I have used kefir as a straight substitute for commercial buttermilk in making cakes and biscuits - without any problems. I do not know the chemical differences/siimilarities, though.
Here is a simple substitute for buttermilk that I have personally used myself: - 1 tbsp lemon juice or vinegar - 1 cup minus 1tbsp whole or 2% milk(room temperature) In a liquid measuring cup, place...
No, your cake will have a strange texture. You can substitute mayo or apple sauce for oil though. You could even use butter, but I am guessing you are trying to get a low fat recipe?
Sourdough is a type of bread made with yeast starter instead of dry yeast. Starter is for yeast bread and baking powder and soda are for quick breads and cookies. They are not interchangeable.