Substituting buttermilk powder for buttermilk I made a banana cake with ripe, pureed bananas and substituted pineapple juice mixed with buttermilk powder instead of the buttermilk in the recipe. I used the same amount of juice as you would water listed on the package. The cake tasted great, but was very dense. Will the powder and juice mixture affect the consistency of the cake? Thanks!
Cakes are chemically leavened with baking powder or baking soda. They have a narrow pH range in which they work. 

Pineapple juice is very acidic. It probably reacted with your leavening and deactivated it. 

You can use buttermilk powder for the cake, but you have to mix it with water, not fruit juice. Then it will work as usual.