Gelatin is derived from the animal protein collagen, which is abundant in muscles, bones, and other tissues of the body. When gelatin is dissolved in water, heated, and then cooled, it forms a semisolid material, such as we've all eaten in the form of gelatin salads, mousse, and other treats.
When gelatin is heated, the hydrogen bonds that hold the shape of the collagen molecule are broken. When it is mixed with water, heated, and then cooled, the bonds begin to reform, and gelatin takes shape. Since gelatin is a protein, it is susceptible to enzymatic activity. The enzyme bromelain is found in fresh pineapple and breaks the bonds of collagen, so gelatin will not gel in the presence of fresh pineapple or fresh pineapple juice. (Bromelain is also used to tenderize meat, since it breaks the bonds of collagen.)
Set up a demonstration in which a Jello (or other brand of gelatin) mold has already been prepared, showing students what the gel looks like. Samples may be passed out if desired (Knox blocks work well this way). Then prepare one box of Jello using hot water and chilled canned pineapple juice and another box using hot water and chilled fresh pineapple juice. Set these both aside in a refrigerator.
During the next class period, bring out the two preparations of gelatin. Students will see that the one containing canned juice is firm as expected, but the preparation containing the fresh pineapple juice is not. Explain that during the canning process, the juice is heated, which destroys the bromelain.