Pasteurization, also known as low-temperature disinfection and cold sterilization, is a disinfection method that can kill germs and keep the flavor of nutrients in articles unchanged at low temperature, and is often widely used to define heat treatment methods that need to kill various pathogens.

Pasteurization is to heat the mixed raw materials to 68 ~ 70℃, keep this temperature for 30 minutes, and then quickly cool to 4 ~ 5℃. Because the lethal point of common bacteria is the temperature below 68℃ and the time below 30 minutes, the mixed raw materials can kill pathogenic bacteria and most non-pathogenic bacteria after being treated by this method. The mixed raw materials are suddenly cooled after heating, and the sharp change of hot and cold can also promote the death of bacteria [2].

In a certain temperature range, the lower the temperature, the slower the bacterial reproduction; The higher the temperature, the faster the reproduction (generally, the suitable temperature for microbial growth is 28℃-37℃). But if the temperature is too high, the bacteria will die. Different bacteria have different optimum growth temperature and heat and cold tolerance. In fact, pasteurization is to use the characteristics of bacteria that are not very heat-resistant, and treat them with appropriate temperature and heat preservation time to kill them all. However, a small amount of harmless or beneficial bacteria or bacterial spores remain after pasteurization, so pasteurized milk should be stored at about 4℃, and it can only be stored for 3 ~ 16 days at most.

There are many kinds of pasteurization methods in use today. "LTLT" treatment is an intermittent process, and now it is only used by small dairy factories to produce some cheese products. High temperature and short time (HTST) treatment is a "flowing" process, which is usually carried out in a plate heat exchanger, and is now widely used in the production of drinking milk. The product obtained in this way is not sterile, that is, it still contains microorganisms and needs to be refrigerated during storage and handling. "Rapid pasteurization" is mainly used to produce yogurt and dairy products. There are two common pasteurization methods in the world:

One is to heat the milk to 62~65℃ and keep it for 30 minutes. This method can kill all kinds of growth-type pathogenic bacteria in milk, and the sterilization efficiency can reach 97.3%~99.9%. After disinfection, only some thermophilic bacteria, thermotolerant bacteria and spores remain, but most of these bacteria are lactic acid bacteria, which are not only harmless to people, but also beneficial to health.

The second method is to heat the milk to 75 ~ 90℃ 15 ~ 16 seconds, which has shorter sterilization time and higher working efficiency. But the basic principle of sterilization is that it can kill pathogenic bacteria. If the temperature is too high, more nutrients will be lost.