To Sour Milk

To sour milk, use 1 T. white vinegar plus milk to equal 1 cup.

From All New Amish Cookbook, p. 78. 

1 c. sour milk, for baking: 1 c. sweet milk mixed with one of the following:
1 T. vinegar. Or 1 T. lemon juice. Or 1-3/4 t. cream of tartar.

From Food Fare, Orlando Sorosis Woman's Club, p. 62.