EP0382399B1 - Procédés et dispositifs utilisés pour le chauffage par micro-ondes d'aliments et autres matériaux - Google Patents

Procédés et dispositifs utilisés pour le chauffage par micro-ondes d'aliments et autres matériaux

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Publication number
EP0382399B1
EP0382399B1 EP90300997A EP90300997A EP0382399B1 EP 0382399 B1 EP0382399 B1 EP 0382399B1 EP 90300997 A EP90300997 A EP 90300997A EP 90300997 A EP90300997 A EP 90300997A EP 0382399 B1 EP0382399 B1 EP 0382399B1
Authority
EP
European Patent Office
Prior art keywords
apertures
microwave
aperture
article
surface layer
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 - Lifetime
Application number
EP90300997A
Other languages
German (de)
English (en)
Other versions
EP0382399A2 (fr
EP0382399A3 (fr
Inventor
Richard M. Keefer
Cindy Marie Lacroix
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
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Filing date
Publication date
Application filed by Alcan International Ltd Canada filed Critical Alcan International Ltd Canada
Publication of EP0382399A2 publication Critical patent/EP0382399A2/fr
Publication of EP0382399A3 publication Critical patent/EP0382399A3/fr
Application granted granted Critical
Publication of EP0382399B1 publication Critical patent/EP0382399B1/fr
Anticipated expiration legal-status Critical
Expired - Lifetime 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
    • B65D2581/3486—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 methods and devices for modifying microwave energy fields, having utility in the microwave heating of bodies of material exemplified by (but not limited to) foodstuffs.
  • a susceptor is a device that incorporates lossy material, i.e. material that absorbs the microwave energy to become heated. This device is then placed close to the surface layer to be browned or crispened so that the heat in the susceptor is transferred by conduction and radiation to this surface layer. This process necessarily requires the temperature of the susceptor to be higher than that of the surface layer in order for the heat to flow into such layer. It has been found that there are practical disadvantages in heating the susceptor to the necessary high temperatures. There is always the risk of overheating and of breakdown of the material of the susceptor, and even the generation of toxic products.
  • the islands act as susceptors, so that, provided the island are closely adjacent to a surface of the food load, the heat generated in the islands will be transferred directly by conduction to the food surface to elevate its temperature and thus achieve a browning or crispening effect.
  • the mentioned patent discloses that the microwave-transparency of the wrapping can be varied in order to adapt to the requirements of a particular food article by modifying the ratio between the dielectric (bulk) heating and the thermal heating generated in the wrapping and transferred therefrom to the food surface.
  • the wrapping described in this prior patent simultaneously acts as a microwave-transparent covering for part of the energy, and as a susceptor, i.e. a structure that absorbs microwave energy and hence becomes heated, for the remainder of the applied energy.
  • susceptor embedded in a cooking utensil, such as a frying pan or baking dish.
  • the utensil can be placed in a microwave oven, initially with or without food in it.
  • the susceptor in the utensil absorbs microwave energy, so that the cooking surface of the utensil becomes heated to a high temperature.
  • the food is added, or if it has been present from the outset, its bulk is heated dielectrically in the usual manner in a microwave oven and its surface is browned or crispened by the cooking surface of the utensil.
  • the present invention provides an arrangement in which the surface layer of the article to be heated as well as its main portion beneath the surface layer continue to be heated dielectrically, i.e. by the microwave energy, without first converting such energy into heat in a susceptor.
  • the microwave energy field is so altered that the dielectric heating effect within the surface layer is enhanced relative to the dielectric heating effect in the main portion of the article. As a result, the surface layer reaches a higher temperature. In the case of a food article, this non-uniformity of heating results in browning and/or crispening of the surface layer.
  • the surface layer of the food article may be a top layer (for example, a pie crust), or a bottom layer (for example, a pizza base), or both top and bottom layers (for example, a breaded fish fillet).
