Jam: Getting a Set

Jam sets because three things come together: pectin from the fruit, acid, and enough sugar boiled to a high enough temperature. If a jam will not set, one of the three is short.

Pectin and acid

Citrus, cooking apples, gooseberries, currants, damsons and quinces are rich in pectin and set easily. Strawberries, cherries, raspberries, rhubarb and pears have little.

For low-pectin fruit, use jam sugar, which has pectin added, or cook it with a high-pectin fruit, or add lemon juice, which supplies the acid pectin needs to work.

Slightly underripe fruit has more pectin than very ripe fruit. A mixture of the two gives both set and flavour.

Cooking

Soften the fruit fully before the sugar goes in: once the sugar is added, the skins stop softening and stay tough.

Dissolve the sugar completely over a low heat before letting it boil. Undissolved sugar crystallises in the jar later.

Then boil hard and fast, at a rolling boil that cannot be stirred down. A short, hard boil gives a fresher-tasting, brighter jam than a long slow one.

Testing for a set

Put a few saucers in the freezer before you start. Take the pan off the heat, drop a little jam on a cold saucer and leave it for a minute. Push it with a finger: if the surface wrinkles, it is set. If not, boil for 2 minutes more and test again.

A sugar thermometer should read 104 to 105°C at the setting point, but the saucer is the more reliable test.

Potting

Sterilise jars by washing them and drying them in a low oven, or on the hottest dishwasher cycle, and fill them while both jars and jam are hot.

Leave jam with whole fruit to stand for 10 minutes before potting, so the fruit stays spread through rather than floating to the top.

A jam that has not set after a day or two can be tipped back into the pan and boiled again.