There is disclosed a system comprising a method and an apparatus for making an air-containing mixture 7, where air 8 is introduced into a viscous mass of raw mixture 5 for forming the air-containing mixture, and where the introduction of air is performed in connection with introducing the mixture in a treating unit 13. In the system, the raw mixture will be pumped under pressure through a nozzle 18 for forming least one jet 19 which is sprayed through an air space 17 against the surface 21 of the mixture, so that air is entrained in the mixture and that the air-containing mixture formed thereby is subsequently introduced in the treatment unit.