Crystallised jam

Sugar out of solution

Jam is a supersaturated sugar syrup — it holds more sugar than water at that temperature normally would. That state is stable as long as nothing gives the sugar a reason to come out.

A single undissolved crystal is enough. It acts as a seed, and other sugar molecules attach to it and grow, spreading through the jar over days or weeks until the jam is gritty throughout.

The same thing happens with fudge, caramel and syrup for exactly the same reason, and the preventions are the same.

The three causes

• Sugar not fully dissolved before boiling. By a long way the commonest. Any grain still solid when the pan reaches a boil becomes a seed crystal. Stir over a low heat until the mixture is completely clear before it ever bubbles.
• Crystals on the side of the pan. Splashes dry on the pan wall, crystallise, and fall back in. Brush the sides down with a wet pastry brush, or put the lid on for a minute early so condensation washes them down.
• Boiled too long. Over-boiling drives off too much water, so the syrup is more supersaturated and more likely to crystallise. This is why over-set jam and crystallised jam so often go together.
• Too much sugar for the fruit, which pushes the same way.
• Stirring during the rolling boil. Agitation encourages crystals to form. Stir to dissolve, then leave it alone.
• Storage in a cold place, which reduces how much sugar the syrup can hold.

Preventing it

1. Warm the sugar in a low oven before adding it. It dissolves much faster, which shortens the pre-boil stage.
2. Add the sugar off a fierce heat and stir over a low heat until the mixture is completely clear. Lift the spoon and look: no grains.
3. Only then raise the heat to a rolling boil.
4. Brush any crystals off the pan sides with a pastry brush dipped in water.
5. Do not stir once it is boiling.
6. Add lemon juice where the recipe calls for it. Acid inverts some of the sugar into forms that crystallise far less readily, which is part of why it appears in so many recipes.
7. Stop at setting point rather than boiling on.

Can it be fixed

Yes, usually

Tip it back into a pan, add a splash of water and warm it gently until the crystals dissolve.

Add lemon juice

A tablespoon per jar helps stop it recurring.

Do not boil it hard again

That risks over-setting and re-crystallising.

Repot into clean sterilised jars

Sterilising and storage are food-safety matters for the Food Standards Agency.

Or use it as it is

Gritty jam is fine warmed as a sauce, in a bakewell, or in a glaze where the texture disappears.

It is not spoilage

Crystallisation is a physical change and says nothing about whether the jam is safe.

Common questions