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Gelatin is a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, bones, and connective tissues of animals such as domesticated cattle, chicken, pigs, and fish. During hydrolysis, the natural molecular bonds between individual collagen strands are broken down into a form that rearranges more easily. Its chemical composition is, in many aspects, closely similar to that of its parent collagen.
Gelatin readily dissolves in hot water and sets to a gel on cooling. When added directly to cold water, it does not dissolve well, however. Gelatin also is soluble in most polar solvents. [Via: wikipedia]