Why is it ok to used canned pineapple but not fresh pineapple when making a gelatin dessert?
Fresh pineapple has protease, which is a chemical that digests protein. Actually, fresh pineapple contains bromelain, with is a combination of two protein-digesting chemicals.
Gelatin is made from the meat protein collagen. The bromelain in fresh pineapples digests collagen preventing the gelatin from gelling.
Since bromelain is rendered ineffective when heated to a temperature of about 158 degrees Fahrenheit (70 degrees Celsius), cooked pineapple does not affect the gelling of gelatin.