The instructions in this fact sheet are for jellies made without added pectin. The proper level of acidity is critical to gel formation. If there is too little acid, the gel will never set. For fruits low in acid, add lemon juice or other acid ingredients as directed. Commercial pectin products contain acids that help ensure gelling.
Sugar serves as a preserving agent, contributes flavor and aids in gelling.
Cane and beet sugar are the usual sources. Corn syrup and honey may 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 or 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.