. Because all chemical and enzymatic reaction rates are affected by temperature, microorganisms will not reproduce basically under freezing conditions, but the taste and flavor are still changing slowly, fat will be oxidized slowly and vitamins will be decomposed slowly. Generally speaking, the lower the temperature, the slower the decomposition of nutrients, the loss of flavor and the oxidation of fat, and the quality of products can be kept stable for a longer time. In fact, the freezer in the supermarket is often open, and people put all kinds of food over and over again, so the temperature can't be kept at-18℃ all the time. On the way home, if the ambient temperature is high, the product will not completely melt, but the temperature will also rise. In China, the freezing room of ordinary refrigerators rarely keeps a constant temperature of-18℃. If there are many things in it, it is even more difficult to reach this level, and some even have only about -8℃. Studies have shown that when food is stored at-1℃-8℃, many vitamins are lost faster than at 0-4℃. That is to say, when people open the frozen food to eat, many products have passed the "shelf life" in advance, and the flavor and quality have changed. Not only do they have no umami flavor, but they may also have a slightly pungent oil flavor because of fat oxidation. Therefore, when choosing frozen food, we should pay attention to the following principles: buy the freshest products. Look at the factory date printed on the package, the closer the date, the better, and it is best not to exceed one month. If it has been more than two months, you must think carefully. Don't toss and turn when choosing, just take the bag below, because the temperature at the lower part of the freezer is relatively stable. After buying, take the frozen food home as soon as possible to avoid being heated and melted on the road. If you can't eat it right away, put it in the refrigerator at once. You know, your refrigerator is not a quick-freezing production line in the factory, and most of it can only be "slowly frozen". In the process of slow freezing, the nutrition and flavor of food will change, and the taste quality of cooking will also decrease obviously. After food is put in the refrigerator, you should eat it as soon as possible. Due to the unstable temperature in the refrigerator, frozen food stored for a long time often appears ice crystal precipitation, cracks, adhesion and other phenomena, and its nutritional value is greatly reduced.