Milk is rich in nutrition and is a favorite drink of people. When visiting the supermarket, did you find that some milk needs to be kept in the refrigerator, while others can be kept at room temperature for several months? Why are they all milk, but the preservation methods are so different?

Have you noticed? In the supermarket, some milk is sold in the freezer, while others are put on ordinary shelves. The same milk, why is there such a different treatment?

So these are two different kinds of milk. Pasteurized milk is in the freezer, and normal temperature milk is on the shelf.

These two different kinds of milk are actually fresh milk from pasture, but the processing methods are different.

Pasteurization of pasteurized milk refers to the famous French microbiologist Pasteur (1822- 1895). Pasteurized milk is pasteurized fresh milk.

Speaking of the origin of pasteurization, there is an interesting past:19th century, French breweries once encountered a headache, that is, beer would turn sour after brewing, so they asked Pasteur to solve this problem. Pasteur soon discovered that the "culprit" that made beer sour was lactic acid bacteria (that is, the "hero" who turned milk into yogurt). The easiest way to kill lactic acid bacteria in beer is to boil the beer, but the cooked beer doesn't even taste of alcohol. Who wants to drink? After repeated research, Pasteur found in 1863 that heating beer at 50 ~ 60℃ for half an hour can kill lactic acid bacteria in beer and keep the flavor of beer. This method is called pasteurization, also known as low temperature sterilization. This "low temperature" is only relative to the high temperature above 95℃.

Pasteurization was later used to sterilize milk. When milk is heated to 61.1~ 65.6℃ for 30 minutes or 7 1.7℃ 15 seconds, the total number of bacteria in milk can be reduced by 90% ~ 95%. This kind of pasteurized milk can be stored for 3 ~ 10 days at about 4℃, so it should be stored in the refrigerator in the supermarket. The advantage of pasteurized milk is that the heating temperature is not high, so it retains most of the nutrients of fresh milk. Now more than 80% of fresh milk is processed into "pasteurized milk" for sale.

Normal temperature milk is to sterilize fresh milk at high temperature, that is, heat it above 95℃ for 20 minutes. Because of thorough sterilization, such milk can be stored for 30 ~ 240 days at room temperature after sealing, so it is sold on ordinary shelves. However, high temperature will lead to a large loss of vitamins and amino acids in milk.