Strawberry jam has a habit of staying soft, syrupy and runny in the jar. The fruit is the reason: strawberries are low in pectin, the natural setting agent, so they need help. This recipe gets its set from lemon juice and a hard boil to setting point, with no bought pectin, and the fixes below cover what to do when a batch won't behave.

Makes
varies with how far it boils down; have 3 or 4 x 450g jars sterilised and ready
Keeps
6 months to a year unopened in a cool, dark cupboard; in the fridge once opened, eaten within a few weeks to a month or two
Ready to eat
once cool and set; leave the jars unmoved for at least 12 hours
In season
from early summer into autumn, depending on the type of strawberry
Time
about 10 minutes to dissolve the sugar, then a rolling boil until it reaches setting point

Ingredients

  • 900g strawberries, washed and hulled (green tops removed)
  • 800g caster sugar
  • 4 tbsp lemon juice

A bigger batch takes longer to reach setting point and sets less reliably, so for more jam make two batches of these quantities one after the other.

Method

  1. Put two or three small plates in the fridge to chill for the setting test.
  2. Sterilise the jars so they're hot and fully dry when the jam is ready: 160°C (gas mark 3) in the oven for 10 minutes, a hot dishwasher cycle, or water above 90°C for 10 minutes. Check the jar maker's instructions; some (Kilner, for example) advise against the oven for their jars. Have new lids ready, prepared as their maker directs.
  3. Put the strawberries in a preserving pan or a large, heavy pan and crush them with a potato masher.
  4. Add the sugar and lemon juice.
  5. Stir over a low heat until the sugar has fully dissolved and no grains are left on the spoon or the side of the pan.
  6. Turn the heat up to high and bring the jam to a rolling boil, one that keeps bubbling hard when you stir it.
  7. Keep boiling and stir all the time so it doesn't catch on the bottom, until it reaches 105°C on a sugar thermometer (a little lower in a hill garden; see Reaching setting point below). Start testing on a plate early rather than waiting for the thermometer.
  8. Take the pan off the heat and test for a set: spoon a little onto a cold plate, wait a minute and push it with your finger. If the surface wrinkles, it has set. If not, boil for a few more minutes and test again. The setting point guide covers the thermometer and plate tests in more detail.
  9. Skim off any foam from the surface.
  10. Let the jam cool only slightly. It must still be very hot when it goes into the hot, dry, sterilised jars. Leave about 1cm of space at the top.
  11. Wipe the rims clean with a damp paper towel.
  12. Fit new lids at once while the jam is hot, so each jar seals as it cools.
  13. Leave the jars where they are, unmoved, for at least 12 hours.
A spoonful of strawberry jam on a cold white plate, its surface wrinkling as a finger pushes it, a pan of boiling jam behind
Test the set on a cold plate: the jam should wrinkle when you push it.

Why strawberries need lemon

Jam sets when pectin, acid and sugar come together at the right concentration. Acid is needed for pectin to gel at all. In this recipe the lemon juice supplies the acid, and the long rolling boil drives off water until the fruit's own small amount of pectin and the sugar are concentrated enough to set. The pectin in fruit guide shows how strawberries compare with other garden fruit.

The sugar does more than sweeten. It's the preservative that stops moulds and other microorganisms growing, and it helps the set. Never cut the sugar in this recipe, or you'll end up with strawberry syrup that spoils sooner.

The jam sugar route

The other way to set strawberry jam is with bought pectin, either as jam sugar (sugar with pectin added) or as liquid or powdered pectin. That's a different method with its own proportions and boiling times. If you go that way, follow the recipe on the pack rather than these quantities, and don't mix the two methods. Jam made with added pectin needs less cooking and generally gives a larger yield. Buy fresh pectin each year, because old pectin can give a poor set.

Picking fruit for jam

Ripeness changes the set. The riper the fruit, the less pectin it contains, and slightly under-ripe fruit has more acid too, so a few firm berries help. The best-quality pectin is in fruit that is just ripe, so most of the pan should still be ripe fruit: about a quarter slightly under-ripe to three-quarters fully ripe works well for jam without added pectin. Overripe, soft berries give the weakest set, so save those for eating fresh.

Pick for jam when there's a good mix of just-ripe and fully ripe berries on the plants; the strawberry growing guide covers when each type crops, and the best strawberry varieties for Scotland covers choosing plants.

Reaching setting point

A sugar thermometer is the most dependable test. Jam sets at around 105°C, but that figure assumes water boils at 100°C. The set really comes at about 4.5°C above the point where water boils, and water boils at a lower temperature the higher up you are: roughly 1°C lower for every 300m above sea level. So in a hill garden the jam can set before the thermometer reads 105°C, and the plate test will show it.

Garden heightWater boils at aboutJam sets at about
Sea level100°C104.5–105°C
300m99°C103.5°C
450m98.5°C103°C

To find your own target, put the thermometer in a pan of boiling water, note the reading and add 4.5°C. That also shows up a thermometer that reads high or low. If the two tests disagree, trust the wrinkle and don't keep boiling.

Batch size matters too. A large batch takes longer to come to the boil and to reach setting point, sets less reliably, and darkens with the extra cooking.

When the jam goes wrong

ProblemLikely causeWhat to do
Runny or softNot boiled long enough; too little acid; batch too large; jars moved too soonWait up to 2 weeks, as some jam firms slowly. If it's still runny, reboil and test again (see the setting point guide), then fill freshly sterilised hot jars and fit new lids
Too stiff or toughBoiled well past setting pointTest early and often next time, and take the pan off the heat to test
Dark or tasting scorchedLong or slow boiling; a large batch; jam stored too long or somewhere warmBoil hard, stir all the time, keep batches small, and store cool
Gritty sugar crystalsSugar not fully dissolved, or grains left on the side of the panDissolve the sugar completely over a low heat before boiling
Foam in the jarsAir trapped in the hot jamSkim before filling
Mould, or a fermented, yeasty or alcohol smellPoor seal or storageThrow the whole jar away

Don't scrape off mould and eat the rest. Moulds can produce toxins called mycotoxins, which have been found in mouldy jams and jellies.

Storing the jars

The lid has to go on while the jam is hot, because that's what forms the vacuum seal as it cools. Always use new lids, never old ones; jars for preserving explains which types seal well. Store the jars in a cool, dark, dry cupboard. Once a jar is open, keep it in the fridge and eat it within a few weeks to a month or two. A low-sugar jam, made from a recipe designed for it, keeps for less time. More on keeping times is in how long preserves keep.

Common questions

Why is my strawberry jam runny?

Usually it hasn't reached setting point, or the batch was too big to set well. Give it up to 2 weeks unmoved before deciding. If it's still soft, reboil it, test again, and pot it into freshly sterilised hot jars with new lids.

Can I use less sugar?

Not in this recipe. The sugar is the preservative and part of the set. Cut it and the jam won't set properly and will spoil sooner.

Can I use jam sugar instead of this lemon-and-boil method?

Yes, but then follow the recipe on the jam sugar pack, not the quantities here. The two methods use different proportions and boiling times.

Should I use under-ripe strawberries?

A few help. About a quarter slightly under-ripe to three-quarters ripe gives more pectin and acid than fully ripe fruit alone. Avoid overripe, soft berries.

Sources

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