Why does fresh pineapple keep gelatin from gelling?
Fresh pineapple contains the enzyme bromelain. This enzyme degrades the protein in the gelatin, which doesn’t allow it to set. Canned pineapple on the other hand does not contain this enzyme. During the canning process the pineapple is heated up thus denaturing the bromelain, so no affect on the gelatin. Interesting fact: bromelain is found in many meat tenderizers.