A new method for making an air-containing mixture introduces air into a viscous mass of raw mixture for forming the air-containing mixture. The introduction of air is performed in connection with introducing the mixture in a treating unit. The raw mixture is pumped under pressure through a nozzle for forming at least one jet which is sprayed through an air space against the surface of the mixture. Air is entrained in the jet of the mixture. The air-entraining jet enters the mixture, forming the air-containing mixture. The air-containing mixture formed thereby is subsequently introduced into the treatment unit.
