EP0271981B1 - Microwave container - Google Patents

Microwave container Download PDF

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Publication number
EP0271981B1
EP0271981B1 EP19870309398 EP87309398A EP0271981B1 EP 0271981 B1 EP0271981 B1 EP 0271981B1 EP 19870309398 EP19870309398 EP 19870309398 EP 87309398 A EP87309398 A EP 87309398A EP 0271981 B1 EP0271981 B1 EP 0271981B1
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EP
European Patent Office
Prior art keywords
container
dielectric
wall portion
thickness
electrical thickness
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP19870309398
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0271981A2 (en
EP0271981A3 (en
Inventor
Richard Mackay Keefer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Rio Tinto Alcan International Ltd
Original Assignee
Alcan International Ltd Canada
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from CA000508812A external-priority patent/CA1279902C/en
Priority claimed from US06/943,563 external-priority patent/US4888459A/en
Application filed by Alcan International Ltd Canada filed Critical Alcan International Ltd Canada
Priority to AT87309398T priority Critical patent/ATE75688T1/de
Publication of EP0271981A2 publication Critical patent/EP0271981A2/en
Publication of EP0271981A3 publication Critical patent/EP0271981A3/en
Application granted granted Critical
Publication of EP0271981B1 publication Critical patent/EP0271981B1/en
Expired legal-status Critical Current

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
    • B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
    • B65D81/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
    • B65D81/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within the package
    • B65D81/3446—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within the package specially adapted to be heated by microwaves
    • B65D81/3453—Rigid containers, e.g. trays, bottles, boxes, cups
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
    • B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
    • B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
    • B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
    • B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
    • B65D2581/3439—Means for affecting the heating or cooking properties
    • B65D2581/344—Geometry or shape factors influencing the microwave heating properties
    • B65D2581/3441—3-D geometry or shape factors, e.g. depth-wise
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
    • B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
    • B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
    • B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
    • B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
    • B65D2581/3471—Microwave reactive substances present in the packaging material
    • B65D2581/3472—Aluminium or compounds thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
    • B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
    • B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
    • B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
    • B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
    • B65D2581/3486—Dielectric characteristics of microwave reactive packaging
    • B65D2581/3487—Reflection, Absorption and Transmission [RAT] properties of the microwave reactive package
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
    • B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
    • B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
    • B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
    • B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
    • B65D2581/3486—Dielectric characteristics of microwave reactive packaging
    • B65D2581/3489—Microwave reflector, i.e. microwave shield

