2 upstream of the trimming heater 10 with respect to the flow direction of the first heating medium m 1 a second conveying device 3 that conveys from the branch point v 1 to the union point v 2 with an additional volume u03b4q is arranged in the return line lr while the first heating medium m 1 circulates in the first circuit line k 1 with the first volume flow q 1 by means of a not shown conveying device a volume flow q combined from the first volume flow q 1 and the additional volume flow u03b4q and thus increased with respect to the first volume flow q 1 results between the branch point v 2 and the union point v 1 and thus in the trimming heater 10 and in the first heat exchanger 100 of the pre heating zone by this volume flow q an accurate and speedy temperature adjustment of the food product p leaving the pre heating zone vz to the temperature conditions at the entrance of the high heating zone hz in the sense of the aims of the present invention is achieved through the cooperation with the additional heating of the first heating medium m 1 in the trim heater 10 as a further flanking measure in the sense of the aims of the present invention improvement of the heat transfer conditions is still proposed at least in the first heat exchanger 100 of the pre heating zone in fact at the side of the first heating medium m 1 and also at the.