from naturalnews.com: Jell-O molds made with pineapple are practically a U.S. cultural icon, so many people may be surprised to learn that if you try to make a Jell-O mold with fresh or frozen pineapple, the mold will never set. Only canned pineapple can be used. In fact, according to the label on a box of Jell-O mix, fresh or frozen pineapple, figs, ginger root, guava, kiwi or papaya will all prevent the gelatin dessert from setting.
This occurs because all these fruits contain enzymes known as proteases, meaning that they break apart protein. Gelatin is primarily composed of the animal protein collagen, the major component of connective tissue. When the collagen in gelatin is heated and mixed with water, it naturally gels.
Proteases such as those found in pineapple, however, break apart the collagen and make gelling impossible. This is why pineapple is sometimes used as a meat tenderizer: the protease bromelain partially dissolves the proteins in meat, making it easier to chew.
Canned pineapple, of course has all its enzymes destroyed by heat. It is a "dead" food, as is all pasteurized foods (which includes all canned foods). The fact that their enzymes are destroyed may be convenient for making Jell-O, but it also extracts a health cost in your own body. To stay healthy, be sure to take in living foods (raw plants) in sufficient quantities.