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Jello is made from gelatin, which has a protein called collagen in it. Fresh pineapple has a protein enzyme that breaks down collagen. Full-sized collagen is a long stringy molecule. The enzyme in fresh pineapple 'digests' the big collagen molecules into little collagen molecules that are too short to get tangled up and make a gel. So the jello would be runny. But when pineapple is canned, it gets heated, and the heat destroys pineapple's enzyme protein so it is inactive and can't 'digest' collagen proteins.

This is a really interesting question because we know that we can put a lot of different fruit in jello, so why can’t we make pineapple jello?

Let’s look at the process by which we make jello. Jello starts out as a liquid and over time will solidify. It does this by forming bridges between jello molecules. Once enough bridges are formed, the jello will solidify. Most fruits can be ground up and added to the jello, but pineapple has a specific property that makes this impossible. Pineapples have an enzyme that breaks the bridges that form to solidify the jello. Enzymes are little biological machines that can perform a specific task: in this case, breaking down jello bridges! Because pineapples have this enzyme, it makes it impossible to form the briges that allow the jello to solidify. Thank you for your question!