Soured milk

Soured milk, sour milk, spontaneously soured milk, or naturally soured milk is the milk product fermented with indigenous microorganisms, as opposed to cultured milk of which the production involves controlled fermentation with starter cultures. Lactic acid produced by the bacteria causes milk to coagulate and thicken, inhibiting the growth of harmful bacteria and improving the product's shelf life.
Examples include amasi from southern Africa, clabber from Ireland, Dickmilch and Herbstmilch from Germany, gruševina from Yugoslavia, prostokvasha from Russia, laban rayeb from Egypt, maziwa lala from Kenya.

In recipes

that has not gone sour is sometimes referred to as "sweet milk", because it contains the sugar lactose and lacks lactic acid. 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.