How to make jam

The three-legged stool

Pectin is a carbohydrate in the cell walls of fruit. Under the right conditions its long molecules link into a mesh that traps liquid, which is what a set is. Some fruits are full of it — currants, gooseberries, quince, apples, damsons, citrus pith. Some have very little — strawberries, cherries, peaches, figs, rhubarb.

Acid does two jobs. It helps break pectin free of the fruit tissue during cooking, and it neutralises the negative charges along the pectin molecules that would otherwise repel each other and keep them apart. Without enough acid, the mesh never forms. This is why lemon juice appears in so many jam recipes and why leaving it out gives syrup.

Sugar competes for water. Pectin molecules only find each other when there is not enough water to keep them apart, and the high sugar concentration in a jam is what forces the issue. It is a structural ingredient, not only a sweetener, which is why low-sugar jams need added pectin or a different setting agent.

Which fruits have what

Read this and most jam recipes explain themselves.

High pectin, high acid

Cooking apples, crab apples, damsons, blackcurrants, redcurrants, gooseberries, cranberries, quince, Seville oranges, kumquats. These set themselves.

High pectin, low acid

Sweet apples, ripe plums. Add lemon juice.

Low pectin, high acid

Raspberries, apricots, sour cherries, rhubarb, pineapple. Add pectin, or a high-pectin partner.

Low pectin, low acid

Strawberries, sweet cherries, peaches, figs, pears, mulberries, melon. Add both — lemon juice and jam sugar.

Ripeness matters

Pectin is highest in slightly under-ripe fruit and falls as fruit ripens. A proportion of firm fruit improves the set of any jam.

The method, whatever the fruit

Ratios and setting point are matters of texture and are given here. FoodHQ does not publish keeping times, jar-processing schedules or the acidity figures that decide whether a sealed preserve is safe on a shelf. Those are safety numbers set by a food safety authority rather than by a cookery writer. In the UK the Food Standards Agency is the authority — follow its guidance on sterilising, sealing and storing, and treat this page as the explanation of what is happening in the pan.

1. Weigh the fruit. Everything else is calculated from that, and a recipe that says "four punnets" is guessing.
2. Prepare the fruit and, for tough-skinned fruit like currants or gooseberries, simmer it in a little water first until the skins are soft. Sugar added before the skins soften keeps them tough for good.
3. Put a couple of small plates in the freezer. This is the setting test and it is worth doing first.
4. Add sugar — commonly equal weight to fruit, less for a softer set and a fresher flavour — and any lemon juice. Stir over a low heat until every grain has dissolved before the pan boils. Undissolved sugar crystallises in the jar.
5. Bring it to a hard, rolling boil that cannot be stirred down. This is where water evaporates and the set forms, and it needs to be fierce.
6. Test from about eight minutes. Take the pan off the heat, put a teaspoon of jam on a frozen plate, wait a minute, and push it with a fingertip. If the surface wrinkles, it is set. If it floods back, boil for two more minutes and test again.
7. Take it off, skim any scum, and let it stand for ten minutes before potting so the fruit does not all float to the top.

When it will not set

Diagnose rather than boil indefinitely — over-boiling gives a dark, caramelised, stiff jam.

• It was never going to. Low-pectin, low-acid fruit with plain sugar and no lemon. Add lemon juice, or commercial pectin, and reboil briefly.
• Not boiled hard enough. A gentle simmer will not do it. The boil must be violent.
• The pan is too small. A wide, shallow pan evaporates far faster than a deep one. A preserving pan is wide for this reason.
• Too much water added at the start. All of it has to boil off again.
• Sugar not fully dissolved before boiling, which gives a grainy jam and interferes with the set.
• Tested too early or too hot. A jam thickens considerably as it cools. Always test on a cold plate.
• If it is still runny in the jar, tip it back into the pan, add lemon juice, reboil, and retest. It can be rescued.

Common questions