Definitions

  • This invention relates to cooking containers which can be used in microwave ovens. More particularly, the present invention relates to a container which provides improved microwave heating distributions when used in a microwave oven.
  • a container for containing a material to be heated in a microwave oven comprising an open-topped tray for carrying the material and a lid covering the tray to form a closed cavity, the container being characterized in that at least one surface of the container is formed with microwave generating means for generating a mode of a higher order than those of the fundamental modes of the container, the microwave generating means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material to thereby locally heat the material.
  • the term "container” as used herein should be interpreted as meaning an individual compartment of that container. If, as is commonly the case, a single lid covers all compartments, then "lid” as used above means that portion of the lid which covers the compartment in question.
  • the container may be made primarily from metallic material, such as aluminium, or primarily from non-metallic material such as one of the various dielectric plastic or paperboard materials currently being used to fabricate microwave containers, or a combination of both.
  • microwave energy In a conventional microwave oven, microwave energy, commonly at a frequency of 2.45 GHz, enters the oven cavity and sets up a standing wave pattern in the cavity, this pattern being at fundamental modes dictated by the size and shape of the walls of the oven cavity.
  • fundamental modes dictated by the size and shape of the walls of the oven cavity.
  • higher order modes are also generated within the cavity and are superimposed on the fundamental modes.
  • these higher order modes are very weak, and in order to promote better distribution of energy within the container, a "mode-stirrer" can be used to deliberately generate or enhance the higher order modes.
  • a container such as a food container
  • microwave energy is caused to propagate into the interior of that container
  • a standing wave pattern is set up within the container, this pattern being primarily in the fundamental modes of the container (as distinct from the fundamental modes of the larger oven cavity) or, in certain circumstances, in the fundamental modes of the foodstuff within the container.
  • the container also contains modes higher than those of the fundamental modes, which higher modes are, for example, generated by irregularities in the interior shape of the container and/or its contents. As before, these higher order modes are generally of much lower intensity than the fundamental modes and contribute little to the heating of the material within the container.
  • the various fundamental and higher order modes set up within the container will normally have a pattern dictated by the physical geometry of the container. However, when the shape of the foodstuff container within the container departs significantly from the cross section of the container, and particularly if the container has microwave-transparent side-walls, the fundamental modes of interest will be predominantly determined by the food shape. If the container has metallic side walls, then fundamental modes due to the container geometry will also exist and it is a matter of degree as to which will predominate. In practice, in such circumstances, the geometry of the multi-moding structures may correspond primarily to the container, primarily to the food shape or to a blend of the two.
  • the material in the central part of the container receives the least energy and therefore, during heating, its centre tends to be cool.
  • this problem of uneven heating is ameliorated by instructing the user to leave the material unattended for a few minutes after the normal microwave cooking time in order for normal thermal conduction within the food to redistribute the heat evenly.
  • the material may be stirred, if it is of a type which is susceptible to such treatment.
  • the shape of these "cold" areas varies according to the shape of the container.
  • the shape of the cold area in the horizontal plane is roughly rectangular with rounded corners; for a container which is circular in horizontal cross section, the cold area will be likewise circular and positioned at the centre of the container.
  • the "cold" area will roughly correspond to the outside contour of the container shape and will be disposed centrally in the container.
  • the container can be notionally considered as having been split into several smaller areas each of which has a heating pattern similar to that of the fundamental modes, as described above.
  • the areas are now physically smaller, normal thermal convection currents within the food have sufficient time, during the relatively short microwave cooking period, to evenly redistribute the heat and thus avoid cold areas.
  • higher order mode heating may take place due to both of the above mechanisms simultaneously.
  • the process for generating the microwave field may take one of two forms:-
  • the plate can be considered as a two-dimensional antenna, the characteristics of which follow from well-known antenna theory.
  • the plate can be considered as receiving microwave energy from the oven cavity, whereupon a microwave field pattern is set up in the plate, the characteristics of which pattern are dictated by the size and shape of the plate. The plate then retransmits this energy into the interior of the container as a microwave field pattern. Because the dimensions of the plate are necessarily smaller than those of the container surface with which it is associated, the order of the mode so transmitted into the interior will be higher than the container fundamental modes.
  • the aperture can be considered as a slot antenna, the characteristics of which again follow from theory.
  • the slot antenna so formed effectively acts as a window for microwave energy from the oven cavity.
  • the edges of the window define a particular set of boundary conditions which dictate the mcirowave field pattern which is formed at the aperture and transmitted into the interior of the container.
  • higher order mode generating means be they plates of apertures - may be provided on each container to improve the heat distribution.
  • the higher order mode generating means may all be provided on one surface of the container, or they may be distributed about the container on different surfaces. The exact configuration will depend upon the shape and normal (i.e. unmodified by the plates and/or apertures) heating characteristics, the object always being to get microwave energy into the cold areas, thus electrically subdividing the container down into physically smaller units which can more readily exchange heat by thermal conduction.
  • the considerations which are to be given to the positioning of the higher order mode generating means will depend upon which of the two mechanisms of operation it is desired to use: if it is desired to enhance or generate a particular higher order mode which is natural to the container, then the above-mentioned cell pattern appropriate to that mode should be used to position the plates or apertures forming the higher order mode generating means.
  • a plate/aperture of approximately the same size as the cell will need to be placed over at least some of the cells - the larger the number of cells which have a plate or aperture associated with them, the better the particular mode chosen will be enhanced.
  • a sufficient space must be left between individual plates/apertures in order to prevent field interaction between them - it is important that each plate/aperture is sufficiently far from its neighbour to be able to act independently. If the spacing is too close, the incident microwave field will simply see the plates/apertures as being continuous and, in these circumstances, the fundamental mode will predominate, which will give, once again, poor heat distribution.
  • a typical minimum spacing between plates would be in the range of 6 to 12 mm, depending upon the particular container geometry and size.
  • a typical minimum spacing between apertures i.e.
  • apertures are separated by regions of foil or other metallised layer
  • the plate/aperture forming the higher mode generating means needs to be placed over the cold area or areas within the container.
  • the plate/aperture in effect, acts as a local heating means and does not (usually) significantly affect the natural modes of the container.
