There are several choices. One is to mix 1-3/4 tablespoon of cream of tarter into 1 cup of milk. Another is to use 1 tablespoon of lemon juice or white vinegar in 1 cup of milk, which you stir and let stand for 5 minutes. Or in many cases you can use plain yogurt or sour cream.
How do I determine how much orange oil to use in place of grated orange peel? What would the ratio be?