Jams, jellies and marmalades all consist of fruit boiled with sugar to jell point. Fruit contains acid and the natural setting agent pectin as well as its own sugar. The acid, pectin and sugar must be in the right proportions for the preserve to set. Jam, or conserve is made with whole or cut fruit, marmalade with citrus fruit, and jelly with strained fruit juice boiled with sugar.
Acid fruits make the best preserves, and barely ripe fruit is to be preferred because at this stage the pectin content is highest. Tart apples, red currants, black currants and plums are all good choices. Bitter Seville oranges make the best marmalade, and crab apples are a natural choice for jelly.