Pectin occurs naturally in fruits like berries and apples. When that pectin is cooked with sugar, it can thicken, forming the gel that makes jam and jelly. Apples have a very good amount of natural pectin, especially when you cook the peelings, cores, and all to make your juice. Store bought juice may not have all the natural pectin you'll get from a good homemade apple juice made by cooking the whole apple.