Ingredient

Jam

Jam sets because pectin, sugar and acid form a network that holds water in place. Get any one of the three wrong and you have fruit syrup. The good news is that all three are easy to control once you know which fruit you are dealing with.

The three requirements

Pectin is a substance in fruit cell walls that forms a gel in the right conditions. Sugar pulls water away from the pectin so the strands can link to each other instead. Acid neutralises the electrical charge that makes pectin strands repel one another. All three are needed and none substitutes for another.

This is why low-sugar jam is genuinely harder: with less sugar there is more free water and the network is weaker. It is doable with added pectin, but it also keeps far less well, because sugar is the preservative as well as the setting aid.

It is also why lemon juice appears in almost every jam recipe. It supplies acid, and it draws pectin out of the fruit more effectively.

Which fruits set easily

High pectin

Cooking apple, quince, redcurrant, blackcurrant, gooseberry, citrus peel, damson, cranberry. These set with no help.

Medium

Plum, apricot, raspberry, blackberry, greengage. Usually fine with lemon juice.

Low pectin

Strawberry, cherry, rhubarb, peach, fig, pear, most tropical fruit. Need added pectin, lemon juice, or a high-pectin fruit blended in.

Under-ripe helps

Pectin degrades as fruit ripens, so a proportion of slightly under-ripe fruit improves the set considerably.

Jam sugar

Ordinary sugar with pectin and acid already added. The simplest answer for low-pectin fruit.

Making jam

1. Put a few small plates in the freezer before you start. This is how you will test the set.
2. Cook the fruit gently with a little water until soft, before the sugar goes in. Sugar added too early toughens fruit skins and they never soften afterwards.
3. Add the sugar and stir over a low heat until it has completely dissolved. Undissolved sugar crystallises in the jar.
4. Now raise the heat and boil hard. A rolling boil is necessary — this is where water evaporates and the concentration rises to setting point.
5. After about five minutes start testing. Drop a teaspoon of jam on a frozen plate, wait a minute, and push it with a finger. If the surface wrinkles, it is ready.
6. Take it off the heat the moment it passes. Skim any scum, and let it stand ten minutes before potting so the fruit does not all float to the top.
7. Pot into sterilised warm jars and seal immediately.

What goes wrong

• Runny — not boiled long enough, or low-pectin fruit with no help. It can be re-boiled with lemon juice or pectin.
• Too stiff — boiled past setting point. Stir in a little boiling water and re-heat gently.
• Crystallised — sugar not fully dissolved before boiling, or too little acid.
• Fruit floating at the top — potted too hot. Let it stand and stir before filling.
• Mould after storage — jars not properly sterilised, seal imperfect, or sugar reduced too far.

Common questions