  • the "forced” mechanism utilises the heating effect of the container fundamental superimposed onto its own heating effect. At certain cricital sizes and positioning of the plates, both mechanisms - forced and natural - may come into play.
  • the present discussion considers matters only in the horizontal plane and for the same reason, the only surfaces which are formed with the higher order generating means in the embodiments which follow are horizontal surfaces - i.e. the bottom of the container or the lid of the container.
  • the teachings of the aforementioned copending application should not be applied to other than horizontal surfaces since the ambient microwave field in which the container is situated is substantially homogeneous.
  • the characteristics of the plate/aperture alternatives are analogous (indeed a particular aperture will transmit an identical mode to that transmitted by a plate of identical size and shape), it is possible to use them interchangeably - in other words, whether a plate or aperture of particular dimensions is used, can be dictated by considerations other than that of generating a particular microwave field pattern.
  • the heating effect of the higher order mode generating means will be greatest in the food immediately adjacent to it and will decrease in the vertical direction.
  • a plate or plates are formed on the lid, while in-registry aperture or apertures are formed in the container bottom. In another embodiment, apertures are provided in both lid and bottom surfaces.
  • the aforementioned copending application also contemplates a method of manufacturing a container as described above for containing a material to be heated in a microwave oven, comprising forming, on at least one surface of the container, microwave generating means for generating a mode of a higher order than that of the fundamental modes of the container, such generating means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material to thereby locally heat the material.
  • Each higher order mode generating means may be so configured and positioned on its surface as to generate or amplify higher order modes which are natural to the container and dictated by its boundary conditions, and/or to generate a mode which is of higher order than that of the fundamental of the container but is not otherwise dictated by the boundary conditions of the container and would not normally exist therein.
  • the present invention embraces the discovery that useful field-modifying or mode-generating effects can be achieved with a dielectric (i .e. electrically nonconducting) wall structure by providing appropriately arranged and configured adjacent or contiguous dielectric portions thereof that differ from each other in electrical thickness.
  • a dielectric i .e. electrically nonconducting
  • the surface comprises a sheet of microwave-transparent dielectric material having a conductive metal plate disposed thereon
  • comparable field-modifying effects are attainable (in accordance with the present invention) by substituting for the metal plate a dielectric portion, in or on the sheet, having a greater electrical thickness than the surrounding portion of the sheet.
  • the higher order mode generating means is a metal sheet defining one or more apertures
  • comparable effects are attainable by substituting for the metal sheet an "aperture"-defining dielectric wall portion of relatively high electrical thickness, with the "aperture(s)" constituted of dielectric wall portions of lower electrical thickness.
  • the dielectric wall structure of the invention serves (generally like the metal plate-dielectric sheet or metal aperture-defining sheet structures of the aforementioned copending application) to establish or generate, within the container, one or more modes of a higher order than a fundamental mode of the container or its contents, so as to achieve a beneficially modified heating distribution in the body of material being heated, as desired (for example) to provide enhanced uniformity of heating throughout the body, or to effect localised intensification of heating in or on selected postions of the body, as for browning or crispening.
  • the present invention affords a new way of overcoming the heat-distribution problems and limitations of conventional microwave heating, wherein significant heating is produced predominantly or exclusively by microwave energy in the fundamental mode(s) of the container holding the body, or the body itself. Indeed, in at least some instances, the mode-generating, heat-distribution-modifying effects of the present inventions may be superior to those afforded by the structures of the aforementioned copending application.
  • the "electrical thickness" of a dielectric wall structure is a function of the actual spatial thickness of the wall (measured, in conventional units of length, between opposed surfaces thereof) and the dielectric constant of the wall material. Stated with reference to microwave energy of a given frequency, having a free-space wavelength W o , and a wavelength W m in the dielectric wall material, for a wall having an actual spatial thickness d equal to n o times the wavelength W o (d being, of course, also equal to n m times the wavelength w m , i.e.
  • the dielectric wall portion(s) of greater electrical thickness are constituted of material having a higher dielectric constant than the material of the dielectric wall portion(s) of lesser electrical thickness.
  • the portion(s) of greater electrical thicness may also have a greater spatial thickness than the portion(s) of lesser electrical thickness, although this is by no means necessary in all cases.
  • dielectric herein is to be understood broadly as embracing conventional dielectric (nonconductive) materials and also so-called artificial dielectrics, such as dispersions of metallic particles in a nonconductive matrix, which are characterised by a dielectric constant significantly higher than that of the matrix material alone.
  • one or more of the aforementioned dielectric wall portions may be so constituted as to undergo a change in dielectric constant when subjected to irradiation by microwave energy.
  • the portion (or one or more of plural portions) of greater electrical thickness is made initially "lossy" (i.e. absorptive of, and thus directly heatable by, microwave energy), and is of such a nature that it exhibits a decrease in dielectric constant when heated, the decrease being either progressive or occurring upon attainment of some particular elevated temperature.
  • the wall portion thus constituted heats up, and its dielectric constant drops either gradually or suddenly as it attains a predetermined elevated temperature, with the result that its electrical thickness (and the difference in electrical thickness between contiguous wall portions) decreases, reducing or terminating the effect of the dielectric wall structure on microwave electrical field patterns within the container and thereby altering the heat distribution within the body being heated.
  • the initially modified heating distribution produced by the wall structure of the invention may be such as to cause locally intensified heating to effect browning or crispening, and this local intensification may then be shut off (by reduction in dielectric constant of the wall portion of greater electrical thickness) while overall heating continues.
  • the wall portion in question is lossy, its heating by microwave energy may be such that it serves as a supplemental source of heat (through radiation and/or conduction) for at least localised regions of the body being heated.
  • a dielectric wall portion of initially greater electrical thickness may be constituted of a porous or other material (e.g. a silica gel) having an initially high moisture (water) content, which enhances its dielectric constant; as heating proceeds, the water volatilises, progressively reducing the dielectric constant.
  • a porous or other material e.g. a silica gel
  • Some edible materials, e.g. pie crusts or layers of heterogeneous composition and/or varying thickness may themselves be capable of functioning as dielectric wall structures in such a manner if appropriately configured.
  • One specific embodiment of the invention especially advantageous for use as frozen food packaging, incorporates a dielectric wall portion of hygroscopic material which takes up moisture when exposed to air at ambient temperatures so as to constitute a or the wall portion of greater electrical thickness, though it may be substantially dry while frozen.
