end wall and two on the rear wall of a rectangular oven other arrangements for mounting a plurality of bag and holder assemblies 34 for simultaneous heating in a microwave oven can be employed and may vary with the shape of the oven cavity the drive mechanism will be arranged to simultaneously oscillate and advance the rotation of each holder assembly for example turning it in one direction about 270 u00b0 then reversing the direction of rotation for about 180 u00b0 then turning it forward again about 270 u00b0 and so on in this way the holder assembly is rocked to ensure mixing of the liquid with the still frozen portions in the bag during the thawing process while also ensuring a constantly varying orientation of the assembly relative to the microwave energy field by the progressive advancement of the average rotational position of the holder assembly in the field it has been found that operating with the standard microwave frequency of 2450 mhz and using a standard 300 ml thin plastic plasma bag the major dimensions of the holder 10 should preferably be held within relatively close limits i e within a few percent of the following a the diameter d1 of the holder 10 without the flange 14 i e the diameter of the base 11 17 5 cm b the depth d2 of the cavity formed by the apron 15 i e the distance from the base 11 to the apron 15 1 8 cm while the full depth d3 of the external dimensions 2 cm c the width d4 of the apron at its center 4 5 cm other dimensions that are less critical but nevertheless should preferably be adopted are e the.