  • a top layer for example, a pie crust
  • a bottom layer for example, a pizza base
  • both top and bottom layers for example, a breaded fish fillet
  • the desired enhanced heating effect within the surface layer is achieved by directing the microwave energy through a plurality of apertures into the surface layer and thence into a main portion of the article in such a manner that modes of said energy are in cut-off in the surface layer to heat said surface layer directly by said energy.
  • the invention also embraces a device for carrying out the above method, said device comprising a sheet of microwave-transparent material, and means defining a series of microwave-transparent apertures for the transmission of microwave energy into said article, characterised in that said apertures are of such dimensions that, at the frequency of the microwave energy and with the sheet located adjacent a surface layer of an article to be heated, the modes of such energy that propagate through the apertures will be in cut-off in said surface layer to heat said surface layer directly by said energy.
  • Cut-off propagation is also referred to herein as "evanescent propagation".
  • mode-filtering is employed to refer to accentuation of the transmission of higher order modes while reflecting fundamental modes.
  • Microwaves that are in cut-off are referred to as propagating evanescently because they decay exponentially. Due to this strong decay of evanescent microwaves, the ratio of surface to bulk field intensities (or heating) is increased. Analogously with the skin effect observed at high frequencies in conductors, more energy is deposited on the surface layer than in the bulk from the modes of microwave energy that propagate through the apertures in cut-off in the surface layer.
  • the device employed for this purpose in accordance with the invention, consists of a sheet of microwave transparent material provided with substantially non-absorptive conductive material defining either an island-aperture array or an array of annuli defining apertures, the dimensions of the annuli or of the gaps between the islands and the apertures being such as to achieve the desired absorption profile.
  • the apertures will be of such dimensions that, at the frequency of the microwave energy and with the sheet located adjacent the surface layer of the article to be heated, the modes of energy that propagate through the apertures will be in cut-off in the surface layer.
  • Such modes may or may not be in cut-off in the main portion (bulk) of the article lying beneath the surface layer.
  • a device according to the invention can be located on a separate sheet of microwave-transparent material, or it can be embodied in a container for the article, e.g. as a bottom wall or lid of such container. In the latter case, small holes for venting steam and/or for draining liquids such as fat can be provided in the container structure.
  • the present arrangement readily lends itself to the provision of such venting and draining holes, whereas it would be difficult to incorporate this feature into a standard susceptor or the devices described by the aforementioned U.S. patent.
  • container as used herein embraces all manner of elements or devices (including, but not limited to, flat sheets, laminar members, pouches, pans, lidded containers, etc.) that at least partially enclose, contain, hold, support, or are supported by, the foodstuff or other material during heating in a microwave oven.
  • the modes of microwave energy that propagate through the apertures will be in cut-off for both the surface layer and the main portion.
  • attenuation will be higher per unit distance into the article, but there will still be a greater heating effect in the surface layer due to evanescent propagation.
  • the fact that the propagation into the main portion is also evanescent will result in less depth of heating in such main portion, but this feature may well be acceptable in practice if the product is a thin one, e.g. pizza, pancake, sliced potato etc.
  • mode is used in this specification and claims in its art-recognized sense, as meaning one of several states of electromagnetic wave oscillation that may be sustained in a given resonant system at a fixed frequency, each such state or type of vibration (i.e. each mode) being characterised by its own particular electric and magnetic field configurations or patterns.
  • the fundamental modes i.e.
  • the fundamental modes predominating in a system that does not include any higher order mode generating means.
  • the fundamental modes are thus defined by the geometry of the container and the contained body of material to be heated, or alternatively by such body itself when it constitutes a separate article that is not placed in a container.
  • a mode of a higher order than that of the fundamental modes is a mode for which the electric field pattern (again, for convenience of description, considered as viewed in a horizontal plane) corresponds to each of a repeating series of areas smaller than that circumscribed by the electric field pattern of the fundamental modes.
  • Each such electric field pattern may be visualized, with some simplification but nevertheless usefully, as corresponding to a closed loop in the horizontal plane.
