Ripe Blackberries are lacking in pectin and you would have problems getting a good set without the apples. You can also add some lemon juice to taste as they are also lacking in acid.
Ingredients for Blackberry and Apple Jam:
- 1 lb (450 g) cooking apples
- ¼ pint (145 ml) water
- 4 lb (1.8 kg) sugar
- 3 lbs (1.35 kg) blackberries
Method for Blackberry and Apple Jam:
- Peel and core the apples. Weigh and cut into slices.
- Put into a preserving pan with just enough water to prevent the apples burning.
- Simmer gently until the apples are soft.
- Pick over and wash the blackberries, drain well and put into a pan with ¼ pint water.
- Stew until tender and add to the cooked apples.
- Test for pectin.
- Add the sugar and stir until dissolved.
- Bring to the boil and boil rapidly for about 10 minutes, until the jam sets when tested.
- Remove the scum.
- Pot and seal at once.
Makes approximately 6 lbs (2.7 kg) of blackberry and apple jam.
General Information on Jam Making