Weighing is more accurate than measuring. Full-flavored fruits are best because the large proportion of sugar you add for good consistency and keeping quality dilutes the flavor. You need a tart juice for good-tasting jelly. Various lots of fruit differ in composition, therefore recipe formulas may not always give the same results. To make a perfect jelly, the pectin, acid, sugar and juice must be in the right proportions.
Fruits vary in the amounts of pectin and acid they contain.
If a fruit is low in either pectin or acid, combine it with one or more fruits rich in the needed pectin or acid. For best results, use a mixture of slightly underripe and ripe fruit. Never use overripe fruit to make jelly. If you are not sure about the fruit, use commercial pectin which takes the guesswork out of making jellies and related products. When you use recipes calling for liquid or powdered fruit pectin, follow proportions and techniques exactly because the recipes are balanced to ensure a correct gel.