Gelatin is a protein extracted from animal bones and connective tissue. Another rich gelatin source is seaweed. Irish moss (carrageen) is usually used whole, but most seaweed-derived thickeners are processed into products such as agar-agar. Gelatin increases the viscosity of liquids because when you moisten it, the gelatin granules swell up to approximately ten times their original size, trapping water molecules in the process. This phenomenon is somewhat similar to the thickening action of starches, but the final results are different. A gelatin-thickened preparation will be finer-textured and will retain its stability under a broader range of temperatures.
The firmness of a gelatin-thickened mass depends on the ratio of gelatin to liquid, the temperature of the mixture, and the presence of any other ingredients you may have added. Use too little gelatin and a limp product will result. Use too much gelatin and your creation may be capable of bouncing into your dining room on its own. The paragon mixture, when chilled and un-molded, will support its own weight, yet quiver slightly if shaken.
Firmness varies inversely with the temperature of the preparation. Once thickened, the preparation can be changed back into a liquid simply by heating it. Re-chill that liquid, and you once again have a solid. This alternating process can be done a number of times, though not indefinitely because repeated temperature extremes partially destroy the gelatin's thickening ability.
Some ingredients, including sugar in excess, inhibit gelatinization. Fresh pineapple is particularly difficult. It contains an enzyme (bromelain) that severely retards thickening. If you want to include fresh pineapple in your mixture, destroy the hindering enzymes by simmering the fruit pieces for several minutes. If you use canned pineapple, you can skip the parboiling step because the enzymes have already been deactivated.
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