In order to make preserves like jams and jellies, you normally cook together fruit, acid, sugar, and pectin, a substance found in certain fruits that gels when heated. Some fruits -- like quinces, gooseberries, tart apples, and sour plums -- contain enough natural pectin that they'll thicken all by themselves into preserves. Others, like cherries and some berries, need an extra boost to firm up. Jam recipes for pectin-deficient fruit normally call for liquid or powdered pectin, which you can find among the baking supplies in most supermarkets. The recipes usually specify what brand of pectin to use, and it's not a good idea to substitute one brand for another, since they have different formulas. Some brands (like Sure Jell and Certo) need acid and sugar to set, some (like Sure Jell for Low Sugar Recipes) need acid and just a little sugar to set, some (like Pomona's Universal Pectin® or Mrs. Wages Lite Home Jell Fruit Pectin®) don't need any sugar to set. Liquid pectin contains sulfite, which can cause an allergic reaction in people with sulfite sensitivites, but powdered pectin does not.