Gelatin is an edible protein made from the skin, bones and ligaments of animals. It is clear, usually colorless or pale yellow, odorless and tasteless, and dissolves in water. The hot solution is liquid, but as it cools, it "gels," forming a semi-solid, which is soft and flexible, yet firm enough to hold any shape into which it may be molded or cut. A familiar example of gelatin is the clear, sticky substance found on parts of a chicken leg after it has been cooked.
Most manufactured gelatin comes from pork skins and the skin and bones of cattle. These contain a tough, fibrous protein called collagen. First they are treated with either acids or bases (alkali) to dissolve hair, flesh, and other unwanted substances. Then they are cooked in hot water. The heat converts the collagen to gelatin, which dissolves in the water. The solution is purified and the water is removed by evaporation. Finally, the pure, solid gelatin is ground into flakes or powder.
Like all proteins, gelatin is a polymer. That is, its molecules are built up of smaller units called amino acids linked together by chemical bonds like beads along a string. In collagen three amino acid strands, each about a thousand amino acid units long, are twisted together as a sort of braid. Individual "braids" are joined by chemical bonds, making very tough web-like structures. Heating in water breaks some of the bonds. Therefore, gelatin consists of shorter strands, with fewer chemical bonds between them. These smaller fragments dissolve in water. As a hot gelatin solution cools, some of the bonds form once again, causing the solution to thicken. Cooling further, the protein strands form a three-dimensional mesh, with water filling the holes of the mesh. The resulting "gel" is soft enough to cut, yet rigid enough to hold its shape.
Gelatin cannot be a major source of protein in the human diet because it lacks tryptophan, one of the amino acids essential for human nutrition. Its main use in the food industry is to provide texture and shape to foods, especially desserts, candies, and dairy products. Gelatin also has many non-food uses. Made into capsules, it encloses drugs for the pharmaceutical industry and microscopic drops of ink for "carbonless" copying papers. A layer of gelatin binds light-sensitive chemicals to the surface of photographic film. Gelatin is also used as glue for objects as diverse as match-heads and the bindings of telephone books.