Why must pasteurized milk in supermarket be put in refrigerator, but UHT milk can be preserved at room temperature?

As we all know, milk contains all the necessary nutrients except dietary fiber, which is the most beneficial food given to human health by nature. It is known as the “closest to perfect food” and the essential food for the current family, known as “white blood”. The drinking history of milk has a long history. Archaeological data confirm that the primitive animal husbandry in China began in the Neolithic age and has a history of at least 8000 years. With the development of animal husbandry, the activities of milking and drinking milk will naturally occur. In the Shang and Zhou dynasties, the “six livestock” had already existed, animal husbandry was developed, and the consumption of meat and milk food was also increasing day by day. Only drinks included pulp, milk and wine. In the classic book of rites of the pre Qin Dynasty, the word “cheese” has been written. At that time, some people thought that drinking fresh milk directly was very dangerous, especially for young children. Some doctors even thought that drinking milk would spread diseases. For example, the health treasure house published in 1607 described the symptoms of lactose intolerance, and reminded people to be cautious about eating dairy food. Milk has become an indispensable part of the household consumption system, and it has only a short history of 200 years. As Deborah wallenz, the author of “the local history of milk”, said in the book, milk is closely linked with the development of modern society and economy, and the two “revolutions” of science and technology played a magic role in the important moment of milk development. < p > < p > in 1864, the French scientist Pasteur found that when the wine was heated to 55 ℃ – 60 ℃ in a short time, the microorganisms causing the wine to turn sour could be eliminated, thus prolonging the storage time of wine. Later, this technology was widely used in the processing of liquid food such as wine and milk. Milk processed in this way is called pasteurized milk. < p > < p > pasteurization greatly improves the safety of milk. Milk is officially separated from the “natural” property and becomes a commodity that can be produced on a large scale. The promotion scope of milk has been expanded. However, pasteurization can only kill the common pathogenic microorganisms in the raw milk, and can not effectively kill other harmful microorganisms contained in the milk. Therefore, this pasteurized milk can only be refrigerated at about 4 ℃ and the shelf life is only about 7 days. In the 1940s, aseptic processing technology and packaging were invented. After the 1960s, Tetra Pak started industrial production in Sweden, which brought a new revolution to milk. < / P > < p > milk using aseptic processing technology, also known as normal temperature milk or high temperature milk, UHT (ultra high temperature instantaneous sterilization) milk, its production process is heating at 137 ℃ – 142 ℃ for 2-3 seconds. After sterilization, the raw milk was filled and sealed with 6-layer paper aluminum plastic composite aseptic material. < / P > < p > the milk processed by this method does not need to be stored in cold storage. It can be stored at room temperature for 6 months without damaging the nutrients. < p > < p > UHT aseptic packaging technology has expanded the transportation distance of milk, and this technology has been widely welcomed once it was launched. In the 30 years after the birth of this technology, the sales volume of UHT milk has doubled in Western Europe. Meanwhile, in countries with long traditional dairy culture such as the Netherlands, Germany and Belgium, UHT milk is more and more favored by the market. In 1989, aseptic processing technology was praised as the most important achievement in Food Science in the past 50 years by the American Institute of food technology. Now this technology is not only used in dairy processing, but also in juice, jam and many other food and beverage processing. < p > < p > the appearance of yogurt is later than milk. As an important member of dairy products, yogurt is favored by people for its unique flavor and high nutritional value. Yogurt has been used as food for at least 4500 years. The earliest yoghurt may be that the milk in sheep skin bag of nomadic people is naturally fermented by bacteria attached to the bag, which may be the earliest Yogurt. At the beginning of the 20th century, when Russian scientist Eliye Ehrlich metchenikov studied why Bulgarians had more longevity, the investigation found that these long-lived people liked to drink yogurt. He also isolated and found lactic acid bacteria in yogurt, named “Lactobacillus bulgaricus”. Metchenikov proposed that lactic acid bacteria is an important element of human health, so he began to promote cheese throughout Europe. After that, a Spanish entrepreneur, Isaac Carrasso, industrialized the production of cheese. In 1919, caraso set up a yogurt factory in Barcelona, and the industrial production of yogurt began. There are many classification methods for yogurt, but they are usually classified according to processing technology and production formula. Yogurt can be divided into solidified yoghurt and stirred yoghurt according to processing technology, and pure yogurt and flavor yogurt according to processing formula. After two revolutions, < / P > < p > milk has gradually become a regular customer on the dining table of residents and has conquered consumers with its rich nutritional characteristics. With the improvement of the quality of life of consumers, milk products with various flavors have come out, and the production technology has been improved on the basis of the two revolutions. < / P > < p > at present, in the liquid milk market in China, the normal temperature milk processed by UHT aseptic packaging technology plays a leading role, accounting for about 70% of the market share.