Soured milk

Soured milk is a general term for milk that has acquired a sour taste, either through the addition of an acid, such as lemon juice or vinegar, or through bacterial fermentation. The acid causes milk to coagulate and form a thicker consistency. Soured milk that is produced by bacterial fermentation is more specifically called fermented milk or cultured milk. 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 US, the acids that may be used in the manufacture of acidified milk are 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.

In recipes, soured milk created by the addition of an acid or by bacterial fermentation can often be used interchangeably. For example, 1 cup of cultured buttermilk, a soured milk produced by bacterial fermentation, can be substituted with 1 tablespoon of lemon juice or vinegar plus enough milk to make 1 cup. The chemically soured milk can be used after standing for 5 minutes.

Sour Milk
Currently, sour milk commonly means (pasteurized) milk that has spoiled and is "in an unpalatable state". However, the meaning of sour milk can be ambiguous since it has been used in different contexts in different time periods.

In more recent years, sour milk has been used both for chemically and biologically soured milk.

Some older recipes use sour milk, but today it is unclear which dairy product was "sour milk". Pasteurized milk was not available in the US until the early 1900s. For example, the South Jersey Milk Company created a poster in 1930 entitled "What is pasteurized milk?" to educate the American public about the safety of its newly pasteurized milk. Thus, older recipes that use sour milk may have been written prior to the availability of pasteurized milk. Before pasteurized milk became widely available, sour milk may have been unpasteurized milk that had naturally acquired a sour taste through bacterial fermentation at room temperature.

At least until the 1920s, there was a clear distinction between sour milk and buttermilk, where buttermilk was the sour tasting thin liquid leftover from making butter. Today, in North America, either cultured buttermilk, also commonly known as buttermilk but not the same product as the aforementioned buttermilk, or milk soured by the addition of lemon juice or vinegar is often used when sour milk is needed in a recipe.