While reading the instructions for a certain gelatinous sweet dessert with my daughter, we wondered why you're not supposed to use fresh pineapple. Here's the deal.
Other fruits like kiwi and papaya have different enzymes that act similarly.
For more information:
On how gelatin gets processed
On how gelatin sets and this
How bromelain breaks down proteins as a meat tenderizer and can make your tongue hurt.
How do experiments with fruit and jello. Canned pineapple is OK because it's been cooked so the bromelain doesn't work.
Have you done this?