Fruit juice is the source of water in jelly. Pectin provides the three dimensional structure which results in a jellied product. Pectin is formed from a parent compound, protopectin, during the ripening of fruit and during the cooking of underripe fruit to extract juice. Fully ripe fruits contain less pectin than partially ripe fruits. For this reason, some jelly recipes specify the use of a portion of underripe fruit.
All fruits contain some pectin.
Apples, crabapples, gooseberries, some plums, and highbush cranberries usually contain enough pectin to form a pectin gel. Measure 1 tablespoon of rubbing alcohol into a small glass. Add 1 tablespoon of extracted fruit juice and let stand 2 minutes. If a good solid mass forms, enough pectin is naturally present in the fruit juice to form a pectin gel. If only a small weak mass forms, there is not enough pectin to form a gel and a commercial pectin should be used in the jelly making.