Milk tastes sour when it is left in open for some time. This happens due to the formation of

Like many other things, milk contains bacteria, Milk also naturally contains a kind of sugar known as “lactose.” The bacteria that live in milk get energy from this special sugar and use it to reproduce to make even more bacteria. When the bacteria use the lactose sugars to reproduce, they change it from “lactose sugar” into “lactic acid,” which tastes sour.