How do I make jam that sets?

Pectin, acid and sugar in balance — and the cold plate test that tells you when to stop.

Difficulty: intermediate
Time: 1 hr
Read: 2 min
Safety: caution

Short answer

Setting needs pectin, acid and sugar together. Use slightly underripe fruit, add lemon juice, and test on a chilled saucer — push the jam with a finger and it should wrinkle. That test is more reliable than any thermometer.

Jam sets when pectin forms a mesh, and pectin only does that with enough acid and enough sugar. Runny jam almost always means one of the three was short, or it simply was not boiled long enough.

Safety

Boiling jam is far hotter than boiling water and causes severe burns. Use a deep pan, keep children away, and never lean over it while stirring. Sterilise jars properly and discard any jar that shows mould or a broken seal.

What you'll need

Fruit
Sugar
Lemon juice
Deep heavy pan
Sterilised jars
Saucers in the freezer

Step by step

Put two or three saucers in the freezer first.This is your setting test and it needs to be genuinely cold. Do it before you start cooking.
Use some slightly underripe fruit.Pectin is highest just before full ripeness and drops as fruit ripens. A mix of ripe and slightly underripe gives flavour and setting power.
Add lemon juice.Acid is essential for pectin to set, and most fruit does not have enough on its own. A tablespoon or two per kilo of fruit.
Use the right sugar for the fruit.Jam sugar contains added pectin and suits low-pectin fruit like strawberries and cherries. Ordinary sugar is fine for high-pectin fruit like blackcurrants, apples and citrus.
Dissolve the sugar completely before boiling.Gentle heat, stirring, until no grains remain. Boiling with undissolved sugar gives a grainy jam.
Then boil hard.A rolling boil that cannot be stirred down. Timid simmering never reaches setting point. Watch it — it rises fast.
Test on a cold saucer after about 8 minutes.A teaspoon on a frozen saucer, back in the freezer for a minute, then push it with a finger. If the surface wrinkles, it is ready. If it floods back, boil for a few more minutes and test again.
Pot it hot into hot sterilised jars and seal immediately.Fill nearly to the top, lid on straight away. The cooling contents create the seal.

Tips

Skim the foam at the end rather than throughout — a small knob of butter stirred in disperses most of it.
Let the jam stand for ten minutes before potting if it contains whole fruit. It stops everything floating to the top.
Runny jam is still perfectly good on yoghurt, ice cream or in a sauce. It can also be reboiled with more lemon juice.

Questions people ask

Why did my jam not set?

Not enough pectin or acid, or not boiled hard enough or long enough. Add lemon juice and reboil, testing on a cold saucer each time.

Do I need a thermometer?

No. The cold saucer test is more reliable in a home kitchen than the 105°C setting point, which varies with sugar concentration.