CA1172317A

CA1172317A - Microwave heating appliance and method

Info

Publication number: CA1172317A
Application number: CA000388725A
Authority: CA
Inventors: George Freedman; Robert F. Bowen
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1980-12-29
Filing date: 1981-10-26
Publication date: 1984-08-07
Also published as: DE3150619A1; JPS6331696B2; JPS57136030A; GB2090510B; DE3150619C2; US4362917A; GB2090510A

Abstract

MICROWAVE HEATING APPLIANCE AND METHOD
Abstract A microwave heating appliance having a metallic member supporting a body comprising ferrite bonded to an outer surface of the metallic member with a grooved inner surface of the metal-lic member contacting a food body to be heated by thermal energy generated from microwave energy absorbed by the ferrite body and transferred through the metallic member to the food body.
An additional metallic member forms a container for the food body, and adjacent peripheral areas of the two metallic members provide substantial microwave sealing between the two metallic members thereby substantially shielding the food body from the microwave energy when the appliance is placed in a microwave oven. Upper and lower surfaces of the interior of the appliance contain grooves to form corrugated surfaces which contact the food body and which permit juices produced by heating the food body to flow away from regions of contact between the corrugated surfaces and the food body.

Description

' ~ '7 ~ 7 Baekground of the Inventlon Attempts have been made to hea-t food bodies by causing mierowave energy to be absorbed by an intermedia-te body such as a block of ferrite which, in turn, trans:Eerred hea-t to the food body. IIowever, such attempts have not been economically prac-tical since the intermedia-te materials chosen, sueh as ferrites, have generally been large and bulky, for example, on the order of a quarter waveleng-th thiek or thieker, and generally the fer-rites available have been sintered material which eraeked due to the differential temperature encountered in the ferrite.
In addition, when microwave energy is absorbed by the food body itself, the interior portion of the food body becomes overcooked eompared with the surfaee eonditions. This is par~
tieularly true for food bodies such as steaks and hamburgers where a browned surface food products preferably has a gradation of cooking extending to rare in the center, Also, heat loss is encountered in eooking foods having juiees sueh as meat fats/ by reason of the juiee whieh drains from the meat eontinuing to absorb microwave energy, This results in a reduction in the eooking rate for a given level of mierowave power and a eorresponding inerease in eooking time as well as changes in the surface texture of the eooked food produet.

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l ~72317 Summary of the Invention In accordance with this invention, there is provided a microwave appliance in which a metal member has a body of ferrite material bonded to an upper surface of said member while a lower surface of said member has a nonplanar surface contacting a food body. The food body is preferably heated predominantly by thermal energy generated in the ferrite material when the appliance is subjected to microwave energy in a microwave oven.
More specifically, when the appliance i5 in a microwave oven, the ferrite material of said ferrite body i5 preferably posi-tioned substantially entirely within an eighth of a wave'ength of the conductive surface of the metal member so that the ferrite material will react with the high magnetic fields produced near said conductive surface due to reflection of the microwaves from the metal member. The dielectric component of the ferrite and/or bonding medium which would react to the electric field component of the microwave energy is substantially shielded by being close to said conductive surface. The appliance thus will absorb microwave energy through the ferrite material until the tempera-ture of the Curie point region of the ferrite material is reached.
The amount of microwave energy absorbed by the dielectric compo-nent of the ferrite body normally increases with temperature, but will generate substantially less thermal energy than that lost from the microwave appliance by radiation, convection and/or conduction at normal cooking temperatures. Preferably, a ferrite material is chosen having a Curie point region which is between 500 and 800F, but below the degradation temperature of thermal insulating materials adjacent the ferrite body.
In accordance with this invention, the microwave appliance is made of two metallic members having substantial peripheral

-2-I ~72~7 areas which oppose each other when a food body is insertecl in the container, and -the metal cover is closed over the me-tal food con-tainer. Due to the substantial surEace area between the surface area and the peripheral opposing regions, the microwave energy is substantially inhibited from passing through the space between the two metallic members of the appliance so that the food body is cooked predominantly by the transfer of -thermal energy from the ferrite material through one or both of the metallic members to the food body. In accordance with this invention, the interior surface regions of the microwave appliance opposite -to the ferrite material are formed with sub-stantially nonplanar contours, such as grooves r forming cor-rugated surfaces so that juices produced by the food body during cooking can flow away from the regions of contact of -the food body with the metal member supporting the ferrite material. As a result ! substantially all the thermal energy goes into heat-i!ng the food body while substantially none is used to heat the juices.
Broadly stated, according to a first broad aspect of the present invention, there is provided a microwave heating appliance comprising: a metallic member having a surface por-tion; a body comprising ferrite material bonded to an upper surface of said metallic member; said ferrite body producing thermal energy when subjected to microwave energy; and said body transferring said thermal energy through said metallic member to a lower nonplanar surface thereof having regions con-tacting a food body.
According to a second broad aspect of the present invention, there is provided a microwave heating appliance com-prising: a food container having an upper metallic member anda lower metallic member for contacting an upper and a lower sur-' ' ! 17231~

