Microwave oven with browning means

Abstract: A microwave oven having browning means comprising a metal browning plate (10), the underside of which is provided with a layer (11) comprising a ferrite material. Via an input opening (6) for supplying microwaves to the oven cavity polarized microwaves are supplied into a space provided under the browning plate. A browning plate of a circular shape and fitted to be carried by the rotating bottom plate (8) in a said microwave oven.

Description: The invention is directed to a microwave oven comprising means for browning of a piece of food, an oven cavity and a microwave source. A general problem when preparing food in microwave ovens is the difficulty to obtain browning or a browned surface of the food. Many proposals to the solution thereof have been developed among others in the shape of accessories to the microwave oven, specifically shaped vessels for use in the oven, as well as specifically shaped packings for prefabricated dishes in the first hand. A specific problem of these accessories, vessels and packings are the influence thereof on the propagation of microwaves in the cavity and a consequent risk of a deteriorated cooking result in comparison with the situation when the cooking takes place without such facilities. Changes of the microwave propagation may also bring with it a decreased microwave efficiency in the oven. In consequence thereof problems will arise with obtaining a rapid and even heating, which may bring with it among other things an overheating of edge areas. DE-B2-25 22 934 discloses a microwave oven according to the preamble of claim 1, with a circular rotating bottom plate and with an electrical heat element in the roof. From the US patent specification no 2.138.162 it is previously known to convert microwave energy to heat energy by using a ferrite material which may be arranged on the underside of a vessel. Measurements in order to speed up and improve the efficiency of the energy absorption are not disclosed in said publication. The SE patent specification no 343742 is also based on the use of ferrite material in a so called absorbator, which may comprise a free plate alternatively being arranged at the bottom of the vessel. In the microwave oven disclosed therein the absorbator is supplied with microwave energy via a so called microwave applicator, being specifically adapted for the object. The disclosed construction of said applicator, as well as the microwave oven comprising the same, however, have a technical structure which is complicated and cost demanding. EP-A1-0317203 describes a plate of a microwave energy absorbing material, in which regions are obtained having different "loss characteristics", partly absorbing microwave energy, partly providing a coupling to the microwaves and initiating thereby higher modes of oscillation propagation in the food piece/load which is placed on said plate. Measurements in order to speed up and improve the efficiency of the heating progress have not been disclosed. One further example of prior art is found in US patent specification no 4.369.346 disclosing a cheramic plate specifically shaped for baking purposes, for example baking of pizzas. In the cheramic material on the under side of the plate a pattern of microwave absorbing areas have been provided by means of ion implantation into the cheramic material. Other kinds of browning plates which comprise a magnetic loss generating material layer are known from GB-A-2165429, EP-A-240235, US 4496815. Examples of the specifically designed packings for use in microwave ovens are disclosed in US patent specifications nos 3.302 632, 4.594.492, 4.676.857. In these packings parts of the packings have been fitted by the use of metal foil etc in order to obtain the desireable heating, browning and crisping effects. Such packings, however, only to a small extent are related to the invention. All of said prior art solutions, except SE 343742, are lack of adaption between the proposed facilities and the propagation of microwaves in the oven in which the same are intended for use. The object of the invention is to provide a microwave oven making possible browning and crisping of a food piece without the drawbacks of prior art, maintaining a desireable microwave propagation in the oven cavity and a good microwave efficiency. The object of invention is obtained by a microwave oven of the type mentioned in the introduction, which according to the invention presents the features of claim 1. The field concentration of polarized microwaves along the material layer makes possible a use of a metal plate, having advantages from the point of heat equalization, and nevertheless an effective absorption of microwave energy in the loss generating material layer and thereby a faster heating of the metal plate. One preferred embodiment of the microwave oven according to the invention, in which said input opening is provided adjacent to the cavity bottom in a sidewall of the cavity, while said matal plate is shaped as a browning plate to be positioned horizontally in the load zone of the cavity, on the upper side of which the food piece is positioned during food preparation, is characterized by having distance means for carrying said browning plate in the load zone at a distance from the cavity bottom, in which said input opening is arranged to supply microwaves with a substantially vertical E-field and propagating in the space provided by a said distance means between the magnetic loss generating material layer of the browning plate and the cavity bottom. The microwave propagation under the browning plate makes possible a stable field concentration and a good heat equalization in the metal of the browning plate. Heating of a metallic browning plate in a traditional oven by incident microwaves on the plate from the upper part of the cavity is practically impossible, because the metal plate reflects the microwaves and the dominating part of the microwave energy is instead absorbed by the food on the browning plate. One further preferred embodiment of the microwave oven according to the invention, comprising a further input opening in said sidewall at the cavity roof, in which the input openings adjoins a vertical centre line of the sidewall, is characterized in that said waveguide device is resonant for the microwaves from the microwave source and shaped to supply said polarized microwaves as being coherent and locked in phace opposition at a respective input openings, whereby the influence from the browning plate and the load on the microwave supply via said input openings is substantially eliminated and the microwave propagation in the space under the browning plate is