Occasionally an older recipe for bread, pancakes, or other baked goods will call for “sour milk” in the ingredients. The date sour milk was first used is unknown, but cookbooks and other records show that it was commonly used in the colonial United States in the 1700s. Back in those days without refrigerators or pasteurization fresh milk spoiled rather quickly. The bacteria in the milk would multiply and convert lactose sugar into lactic acid. This would give the milk a sour, acidic taste. The lactic acid also caused the milk proteins to coagulate, causing the milk to turn thick and lumpy. Sour milk was often mixed with a base such as potash or baking soda to help baked goods rise. The sour taste and thickness of the milk also added richness to the finished product.
There are two ways to make sour milk. The first is simply to let regular milk sit beyond its selling date. (This does not appear to be entirely accurate, see heppigirl's addition below). The milk will slowly become sour and thick as the bacteria produce more lactic acid. Interestingly, using spoiled, sour milk in baked goods does not appear to pose a health hazard. Many people apparently swear by this method but I haven’t worked up the nerve to try it. I prefer the second, simpler method where an acidic liquid is added to fresh milk. To make your own sour milk, put a tablespoon of lemon juice or plain vinegar into a liquid measuring cup. Add milk until you have a cup of liquid. Let the mixture sit at room temperature for about 15 minutes. After this time the milk should be somewhat thick and lumpy when stirred and taste mildly sour.
Buttermilk and sour milk are often used interchangeably in recipes and I regularly substitute sour milk when I didn’t remember to purchase buttermilk. Sour milk tends to be thinner and have a milder acidic taste than buttermilk, which slightly alters the taste and texture of the finished baked good.
heppigirl says : milk that goes off in the fridge is different to sour milk. My folks eat sour milk with a plate of boiled potatoes - it's a Polish thing I guess, but you are supposed to pour the fresh milk in a large bowl and leave it on the bench top (in summer for example) until it just turns, then stick it in the fridge. The first lot is never that good, so you then use a bit of the first lot as a starter culture for the next batch which tastes much better :)
jmpz says : in the Caribbean there is a dessert made entirely of sour milk, sugar, eggs and cinnamon. it is a granular sweet concoction that is extraordinarily tasty...