The essential thickening agent in jelly is pectin, a carbohydrate that occurs naturally in most fruits. If your jelly does not set, you can safely assume that either too little pectin or an incorrect proportion of other ingredients went into it, or that cooking conditions interfered with the thickener's job. Two main factors determine a jelly's pectin content. The first is the type of fruit used. For instance, apples, citrus fruits, cranberries, sour blackberries, and quinces have a high pectin content. The opposite is true for apricots, pineapples, sour cherries, peaches, nectarines, raspberries, and strawberries.
The second pectin-content determinant applies to all fruits: the stage of ripening. An almost ripe fruit is more appropriate for making jelly than a fully ripe or unripe one because useful pectin is at its maximum just before the fruit reaches its peak of ripeness. To compensate for a pectin-deficient fruit, you can add a commercial pectin concentrate in liquid or powder form. Only a small quantity of this concentrate is necessary; beware of an overdose, which will give the jelly a tough, rubbery consistency, making it difficult to spread on toast - assuming someone would want to eat such an unappetizing product in the first place. The proportion of pectin can also be increased by reducing (boiling down) the fruit juice used in the jelly-making process.
Pectin alone will not set a jelly, for it requires both acid and sugar to thicken properly. Most fruits contain acid, but acid content also changes with the ripening process. Here is one more variable that makes an almost ripe fruit preferable to a fully ripe one: higher acid content. If you are using ripe fruit and a commercial pectin, you can supply the necessary acid by adding approximately 1 tablespoon of lemon juice per cup of fruit juice. Since jelly recipes call for fistfuls of sugar, deficiency of it is unlikely to be the cause of any setting problems.
Heating makes the pectin in fruit water-soluble, a condition necessary for jelling. However, you must be careful not to destroy the pectin in your mixture by cooking it at too high a temperature or for too long.
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