Advanced options
Classic ratio: equal weights fruit and sugar. Low-pectin fruits set better with jam sugar (added pectin) or lemon juice (about 1 lemon per kg). Setting point is 105C on a thermometer, or when a blob on a chilled saucer wrinkles when pushed. Sterilise jars at 140C for 15 minutes and fill hot.
What The Jam Calculator Works Out
- Sugar needed: the one-to-one rule, equal weights of fruit and sugar, so 2kg of picked fruit takes 2kg of sugar. Sugar is not just sweetness but the preservative and half the set.
- Expected yield: fruit plus sugar less the 17% of water the rolling boil drives off, so 2kg of each boils down to about 3.3kg of jam. Setting point is 105C on a sugar thermometer, cross-checked by the saucer test (a teaspoon on a freezer-chilled plate wrinkling when pushed); overboiling past set costs colour and freshness.
- Jars (454g / 1lb): the yield divided into traditional one-pound jars, with a part jar flagged when there is more than a third of a jar over. Sterilise jars in a 140C oven and fill hot to the brim; the popped-down lid is the seal certificate.
- Fruit type: set is chemistry with two levers, pectin and acid. Currants, gooseberries, cooking apples and plums carry their own; strawberries and cherries want jam sugar (pre-blended with pectin), generous lemon juice, or a muslin bag of apple cores the old way. Fruit picked slightly underripe sets better than fully ripe.
- Sugar share and water boiled off (advanced): cut the sugar below 100% of the fruit and the finished jam drops under 60% sugar, which shortens shelf life and softens the set; proper reduced-sugar jams are a different recipe (more pectin, fridge storage). Adjust the evaporation figure if your batches come out heavier or lighter.
- Jars at your size (advanced): the same yield in 340g, 250g or 1kg jars, with how many to sterilise including the part jar.
- Lemons (advanced): about one per kg of medium-pectin fruit, half as much again for low-pectin fruit, none for high-pectin fruit that sets itself.
- Pan size (advanced): a preserving pan should be no more than a third full before the boil, so allow about 3 litres of pan per kg of fruit plus sugar. Warm the sugar before it goes in so the boil resumes fast, and a knob of butter settles the scum.
- Cost per jar (advanced): fruit and sugar at your prices divided by the jars. With foraged or home-grown fruit and sugar at about GBP 1.10 per kg, a 1lb jar costs under 50p against GBP 2 to GBP 4 for a premium shop jar. Sealed jam keeps a year or more in a cupboard and a month in the fridge once opened.