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.
Raw milk that has not gone sour is sometimes referred to as "sweet milk," because it contains the sugar lactose. Fermentation converts the lactose to lactic acid, which has a sour flavor. Before refrigeration, raw milk commonly became sour before it could be consumed, and various recipes incorporate such leftover milk as an ingredient. Sour milk produced by fermentation differs in flavor from that produced by acidification, because the acids commonly added in commercial manufacture, such as acetic acid and citric acid, have different flavors from lactic acid, and also because fermentation can introduce new flavors. Modern food safety standards mean that milk that has gone sour is considered safe to drink.
Buttermilk is a common modern substitute for naturally soured milk.
- "TITLE 21--FOOD AND DRUGS: CHAPTER I, PART 131 MILK AND CREAM". Electronic Code of Federal Regulations (e-CFR). 2007-04-01. Retrieved 2007-11-18.
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- US patent 3625702, Exler, Heinrich, "PREPARATION OF SOUR MILK DRINKS", published 1971-12-07, issued 1971-12-07, assigned to Heinrich Exler
- US patent 3978243, Pedersen, Jens Kristian, "Process for preparing gelled sour milk", published 1976-08-31, issued 1976-08-31, assigned to Kobenhavns Pektinfabrik
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