face of a food body with th.e opposing periphe.ral portion of saidmetallic members having substantial areas for shielding said food body from microwave energy supplied to said appliance; and a body comprising ferrite material supported by an ou-ter sur-face of said upper metallic member and produciny thermal energy by absorbing portions of said microwave energy and for trans-ferring portions of said thermal energy through said upper mem-ber to said food body, According to a third broad aspect of the present in~
vention, there is provided a microwave heating appliance com-prising: a food container having a lower metallic member for supporting a food body; an upper metallic body pivotally sup-ported with respect -to said lower metallic body and engageable with an upper surface of said food body; and a body comprising ferrite material supported by an outer surface of one of said metallic members for absorbing portions of said microwave energy at a substantial rate at tempera-tures below the Curie point region of said ferrite material while absorbing microwave energy at a substantially lower rate at temperatures above said Curie point region.

-3a-' 1 723~
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Brief Description of the Drawin~
Other and further objects and advantages of the invention will be apparent as the description thereof progresses, reference being had to the accompanying drawings wherein:
FIG~ 1 illustrates a top plan view of a microwave appliance embodying the invention;
FIG~ 2 illustrates a front elevation view of the microwave appliance illustrated in FIG~ l;
FIG~ 3 illustrates a side elevation view of the microwave appliance illustrated in FIGS~ 1 and 2;
FIG~ 4 illustrates an exploded sectional view of portions of the microwave appliance of FIGS~ 1-3 taken along line 4-4 of FIG~ l;
FIG~ 5 illustrates a detailed view of a portion of the hinge used in the microwave appliance of FIGS~ 1 3 taken along line 5-5 of FIG~ ~ with the microwave appliance cover closed;
FIG~ 6 illustrates the same detail as FIG. 5 but with the microwave appliance cover open and with the cover and dish shown in cross-section;
FIG~ 7 illustrates a detail of the cover of FIGo 6 taken along line 7-7 of FIG~ 6;
FIG~ 8 illustrates the same detail as FIG~ 5 but with the cover elevated to accomodate a large food body and with the dish and cover in cross-section; and FIG~ 9 illustrates the dish of FIGS~ 1-3 being used in a microwave oven to heat a food body in accordance with this invention.

