Buttermilk substitution: does the milk have to curdle before use?
If I substitute 1 cup milk + 1 tablespoon lemon juice for 1 cup of buttermilk in a cake, do I have to wait for it to curdle before using?
In most cases, it's the acid in the buttermilk that's needed for the leavening (when mixed with baking soda). Can't I just add milk and lemon juice straight into the batter?
No, the milk does not have to curdle before you use it in your cake batter. But I'd let the mixture rest in the refrigerator for fifteen or twenty minutes just the same.
Buttermilk is made using lactic acid, lemon juice contains citric acid. Buttermilk is also lower in fat than whole milk. You won't make "buttermilk" by mixing lemon juice and milk but you'll get a chemical reaction in your cake batter that closely resembles what you might expect from buttermilk. Because buttermilk has less fat than whole milk, lemon juice added to 1% or2% milk will come closer to the acidity balance you might prefer for your recipe.