  • the material of the wall portion of greater electrical thickness may be a ferroelectric substance having a high ambient-temperature dielectric constant but underoging a marked drop in dielectric constant when its Curie temperature is reached.
  • some high-performance ferroelectric materials e.g. titanates based on heavy metals
  • the dielectric wall structure of the invention is the container lid, and may be associated with a container tray of any convenient or desired type, e.g. fabricated of metallic and/or dielectric material.
  • the container bottom (for example) may have a higher-mode-generating metal plate or aperture structure as described in the aforementioned copending application, and may be designed to co-operate with the dielectric wall portions provided in the lid, in a manner analogous to the co-operation between a plate or aperture-type bottom and a plate or aperture-type lid described in that application.
  • the dielectric wall structure of the invention may alternatively be provided as or in the container bottom, or as or in another wall of the container.
  • the contiguous portions of respectively greater and lesser electrical thickness, in the dielectric wall structure of the invention may be sharply demarcated; i.e. there may be an abrupt discontinuity or stepwise variation in dielectric properties between them.
  • the specific embodiments of the invention to be described will be shown as having such stepwise variation.
  • the variation between the contiguous portions may be more or less smooth, gradual, and continuous, with respect to spatial thickness and/or dielectric constant.
  • the material of said first dielectric wall portion is the same as the material of said second dielectric wall portion, and is integral therewith, and that wall portion having the greater electrical thickness has a greater spatial thickness than the said portion of lesser electrical thickness.
  • that dielectric wall portion having a greater electrical thickness comprises a multilayer structure having a total spatial thickness greater than said portion of lesser electrical thickness.
  • said dielectric wall structure comprises a wall having attached to one surface thereof a block of dielectric material, the arrangement being such that that part of the wall covered by the block constitutes said dielectric wall portion having a greater electrical thickness, while that part of the wall not covered by the block constitutes said dielectric wall portion having a lesser electrical thickness.
  • the circular surface shown may comprise the bottom surface or the lid surface of circular cylindrical container 8.
  • the surface, shown under reference 10 is made principally from microwave-transparent material and is substantially planar (although this is not essential).
  • the remainder of the container 8, which is not shown, may be of metal, such as aluminium foil, or one of the microwave-transparent plastic, cellulosic and composite materials currently available. Attached to the surface are three similar segmental plates 12 of metal foil.
  • Each of the plates 12 acts as a source of a higher order mode wave pattern which propagates into the container and acts to generate a higher order mode harmonically related to the fundamental of the container and defined, in essence, by the boundary conditions of the cylindrical wall of the container.
  • the area 14 bounded by the three plates 12 is of microwave-transparent material and is thus a route by which microwave energy enters the container.
  • Figure 2 is similar to Figure 1, except that the plates now shown under reference 16, are substantially semicircular in plan view and are separated by a gap 18.
  • This embodiment operates in the same way as the Figure 1 embodiment in that it generates a higher order mode harmonically related to the fundamental of the container and defined by the boundary conditions of the container.
  • the difference between Figures 1 and 2 is simply in the order of the particular higher order mode generated: in Figure 1 a third order mode is being generated; in Figure 2 a second order mode.
  • Figures 3 and 4 show a container bottom or lid surface 10 for a rectangular container 8.
  • the surface 10 is made of conducting material such as metal in which are formed two rectangular apertures 22 covered with microwave-transparent material.
  • each aperture 22 acts as a window, allowing through it microwave energy from the oven cavity.
  • the shape and dimensions of the edge of the aperture create boundary conditions which establish a microwave field pattern which propagates into the container.
  • the wave thus transmitted into the container is of a higher order than that of the container fundamental and acts to accentuate or amplify a higher (second) order mode - the E12 or E21 mode - which is almost certainly already present within the container but at a low power level.
  • this mode is harmonically related to that of the container fundamental and is therefore essentially determined by the geometry of the container.
  • the amplification of the second order mode effectively electrically splits the rectangular dish into two identical cells divided roughly by the dividing line 24 between the two apertures 22.
  • Each of these cells can, as explained above, be considered as a notionally separate container operating in the fundamental mode.
  • a relatively cool area is found at the centre of each of the notionally separate containers, because the containers are physically only half the size of the actual container, the problem of redistributing heat by thermal conduction from the hotter areas into the cooler areas, is greatly reduced.
  • the preferred higher order mode is that which is as low as possible consistent with giving an acceptable distribution of heating within the food.
  • the exact value of the order which is decided on will also depend upon the physical size of the container in the horizontal plane - clearly large containers will have to be operated in higher modes in order to keep down the physical size of each heating cell.
  • container modes between the first order and the fifth order (the fundamental being regarded as the zeroth order) will be used.
  • the antenna It is necessary to keep the linear dimensions (length and width) away from those values causing resonance and sub-multiples of those values. The reason for this is that, at resonance, the antenna generates high field potentials which are capable of causing electrical breakdown and overheating in adjacent structures. Also, the antenna radiates strongly in the direction of the food, and can cause burning before the remainder of the food is properly cooked.
  • the resonance of concern in this regard is "one-dimensional" resonance, as exemplified by a plate, the longest dimension of which is close to one-half of the free-space wavelength of the microwave energy (or close to an integral multiple of that half wavelength value), and the shortest dimension of which is much smaller, e.g. (for a microwave frequency of 2.45 GHz) a plate about 6 cm. long and 1 cm. wide.
  • Two-dimensional reasonance creates no problem, because the field intensity is much more distributed.
  • the higher order mode generating means is now formed of a pair of plates 26. These act in the same way as the windows 22 of the Figure 3 embodiment and will amplify the E12 or E21 mode already in the container.
  • test results carried out on circular and rectangular metal foil containers comprised metal foils attached to thermoformed 7 mil polycarbonate lids.
  • the test oven was a 700 watt Sanyo (trademark) microwave oven set at maximum power.
  • a thermal imager was an ICSD model No. 320 thermal imaging system and video interface manufactured by ICSD (trademark) Corporation. The load to be heated was water saturated into a cellular foam material.
  • the test container was heated for 40 seconds and its thermal images recorded. Heating was concentrated around the edge of the load with a temperature differential of about 10° between the edge and the centre of the container. With a 6 cm foil disk on the cover as described above, the thermal images indicated heating both at the centre and edge of the container, showing a better thermal distribution. With the 1.5 cm diameter aperture, a slightly more even thermal image was obtained for a 40 second test.
  • the container comprises a generally rectangular metal foil tray 40 having a lid 42 of microwave-transparent material located thereon.
