For my mom's birthday, I want to make an angel food cake.   I am against using any type of processed sugar (even cane sugar) but I am okay with fruit so I am looking for a way to substitute sugar with fruit.  The only thing I could come up with was crushed pineapple. I found a recipe for a pineapple angel food cake online but it requires Betty Crocker cake mix, and I am planning to make mine from scratch.    Do you think it would work if I use a basic angel food cake recipe, but add crushed/ pureed pineapple chunks instead of sugar and add more flour so it's not too wet?  Has anyone tried adding crushed pineapple as a sugar substitute?  Also, is a tube pan necessary? Cause I don't own one.   Thank you.  
Angel food cake gets it's characteristic texture from the egg foam on which it's structure is built. The flour and, to a lesser degree, the sugar stabilize the foam so it's less fragile.  Adding more flour will change the texture of the cake to be more... cakey. That is, not as springy as we expect from an angel food cake. I don't know what the texture of the foam will be without any sugar. I suspect it will be drier.  Pineapple won't be able to replace the function of sugar because it can't be fully Incorporated. In fact, many angel food cake recipe even use powdered sugar as regular sugar is too coarse.  If you are satisfied with the texture changes of no sugar, you can certainly fold in pineapple chunks when the batter is folded together. I would recommend canned to avoid the risk of protease enzymes in fresh pineapple damaging your structure-- egg foam is all protein after all.  I have never tried this but if I were to do this experiment, I would cut out part of the sugar but leave some for the texture. Then I'd place canned pineapple slices on the bottom of the pan and make an angel food pineapple upside down cake.  You do need a tube pan for egg foam cakes like angel food and chiffon because they are mostly air and don't conduct heat well to their interiors. Tube pan are cheap and easy to find at thrift stores. In a pinch I've had success using a regular cake pan with a clean, empty can or canning jar in the middle. Made it tricky to invert the cake at the end but it worked. 