Q: I made a jellied fruit salad recently that didn't set.  Did I do something wrong?  Would it help to add more gelatin?

Chances are that you used fresh pineapple, kiwi, or papaya in your salad.  Each of these fruits, and a few others, contains high enough levels of an enzyme that breaks down protein to stop the jelly from setting.  The exact enzyme varies from fruit to fruit, but the result is the same. The gelatin used to make jellied salad is a protein which is destroyed by the enzyme and then no longer has the ability to set up.

Because the enzymes are not themselves consumed in the reaction that breaks down the gelatin, adding more gelatin will not make it set.  The new gelatin will simply be destroyed as well.

For any of these, if you use fruit that is canned, the gelatin should set properly.  Enzymes are sensitive to heat and lose their chemical properties when heated above about 150°F (65°C).  The heat used during canning is sufficient to make this happen.

The same is not true if the fruit has been frozen.  In that case, the enzyme will still be active, and the fruit will stop a jellied salad from setting.