Ingredient

Pectin

Pectin is the substance in plant cell walls that holds fruit together, and — extracted and concentrated — the thing that sets jam. A jam that has not set is nearly always missing one of three conditions.

The three conditions for a set

Ordinary pectin sets only when pectin, sugar and acid are all present in the right proportions. Miss any one and the jam stays a syrup.

Pectin is the network itself. Sugar competes for the water, freeing the pectin chains to associate with each other rather than staying dissolved. Acid reduces the electrical repulsion between the chains so they can approach and bond.

That is why a low-sugar jam made with ordinary pectin does not set, and why sweet, low-acid fruits like strawberries and pears need lemon juice added. It is also why special low-sugar pectins exist: they are chemically modified to set with calcium instead of sugar, which is an entirely different mechanism.

Fruit by pectin content

High-pectin fruit sets on its own; low-pectin fruit needs help.

High

Cooking apples, crab apples, quince, redcurrants, blackcurrants, gooseberries, citrus peel and pith, cranberries, damsons.

Medium

Plums, apricots, raspberries, blackberries, greengages.

Low

Strawberries, cherries, rhubarb, pears, peaches, figs, elderberries, most tropical fruit.

The classic fixes for low-pectin fruit

Add lemon juice, add grated apple or apple juice, use preserving sugar with added pectin, or combine with a high-pectin fruit.

Ripeness matters

Under-ripe fruit has more pectin than ripe. A mixture of slightly under-ripe and fully ripe fruit gives set and flavour together.

Reaching a set

• Boil it hard. A rolling boil is needed to reach the setting temperature and to evaporate enough water. A gentle simmer will never set jam.
• Test properly. Chill a saucer in the freezer, put a spoonful on it, wait a minute, and push it with a finger. If it wrinkles, it is set.
• Do not overcook. Boiled far past the setting point, pectin breaks down again and the jam gets darker, more caramelised and less likely to set.
• Add lemon juice to any low-acid fruit — a tablespoon or two per kilogram.
• Use a wide pan. Evaporation happens at the surface, and a jam in a tall pan takes far longer and darkens.
• Skim the foam at the end, or add a small knob of butter, which disperses it.

When it will not set

• Boil it again with added lemon juice, and test again. This works more often than people expect.
• Add commercial pectin and re-boil briefly.
• Add grated apple or apple juice, boil, and re-test.
• Accept a syrup. Unset jam is an excellent sauce for ice cream, yoghurt and pancakes, and it is not a failure worth another hour.
• Do not just keep boiling. Past the setting point pectin degrades, and you will get dark, over-cooked syrup rather than jam.
• Give it time. Some jams take a day or two to reach their final set in the jar.

Where pectin turns up besides jam

• In every fruit and vegetable, as part of the cell wall. It is what holds plant cells to one another.
• Softening as things cook. Pectin breaks down with heat, which is a large part of why cooked vegetables go soft — see cell wall breakdown.
• Acid slows that breakdown, which is why potatoes in a tomato sauce stay firm for far longer than potatoes in plain water, and why adding lemon to cooking beans keeps them stubbornly hard.
• Alkali speeds it up, which is why a pinch of bicarbonate of soda in the water softens chickpeas and pulps onions faster.
• As an added setting agent, sold as powder or liquid, for jams made from low-pectin fruit.
• In commercial food as a gelling and thickening agent, where it does the job gelatine does but sets firmer and is plant-based.

Common questions