Acidification, which gives the milk a tart taste and unpleasant smell, is achieved either through bacterial fermentation or through the addition of an acid, such as lemon juice or vinegar.

The acid causes milk to coagulate and thicken, inhibiting the growth of harmful bacteria and improving the product's shelf life.

Soured milk is commonly made at home or is sold and consumed in Eastern Europe, the Balkans, the Baltic states and Scandinavia.

It is also made at home or sold in supermarkets and consumed in the Great Lakes region of Somalia and Eastern Africa (Kenya, Uganda, Rwanda, Burundi and Tanzania).

Fermentation converts the lactose to lactic acid, which has a sour flavor.