A METHOD OF THAWING FROZEN FOOD AND AN EQUIPMENT FOR CARRYING OUT THIS METHOD

Abstract: A method of and an equipment for thawing large pieces, such as parts of bodies of food or the like, by means of microwave energy, preferably at a frequency within the ISM-band of 915 MHz in a large, closed, metallic and microwave sealed room (10) with dimensions exceeding four wavelengths in all directions, simultaneous with convective air cooling of the objects (16), whereby the microwave power density and the cooling effect are so adjusted that a frozen surface layer is maintained during the thawing until the inner parts are thawed.

Description: A method of thawing frozen food and an equipment for carrying out this method The present invention relates to a method of thawing frozen food in the shape of big pieces, e.g. parts of or whole animal bodies, and to an equipment for carrying out this method. It is nowadays more and more frequent to use microwaves as a heating source for thawing, which preferably in household microwave ovens offers the advantage of a short thawing process. In industrial applications, various types of microwave equipment are mainly used for so-called tempering of frozen animal products, i.e. to perform a process where the phase transition from the frozen state is only partial. In this semi-solid state of the meat it is possible to cut and chop it while it keeps its liquid and freshness. However, by thawing, the phase transition is complete so that the final temperature exceeds 0 C. A problem in microwave thawing appears in that the depth of penetration of the waves is essentially reduced, when the temperature of the object is increased, e.g. the depth may be 150 mm at -20 C, 75 mm at -10 °C and 20 mm at 0 C for microwave frequences about 1 GHz. The depth of penetration is defined as the perpendicular distance from the plane surface to a point where the intensity of the plane wave in the object is decreased by the ratio l:e of the value at the surface. The heat conduction (heat transmission coefficient) is comparatively large in a frozen object but small in a thawed object. By the present invention a method of thawing a frozen object with high water content is presented, in which the penetration depth during most of the treatment is maintained on a large value owing to the measure of keeping a surface layer of the object at a low temperature. If a big object is being cooled so that the surface layer remains frozen (so-called surface freezing) and the object is at the same time heated by a medium or low microwave power density, the thawing will start below the surface layer. Thus, the absorption ability is increased in the region under this layer and somewhat decreased in the surface layer, and as the total incident microwave power is assumed to be constant, the surface la R will in practice mainly transmit without absorption the microwaves so that a greater portion of the absorption takes place deeper in the object than without surface cooling. The surface layer can thus remain frozen and maintain its high heat conductivity. This means that the layer will not be very thin and that there will be an equilibrium thickness, maintained on one hand by the difference of microwave absorption between the frozen and thawed regions, OMPI IPϋ respectively, and on the other hand by the heat conduction between these regions which tends to reduce temperature differences. If the microwave power density is low, the cooling will dominate and the temperature rise will proceed slowly, preferably deep below the surface. The difference in microwave absorption between the regions will not be so high that the difference in temperature can be increased. If the microwave power density is high, the frozen layer will successively become thinner, which in its turn will result in a reduction of the temperature differences in the transient region due to the shorter heat con¬ duction distance - the frozen layer will be thawed. The consequence will be that there are microwave power densities and surface cooling rates which in combination will create conditions where there will exist a frozen surface layer and an efficient microwave heating deep below the surface. Among other advantages of microwave thawing achieved by the invention over conventional methods (warm air thawing or water immersion thawing ) the following may be noted: The process time will decrease to considerably less than the half. The process is suitable also for prepacked objects or objects in containers. The microwave heating power can be controlled and rapidly adjusted to avoid damages during the process. The losses of meat juice, so-called drip losses, are essentially decreased and the wastage caused by damage by bacteria and the like is essentially decreased. The sanitary problems are largely reduced. There will be very small differences in the final temperature between different regions, also when the geometry of the object is irregular. This compensation is caused by the favourable variations in the thickness quotients frozen/thawed between thick and thin regions. It is apparent from the foregoing that within a region with a certain ratio 2 between microwave power density (per kg) and cooling effect (per m ) a frozen surface layer will be established and maintained. However, the entire region is not of practical interest, as the total energy demand for thawing is determined by the difference between the supplied microwave power and the supplied cooling effect, whilst the total energy consumption for the process is the sum of the input energy to the microwave and cooling systems together. In order to obtain, a high efficiency it is therefore necessary to try to achieve a good technical result by using as low total cooling effect as possible. A further requirement is that the frozen layer shall have a proper thickness - it should not be so thin that the improved penetration of the OMP ' microwaves deep into the object through the frozen layer is reduced, it should not exert a bad influence on the hygienic conditions, contamination or drip losses; it should not be so thick that the final parts of the thawing procedure are delayed by a time consuming thawing and temperature equilibrization in the surface layer. The proper thickness for lean meat seems to be 10 - 20 mm. Using these basic theoretical considerations it is possible to optimize the different parameters to carry out microwave thawing during surface cooling. The method and equipment therefor are defined in more detail in the following Claims 1 and 5. An example of a method according to the invention will be described in the following with reference to the accompanying Fig. 1 showing a diagram. Also an equipment according to the invention for carrying out the method is described with reference to the accompanying Fig. 2 showing such an equipment in a vertical projection and Fig. 3 showing the equipment seen from above. Fig. 4 is a view of a modified equipment seen from above. For carrying out the method according to the invention three variable physical parameters are used, i.e. the microwave power, cooling effect and time. The microwave power is supplied by a microwave source operating in an established IS -frequency range, preferably 915 MHz. The cooling effect is obtained from a cooling apparatus with a fan device. The method is preferably carried out in a treatment room 10 with a microwave source 11 and a cooling apparatus 12 connected thereto by means of waveguides 13 and air conduits 14. In the room there are also carrying members 15 for the objects 16 to be treated. The inner walls of the room are designed for high reflectivity of microwave energy and are carefully sealed for prevention of microwave leakage. The carrying members can be movable on a horizontal track 17, e.g. in a meander pattern (Fig. 3). To avoid microwave leakage through the track from the members in the ceiling of the room, the members may be made of non-metallic material. Flanges 18 are secured to the carrying members at the ceiling in order to bring about a capacitive sealing against the ceiling so that the waves are short-circuited and do not escape. The diameter of the members at the passage through the ceiling is chosen so that wave propagation through the opening cannot occur. In another modification (Fig. 4) the shafts of the carrying members 15 are journailed so that the members can rotate. The purpose of making the carrying members movable is to make it possible for the microwaves to reach the objects from several directions and thus bring about a uniform absorption. O P As in most cases of microwave heating, the main parameters are time and temperature. In the beginning of the process the object to be treated is a fully-frozen body, the thickness of which is of the same order as the penetration depth of the microwaves, under the assumption that the waves enter from several directions. The object to be treated is subjected to microwave irradiation and cooling air in combination, and the respective effects are determined by means of a diagram based on practical experience (Fig. 1). The equilibrium between cooling effect (horizontal axis) and heating effect (vertical axis) is obtained in the shaded area of the diagram. The field to the left of this area indicates a dominating heating effect so that no frozen surface layer can be maintained, and the field to the right of the area indicates too slow thawing, i.e. the cooling effect dominates. By inclined lines in the diagram, the summed amounts P. . of the cooling and heating powers (both with positive signs) are indicated. It is desirable to keep P. as low as possible and thus to position the point of operation in the diagram in the neighbourhood of the lower end of the shaded area. It is further to be noted when choosing the point of operation that a point at the top of the shaded area (high microwave power) brings about overheated regions in the body. For lower effects the thawing time will be longer, and temperature differences in the body will be reduced by heat conduction. In the central part of the body, to which the microwaves at the end of the process do not reach, the final thawing is obtained by heat conduction. At the final part of the process, the cooling effect is decreased so that the surface cooling of the object diminishes, i.e. the point of operation is moved to the left in the diagram. The frozen surface keeps the object in a good condition during the first part of the process and will at the end be thawed essentially by heat conduction from inside regions. By experiments, the temperature in various parts of the object can be measured by temperature sensors. As should be expected, any microwave heating after the final thawing of the surface layer proceeds only in the surface region of the object, because of the smaller penetration depth of the waves in a thawed state. It can therefore be said that thawing of thick objects by means of microwaves will be technically feasible only in combination with surface cooling. It should be noted that the diagram in Fig. 1 only shows the relations in principle and that the shape of the shaded area may vary and that there may be objects for which it does not even exist. A series of conditions must be satisfied if the method shall be possible to carry out. It is, however, interesting to note OMFI that it is possible in many instances to optimize the parameters microwave power, cooling effect and time. As an addition to the method, it could also be mentioned that a preprepara- tion of the objects comprising dipping or spraying of the same with a water - containing liquid may be made. This liquid will freeze on the surface of the object and contribute to keeping it fresh during the thawing procedure and prevent an evaporation of the meat juice. In contrast to the case of conventional thawing (by surface heating) the object in microwave thawing is warmer than the air, which may give a tendency of surface evaporation.