safeguarded. The microwave oven according to the invention may further be characterized in that said browning means comprises an electric heat element provided at the roof of the cavity in order to provide, in cooperation with the browning plate, a simultanous browning of the upper side and the underside of the food piece. Because of the effective microwave absorbtion and thereby the rapid heating of the browning plate according to the invention, a mutual adaption of the browning effect of the browning plate respectively the browning element is simplified, and thereby a desireable browning of both sides may be obtained at the same time without requiring a turn around of the food piece. The browning plate used in the oven must have a good heat equalization, and its design must be adequate for use as an easily applicable accessory of a microwave oven with a rotating bottom plate and an input opening for microwaves adjoining a cavity bottom. The browning plate comprises a substantially circular metal plate of good heat conductivity, the underside of which is provided with a layer comprising a ferrite material and covering substantially the complete underside of the plate, said ferrite material having a selected Curie-point lower than the desirable maximum temperature of the metal plate during food preparation. The underside of said metal plate is designed to be stably and detachably carried by the rotating bottom plate alternatively the rotation mechanism of the bottom plate, said browning plate being thereby heated substantially by the absorption in the ferrite layer of H-field energy from microwaves propagating in the space between the cavity bottom and the browning plate. The metal of good heat conductivity in combination with the fact that the browning plate according to the invention has a design which is adapted to the rotating bottom plate or the rotation mechanism of the same, the desirable good heat equalization in the plate is obtained. By allowing the browning plate to be carried by the rotating bottom plate or the rotation mechanism of the same is obtained in simple manner a wave propagation space under the browning plate and thereby rapid heating of the same by the absorption of microwave energy in the ferrite layer. By these measures the need is eliminated of a separate preheating of the browning plate before the food is placed on the same. Said choice of the Curie-point of the ferrite material has the consequence that the energy absorption will substantially be stopped in areas of the ferrite layer having reached the Curie-point, and thereby also that the conduction of heat to corresponding parts of the metal plate is substantially stopped, which contributes to the heat equalization in the metal plate and eliminates the risk of overheating of its empty parts, e.g. the edge areas thereof. The invention and its advantages will be closer described in the following in connection with the preferred embodiment which is shown in the drawings, in which: Fig 1 schematically shows a frontal view of the oven cavity in a microwave oven according to the invention with said browning plate eliminated; and Fig 2 shows a partly sectioned blow up of a part of Fig 1 with the browning plate according to the invention arranged in its use position. Fig 1 shows schematically the oven cavity in a microwave oven according to the invention, in which the door for closing the cavity has been eliminated. The cavity has the shape of a rectangular parallelepiped or a cube with sidewalls 1 and 2, a roof 3 and a bottom 4, and a backwall 5. In the sidewall 1 are provided two input openings 6 and 7 for the supply of microwaves into the cavity from a microwave source (that is not shown) via a waveguide device 16 (see Fig 2). The input openings and the waveguide device are arranged to supply microwaves which are coherent and locked in phase opposition and having a substantially vertical E-field in the cavity. For a closer description of the microwave input system is referred to the Swedish patent application no 9003012-3 (PHZ 90007). Fig 1 shows also schematically an electric browning element 15 at the cavity roof 3 for browning of the upper side of a food piece. As is clear from Fig 1 the input opening 6 is positioned at some distance above the cavity bottom 4, while the input opening 7 is positioned adjacent to the roof 3. In the cavity is provided a circular bottom plate 8 of a microwave penetratable material, e.g. glass or cheramics. The bottom plate may in turn be carried by a rotation mechanism 9, as is desclosed more in detail in Fig 2. Fig 2 shows a blow up of a part of Fig 1 with the browning plate 10 placed on the bottom plate 8. According to Fig 2 the browning plate 10 has an underside of a shape which is adapted to the upper side of the bottom plate 8 in order to obtain a stable contact between the same. Alternatively the bottom plate 8 may be removed and the browning plate 10 placed directly against the rotation mechanism. The underside of the browning plate 10 is provided with a layer 11 comprising a ferrite material, said layer covering substantially the complete underside of the browning plate. The browning plate itself is manufactured from aluminium or a similar metal having a good heat conductivity and fitted for carrying a piece of food lying directly thereon, for example a pie. In a different embodiment the contact surface of the browning plate may consist of a thin layer of stainless steel the underside of which is covered with a thicker aluminium layer, in turn carrying the ferrite layer. The outer edge of the underside of the browning plate 10 rests against a circumferential edge 12 of the bottom plate 8. By the fact that the browning plate in this manner rest freely against the bottom plate the same is easily applicable in the microwave oven in cases when the food preparation comprises browning, and simply removed from the oven when this is not the case. In Fig 2 a rotation mechanism for the bottom plate has been partly shown by a leg 9 the outer end of which is provided with a wheel 13. A commonly used rotation mechanism comprises three legs of this kind arranged on a centre part at mutual angles of 120°. The centre part is rotated by means of an electric motor provided under the cavity bottom and having a through shaft connected to the central part. By the rotation the wheels 13 are brought to roll on the cavity bottom, thereby providing rotation of the bottom plate 8 resting against the wheels. The browning plate 10 rests on the bottom plate and follows the rotation thereof. The rotation mechanism and the bottom plate forms together distance means carrying the browning plate 10 at a distance from the cavity