Why does jelly not set with fresh pineapples but it sets with tinned pineapples?
I guessing you mean what Americans call Jello, i.e. set with gelatin rather than cooked with added pectin to set the fruit. The reason a pineapple gelatin won’t set with fresh pineapple is due to an enzyme – bromelain (sp?). It’s called that because pineapple belongs to a group of plants called bromeliads. This enzyme, like all enzymes, can be inactivated with the application of heat. Bromelain breaks down the amino acids (proteins) in the gelatin. Tinned pineapple, or canned as Americans call it, has been heated to a high enough temperature to inactivate the enzyme. You can inactivate the enzyme by boiling the fresh pineapple in some of its own juice or a small amount of water for a few minutes.
