The pan has been at a rolling boil for a while and you can't tell whether it's ready. A spoonful on a cold plate or a quick look at a thermometer will settle it in a minute or two. Here's how each test works, what to do when a batch refuses to set, and why some fruit sets far more easily than others.

What makes jam set

Jam and jelly set when pectin, acid and sugar work together. Fruit supplies at least part of the pectin and acid, and acid is needed for the gel as well as for flavour. Sugar helps the gel form and is also the preservative, so never cut the sugar a recipe calls for. Boiling drives off water until the mixture is concentrated enough to gel, and the tests below tell you when that point has arrived.

The three setting tests

Take the pan off the heat whenever you test, or it keeps cooking while you look.

TestHow to do itSet when
Wrinkle (cold saucer)Chill two or three small plates in the freezer beforehand. Spoon a little jam onto one and leave it a minute. (Or put a spoonful on a plate in the freezer for a few minutes.)You push it with a finger and the surface wrinkles instead of flowing back.
Sheet (flake)Dip a cool metal spoon in, lift it out of the steam and tip it so the syrup runs off the side.The drops run together and slide off as one sheet. Light, runny drops mean it's early; two separate heavier drops mean it's close.
ThermometerMove it through the mixture rather than leaving it in one spot.Jam: roughly 104°C to 105.5°C, with 105°C the usual target. Jelly without added pectin: about 104.5°C.

The thermometer is the most dependable of the three, as long as it reads true. The saucer test needs no equipment and makes a good cross-check.

Checking your thermometer

Setting point is about 4.5°C above the boiling point of water. Before you start, stand the thermometer in a pan of boiling water and note the reading. If it shows a degree under or over 100°C, move your target by the same amount. Height works the same way: water boils at a lower temperature the higher you are, so the setting point drops by about 1°C for every 300 m above sea level.

Why some fruit sets easily and some needs help

Pectin and acid levels vary by fruit and by ripeness. The riper the fruit, the less pectin it contains, and acid is also higher in under-ripe fruit. For jelly made without added pectin, a mix of about a quarter slightly under-ripe fruit to three-quarters fully ripe works well. Slightly under-ripe means just short of ripe, not hard and green. Overripe fruit loses its set, even in the fruits that normally set easily.

FruitSets with sugar alone?What to do
GooseberriesYes, if not overripeNothing extra
Currants (blackcurrants, redcurrants)YesNothing extra
Quince, crab apples, sharp applesYesUse apples that are sharp, not fully ripe
Sharp plumsYesUse fruit that isn't overripe
Blackberries (brambles)Yes if some are slightly under-ripe; fully ripe ones are low in acid, pectin or bothMix in some under-ripe berries, or use a tested recipe that adds acid or pectin
RaspberriesSources differ: some class them as high in pectin, others as needing added pectin or acidStart testing early and let the test decide
StrawberriesNo: need added acid, pectin or bothUse a tested strawberry recipe that adds them
RhubarbNo: low in pectinUse a tested recipe that adds pectin
RowanNoCook with a pectin source such as crab apples or Bramley cores

Rowan and bramble are Scottish autumn staples, and both need help for different reasons. Rowan berries lack the pectin to set alone. They are also bitter and not for eating raw; cooking makes them safe. Brambles lose their set as they ripen, so a late, fully ripe picking benefits most from a few sour berries mixed in or a little lemon juice. To pair a low-pectin fruit with a high-pectin one, see mixing fruit and veg in preserves.

If it won't set, or sets too hard

Give it time first

A jar that looks runny may not be finished. Leave jellied preserves unmoved for at least 12 hours after potting. Some take up to 2 weeks to set completely, and plum jelly is one of the slower ones.

Reboil a soft batch

If it still hasn't set, tip it back into the pan and reboil until it wrinkles or reaches setting temperature. For jelly made without added pectin, add 2 tablespoons of bottled lemon juice for every 950 ml of jelly, bring it to the boil, boil for 3 to 4 minutes, then test again. Skim, then pot into freshly sterilised jars.

Common causes of a soft set are too little acid, not boiling long enough, and making too large a batch at once.

Avoid boiling too far

Overcooking gives a stiff, tough set and a darker colour, so start testing early. Small batches boiled rapidly reach setting point sooner than big ones cooked slowly, so never double a tested recipe.

Jelly: the extra steps

Jelly uses only the strained juice, so the setting starts with how you extract it.

  • Cook the fruit until just soft. Berries need 10 minutes or less; apples and other hard fruit may need 20 to 25 minutes. Overcooking the fruit at this stage lowers the pectin's ability to gel, and too much water dilutes it.
  • Let it drip. Pour the cooked fruit into a jelly bag and let the juice drip through on its own. Squeezing or pressing the bag makes cloudy jelly, and so does using green fruit.
  • Keep batches small. Use about 1 to 1.4 litres of juice per batch.
  • Pot it quickly. Jelly left standing in the pan, or poured too slowly, turns cloudy. Skim off the foam and fill the jars straight away.
Dark red fruit pulp inside a cloth jelly bag suspended from a stand, clear juice dripping through into a glass bowl below
For a jelly, the cooked fruit drips through a jelly bag before the juice is boiled to setting point.

Whether it's jam or jelly, pour it hot into clean, fully dry, sterilised jars and put new lids on at once, while it's still hot, so the jar seals as it cools. For the oven, dishwasher and hot-water methods, see sterilising jars; for shelf life, see how long preserves keep.

Related reading:

Common questions

Does the wrinkle test work for jelly as well as jam?

Yes. Spoon a little jelly onto a chilled plate, wait a minute and push it. If the surface wrinkles, it has set.

Why has my jelly turned dark?

Long boiling of the sugar and juice darkens it. Jars kept too long or somewhere too warm also darken over time, so store them somewhere cool and dark.

Can I use less sugar so it tastes fruitier?

Not by cutting the sugar in an ordinary recipe. Use a tested recipe written for low-sugar jam, whose setting point differs. Low-sugar jam keeps less well and deteriorates sooner in the jar, so store even sealed jars exactly as that recipe directs. Any jam goes in the fridge once opened and is best eaten within a few weeks to a month or two; low-sugar jam needs eating sooner.

A jar has grown mould on top. Can I scrape it off?

No. Throw away any jar with mould, or with a yeasty, fermented or alcohol smell. Mycotoxins (poisons made by some moulds) have been found in mouldy jelly, so removing the visible mould doesn't make the rest safe.

Sources

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