Apples, crab apples, gooseberries, and some plums and grapes usually contain enough natural pectin to form a gel. Other fruits, such as strawberries, cherries and blueberries, contain little pectin and must be combined with other fruits high in pectin or with commercial pectin products to obtain gels. The proper level of acid is critical to gel formation. For fruits low in acid, add lemon juice or other acid ingredients as directed. Commercial pectin products contain acids that help to ensure gelling.
Sugar serves as a preserving agent, contributes flavor and aids in gelling.
Cane and beet sugars are the usual sources of sugar for jelly or jam. Corn syrup and honey may be used to replace part of the sugar in recipes, but too much will mask the fruit flavor and alter the gel structure. Use tested recipes for replacing sugar with honey and corn syrup. Do not try to reduce the amount of sugar in traditional recipes. Too little sugar pre-vents gelling and may allow yeast and mold growth.