Claims: Claims 1. A method of thawing deep-frozen objects by means of a microwave source in a treatment room, characterized in that the object is subjected to a combination of microwave irradiation and a cooling air stream, the microwave power thus penetrating a frozen surface layer of the object which is maintained by the air stream, the microwaves being substantially absorbed in and inside a frozen and partly thawed region under the surface layer. 2. A method according to Claim 1, characterized in that the object is preprepared before the treatment with microwaves by a layer of water- containing liquid, which rapidly freezes in contact with the object. 3. A method according to Claim 2, characterized in that the layer is applied by spraying. 4. A method according to Claim 2, characterized in that the layer is applied by dipping. 5. An equipment for carrying out the method according to Claim 1, com- prising a treatment room with walls having high microwave reflectivity, good microwave sealing and dimensions in all directions exceeding four wavelengths of the used microwave length, generated by a microwave source connected to the treatment room, characterized by a cooling apparatus (12) and a fan device for generating a cooling air stream through the treatment room (10). 6. An equipment according to Claim 5, characterized in that a conveying arrangement with carrying members (15) for the objects (16) to be treated is disposed in the ceiling of the room and in that the arrangement is carried out in a meander pattern. 7. An equipment according to Claim 5, characterized in that the carrying members (15) for the objects (16) to be treated are arranged in the ceiling of the room and provided with a drive arrangement for the rotation of the objects.

Inventors: IVERLUND B, RISMAN P, UDDBORN H
Applicants: IVERLUND B, MIKROVAGSAPPLICATION AB, RISMAN P, UDDBORN H
Application year: 1981
Main classification: A23B4
IPC codes: A23B4, A23L3