Why does fresh pineapple keep gelatin from gelling?

The bromelain enzyme present in fresh pineapples breaks down the protein structure in gelatin. This enzyme activity prevents gelatin from forming a semi-solid gel. To avoid this problem, you can either use canned pineapple which contains much lower levels of bromelain or heat fresh pineapple to about 180°F (82°C) to denature the enzyme before adding it to the gelatin mixture.