Wash all the blueberries thoroughly. Mix the packet of Gelfix Extra with 2 tbsp sugar and add this to the fruits. Stir nonstop on moderate heat. Once the mixture comes to a boil, add the rest of the sugar.
Boil about 5 min., while removing the foam. Add the citric acid and remove from the heat.
To vacuum-seal the mixture, distribute it into dry, warm jars, cap and turn them upside down.
Personally, I cool the jam, distribute it into jars, close them and put them to boil for another 5 min.