  • the preferred embodiments of the present invention combine the uniformity of heating of the load in the lateral dimensions that can be achieved with the generation of higher order modes, with the desired disuniformity of heating in the direction perpendicular to the lateral dimensions of the load, i.e. in the direction perpendicular to the surface of the load, as is required for the surface to be browned or crispened.
  • the embodiment of the invention illustrated in Figs. 1 and 2 is a mode-filtering structure including a flat sheet 20 (shown as rectangular) of microwave-transparent material such as a suitable plastic, which may, in one illustrative example, be the flat top portion of a microwave heating container lid.
  • a single mode filter 22 is mounted on a flat surface of the sheet 20.
  • an electrically conductive plate 24 e.g., a plate of household-gauge aluminum foil, or of so-called converter gauge foil, the thickness of which would typically range between 6 and 7 microns
  • the outer dimensions of this plate being about equal to the latter surface so that the plate 24 extends substantially over the entire sheet in a horizontal plane.
  • This plate is formed with a plurality of apertures 26 each with a closed periphery of generally rectangular configuration; a 5 x 4 array of twenty apertures is shown, with all of the apertures being identical in size and spaced apart from each other and from the outer periphery of the plate by strip or mullion portions 28 of the plate.
  • the apertures are equidistantly spaced in an arrangement that is symmetrical with respect to the plate 24 and sheet 20.
  • the mode filter 22 also includes a plurality of electrically conductive islands 30, which in this particular embodiment are identical to each other in shape and dimensions and are again conveniently fabricated of household-gauge aluminum foil, bonded to the same sheet surface as the plate 24.
  • These islands 30 are equal in number to the apertures 26, and have closed generally rectangular peripheries substantially conforming in shape to the aperture peripheries but are smaller in area than the apertures; the islands 30 are respectively disposed within (and in register with) the apertures 26, so that the periphery of each island 30 is substantially uniformly spaced from the surrounding aperture periphery, and defines therewith a rectangular annular gap 32 (which, in this embodiment, is of substantially uniform width) closed or spanned by the microwave-transmissive dielectric material of the sheet 20.
  • each of the apertures 26 is a 2.20 x 1.8 cm rectangle, the strips 28 between apertures (on both the long and short sides of the apertures) being 0.5 cm in width.
  • Each of the conductive islands 30 is a 1.70 x 1.3 cm rectangle, and is centered in its associated aperture so as to define therewith a rectangular annular gap 32 having a uniform width of 0.25 cm on all sides.
  • the outer end columns of four apertures are spaced 1.0 cm from the short side edges 24a of the plate 24, and the outer side rows of five apertures are spaced 1.5 cm from the long side edges 24b of the plate 24, which has 2.0 cm-radiused corners.
  • a single mode filter as exemplified by the above-described embodiment of Figs. 1-2 may be used (by way of non-limiting example) with metallic, composite, or microwave-transparent containers, including those described in one or more of the aforementioned copending applications.
  • metallic containers are preferred, because radiation entering the container is then forced to interact with the mode filter.
  • a mode filter when a mode filter is used with a microwave-transparent container, it exerts little influence on radiation entering the container through the other surfaces not adjacent to it.
  • an array of one or more metallic islands (“island array”) is superimposed on an array of one or more corresponding apertures (“aperture array”) of a metallic area or plate.
  • Both the island and aperture arrays may be constructed (as by die-cutting) from aluminum foil, for example.
  • the mode filter is preferably positioned over the container, in electrical isolation from it, as in the embodiment of Figs. 5-6 described below.
  • the mode filter may also be in close mechanical and electrical contact with the container, or may be integral with it (as in a pouch type of construction, also described below, with reference to Fig. 23).
  • the aperture array is fabricated from rigid foil, a container or pan for holding the food to be heated may be constructed from the same foil, using similar techniques to those employed in the manufacture of "wrinkle-wall" or “smoothwall” pans.
  • Dielectric material is used to maintain the spatial relationship between the island and aperture arrays.
  • Suitable dielectric material (typified by such plastics as polypropylene, polyester, or polycarbonate resins) shows good resistance to dielectric breakdown, has low dielectric losses, and maintains its strength properties at the service temperatures imposed by the heating of the food.
