Most enzymes are proteins that have evolved to function as catalysts for chemical reactions in living systems. Some fruits contain proteases, a group of enzymes that catalyze the breaking of peptide bonds in proteins. This can have an undesirable effect when fruit is prepared with some foods such as gelatin, whose main component is collagen, which can be degraded by the fruit's proteases and prevent it from solidifying. This is the reason for the warning on many gelatin packages "Do not use with fresh or frozen pineapple." Other fruits may also be included in the warning.
Fresh pineapple, papaya, kiwi, and figs contain the proteases bromelain, papain, actinidin, and ficin, respectively. When enzymes are denatured by cooking or canning, they lose their enzymatic capacity.
In this activity, students observe gelatin samples treated with substances that may or may not have an enzymatic effect on the protein present in the gelatin. Substances used are fresh pineapple, canned pineapple, fresh frozen and frozen and microwaved pineapple.