How Pectin Goes to Work to Make a Jelly

Gelatin occurs when a mesh forms, trapping a liquid. When a fruit or berry is heated by itself for jams or with water for jellies, the pectin, a long stringy molecule, is extracted from the pulp, skin, and, in pomes, from the cores. These long strands of pectin do not bond to one another, but rather to water. As you add quite a large amount of sugar, the dissolving sugar attracts the water (present in the fruit juices or as added water), leaving the pectin somehow stranded. The pectin would be in danger of separating (precipitating) from the mixture were it not for the natural fruit acids and/or added lemon juice. The acids prevent the pectin from precipitating, and as the latter absorbs water, a solid mass of jelly forms.