Why has my jam gone sugary and crystallised?
The three causes of gritty, crystalline jam, which of them you can undo, and how to stop it happening again.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 2 min
Short answer
Sugar crystals form when there is more sugar than the liquid can hold, or when undissolved grains seed the mixture. Warm the jar gently in a pan of hot water with a splash of water or lemon juice and stir until the crystals dissolve, then keep that jar in the fridge. Prevent it by dissolving the sugar completely before the boil and adding lemon juice.
Jam is a supersaturated sugar solution held in a pectin mesh — there is more sugar dissolved in it than there strictly should be. That is a slightly unstable state, and any excuse the sugar gets to come back out of solution, it takes. Grittiness is that happening.
Step by step
- Identify which crystals you have.Fine grittiness throughout is usually sugar. Hard, sharp, glassy crystals in grape, plum or damson jam are more likely to be tartrate crystals from the fruit itself, which are harmless and mostly settle out.
- Warm the jar to redissolve.Stand the open jar in a pan of hot — not boiling — water, and stir until the crystals go. Add a tablespoon of water or lemon juice per jar if it is very stiff.
- Put the rescued jar in the fridge.You have opened it and added water, so it is no longer a sealed shelf-stable preserve. Treat it as an opened jar and eat it within a few weeks.
- Dissolve the sugar fully next time, before the boil.Sugar goes in over low heat and is stirred until you cannot feel any grains on the base of the pan with a spoon. Only then does the heat go up. Any undissolved grain that survives into the jar becomes a seed crystal that grows.
- Add acid.Lemon juice inverts some of the sucrose into glucose and fructose, which crystallise far less readily. This is the same reason a splash of lemon goes into a sugar syrup for sweets.
- Do not overboil.Boiling well past setting point drives off water, which raises the sugar concentration past what the remaining liquid can hold. Overcooked jam and crystallised jam are usually the same jam.
- Wipe the pan sides down during the boil.Sugar splashed up the side of the pan dries, crystallises, and falls back in as seed crystals. A pastry brush dipped in water run round the inside of the pan prevents it.
Tips
- Storing jam somewhere that swings between warm and cold speeds crystallisation. A stable cool cupboard is better than a shelf beside the oven.
- Marmalade crystallises more readily than most jams because it has a long boil and a high sugar ratio. Extra lemon in the pan helps.
- If a jar has both crystals and any fuzzy growth, ignore the crystals and throw it away — mould decides.
Questions people ask
Is crystallised jam safe to eat?
Yes. It is a texture problem, not a spoilage problem — the sugar has simply come out of solution. Provided the jar sealed properly, has no mould and smells right, it is fine to eat as it is or to warm through and redissolve.
What are the hard glassy bits in my grape or damson jam?
Almost certainly tartrate crystals, which are natural to those fruits and come out of solution in the cold. They are harmless. Straining the cooked fruit and letting the juice stand cold overnight before making the jam lets most of them settle out first.