“Sassy Suzanne” asks: How do you insure a moist cake or cupcake? Is it the oil or butter? When can you switch to yogurt? is it interchangeable? Also, when adding fresh fruit - How do you determine the amount? I was concerned about the rise and moisture of the end product.
Baking S.O.S. says: Thank you for the challenging questions!
There are a number of factors that contribute to the moistness of a cake or cupcake, including the fat (oil or butter, as you mentioned), the amount of sugar, the amount and type of liquid called for in the recipe, and even the eggs! Although eggs help provide structure to hold all of the other ingredients together when the proteins set during baking, eggs also contain water, so they contribute to the overall moisture content of baked goods.
So when making substitutions in a cake recipe, the moisture content will be affected, as well.
In the case of yogurt specifically, I have found recipes that substitute yogurt for the eggs, the liquid, sour cream, and the oil or butter. So depending on which ingredient you want to substitute the yogurt for, it will affect the outcome of the finished cake, but it is difficult to predict exactly HOW the finished cake will be affected. You simply have to find out through experimentation.
To determine how much yogurt to substitute for various ingredients, check out this article from Taste of Home. According to the article, it is not a 1-for-1 exact substitution when using yogurt for lower-fat baking, so you will need to do some calculations depending on where you want to substitute the yogurt. Good luck!