Originally Posted by jet
The recipe actually calls for soured milk? How odd...
Any recipe that calls for soured milk is going to be an old one from back before all milk was pasteurized - unpasteurized milk will sour (bacteria would produce lactic acid giving the milk a tart flavor and thickening it a bit), pasteurized milk just spoils (the bacteria that caused souring is killed in the pasteurization process). Buttermilk has a similar flavor so many old recipes that have been updated now call for it instead of sour milk.
Buttermilk/sour milk does two things in a recipe: 1) gives a distinctive flavor, 2) acts as a tenderizer.
To simulate the old sour milk, or as a substitute for buttermilk, to 1 tablespoon lemon juice or white vinegar add enough room temperature sweet milk to make 1 cup - and allow to sit for 5-10 minutes to "sour".