Wash the blueberries, drain and place in a saucepan. Lightly crush the blueberries with a potato masher and mix with gelling sugar "2:1". Bring the blueberry jam to the boil while stirring. Cook for about 4 minutes until bubbly. Finally stir in the juice of a lemon. Immediately fill into twist-off glasses. Pull the transparent film through the rum and place it on the glasses. Screw the lid on and place the blueberry jam with lemon on the lid for approx. 5 minutes. Then turn around and allow to cool. It is best to use twist-off glasses because they close airtight. Boil the glasses and lid beforehand. Place transparent film ready. The amount is enough for about 3 glasses of blueberry jam with lemon juice. The calories are calculated per glass. If one eats approx. 20 g blueberry jam per portion, that is approx. 40 kcal.