bottom. Thereby is obtained a microwave propagation space between the ferrite layer of the browning plate and the bottom plate. The input opening 6 is provided at the level of this space, which means that a defined part of the microwaves from the input opening will propagate through said space. The microwave propagation in the space between the browning plate and the cavity bottom has been shown by dashed circular archs 14 in Fig 2. The input opening 6 provides microwaves having a substantially vertical E-field and a substantially horizontal H-field. By the direct input of microwaves into the space under the browning plate and the fact that the H-field substantially coincides with the plane of the ferrite material layer, is obtained a very good energy absorption in the ferrite material and consequently good transfer of heat to the aluminium plate and a rapid heating of the same. Dependent on the intended use of the browning plate a maximum temperature of the same is decided corresponding to the desirable browning temperature. Thereafter a ferrite material having a Curie-point under said maximum temperature is chosen. By this choice the microwave absorption in the ferrite layer will substantially be interrupted in areas in which the Curie-point is reached, which will first take place in areas in wich the heat transfer from the metal plate to the piece of food is small. When the complete ferrite layer has reached the Curie-point the energy absorption in the layer will substantially be stopped, and thereby the microwaves in the space under the browning plate will be reflected against the metal plate of the browning plate, and thereafter the propagation will continue out and into the cavity past the edges of the browning plate. In this condition, when the browning plate has reached its maximum temperature and thereby generates the desirable high underheat to the piece of food on the browning plate, microwave energy will be absorbed only by the piece of food in itself. When thereafter the heat dissipation from the browning plate to the piece of food decreases the temperature thereof in some part, a temperature decrease will appear also in the corresponding part of the ferrite layer, which will then again start absorbing microwave energy and transferring heat to the browning plate. Thereby is obtained that the complete surface of the browning plate will maintain a substantially even temperature equal to the desirable browning temperature, and simultanously a good microwave efficiency because the energy which is not absorbed by the browning plate will mainly be absorbed by the load. The substantially constant temperature of the browning plate provides for a well defined browning process, which in turn simplifies an adaption of the heat effect and the design of the browning element 15 of the cavity roof 3, such that a corresponding browning of the upper side of the piece of food may be obtained within the same elapse of time. Conclusively, mainly the following features contribute to a good browning result using the browning plate according to the invention, namely: direct input of microwaves into a wave guide space under the browning plate, giving an efficient and rapid heating of the plate; a plate of aluminium or other metal of good heat conductivity, providing heat equalization between warmer and colder areas of the plate; a rotating browning plate, contributing to an improved heat equalization in the plate; a Curie-point of the ferrite material layer of the browning plate which is so selected that the desirable browning temperature in the area of contact with the piece of food. In the description and the patent claims the denomination "ferrite material" has been used for the microwave energy absorbing layer 11. This is understood to mean generally a material having the ability to generate magnetic loss energy in a microwave field, in the way which is closer described in the US patent specification 2.830.162.

Claims: A microwave oven comprising an oven cavity, a microwave source and means (11) for browning of a piece of food of the type comprising a metal plate (10) of good heat conductivity, one side of which is intended to be in contact with the piece of food and the other side of which is provided with a magnetic loss generating material layer (11), characterised in that it comprises an input system (16, 6, 7) for feeding of polarised microwaves into the cavity, such system having a wave guide device (16) with at least one input opening (6) arranged to establish a field concentration of polarised microwaves along said material layer (11) the magnetic field vector of which is directed substantially along said material layer (11) in order to generate magnetic losses in the material layer (11) and thereby heating of the metal plate. A microwave oven as claimed in claim 1, in which said input opening (6) is provided adjacent to the cavity bottom (4) in a sidewall (1) of the cavity, while said metal plate (10) is designed as a browning plate to be positioned horizontally in the load zone of the cavity, on the upper side of which the piece of food is placed during food preparation, characterized by having distance means (8, 13) for carrying the browning plate (10) in the load zone at a distance from the cavity bottom (4), in which said input opening (6) is provided for supplying microwaves with a substantially vertical E-field and propagating in said space provided by said distance means (8, 13) between the magnetically loss generating material layer (11) of the browning plate (10) and the cavity bottom (4). A microwave oven as claimed in claim 2, comprising a further input opening (7) in said sidewall (1) at the cavity roof (3), in which said input openings (7) coincide with a vertical centre line of the sidewall (1), characterized in that said wave guide device (16) is resonant for the microwaves from the microwave source and shaped for supplying said polarized microwaves as being coherent and locked in phase opposition at the respective input openings (6, 7), whereby the influence by the load on the microwave supply via said input openings (6, 7) is substantially eliminated and the microwave propagation in the space under the browning plate (10) is safeguarded. A microwave oven as claimed in anyone of the preceeding claims, characterized in that said browning means comprises an electric heat element (15) provided at the roof (3) of the cavity in order to provide in cooperation with the browning plate (10), browning of the upper side and underside of the food simultaneously.

Inventors: BERG LARS-ERIK, BERG, LARS-ERIK
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1991
Main classification: H05B6
IPC codes: H05B6, F24C7