The working principle of the pasteurizer is to heat the mixed raw materials to 68～70℃, keep this temperature for 30min, and then rapidly cool to 4-5℃. Generally, the lethal point of bacteria is below the temperature of 68℃ and the time of 30min. Therefore, after the mixed raw materials are processed by this method, the pathogenic bacteria and most of the non-pathogenic bacteria can be killed; the mixed raw materials are cooled instantly after heating, such abrupt changes in heat and cold  can also promote the death of bacteria.

    

    Within 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°C-37°C). But if the temperature is too high, the bacteria will die. Different bacteria have different optimum growth temperature and heat and cold tolerance. Pasteurization takes advantage of the pathogen's characteristics that it is not very heat-resistant, so that the pathogens are treated with appropriate temperature and holding time to kill them all. But after pasteurization, a small part of harmless or beneficial, more heat-resistant bacteria or bacterial spores are still retained. Therefore, pasteurized milk should be stored at a temperature of about 4°C, which can only be stored for 3 to 10 days, and the longer is 16 days.

    There are many types of pasteurization procedures today. The "low temperature and long time" (LTLT) process is an intermittent process that is now only used by small dairy factories to produce some cheese products. The "High Temperature Short Time" (HTST) process is a "flow" process, 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. "Quick pasteurization" is mainly used in the production of yogurt and dairy products. There are mainly two types of high-temperature sterilization methods for pasteurization machines commonly used at home and abroad:

    One is to heat the milk to 62~65°C and keep it for 30 minutes. Using this method can kill various growth-type pathogenic bacteria in milk, and the sterilization efficiency can reach 97.3% to 99.9%. After disinfection, only some thermophilic bacteria, heat-resistant bacteria and spores remain, but most of these bacteria are lactic acid bacteria, which are beneficial to human health.

    Another method is to heat the milk to 75~90℃ for 15~16s. The sterilization time is shorter and the work efficiency is higher. But the basic principle of sterilization is that it can kill pathogenic bacteria. If the temperature is too high, there will be more nutrient loss.