! 17~317 Description of the Preferred Embodiment Referring now to FIGS. 1-8, there is shown an appliance 10 for cooking a food body such as a beef steak in a microwave oven.
The appliance 10 comprises a base portion 12 of thermally insu-lating material such as high temperature plastic formed, for example, by molding in accordance with well-known practice.
Four legs 14 extend downwardly and are molded inte~rally with base portion 12. Base portion 12 supports a food container dish 16 made, for example, of metal and having rib-like members 18 forming a corregated region in its lower surface on which a food body 20 may rest, Preferably, dish 16 is made of thin metal such as aluminum and the rib-like members in the bottom serve the added function of stiffening the dish structure. Dish 16 also has a depressed trough region 22 formed around the peri-phery of the ribbed bottom of the dish with the bottom of the trough being subs,tantially below the bottoms of the ribbed grooves so that juices and fats which drain from the food body 20 during heating can drain along the grooves between the ribs 18 and into the trough 22. A perimeter wall 24 of the dish extends around the periphery of the dish from the bottom of the trough region to a point above the ribs 18 so that juices and fats from the food body will be drained away from the food body to the trough region 22 where they will no longer be heated during the cooking process thereby eliminating their absorption of additional heat from the food b,ody.
The surfaces of dish 16 are preferably coated with a non-stick layer 26 of high temperature plastic such as Teflon in accordance with well~known practice.
~n order to minimize the transfer of heat from dish 16 to the base 12, the dish 16 contacts the base 12 only in the regions ~ 172317 of four small bosses 28 spaced around the periphery of the dish and formed integrally with the molded base 12. ~osses 28 contact the dish 16 as shown at the bottoms of the trough 22 and at the sloping interior peripheral wall of the trough 22 so that such points of contact are separated Erom the ribbed members 18 sup-porting the food body by substantial distances of the thin metal dish 16. These substantial distances act as thermal chokes which reduce the amount of thermal energy flowing from the dish into the base portion 12, thereby reducing cooling of the food body and preventing the base 12 from overheating. In addition, the region of the base portion below the ribbed dish members 18 has a substantial aperture 30 so that air may circulate past the regions of the base 12 closest to the dish to ensure that no portion of the plastic base portion 12 exceeds a temperature of, for example, 200C above which the base 12, which may be rigid plastic such as a polycarbonate, might become weakened or in time deteriorate.
A cover member 32, formed of thin metal such as aluminum, has ribs 34 forming a corregated region therein above the ribbed members 18 in dish 16. Ribs 34 preferably engage the upper surface of the food body 20 during cooking.
Cover 32 has a peripheral metal wall 36 extending substan-tially vertically downwardly from its upper surface outside, and spaced from, the metal wall 24 of dish 16. Wall 36 forms an overlapping region with wall 24 which acts as a microwave seal so that steam and other vapors may exit from the dish through the space between the walls 36 and 24 while microwave energy is substantially prevented from entering the dish.
A flexible plastic bonded ferrite microwave absorbing body 38 is supported on top of cover 32 contacting the surface of 1 1'7~31~
the corregated region of ribs 34. Region 38 acts as a heating element by absorbing microwave energy when the appliance 10 is subjected to microwave fields, for example, in a microwave oven and the thermal energy generated thereby is transferred from ferrite region 38 through the ribbed region 34 of cover 32 to cook the food body 20 in contac~: with the cover 32.
The ferrite region 38 may be, for example, formed of par-ticles of a standard ferrite, such as the ferrite Ql supplied by Indiana General imbedded in a flexible high temperature plastic so that as the element 38 heats and the aluminum ribbed regions of the cover 32 heat, cracking will not occur of the ferrite region 38 due to stetching of the plastic. Preferably, the ferrite material is chosen so that the center of its Curie point region is between 250C and 350C so that microwave energy absorption by the ferrite will be substantially reduced before the plastic binder material of heater element 38 reaches a temperature substantially in excess of 300C. As used throughout the specification and claims, the term "Curie point region" is intended to mean the temperature range in which a ferrite has its value of magnetic permeability reduced from 90~ of its room temperature value to 50% of its room temperature value as the ferrite is heated from room temperature through said temperature range. Thus, in a conventional microwave oven supplying a maxi-mum o~ 800 watts of microwave energy, a surface of ferrite element 38 exposed to the microwave energy of, for example, 4 1/2 inches by 8 inches will not reach a temperature in excess of 500-550F when heated in a microwave oven. This invention takes advantage of the fact that ferrite material responds to the high magnetic fie~ds associated with the reflective surace of the cover 32 to couple the microwave energy into the ferrite 38.
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3 ~ 7 This effect is at a maximum in the low impedance region adjacent the highly conductive surface of the cover 32. The flexible plastic binder of the ferrite heating element 38 being very close to the highly conductive surface, couples relatively poorly to the electric field of the microwave energy adjacent the surface of the cover 32 since this electric field is weak adjacent the highly conductive surface. Thus, flexible high temperature plastic whose degradation temperature is above 600F, such as the commercially available silicone based plastic SILASTIC, can be used to bond the ferrite particles together and to bond to the Teflon coating 26 on cover 32.
For preferred results~ this invention teaches that the dis-tance of the exposed surface of the ferrite body 38 is preferably within a distance of 1/8 of an inch to 3/8 of an inch from the closest metal surface of the cover 32. Thinner ferrite bodies 38 do not provide enough ferrite material to efficiently absorb all the microwave energy into thermal energy whereas thicker ferrite bodies 38 have portions of the ferrite body sufficiently far from the conductive surfaces of the cover that substantially dielectric heating of the flexible plastic binder in the body 38 occurs.
The cover 32 has two plastic pivots 40 attached to the exterior rear corners thereof, for example, by metal rivots 42 or by any other desired means such as gluing or plastic bonding.
Since the pivots 40 are separated from the ferrite heating element 38 by substantial distances of the thin metal of the cover 32, they do not approach the temperature of the ferrite material 38 due to the thermal choking action of the thin metal regions of the cover 32. Plastic pivots 40 pivotally slide in grooves 44 vertically molded into projections formed integrally with and * fie ~ r~ --8--' 17~317 upstanding from the rear corners of base 12. Movement of cover 32 is thus restricted by the action of pivots ~0 in grooves 44 to vertical movement and to pivoting motion for opening the cover 32 to expose the food body 20 and the wall 36 of cover 32 is maintained in spaced relationship to the wall 24 of dish 16.
Preferably, such spacing between the walls 24 and 36 is substan-tially less than a quarter wavelength of the 2.~5 KMH mlcrowave energy conventionally used in domestic microwave ovens. For example, as shown in the present dish, a spacing of approximately 3/8 of an inch is formed between the walls. Pivots ~0 are also positioned so that when no food body is in the dish, they will support the cover 32 by engaging the bottoms of the grooves 44.
A handle 46 is attached to the front region of the cover wall above a similar handle 48 molded integrally with base 12 and supports the front of cover 32 from the base 12 so that the upper edge of dish wall 24 does not touch the interior of cover 32. Otherwise, deterioration by abrasion of the dish and cover by microwave energy arcing might occurq Referring now to ~IG. 9, there is disclosed an alternate embodiment of the appliance in a microwave oven. A microwave oven 50 may be of any desired type such as the commercially available domestic microwave oven having a heating cavity 52 supplied with microwave energy from a waveguide 56 coupled to magnetron 58. A mode stirrer 54 rotated by air or a motor (not shown) reflects microwave energy in cavity 52 to vary the mode pattern. Microwave oven 50 may have conventional timing controls (not shown) in accordance with well-known practice. A door 60 swings down to provide access to the enclosure 5~ so that the microwave appliance 10 may be inserted in the oven and removed therefrom.
_g_ ~ 1~23:~7 Different maxirnum temperatures of the ferrite bodies may be achieved by using different percentages of ferrite material in the ferrite body. However, a preferred percentage of ferrite is in the range between 75 and 80% by weight with the remainder comprising a high temperature binder which binds the particles together and is, in turn, bonded to the metal member or membersO
The microwave appliance 10 is an alternate embodiment of the microwave appliance disclosed in FIGS. 1-8. Microwave ap-pliance 10 comprises upper and lower halves 68 and 70 of high temperature plastic which are hinged together by a hinge 72 and held together when closed by a plastic hasp 74. An upper plate 76 of aluminum has a lower surface with grooves 78 to form a nonplanar surface contacting a food body 80, such as a body of chopped steak. A plurality of bodies of ferrite 82 are bonded to the upper surface of metallic member 76, and intervening spaces between the ferrite members 82 are filled with plastic regions which are part of plastic cover 68. Ferrite members 82 are bonded to the inner surface of the plastic cover 68. A
metallic plate 86 similar to metallic plate 76 is positioned below food body 80 with grooves 88 therein contacting ~he lower surface of food body 80. Ferrite members 90 are bonded to the lower surface of plate 86. Regions of lower plastic member 70 extend between ferrite members 90 with both ferrite members 90 and plastic members 70 being bonded to the lower surface of metal member 86.
Plastic feet 94 extend from the bottom of lower plastic member 70 and a plastic handle 96 extends from a wall of the member 70 on the opposite side thereof from hinge 72. Feet 94 and handle 96 are preferably molded integrally with lower member 70. Hinge 72 is preferably formed as extensions of 3 1 ~7 metal members 76 and 86. The peripheral regions of metallic members 86 and 76 substantially inhibit microwave energy from entering the container so that l:ittle or no substantial heating of food body 80 occurs directly by microwave energy. Rather, the thermal energy for cooking food body 80 is derived pre-dominantly from microwave energy being absorbed by the ferrite members 82 and 90. The embodiment of microwave appliance 10 shown in FIG. 9 preferably has the upper surface area containing ferrite members 82 sized to approximately half the area of that shown in the appliance of FIGS. 1 and 2 or approximately ~
inches by 4 inches so that microwave energy may simultaneously supply ~hermal energy to both the lower and upper surfaces of the food body 80.
As illustrated herein, ferrite bodies 82 and 90 are pre-ferably entirely within an eighth of a wavelength of the micro- .
wave energy supplied to cavity 52 so that their primary heating effect is due to the microwave frequency magnetic fields adjacent the surfaces of the metallic support members 76 and 86.

