The fermentation of milk using micro-organisms was the very first way of preserving milk. Even then, however, it was only possible to extend the keeping qualities of milk by a few days. Originally the milk soured spontaneously, but later on souring was carried out in a controlled manner by enriching and growing suitable lactic acid bacterial cultures and maintaining optimum growing conditions. This resulted in a reliable operating sequence in the technological process and hence to the manufacture of defined products.

The lactic acid bacteria for making sour milk products were also selected to match the climatic conditions. Thus thermophilic lactic acid bacteria were used for products originating in southern countries with warmer climates, while mesophilic lactic acid bacteria were used for products from the north. Of course climatic conditions no longer play a role in the manufacture of sour milk products in today's state of the art.

Souring, i.e. fermentation, of the milk has the following advantages:

• limited preservation of the perishable milk, pathogenic bacteria largely killed off (pH)
• increased consumer enjoyment due to the formation of lactic acid and aromatic substances
• improved digestibility due to many of the milk constituents changing during fermentation

Sour milk products are subdivided by fermentation type, as shown below:

^{Trans. note: a type of curdied milk from Lapland, fermented with bacillus acidophilus}