Irregular and imperfect fruit can be used as long as it is not spoiled, moldy or overripe. Pectin, a substance necessary for gel formation, occurs naturally in most fruits. It is concentrated in the skins and cores of various fruits. This is why many recipes include the skins and cores in preparing fruit for juice or pulp. The amount of pectin varies with the kind of fruit and degree of ripeness.
Underripe fruit has a higher pectin content.
As fruit ripens, the pectin is changed to a nongel-forming substance. Apricots, blueberries, cherries, figs, peaches, pears, pineapple, Italian prunes, raspberries, rhubarb and strawberries are low in pectin. Commercially canned or frozen fruit juices also are low in pectin. Commercial pectins, made from apple or citrus fruits, are marketed in liquid and powder form. They cannot be used interchangeably in recipes.