Choose fruits high in pectin, instead of using commercially produced pectin to naturally thicken jam. Pectin is a naturally occurring thickener found in varying levels in most fruits. For pectin to properly gel, you need to achieve the correct balance of both sugar and acid by using the exact quantity of the ingredients mentioned in your jam recipe. Also consider other factors such as correct handling and cooking techniques to prepare naturally thickened jam.
Select a recipe using high-pectin fruits such as mildly tart apples, gooseberries, crabapples, oranges, plums or grapes. If using low-pectin fruits like strawberries, blueberries or peaches, combine with high-pectin fruits. Fully ripened fruits have a lesser pectin content than unripened fruits. When using fully ripened fruit, combine with approximately one-quarter the quantity of unripened fruit.
Place the fruit in a bowl and wash under running tap water. Do not soak. Rinse well. Discard any stems. Peel the skin off the fruit, chop it with a knife over a cutting board and remove pits, if any.
Crush the chopped fruit in a food processor or with a potato masher. Do not puree, as this alters the acid level of the fruit and prevent the jam from thickening. If using berries to prepare jam, use a sieve or food mill to strain berries that have a lot of seeds.
Place a saucepan on medium heat. Add the crushed fruit, granulated sugar, water, lemon juice and other ingredients exactly according to the quantities mentioned in the recipe you're following. Use a large wooden spoon to stir the mixture rapidly and continuously, to prevent it from burning while cooking. Bring to a boil and cook until the mixture thickens.
Test for readiness by checking if the jam has thickened. Remove the saucepan from the heat. Pour 1 teaspoon of the cooked mixture on a small plate. Place it in the freezer compartment of the refrigerator for around 5 minutes. Remove the plate and check if the mixture has thickened. Your jam is ready if the mixture has thickened. If not, set the saucepan back on the heat source to cook. Re-test every 5 minutes to check if it's ready.