) ~72~7 Description of the Preferred Mode of Operation of the Invention In operation, a food body s~Jch as a beef steak 20 or chopped steak 80 is placed on the rib-like members 18 in the dish 16, and the dish 16 is in the appliance and the cover is closed.
The microwave appliance 10 is then placed in the oven 50 and the door 60 is closed. An appropriate time such as 5 to 10 minutes is set on the microwave oven controls with the oven power setting preferably at full power. The oven start button is then actuated and microwave energy is supplied from the ma~ne-tron 58 through the waveguide 56 and through the aperture between the waveguide and oven wall to cavity 52.
Microwave energy in enclosure 52 impinges on ferrite bodies 82 and 90 where it is converted to thermal energy which is trans-ferred by conduction through the metal members 76 and 86 to food body 80. Thermal energy is then transEerred to the food body 80 by conduction and/or radiation to heat and brown the surface of the food body. The browning will appear more predominantly as a series of bars formed by the points of contact of the metal members 76 and 86.
Vapors given off by the food body 20 pass through the spaces between the points of contact of the food body 20 with said metal members 76 and 86 so that they cease to be heated by con-duction from said metal members.
When the preset time has elapsed, the door 60 is opened and the appliance 10 pulled out from the oven by the base handle 96.
Thus, it may be seen that a food body may be cooked and/or browned on~both sides to any desired degree in accordance with the invention by simply setting the timing of the microwave oven to supply the clesired total amount of energy to the appliance 1~. The appliance may be used in general with any microwave ~ ~72317 oven without overheating and with good results~
This completes the description of the embodiments of the invention illustrated herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of the invention. For example, different sizes and thicknesses of food bodies as well as different types of food bodies may be cooked and browned in the microwave appliance. More specifically, sausages, fish, poultry and other similar food products may be cooked with microwave energy being first converted by the ferrite heating element to thermal energy which is then transferred by con-duction to the interior of the applianceO Different kinds of ferrite materials may be used for the heating element, and materials other than plastic can be used for the base and handles. Accordingly, it is desired that this invention be not limited to the specific embodiments of the invention illustrated herein except as defined by the appended claims.