  • a skirt 44 elevates the top surface 46 of the lid above the top of the tray 40 and therefore above the top surface of the foodstuff contained within the container.
  • a plate 48 of conducting material is centrally located on the top surface 46 of the lid 42. The plate 48 has a shape approximately corresponding to the shape of the top surface 46 of the lid, although strict conformity of shape is not essential.
  • the size of the plate 48 was varied in relation to the size of the surface 46 and the results plotted graphically (Figure 5).
  • the Y-axis represents the amount of microwave energy entering the container from the oven cavity, with an unmodified lid (i.e. no plate 48 present) shown as a datum.
  • the X-axis represents the ratio of the area of surface 46 to that of plate 48.
  • the size of plate 48 was reduced in steps by increasing the width of the microwave-transparent border area by equal amounts. When the size ratio is 100%, the energy entering the container is substantially zero because energy can only enter via the skirt 44 and is greatly limited.
  • the effect of the higher order mode generated by the plate becomes more distinct from that of the container fundamental and thus more significant.
  • the most favourable area is reckoned to be a ratio of between 40% and 20%. Below 20% the order of the mode generated by the plate becomes high and the wave transmitted from the plate is, as explained above, attenuated so quickly in the vertical direction as to have little effect on the overall heatig characteristic, which thus returns to being that of the fundamental mode within the container.
  • the plate 48 of the Figure 6 embodiment operates by a different mechanism to that of each of the areas, be they plates or apertures, in the embodiments of Figures 1 to 4.
  • the plate 48 of Figure 6 "forces" into the container a mode in which the container, due to its physical characteristics, would not normally operate.
  • the mode in this case is dictated by the size and shape of the plate 48 which in essence sets up its own fundamental mode within the container.
  • a fundamental mode of the plate 48 is necessarily of a higher order than the fundamental modes of the container itself, because the plate 48 is physically smaller than the container.
  • This fundamental mode (of the plate 48) propagates into the interior of the container and has a heating effect on the adjacent food.
  • the central location of the plate 48 causes this heating effect to be applied to that part of the container which, when operating simply in the fundamental modes of the container, would be a cool area.
  • the object is not, as in Figures 1 to 4, to accentuate the higher modes at the expense of the fundamental of the container, but rather to give a uniform heating by utilising the aforementioned fundamental mode of the plate 48 in conjunction with the fundamental modes of the container. No attempt is made to generate or amplify naturally higher order modes of the container. However, it is likely that in some circumstances both mechanisms will operate together to provide an even distribution of microwave power within the container.
  • the mechanism which utilises amplification of naturally higher order modes of the container becomes predominant. If we notionally divide the rectangular top surface 46 into a 3 ⁇ 3 array of equal size and shape (as far as is possible) rectangles, then a plate 48 positioned over the central one of these, having an area of approximately one ninth of the area of surface 46 will have a size and shape such that it will generate a third order mode (E33) with respect to the fundamental of the container. This is a mode which may well be naturally present within the container, but at a very low power level.
  • the power distribution pattern of the mode in the horizontal plane comprises a series of nine roughly rectangular areas corresponding to each of the nine areas notionally mapped out above.
  • FIG. 7 shows a multi-compartment container 40 in which each compartment is treated separately.
  • the container has a series of metallic walls (not shown) which form compartments directly under regions 50, 52, 54 and 56 in a lid 58.
  • the lid is made of a microwave dielectric material and is basically transparent to microwave energy.
  • Each compartment has a corresponding top surface area in lid 58 and each top surface area has an approximately conformal plate of metallic foil.
  • Such conformal plates are shown in Figure 7 at 60, 62, 64 and 66.
  • the area of each conformal plate is dimensioned so as to provide the proper cooking energy and distribution to the foodstuff located in the compartment in question.
  • conformal plate 60 is large with respect to this compartment and shields the foodstuff located in region 50.
  • the foodstuff in that compartment does not need much heating, and distribution is not a consideration.
  • the foodstuff in region 56 requires an even distribution of heating and so conformal plate 66 is appropriately dimensioned.
  • a can-type cylindrical container 80 which has metallic side walls 82 and a metallic lid 84 and a metallic bottom 86.
  • the container can be made from any metallic material such as aluminium or steel.
  • Circular aperture 88 which is coaxial with the circular bottom 86, is centrally located in bottom 86.
  • the aperture 88 is covered with a microwave-transparent material 90.
  • a similar aperture 92 and microwave-transparent covering 94 is located on the lid 84.
  • the apertures 88 and 92 will be seen to act as windows to a particular higher mode of microwave energy, the order of this particular mode being dictated by the diameter of the apertures. Because the apertures are located top and bottom, the vertical heat distribution is improved, as explained above.
  • the vertical height "h" of the container can be large and still result in good heating of the foodstuff.
  • the diameter of each of the apertures in relation to that of the adjacent top or bottom surface dictates the mechanism of operation - i.e. whether natural container modes are generated or enhanced, or whether a "forced" mode, dictated solely by the characteristics of the aperture 88 or 92, is force into the container to heat, in conjunction with the heating effect of the container fundamental.
  • Figure 9 is a further embodiment in which higher mode generating sources are located both in the lid and in the bottom of the container for better vertical heat distribution.
  • the container consists of a metal foil tray 100 having a bottom 102 and sides 104. Bottom 102 includes two rectangular apertures 106 and 108. the container also includes a microwave-transparent lid 110 which has two metallic plates 112 and 114 located thereon. The plates 112 and 114 are located in registry with apertures 108 and 106, respectively.
  • This embodiment operates essentially in the same manner as Figures 3 and 4 above and further explanation is thus omitted.
  • Figure 10A and 10B are plan views of, respectively, the container bottom 120 and lid 140 of a further embodiment. From the microwave point of view, it will be understood that the lid and bottom could in fact be interchanged as between Figures 10A and 10B.
  • the bottom is shown as being primarily metallic which is obviously convenient if the rest of the container tray is metallic.
  • the bottom is formed with a 3 ⁇ 3 array of nine apertures 122 to 138, each of which is covered with microwave-transparent material.
  • the lid 140 is primarily of microwave-transparent material and is formed on its surface with a 3 ⁇ 3 array of nine plates 142 to 158 of conductive material such as metal. It will be seen from the pattern of plates/apertures in this embodiment that the mechanism of operation is by way of amplification of the third order (E33) mode.
  • FIGS 10A and 10B also illustrate the "tailoring" of the plate sizes to improve heat input to particularly cold areas: in this invention it will be noted that the size of the central aperture 130/plate 150 is slightly greater than that of the remainder. The reason for this is to cause the central plate aperture, overlying the coldest central area of the container, to operate not only to encourage amplification of the third order mode of the container, but also to act by the "forcing" mechanism by imposing its own field pattern on the central area. Such tailoring and shaping of particular areas is particularly useful for irregularly shaped containers or, as here, to enhance the heat input to particularly cold areas.
