Sour milk, or soured milk, refers to foods made by the acidification of milk. This can be the result of bacterial fermentation or of the addition of an acid, such as vinegar or lemon juice. The acid causes proteins in the milk to coagulate, which changes the texture of the milk and extends its shelf life.
Soured milk that is produced by 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.
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. Modern techniques frequently involve the introduction of a specific bacterial culture to pasteurized milk, in order to control the population of organisms more precisely.
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 the invention of 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 have different flavors from lactic acid, and also because fermentation can introduce new flavors.
Buttermilk is a common modern substitute for naturally soured milk.