Claims

What is Claimed is:

1. A microwave heating appliance comprising:
a metallic member having a surface portion;
a body comprising ferrite material bonded to an upper surface of said metallic member;
said ferrite body producing thermal energy when subjected to microwave energy; and said body transferring said thermal energy through said metallic member to a lower nonplanar surface thereof having regions contacting a food body.

2. The appliance in accordance with Claim l wherein:
said nonplanar surface comprises a plurality of grooves in said metallic member.

3. The appliance in accordance with Claim 1 wherein:
substantially all regions of said ferrite material are within a tenth of a wavelength of said upper surface of said metallic member at the frequency of said microwave energy.

4. A microwave heating appliance comprising:
a food container having an upper metallic member and a lower metallic member for contacting an upper and a lower surface of a food body with the opposing peripheral portion of said metallic members having substantial areas for shielding said food body from microwave energy supplied to said appliance; and a body comprising ferrite material supported by an outer surface of said upper metallic member and producing thermal energy by absorbing portions of said microwave energy and for transferring portions of said thermal energy through said upper member to said food body.

5. The microwave heating appliance in accordance with Claim 4 wherein:
said container is supported by a base of low thermal con-ductivity material.

6. A microwave heating appliance comprising:
a food container having a lower metallic member for sup-porting a food body;
an upper metallic body pivotally supported with respect to said lower metallic body and engageable with an upper surface of said food body; and a body comprising ferrite material supported by an outer surface of one of said metallic members for absorbing portions of said microwave energy at a substantial rate at temperatures below the Curie point region of said ferrite material while absorbing microwave energy at a substantially lower rate at temperatures above said Curie point region.

7. The microwave heating appliance of Claim 6 wherein said Curie point region is above 500°F.

8. The microwave heating appliance of Claim 7 wherein said metallic members contact plastic members whose temperatures do not exceed their degradation temperature when said ferrite material is raised to temperatures in said Curie point region.

9. The microwave heating appliance in accordance with Claim 8 wherein one of said plastic members supports said appliance by contacting said lower metallic member.

10. The microwave heating appliance in accordance with Claim 9 wherein said plastic member contacting said lower metallic member has a plastic handle molded integrally therewith.