Preserving fruit pulp. Jam is mashed fruit pulp combined with water and sugar and cooked until thick. It should have a bright color end a soft consistency that spreads easily, but has no runny juice.
The thickening occurs when the correct proportions of pectin and acid (which are present in all fruits in varying degrees) and sugar are combined and cooked to a certain temperature. If sufficient pectin is not present for a good jell, you’ll need to add commercial pectin, available as a liquid or a powder. (The powder must be added to fruit before cooking.) Some people prefer to use pectin in all their jam and jelly making; the cooking time is less and a jell is guaranteed. However, pectin does affect flavor and generally necessitates the use of more sugar.