  • Typical dimensions for the embodiment of Figure 10 are as follows:- container overall width 115 mm container overall length 155 mm container overall depth 30 mm length of central aperture 130/plate 150 41 mm width of central aperture 130/plate 150 27 mm length of remaining apertures/plates 35 mm width of remaining apertures/plates 22 mm The distance between adjacent apertures/plates is 12 mm, except for the central aperture/plate which is 9 mm.
  • Figures 10A and 10B have been described as showing, respectively, a container bottom and lid for use together, it will be appreciated that either could be used alone.
  • the lid 140 of Figure 10B could be used with a metallic container wherein the bottom has no apertures, or with a container of a dielectric plastic material.
  • the apertured bottom 10B since the apertures are closely proximate to the contained food article, the aperture dimensions are not such as to cut off the propagation of the modes so formed, but this array of apertures could not be effectively used in a lid if there is substantial spacing between the apertures and the contained foodstuff.
  • a ring-shaped plate of metal on a microwave-transparent surface will result in the generation of two higher-order modes, one due to the exterior perimeter of the plate, and one still higher mode due to the interior perimeter of the plate. It is even possible to conceive a whole series of coaxial rings each one smaller than the last, and each generating two modes.
  • Such ring-shaped plates could be circular, or could be rectangular or square. Other shape and configurations of plate/aperture will be apparent to those skilled in the art.
  • the container of the present invention are generally of the types shown in Figures 1-4 and 6-10B, but in place of lids or other surfaces constituted of microwave-transparent sheets bearing metal plates or metal sheets defining apertures covered with microwave-transparent sheet material, there are provided dielectric lid or other wall structures having contiguous dielectric wall portions of respectively different electrical thicknesses. More particularly, in accordance with the invention, the metal plates or sheets in the lids of the containers of Figures 1-4 and 6-10B are replaced with similarly configured dielectric wall portions of electrical thickness substantially greater than that of the microwave-transparent dielectric sheet material extending over the apertures or around and/or between the plates. Similarly, where combinations of metal plates or aperture-defining metal sheets and microwave-transparent material are provided in the base or bottom of these containers, the same substitution is made.
  • containers 8 having such lids or bottoms 10 represent embodiments of the present invention, provided that the difference in electrical thickness between the contiguous wall portions (12 and 14, or 16 and 18, or 20 and 22, or 26 and 28), and the electrical thicknesses of the portions 12, 16, 20 and 26, are sufficiently large to effect modification of the microwave electric field patterns (i.e. to generate higher order modes) within the container.
  • the region 48 is a dielectric wall portion of relatively high electrical thickness and the surrounding part of lid surface 46 is constituted by a dielectric wall portion of relatively low electrical thickness.
  • the remaining containers illustrated in Figures 7-10B may be regarded as embodiments of the present invention if the regions identified in the above description of these figures as metal plates or aperture-defining metal sheets are deemed to be constituted instead of dielectric wall portions of electrical thickness substantially greater than that of the microwave-transparent dielectric material of the regions which surround, or are surrounded by them.
  • either the lid or bottom may be a dielectric wall structure in accordance with the present invention (i.e. constituted of contiguous dielectric wall portions of respectively greater and lesser electrical thickness) and the other co-operating higher-mode-generating means (bottom or lid) may utilise metal plates or aperture-defining metal sheets as set forth in the initial description of these features above.
  • Metal foil tray 40 in Figure 11 contains a body of foodstuff 160 to be heated, and is convered by a lid 161 of dielectric material which is spaced by a gap 162 above the upper surface of the foodstuff body.
  • the upper surface 163 of this lid is divided into a centrally disposed region 165 and a second region 164 contiguous to and completely laterally surrounding region 165, i.e. in the same manner that region 48 of Figure 6 is surrounded by the remaining area of surface 46.
  • the region 165 is defined by a first dielectric wall portion 166, while the region 164 is defined by a second dielectric wall portion 168 contiguous to and completely laterally surrounding portion 166.
  • the wall portion 166 has a substantially greater dielectric thickness than the wall portion 168.
  • the lid 161 is similar to lid 161 of the Figure 11 embodiment, but has a central wall portion 166 of relatively high electrical thickness that is somewhat differently positioned, in a vertical sense, relative to the surrounding low-electrical-thickness wall portion 168.
  • the foil tray 40 is replaced by a tray 170 of dielectric material, having a bottom wall structure constituted of a central dielectric wall portion 172 (in register with wall portion 166, and conforming thereto in plan outline) completely laterally surrounded by a contiguous dielectric wall portion 174.
  • the surrounding wall portion 174 has a substantially greater electrical thickness than the central wall portion 172.
  • containers incorporating the described dielectric lid and/or other dielectric wall structure in accordance with the present invention function, like the containers described in the aforementioned copending application, to modify microwave electric field patterns within the container, i.e. when the container, holding a body of material to be heated, is placed in a microwave oven and irradiated with microwave energy.
  • these dielectric wall structures comprising appropriately arranged contiguous wall portions of respectively different electrical thicknesses generate modes of a higher order than the fundamental modes of the container, and the higher order mode or modes so generated propagate into the body of material to thereby locally heat the material. In this way, desired heating distributions may be achieved in the body.
  • the dielectric wall portions 168 and 172 which have relatively low electrical thickness may, like the microwave-transparent sheets in the structures of the aforementioned copending application, be fabricated of conventional electrically nonconductive container lid or packaging materials such as paperboard or plastic. Typically, such materials have a dielectric constant less than 10, e.g. a dielectric constant in a range of about 3 to about 7.
  • the dielectric wall portions 166 and 174 which have relatively high electrical thicknesses, are so constituted that their dielectric constants are substantially greater than the dielectric constant of the low-electrical-thickness material constituting wall portions 168 and 172.
  • a useful exemplary (but non-limiting) range for the dielectric constant of the higher-electrical-thickness wall portions is about 25 to 30, where the dielectric constant of the low-electrical-thickness wall portions is below 10.
  • the portions 166 and 174 of greater electrical thickness may also have a spatial (physical) thickness greater than the wall portions of low electrical thickness; this is true of portions 166 in Figures 11 and 12, which exemplify two of the possible relative vertical dispositions of the thicker and thinner wall portions in a dielectric wall structure extending substantially horizontally.
  • the wall portions of greater electrical thickness need not be physically thicker than the portion or portions of lesser electrical thickness, provided that there is a sufficient difference in dielectric constant, as exemplified by the bottom wall structure constituted of portions 172 and 174 in Figure 12.
  • the material of the electrically thicker dielectric wall portions 166 and 174 may be homogeneous dielectric material with an appropriately high dielectric constant.
