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Soured milk is a food product produced by the acidification of milk. Acidification, which gives the milk a tart taste, is achieved either through the addition of an acid, such as lemon juice or vinegar, or through bacterial fermentation. The acid causes milk to coagulate and thicken, inhibiting the growth of harmful bacteria and improving the product's shelf life. Soured milk that is produced by bacterial fermentation is more specifically called fermented milk or cultured milk.

Modern commercial soured milk differs from milk that has become sour naturally, though the latter is also commonly known as "soured milk." Traditionally, soured milk was simply fresh milk that was left to ferment and sour by keeping it in a warm place for a day, often near a stove. The milk would become sour and then ferment.

Soured milk that is produced by the addition of an acid, with or without the addition of microbial organisms, is more specifically called acidified milk. In the United States, acids used to manufacture acidified milk include acetic acid (commonly found in vinegar), adipic acid, citric acid (commonly found in lemon juice), fumaric acid, glucono-delta-lactone, hydrochloric acid, lactic acid, malic acid, phosphoric acid, succinic acid, and tartaric acid.

Soured milk is commonly made at home or is sold and consumed in Europe, especially in Eastern Europe (Belarus, Poland, Slovakia, Russia, Ukraine), all over the countries of the former Yugoslavia (Macedonia, Serbia, Montenegro, Bosnia and Herzegovina, Croatia, Slovenia), Bulgaria, Hellas, Finland, Germany, and Scandinavia.

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

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Wikipedia