Why won't jelly set with fresh pineapple, but works with canned?

Because fresh pineapple is full of an enzyme that cuts protein into pieces, and jelly is made of protein. Gelatin is processed collagen, long protein strands that trap water and turn a liquid into a wobbly gel. Fresh pineapple contains bromelain, a protease, literally a protein-cutting enzyme. Drop fresh chunks into your jelly and the bromelain snips the gelatin into fragments too short to hold a gel together. The jelly simply never sets.

Canned pineapple is different because it has been heated. Cooking or canning denatures the enzyme and switches it off, so canned pineapple sets beautifully in jelly. Pineapple isn't alone: kiwifruit, figs and papaya all carry their own protein-cutting enzymes and can sabotage a dessert the same way.

The same trick that ruins jelly is useful elsewhere. Bromelain can tenderize meat by breaking down its proteins, and papaya's enzyme was used to tenderize meat in South America long before Europeans arrived. In 1959 a scientist named A.C. Arcus set out to understand why kiwifruit jellies refused to solidify, and in doing so identified kiwi's own enzyme, actinidin.