  • it may be a so-called artificial dielectric, such as a dispersion of metal particles in a plastic or other dielectric matrix, wherein the metal particles serve to enhance significantly the effective dielectric constant of the material; such aritificial dielectrics are known in the art and accordingly need not be further particularised.
  • the dielectric wall portion or portions of greater electrical thickness may be more or less lossy in character, at least initially (i.e. absorptive of microwave energy, and heatable thereby), and may further be so constituted as to undergo a gradual or abrupt decrease in dielectric constant during the course of a microwave heating operation.
  • the change in dielectric constant that occurs in the latter portion or portions during heating in a microwave oven reduces or substantially eliminates the higher-mode-generating differences in electrical thickness between contiguous dielectric wall portions.
  • the effect of the dielectric wall structure in modifying the field pattern within the container is self-limiting. This enables the distribution of heating in the contained body of material to be altered, at an intermediate stage of the microwave heating operation, as is frequently desirable for particular cooking or other heating purposes.
  • any dielectric body e.g. having the shape of wall portion 166 in Figure 11
  • a relatively high dielectric constant e.g. above 20
  • the wall portion 166 may be formed of a porous plastic material that absorbs water, and may initially be provided with a relatively high moisture content, which imparts to the portion 166 a relatively high dielectric constant.
  • the body portion 166 may comprise a silica gel or other gel structure, or a plastic with glycol or material to hold water, such that again, upon heating, an initially high water content is reduced progressively by volatilisation with resultant decrease in dielectric constant.
  • the portion 166 may be formed of a hygroscopic material, initially substantially dry (e.g. if the container of Figure 11 is a frozen food package, stored in a freezer); this material, when exposed to ambient temperatures before heating, takes up atmospheric moisture sufficient to initially elevate its dielectric constant and to achieve the requisite decrease in water content upon heeating.
  • variable dielectric wall portion 166 such portion may incorporate a suitable ferro-electric material having a dielectric constant which is high (e.g. 150) at ambient temperature but drops to a low value (e.g. 7 or 8) upon heating of the ferro-electric to its Curie temperature.
  • ferro-electrics such as titanites based on heavy metals, may present toxicity problems precluding their use in or on food containers, though they would be suitable for use in accordance with the invention to heat bodies of material not intended for human consumption.
  • Other ferro-electrics, such as Rochelle salts are acceptable for the described use in food packaging.
  • a gradual or abrupt change in electrical thickness may also be achieved where the wall portions 166 in the embodiments of Figures 11 and 12 have an equal dielectric constant to the surrounding wall portions 168. This is illustrated in the arrangements of Figures 13 to 15 which all show container wall surfaces incorporating structures exhibiting a different electrical thickness to that of the surrounding wall portions.
  • FIG 13 there is shown an integral stepped structure 826 filled with material 827.
  • the arrangement shown may be oriented so that the structure protrudes either out of or, preferably, into the container.
  • this filling material 827 can be different from the material of the surrounding wall portion 824, it may be convenient to use the same material for both purposes, thus enabling the filling material and the surrounding wall to be moulded as a unitary structure, in the manner shown.
  • the structure 826 will have a different electrical thickness (due to its greater spatial thickness) than the surrounding material of the wall 824.
  • the local heating effect of the structure 826 can be enhanced by choosing as the filler a material having a dielectric constant greater than 10. Some local heating effect can nevertheless be obtained with material having a dielectric constant below 10. For example, if the container and the filling material were to be formed integrally and made of glass or ordinary ceramics, the dielectric constant of such material would typically be in the region of 5 to 10.
  • the entire container can be made out of a material having such a relatively high dielectric constant, that is a material that is non-standard as far as the usual manufacture of such containers is concerned.
  • a non-standard material might be a foam or a gel material containing water; a ceramic material, including titamates; or a plastic or ceramic material impregnated with metal particles, e.g. polyethylene terephthalate impregnated with small particles of aluminium.
  • the container can be made of a standard plastic material, e.g. having a dielectric constant less than 10, while the filler material has a higher dielectric constant, i.e. achieving a different electrical thickness from two sources, the different spatial thickness and a different dielectric constant (see the description relative to Figure 11 above).
  • the above-mentioned upper limit of 30 for the dielectric constant has been chosen somewhat arbitrarily, having been determined primarily by the fact that some materials with still higher dielectric constants tend to be more exotic and expensive. However, from the electrical point of view, materials with dielectric constants above 30 would be desirable, and such materials may prove economically viable, especially if the container is a utensil, i.e. a container that is designed to be re-used many times, in contrast to a disposable, single-use article.
  • Figure 14 shows a modification to this latter arrangement, wherein an integral stepped structure 926 is filled, while protruding both into and out of the surrounding container wall 924.
  • Figure 14 provides an example of an arrangement in which, by arranging for the filler material to project both upwards and downwards simultaneously, each projection can be kept relatively slight.
  • Figure 15 shows a sloping wall feature with the use of filler material 1127 to form a stepped structure 1126.
  • Figure 15 shows the sloping sidewalls 1132, 1134 inclined at about 60° to the plane of the wall 1124, but this angle can be increased or decreased as desired, including being reduced to about 45° or below while still achieving the desired electrical effect of acting as higher order mode generating means.
  • a slope of less than about 45° would make the walls so gradual in their inclination, that the electrical performance would fall of appreciably. Therefore this angle of 45° can be taken as an arbitrary preferred lower limit, although lower angles (e.g. 30° or even lower) may be operable.
  • Figure 16 shows a modification of Figure 13 wherein the filling material 827 is replaced by a block 1227 that is formed separately from the wall 1224 of the container and secured in place by suitable means, e.g. glue, or even by the material in the container, assuming that the latter will be rigid, e.g. by freezing, and hence able to retain the block 1227 in the desired locations on the container wall 1224 where it will constitute a structure in the same manner as that of Figure 13.
  • suitable means e.g. glue
  • the block 1227 may have a dielectric constant which is the same as or different to that of the surrounding wall 1224, as circumstances dictate.
  • the higher order mode generating means in the form of the structures described herein need not necessarily be located in the bottom wall of the container.
  • such means can be located in any surface of the container, e.g. in a lid or in one or more of the side walls, with due regard to the avoidance of entrapment of the food or other contents.
  • a microwave cooking container for a pot pie of 5-inch nominal diameter, with a plastic lid having a nominal height of 1/2 inch, is provided (in accordance with the aforementioned copending application) with an aluminium foil disc of 5.5 cm diameter centrally mounted on the lid. This arrangement has been found highly effective for browning and cooking a pot pie.
  • the metal foil disc was replaced with a "Polyfoam” porous plastic disc 0.24 inch thick and 5.5 cm in diameter, mounted on the lid in the same centred location.
  • the "Polyfoam” disc was used as a matrix to hold water.
  • a plurality of such containers were prepared, and the "Polyfoam” discs were loaded with water.

