A process and a device are disclosed for mixing-in liquid, pourable or lumpy substances into masses of foodstuffs, in particular foams or doughs. In order to ensure that neither the structure nor the form of the substances or of the foodstuff masses are modified during the mixing process, an elongated mixing container (1) contains at least two horizontal, spiral helices (4a, 4b) that turn in opposite directions, the diameters of which partially overlap without contact and the pitches of which are inclined in the direction of transport of the material to be mixed, so that a mixing section (4d), along which the substances can be mixed without pressure into the foodstuff mass, is formed between an input (9) and an output (3).