Pectin and jam

Why

Pectin is a component of fruit cell walls that is released into the juice during cooking; its long molecules repel each other in water, but sugar draws the water away from them and acid suppresses their charge, so they link into a network that holds the juice like a gel. Classic pectin therefore needs roughly 60% sugar in the finished jam and an acidity around pH 3; apples, redcurrants or gooseberries have plenty of pectin, strawberries, cherries or blueberries little, and fully ripe fruit less than less ripe fruit. With too little acid the jam will not set, with too much it weeps juice, too short a boil leaves it runny and too long a boil makes it gummy and dark, and very long boiling breaks the pectin down so that the jam may not set at all. Sugar also preserves the jam, which is why it is not reduced in a classic recipe: jam with less sugar needs a special pectin, possibly with calcium, and keeps for a shorter time in the fridge. Jam without added pectin is done at about 104 °C, roughly 4 °C above the boiling point of water, or when a drop sets on a chilled plate.

How to do it

• Combine low-pectin fruit such as strawberries or blueberries with apples or redcurrants, or use pectin or jam sugar exactly as the packet says.
• Always cook just one batch of the recipe; a doubled batch takes longer to cook and tends to be runnier.
• For the test, chill a small plate in the freezer, drip some jam onto it and take the pan off the heat meanwhile; if the drop sets and wrinkles when pushed with a finger, it is done.
• With a thermometer, cook jam without added pectin to about 4 °C above the temperature at which water boils where you are, which is 104 °C at sea level.
• For storage, fill jam hot into sterile jars with 6 mm headspace and process for 5 minutes in boiling water, 1 minute longer per 300 m above sea level (Home canning and botulism card).
• To glaze a cake such as a Sachertorte, bring the jam to the boil, sieve it and brush it on hot in a thin layer; let it dry before pouring over the icing.

If it goes wrong

Leave runny jam undisturbed for a few days, as some set slowly; if it stays runny, reboil it with pectin following the manufacturer's instructions, or use it as a fruit sauce for yoghurt or pancakes. Thin gummy, dark jam with a little water or fruit juice while warming it. Dissolve sugar crystals by gentle warming, and next time stir the sugar in completely before boiling. Throw mouldy jam away entirely: scraping off the mould is not enough, because its threads and toxins can reach the rest of the jar.