Question: Pineapple and Jell-O
Have you heard that adding pineapple to Jell-O or other gelatin will prevent it from gelling? It's true. The reason pineapple prevents Jell-O from setting is because of its chemistry.
Answer: Pineapple contains a chemical called bromelain, which contains two enzymes capable of digesting proteins, which are called proteases. Jell-O and other gelatins get their structure from links formed between chains of collagen, which is a protein. When you add pineapple to the Jell-O, you break the links as fast as they form, so the gelatin never sets up. The enzymes in bromelain are inactivated once they have been heated to about 158° F (70° Celsius), so while fresh pineapple prevents Jell-O from gelling, gelatin made using canned pineapple (which was heated during the canning process) won't ruin the dessert.