Good jam begins in the fruit bowl

No amount of stirring will rescue jam made from pale, hard fruit. Pectin — the natural gelling agent that lets you skip the shop-bought stuff — is highest in fruit that is just ripe, and flavour peaks at the same moment. Strawberries, raspberries, blackcurrants, gooseberries, damsons and plums all set happily on their own if you pick or buy them when they smell of themselves. A handful of slightly underripe berries helps a soft-set fruit along; fruit that is actually rotten will taste of nothing much.

Small batches are your friend. Two or three kilograms of fruit makes four or five jars, boils quickly, and lets you test and adjust without risking a whole day's picking.

The three things that decide whether jam sets

Pectin, acid and sugar. Get the balance right and the set happens by itself; get it wrong and you will boil for an hour and still have compote.

• Pectin comes from the fruit — and from pips, skins and cores. Raspberries, blackcurrants, gooseberries and cooking apples are rich in it; strawberries, cherries and rhubarb are poor and need help.
• Acid makes pectin set. One to two tablespoons of lemon juice per kilogram of fruit sharpens the flavour and firms the gel; for low-pectin fruit, use the juice of a whole lemon.
• Sugar preserves the jam and helps it set, but too much prevents gelling and dulls the fruit. As a rough guide: 1kg blackcurrants or gooseberries to 750g sugar; 1kg strawberries or raspberries to 900g–1kg.

Granulated or preserving sugar is all you need. Jam sugar with added pectin is fine, but rarely necessary when the fruit is ripe and the acid is there.

Equipment that earns its place

A wide, heavy-based pan is the single most useful thing you can own for jam. The wider the surface, the faster water evaporates — which means a shorter boil, a brighter colour and a fresher taste. A tall narrow saucepan will do at a push, but expect a longer, duller cook.

• A preserving pan, or the widest deep pan you have
• A long-handled wooden spoon, so you can stir without scalding your knuckles
• A jam thermometer — useful, but never a substitute for the saucer test
• Two or three saucers in the freezer
• A jug or jam funnel, and a square of muslin if you want a clear jelly
• Clean jars, sterilised and kept hot

Dissolve, then boil hard

Tip the prepared fruit into the pan with the lemon juice and warm it gently until the juices run. If you have time, layer strawberries or raspberries with the sugar and leave them overnight — the sugar draws out the juice and the fruit holds its shape better. Otherwise add the sugar now and stir over a low heat, without letting it bubble, until every grain has dissolved. Rub a little between your fingers: it should feel smooth, not gritty.

Then turn the heat up and bring it to a rolling boil — one that bubbles furiously all over the surface and cannot be stirred down. Start timing now: most small batches reach setting point in 8–20 minutes. Stir occasionally to stop it catching, and skim off the foam if you want a clear jam. Take the pan off the heat before every test.

Testing for a set

Fiddling with a thermometer mid-boil is how jams get overcooked, so rely on your senses. The wrinkle test is the most reliable: drop a teaspoon of jam onto a chilled saucer, leave it for a minute, then push it with your fingertip. If it wrinkles and holds its shape, it is ready. If it slides about like syrup, boil for another three or four minutes and test again.

A thermometer should read 104–105°C, and the flake test — letting jam drip from the spoon, where it should fall in a sheet rather than a stream — makes a good second opinion. Remember that jam thickens as it cools, so stop slightly sooner than you think you should. Overcooked jam is dark, stiff and tastes of caramel rather than fruit.

Potting up, and what to do if it stays runny

Fill hot, sterilised jars to the very brim while the jam is still hot, cover with a wax disc if you like, and screw the lid on tightly. As it cools the jam contracts and pulls the lid down — that seal is what keeps it for a year in a cool, dark cupboard. Once opened, keep it in the fridge and use it within three or four weeks.

If your jam refuses to set after cooling overnight, don't panic. Empty it back into the pan, add the juice of half a lemon and a couple of tablespoons of sugar, and boil hard for five minutes with a fresh saucer test. If it still slouches, call it fruit sauce: it is superb spooned over yoghurt, swirled through porridge, or loosened with a little water and poured over ice cream.