Raw cow (sheep) milk or milk powder is used as raw material, and the PH value is reduced after sterilization and fermentation. Fermented milk refers to a large class of dairy products made from animal milk such as milk by lactic acid bacteria fermentation process, mainly including yogurt, active milk, cheese, sour cream and so on. Fermented milk containing a large number of beneficial active bacteria is one of the ideal foods for modern human beings with both nutrition and health care functions.

Fermented milk has a very long history. The early production is made by boiling the milk of domesticated animals (cows, sheep, goats, buffaloes and camels), cooling it to 40 degrees and naturally fermenting it. There is no strict definition of fermented milk in China, but according to foreign materials, fermented milk can be described by the following explanations: various dairy products fermented by non-Bulgarian Lactobacillus (which can undergo enzymatic hydrolysis).

Fermented milk is a comprehensive name for a class of dairy products, including yogurt, kefir, fermented buttermilk, sour cream, milk wine (mainly horse milk) and so on. Oberman and Libudzisz( 1998) classified traditional fermented dairy products into four categories based on the dominant microbial types in fermented milk.

(1) type I fermented milk

This type of fermented milk is a typical fermented food in Scandinavia and Central and Eastern Europe. The dominant microbial flora are thermophilic Streptococcus lactis and Leuconostoc, which usually grow in the temperature range of 10-40℃, and the optimal growth temperature is about 30℃. Dairy products fermented by mesophilic microorganisms are characterized by their viscosity and aromatic flavor (formed by fermentation of citric acid and lactose).

Type I fermented milk contains Lactococcus lactis subsp.? 1actis), Lactococcus lactis subsp.? Cremoris), Lactococcus mutans (Lactococcuslactisssp.? 1actis var..? Diacetylactis), Leuconostoc mesenteroides subsp. Creamy, Leuconostoc mesenteroidesssp., etc. The first two species mainly produce lactic acid and are called organic acid producers; The latter is mainly used to ferment citric acid to form some important metabolites, such as acetaldehyde, CO2 and butanedione, etc., and is called the producer of flavor substances.