Assumptions:
I've got a box of Aeroplane Jelly that states:
DO NO ADD FRESH PINEAPPLE, KIWI FRUIT OR PAW PAW AS JELLY WILL NOT SET
My question is: Why does my jelly crystal box single out pineapple, kiwi fruit and paw paw as preventing setting? Why not orange or watermelon or plum?
Certain enzymes (proteases) cut the protein bonds that create the mesh that causes the jelly (or Jello, or gelatin) to, well, gel.
Orange, watermelon and plum do not contain enough of those enzymes to interfere with gelling. In addition to paw paw (more commonly known as papaya in the US), pineapple and kiwi; mango, ginger, figs and guava also contain enough of those enzymes to interfere with gelling.
The application of high heat will inactivate those enzymes to the point that they will no longer interfere with gelling. That's why you can use canned pineapple, but not fresh, in gelatin desserts.
From The naked scientist
Why does it happen? Jelly is made up of long thin protein molecules. The reason that jelly sets is that as the gelatin molecules tangle up as they cool creating a huge intertwined tangle which traps the water and makes a flexible solid.
The pineapple contains an enzyme called bromelain and kiwi fruit another enzyme called actinidin - both of these enzymes are proteases, which means that they will chop up protein molecules. In the same way that the digestive enzymes in your intestines break up proteins to allow you to digest them. This means that when they are mixed with gelatin, the proteases chop the gelatin protein up into pieces which are far too short to tangle, so the jelly doesn't set.
No mention on that website about Paw Paw but I would assume it is for the same reason.
Pineapple, kiwi and paw paw all contain enzymes that break down proteins (bromelain, actinidin and papain respectively). Since the setting agent in jelly is gelatin, which is mostly protein, using any of these fruits will interfere with the jelly setting.
