A little vinegar added to milk will sour it when the recipe you have calls for sour milk.
By Robin
Why doesn't raw milk turn sour?
By jERZY from Ontario
Raw milk does go sour, but it takes longer. If you want to make it sour just pour a little bit of lemon juice or vinegar into it, let it set for a few seconds and use it!
And the lemon juice or vinegar ratio is 1 tablespoon per one cup of milk. It takes about fifteen minutes for it to begin to curdle.
What is considered sour milk in an old recipe?
By C Fralick
Whenever my late mother made a recipe calling for sour milk she would measure out the amount of milk called for, minus a few drops, then add just enough vinegar to make a full measure. However, milk does sour on it's own.
I have an old recipe for Banana Bread that calls for 1/4 cup soured milk and it is made by putting 1 Tablespoon of white vinegar in the measured milk, stir, and let sit till it thickens, only takes a few minutes and there is no vinegar taste to the Bread.