Pectin

Pectin is the naturally occurring jelling agent contained in the skins, pith, cores, pips, and cell walls of fruits. The amount of pectin in fruits varies considerably, and before making jams or jellies, its level needs to be considered and the recipe adjusted accordingly. More pectin can be added to lower-pectin fruits, either by using jam-making sugar with pectin already added to it, or by adding apple juice.