A tiny bit of lemon juice adds enough pectin to bring this jam together without commercial pectin, and the citrus brings out the tropical flavor notes in the kiwi fruit. Kiwis are full of natural pectin, and there’s absolutely no need to add storebought powdered pectin to a kiwi jam recipe.
This kiwi jam recipe includes lemon juice, both for flavor and added pectin. Lemon juice is high in pectin and will help the jam set. It also brings out the tropical flavor of the kiwis and helps to round out the jam. Lime juice would also be lovely, and I often use it for a unique flavor profile.
Mashed kiwi, pineapple juice, lemon juice and sugar, with apples as a pectin source. In a large saucepan, combine 3 cups mashed kiwi, pineapple juice, lemon juice and apples. Bring to a boil and then add the sugar; stir to dissolve, reduce heat and simmer for 30 minutes. Sterilize the jars and lids in boiling water for at least 5 minutes.
Food 52 actually suggests using kiwis as a pectin source for other jams, simply adding a single kiwi to jam recipes to help them gel. They use them to thicken up strawberry jams since strawberries are naturally low pectin fruits.