Lots of recipes for cookies and breads call for sour milk. Originally, these recipes may have used milk that had been left sitting out. But nowadays, there is a better way to get sour milk. Just a few simple ingredients and you'll be ready to bake in no time.
Measure 1 cup milk. Whole or 2 per cent works best in baking.
Add 1 tsp plain vinegar or lemon juice. Stir the ingredients.
Let the mixture sit for 10 to 15 minutes. The milk should began to curdle slightly. You now have sour milk for your baking needs.
If you need to change the measurements, remember to keep the milk-to-vinegar ratio the same: 1 cup to 1 tsp.