ZoeAnn Holmes in the department of Nutrition and Food Management at Oregon State University offers a short reply: "Jell-O, a brand name, is in general made by heating gelatin (processed collagen) in water. The large, stringlike protein molecules of the gelatin wiggle around in the hot water solution. As the gelatin mixture begins to cool, the protein strands have less and less energy to wiggle, until eventually they eventually bond together. If everything happens correctly, bonding occurs at points along the strands, forming pockets that trap the surrounding liquid. When the process is done, the collagen forms a three-dimensional structure or matrix, known as a semisolid colloidal gel. It is this matrix that gives Jell-O its structural integrity.
"Processed collagen is removed and purified from cattle hides, the same substance that may make your shoes. The cattle hides are high in collagen and a variety of other substances."
David J. McClements, an assistant professor in the Biopolymers and Colloids Research Laboratory at the University of Massachusetts at Amherst, provides this more extensive overview of dessert science:
"The principal component responsible for the transition from a liquid to a semisolid gel on cooling is gelatin. Gelatin is a protein derived from collagen, the major component of the connective tissue of animals. Jell-O and other, similar products consist of powdered gelatin mixed with sweeteners, flavorings and coloring agents.
"In its natural state, collagen exists as fibers that contain three polypeptide chains entwined into a helical structure. Collagen is converted to gelatin by heating it in the presence of water. This procedure breaks down the relatively weak (noncovalent) bonds holding the three polypeptides together, as well as some of the stronger, covalent bonds, and produces a solution in which the polypeptides are arranged into a predominantly amorphous structure.
"When the solution of gelatin cools below a certain temperature, the molecules tend to associate with one another in order to regain some of their original helical structure. In this way, junction zones are formed. The junction zones mark a local return of the original form: three polypeptide chains in a helical formation. If there is enough gelatin present, a gel will form. The gel consists of a three-dimensional network of gelatin molecules linked by these junction zones, which is capable of entraining large amounts of water through capillary forces. This gel has solidlike characteristics, although it is really a viscoelastic material.
"Gelatin is a thermoreversible, cold-setting polymer: if the gel is reheated, it will convert back to a liquid because the forces favoring the amorphous state (mainly configurational entropy) outweigh those favoring the aggregated state (mainly hydrogen bonds). For this reason, gelatin cannot be used in 'cook and serve' products such as puddings that are supposed to form gels when heated. These products must incorporate a heat-setting polymer, such as starch."
For those interested in history, we add the following details: flavored gelatin was invented by Peter Cooper, the founder of the Cooper Union for the Advancement of Science and Art, in collaboration with his wife, Sarah. General Foods began marketing flavored gelatin under the trademarked Jell-O name in 1897.