Pectin is a complex carbohydrate found in many fruits and vegetables which, when boiled with sugar in the presence of acid, causes the liquid released from fruit or vegetables to become jelly-like.

Pectin is found in comparatively large quantities in cooking apples, quinces, black- and redcurrants, damsons, some plums and citrus fruit – thus these fruits make the most successful jams and jellies.

As the skins and pips, particularly of citrus fruit, apples and quinces, contain much of the pectin, these are sometimes tied in a cheesecloth bag and added to other fruit jams to give them a firm set.