Peel and halve the pineapple and cut out the hard stalk. Cut the fruit flesh into 1/2 - 1 cm pieces. Wash plums, drain well, stone and cut into small pieces. Mix fruit pieces and jam sugar in a pot. Squeeze lemon and add the juice. Let the fruit mass soak for 3-4 hours.
In the meantime wash lemon balm, dab dry, pluck leaves and cut into strips. Bring the fruit to the boil while stirring and let it boil for 4 minutes until it becomes bubbly. Remove from the stove. Stir in lemon balm. Fill the jam hot into the prepared glasses. Close well