- 1kg plums, halved and pitted (keep the stones)
- 300g blackberries
- 1.3kg preserving sugar
- Put the plums and blackberries into a preserving pan or very large saucepan with 350ml water. Add the plum stones, as the pectin they contain helps the jam to set. Bring to the boil, then reduce the heat and simmer gently for 25-30 minutes, until the plums are tender.
- Remove from the heat and add the sugar, stirring gently until dissolved. Return to the heat and gently boil over a medium heat for 20-30 minutes, until setting point is reached. To check, remove the saucepan from the heat, spoon a little preserve onto a cold plate or saucer and leave for 2 minutes – it should wrinkle softly when your finger is pushed over the surface. If this point has not been reached, return the saucepan to the heat and continue to boil for another 2-3 minutes. Test as before. (You may need to test several times, though be patient, as this testing is crucial to achieve the correct consistency).
- Turn off the heat and allow the jam to settle for 10 minutes. Meanwhile, sterilise six 250g clean jam jars by placing them in a low oven at 150°C/Gas 2 for 10 minutes.
- Skim off any scum from the jam and remove as many plum stones as possible. Stir the jam, then ladle it into the hot sterilised jars. Seal with the lids. Leave to cool, then label. Store in a cool place for up to 1 year.
Tip: Put two or three small plates or saucers in the fridge when you add the sugar to the jam, so that you have them already chilled for checking the setting point.