Pectin does not form a gel on its own; it needs acid and sugar. Pectin chains repel one another in water and cannot come together, while acid reduces this repulsion. Sugar binds water so the chains meet each other instead of the water. If any part of the trio is missing, the jam will not set. That is why low-sugar jams use either more acid or a different type of pectin.
Pectin is a long-chain substance found in the cell walls of fruit. When these chains dissolve in water, they carry an electric charge; because they have the same charge, they repel one another. As long as this repulsion continues, the chains cannot come together and form a network, so the mixture stays runny, no matter how long it boils. For jam to set, this repulsion must be overcome so the chains can link together. The fruit’s ripeness also affects its pectin content: underripe fruit has more pectin, while overripe fruit has less.
That is exactly what acid does. When lemon juice or another acidic fruit is added to the mixture, it reduces the charge on pectin chains and weakens the repulsion. The chains can then move closer together. This is why lemon juice is added to jams made with low-acid fruit; what it adds is not flavor but the gel itself. For the same reason, it is difficult to work with fruit that has passed its peak of ripeness and has lost acidity. Add lemon juice in moderation: too much makes the jam tart and masks the fruit’s flavor.
Sugar has a less familiar role. Pectin chains attract water; as long as water surrounds them, they bind to the water instead of one another. Sugar binds much of this water, forcing the chains to compete with the water. As the amount of water decreases, the chains tangle together and form a network. So in jam, sugar is not only a sweetener; it is one of the 3 components that make the gel form. This is why reducing sugar in a recipe lowers not only sweetness but also the jam’s ability to set. That is why jam recipes specify the amount of sugar in proportion to the weight of the fruit, rather than leaving it to guesswork.
It is predictable what happens when the balance within the trio is disrupted. If there is too little pectin, the network is sparse and the jam runs. If there is too little acid, the chains keep repelling one another. If there is too little sugar, water remains between the chains. So when jam does not set, the first step is not to boil it longer but to find which component is missing; adjusting the wrong one will not solve the problem. Boiling it too long does not make up for the missing component; it only evaporates water and turns the jam into a thick, heavy mass.
There are 2 ways to make low-sugar jam. First, increase the acid a little and use fruit that is high in pectin. Second, use pectin made for low-sugar recipes; this type of pectin forms a network with calcium instead of sugar, as stated on its package. Whichever approach you choose, low-sugar jam has a shorter shelf life and should be refrigerated. Use smaller jars too, because the jam needs to be eaten soon after opening.