Then it must be sterilized to achieve three purposes.

One is to kill pathogenic microorganisms, such as mycobacterium tuberculosis and Escherichia coli;

The second is enzymes that kill microbial metabolism in milk;

The third is to ensure the nutrition of raw milk to the maximum extent.

At present, there are two main sterilization methods: pasteurization and ultra-high temperature sterilization. Pasteurization process is heated at 72-75℃15-20 seconds, and ultra-high temperature sterilization (uht) is heated at138-142℃ for 2-4 seconds. There are 7 physical changes and 25 chemical changes in milk after heating.

Because the pasteurization process is mild, only the bacteria and some enzymes harmful to human body are killed in raw milk, and at the same time, the nutrients are retained to the greatest extent. The pasteurized milk is called "pasteurized milk" and is internationally recognized as real fresh milk. The ultra-high temperature sterilization rule instantly destroys all bacteria and enzymes, and the nutrients are also destroyed. The milk sterilized by this method is packaged in a sterile state, which greatly prolongs the shelf life, so it is called normal temperature milk. Pasteurization technology requires high quality of milk source. The bacterial content per ton of raw milk should not exceed 200,000 in domestic standards and 50,000 in international standards. The ultra-high temperature sterilization method has no requirement for milk source. Moreover, "Pasteurized Fresh Milk" puts forward higher requirements for transportation, and it needs to be preserved at 4-10℃ in the whole process, and the shelf life does not exceed 15 days, so for consumers, "Pasteurized Fresh Milk" is fresher and more nutritious. Normal temperature milk does not need to be refrigerated, and the shelf life can reach 6-12 months, which is called "canned milk" in the industry.

Du Qunjun, who was born with a master's degree in microbiology and biochemical pharmacy, has worked in Shenyang Huishan Dairy, Kangnai New Milk House Chain, Mengniu Dairy Shenyang Branch and Mengniu Dairy Group R&D Department for more than a year after retiring from the army, and has his own opinions on the nutritional structure of milk. He released the nutritional value list of milk on October/October/October/April, 2004, and thought: "Fresh milk is gold, yogurt is silver, milk powder is copper, and normal temperature milk is iron".