If you're making a recipe using a very old cookbook, a reference to "sweet milk" stands for regular milk, while the term "sour milk" is another way of saying buttermilk. Rather than using fattening, store-bought buttermilk, you can simply make the meal or dessert by mixing up your own sour milk with common household ingredients.
Cut a lemon in half and squeeze out 1 tbsp. of juice into a measuring cup. As an alternative, use distilled white vinegar.
Mix in milk with a 2 percent fat content or greater until you get 1 cup. Then stir.
Let the mixture stand for a quarter of an hour. It will be ready when it appears to start curdling.