Landscapes

  • Engineering & Computer Science (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Food Science & Technology (AREA)
  • Mechanical Engineering (AREA)
  • Cookers (AREA)
  • Constitution Of High-Frequency Heating (AREA)
  • Package Specialized In Special Use (AREA)
  • Electric Ovens (AREA)
EP19870309398 1986-05-09 1987-10-23 Microwave container Expired EP0271981B1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT87309398T ATE75688T1 (de) 1986-05-09 1987-10-23 Mikrowellen-behaelter.

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
CA000508812A CA1279902C (en) 1986-05-09 1986-05-09 Microwave container including higher order mode generation
US943563 1986-12-18
US06/943,563 US4888459A (en) 1986-12-18 1986-12-18 Microwave container with dielectric structure of varying properties and method of using same
CA508812 1986-12-18

Publications (3)

Publication Number Publication Date
EP0271981A2 EP0271981A2 (en) 1988-06-22
EP0271981A3 EP0271981A3 (en) 1988-11-30
EP0271981B1 true EP0271981B1 (en) 1992-05-06

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ID=25670995

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EP19870309398 Expired EP0271981B1 (en) 1986-05-09 1987-10-23 Microwave container

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EP (1) EP0271981B1 (da)
JP (1) JPH0825584B2 (da)
AU (1) AU597742B2 (da)
BR (1) BR8705880A (da)
DE (1) DE3778853D1 (da)
DK (2) DK231487A (da)
ES (1) ES2031910T3 (da)
MX (1) MX161944A (da)
NZ (1) NZ222374A (da)

Families Citing this family (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA1306509C (en) * 1988-06-22 1992-08-18 Bryan C. Hewitt Microwave heating
US4992638A (en) * 1988-06-22 1991-02-12 Alcan International Limited Microwave heating device with microwave distribution modifying means
JP2790184B2 (ja) * 1989-01-27 1998-08-27 ゴールデン バレー マイクロウエーブ フーズ インコーポレーテツド マイクロウェーブ調理用パッケージ
CA1339540C (en) * 1989-02-09 1997-11-11 Richard M. Keefer Methods and devices used in the microwave heating of foods and other materials
US5424517A (en) * 1993-10-27 1995-06-13 James River Paper Company, Inc. Microwave impedance matching film for microwave cooking
FR2745398B1 (fr) * 1996-02-28 1998-05-07 Sundstrand Corp Dispositif de verrouillage de la position angulaire d'un bouton de manoeuvre
FR2813583B1 (fr) * 2000-09-01 2003-07-18 Microondes Syst Sa Recipient pour un chauffage homogene de son contenu par micro-ondes
US7482560B2 (en) 2004-08-06 2009-01-27 Pactiv Corporation Microwaveable laminate container having enhanced cooking features and method for the manufacture thereof

Family Cites Families (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS513419A (ja) * 1974-06-29 1976-01-12 Kenzo Nakamura Atsuryokuhaikanrohasonjijidoheishiben
GB1593523A (en) * 1978-05-25 1981-07-15 Metal Box Co Ltd Food containers
US4230924A (en) * 1978-10-12 1980-10-28 General Mills, Inc. Method and material for prepackaging food to achieve microwave browning
US4416906A (en) * 1979-04-27 1983-11-22 Golden Valley Foods Inc. Microwave food heating container
JPS573668U (da) * 1980-06-09 1982-01-09
US4656325A (en) * 1984-02-15 1987-04-07 Keefer Richard M Microwave heating package and method
NZ210921A (en) * 1984-02-15 1988-07-28 Alcan Int Ltd Package of foodstuff for microwave oven
JPS613264U (ja) * 1984-06-11 1986-01-10 尾池工業株式会社 電子レンジ用包装体
CA1239999A (en) * 1985-06-25 1988-08-02 Richard M. Keefer Microwave container and package comprising said container and a body of material to be heated, and method of making same
JPS62252831A (ja) * 1986-04-25 1987-11-04 Naoya Ichiko 高周波加熱器における食品の加熱制御方法及びこの方法に使用する食品包装材料

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ES2031910T3 (es) 1993-01-01
EP0271981A2 (en) 1988-06-22
DK563287A (da) 1988-06-19
JPS63178971A (ja) 1988-07-23
JPH0825584B2 (ja) 1996-03-13
BR8705880A (pt) 1988-07-05
DE3778853D1 (de) 1992-06-11
DK231487A (da) 1987-11-10
EP0271981A3 (en) 1988-11-30
MX161944A (es) 1991-03-08
DK563287D0 (da) 1987-10-27
AU597742B2 (en) 1990-06-07
AU8051187A (en) 1988-06-23
NZ222374A (en) 1990-02-26
DK231487D0 (da) 1987-05-06

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