Pectin is a "gum" found naturally in fruits that causes jelly to gel. Tart apples, crab apples, sour plums, Concord grapes, quinces, gooseberries, red currants and cranberries are especially high in pectin. Apricots, blueberries, cherries, peaches, pineapple, rhubarb and strawberries are low in pectin. Underripe fruit has more pectin than fully ripe fruit. Jellies and jams made without added pectin should use 1/4 underripe fruit. Many recipes call for the addition of pectin. Pectin is available commercially either in powdered or liquid form. These two forms are not interchangeable, so use the type specified in the recipe. Powdered pectin is mixed with the unheated fruit or juice. Liquid pectin is added to the cooked fruit and sugar mixture immediately after it is removed from the heat. When making jellies or jams with added pectin, use fully-ripe fruit. Pectin is concentrated in the skins and cores of fruit; that is why some recipes call for those to be included. Commercial pectins may be used with any fruit. Many homemakers prefer the added-pectin method for making jellied fruit products because fully ripe fruit can be used, cooking time is shorter and more precise and the yield from a given amount of fruit is greater.