Pectin

The natural gelling agent in fruits. Some fruits, such as apples, redcurrants, plums, damsons and blackcurrants, are high in pectin. Others, such as blackberries, blueberries, peaches, cherries, mulberries and strawberries, are low in pectin so you will need to add pectin or lemon juice to the fruit to facilitate setting. You may also want to combine low- and high-pectin fruits, e.g. blackberry and apple, strawberry and redcurrants. Under-ripe fruit tends to have more pectin.