Because fully ripened fruit has less pectin, one-fourth of the fruit used in jellies without added pectin should be underipe. Commercially frozen and canned juices may be low in natural pectins and make soft textured spreads. The proper level of acidity is critical to gel formation. For fruits low in acid, add lemon juice or other acid ingredients as directed. Commercial pectin products contain acids which help to ensure a gel.
Sugar serves as a preserving agent, contributes flavor, and is necessary for gel formation.
Cane and beet sugar are the usual sources of sugar for jelly or jam. 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 prevents gelling and may allow yeasts and molds to grow.