what is sour milk

For a half cup of sour milk, simply use a splash of lemon juice in your measuring cup, then fill with regular milk to the desired level amount. Let it sit at room temp and you'll have sour milk that works fine in baking. Buttermilk works well, too.

Sour milk is raw (unpasteurized) milk that has turned sour because it has sat too long at room temperature. What sour milk is NOT is the typical milk you get in the grocery store that has been around too long. That is spoiled milk and needs yo be pitched out--not recycled in food.

Buttermilk or milk w/ vinegar. http://food52.com/blog/9624-how-to-make-makeshift-buttermilk