Substitutes for sour milk or buttermilk

from Successful Baking for Flavor and Texture by Martha Lee Anderson, 1936

Sweet milk can be artificially and quickly clabbered or soured by the acids present in citrus fruit juices, that is, lemon, orange, or grapefruit juice, or by vinegar. To change sweet milk to sour milk, equaling it as a leavening agent with baking soda, use the acid juices in the following quantities:

1 1/3 tbsp vinegar (4tsp)
1 1/2 tbsp lemon juice (4 1/2 tsp)
1/4 c grapefruit juice (4 tbsp)
3/4 c orange juice(12 tbsp)

For example: when vinegar is used to clabber sweet milk, place 1 1/3 tbsp vinegar in a measuring cup, add sufficient sweet milk until the combination of milk and vinegar equals the quantity of sour milk required. Mix well. In any case, the resulting sour milk will react with 1/2 tsp baking soda. It can be used in place of sour milk or buttermilk in any baking soda recipe.