Chemistry
Gelatin consists of partially hydrolyzed collagen, a protein which is highly abundant in animal tissues such as bone and skin. Although many gelatin desserts incorporate fruit, some fresh fruits contain proteolytic enzymes; these enzymes cut the gelatin molecule into peptides (protein fragments) too small to form a firm gel. The use of such fresh fruits in a gelatin recipe results in a dessert that never 'sets'.
Specifically, pineapple contains the protease (protein cutting enzyme) bromelain, kiwi fruit contains actinidin, figs contain ficain, and papaya contains papain. Cooking or canning denatures and inactivates the proteases, so canned pineapple, for example, works fine in a gelatin dessert.
Read more about this topic: Gelatin Dessert
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