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A mesh built from fruit

Why Does Jam Set?

Boiling fruit and sugar builds an invisible net that holds the liquid still.

A set jar of jam A mesh of pectin holdingthe liquid
It is still mostly liquid — it just cannot flow.

  1. Step 1 of 5

    Fruit contains a long stringy molecule.

    1 Fruit cellwalls2 Boilingreleases pectin3 Long chainsfloat free

    Pectin is part of the fruit's cell walls. Boiling breaks the fruit down and releases these long chains into the liquid.

  2. Step 2 of 5

    Loose chains repel each other.

    Chains apart Runny juiceChains linked A set gel

    In plain fruit juice, the chains carry a slight charge and push each other apart, so they never join and the juice stays runny.

  3. Step 3 of 5

    Acid switches the repulsion off.

    A squeeze of lemon Chains stoprepellingNow they can link up

    Lemon juice neutralises the charge, so the chains can finally approach each other. That is why almost every jam recipe adds acid.

  4. Step 4 of 5

    Sugar takes the water out of the way.

    Water in the panWater clingingto pectinSugar takingthe waterinstead

    Water molecules cling to pectin and keep the chains apart. Sugar grabs that water for itself, leaving the chains free to meet and bond.

  5. Step 5 of 5

    Cooling locks the mesh in place.

    1 Boil toconcentrate2 A net forms3 It sets as itcools

    The chains join into a three-dimensional net while hot, and as it cools the net stiffens and holds the liquid where it is.

    Strawberries are low in pectin, which is why strawberry jam so often needs added pectin or an apple thrown in.

The short version

Jam sets when acid and sugar let fruit pectin chains link into a mesh that traps the liquid.

Try it yourself

Drip hot jam onto a cold plate and push it with a finger. If the surface wrinkles, the mesh has formed.

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