  • Each island is generally centered on each aperture, but may be either coplanar with the aperture, or (as hereinafter further explained, for example with reference to Figs. 3-4 and 8-10) displaced vertically, so as to be approximately plane-parallel with the aperture.
  • the island array is coplanar with the aperture array, the area of each island is constrained to be less than that of the corresponding aperture.
  • the island and aperture arrays are vertically displaced (as, for example, in Figs. 3-4), the islands may be of greater or lesser area than the corresponding apertures.
  • FIGs. 5 and 6 show a microwave heating container having a generally rectangular, upwardly opening tray 10, with a bottom 11 and side walls 12, fabricated of metal (e.g. stiff, formed aluminum foil), for receiving and holding a body of foodstuff 14 to be heated.
  • a molded plastic (dielectric material) lid 16 transparent to microwave energy and having a downwardly extending portion 18 and a flat top or sheet portion 20, covers the upward opening of the tray, the downwardly extending portion seating on the tray rim.
  • the upper surface of the contained foodstuff is spaced below the top of the lid.
  • the mode filter 22, as described with reference to Figs. 1 and 2 (but here shown as having a 4 x 4 array of apertures and islands) is mounted on the upwardly facing flat surface of the lid top 20.
  • the electrically conductive plate 24 e.g., of household-gauge aluminum foil
  • the lid top surface extending substantially over the entire lid in a horizontal plane, though the plate is electrically isolated from the metallic tray 10 by the downwardly extending portion 18 of the dielectric material lid.
  • the islands 30 are likewise bonded to the lid top surface.
  • the arrangement of apertures and islands is symmetrical with respect to the rectangular lid top and thus with respect to the container as viewed in a horizontal plane.
  • a 4 x 4 array of sixteen spaced, uniformly and symmetrically distributed rectangular annular gaps 32 is provided in the lid top, constituting essentially the only microwave-transmissive areas or windows in the entire lid top, which is otherwise covered by the conductive plate 22.
  • Fig. 7 illustrates a modified form of container in which the tray 10a, like the lid, is formed of microwave-transparent material rather than metal, and in which a second mode-filtering structure 122a (which may be identical to, and in register with, the above-described mode filter 22) is mounted on the downwardly-facing flat bottom surface of the microwave-transparent tray 10a.
  • Both the plate 124a and islands 130a of the structure 122a may be constituted of household-gauge aluminum foil bonded to the tray bottom surface.
  • the plate 124a defines an array of apertures 126a, the peripheries of which in cooperation with the peripheries of the islands 130a define an array of gaps 132a equal in size and number to, and respectively in register with, the gaps 32 of the upper mode filter 22.
  • the contained body of foodstuff 14a is self-sustaining in shape and smaller than the internal dimensions of the tray, so that it is spaced inwardly away from the side walls of the tray. Since the container tray is microwave-transparent, the body 14a acts as a dielectric resonator, in determining the fundamental modes of the system. Stated more precisely, in the system of Fig. 7 the overall resonant boundaries are determined both by the body of foodstuff and by the mode-filtering structures, but power absorption is confined to the food cross-section.
  • the effect of the microwave-transparent tray side walls in determining the resonant boundaries would be simply the consequence of the effect of the tray side walls in defining the food body geometry, in contrast to the situation that obtains when the tray is electrically conductive (microwave-reflective) and constitutes a cavity resonator.
  • vertical displacement of the island and aperture arrays may be obtained by locating each array at opposite faces of a dielectric sheet or, alternatively, by locating the islands on dielectric protrusions (which may be filled or unfilled).
  • Dielectric protrusions may be obtained in the thermoforming of plastic film, for example.
  • Figs. 3 and 4 show a rectangular flat plastic microwave-transparent sheet 20 of the type described above with reference to Figs. 1 and 2 (e.g. the flat top of a microwave heating container lid 16 as shown in Fig. 5), but bearing a mode filter constituted of a conductive plate 34 (defining a 5 x 4 array of twenty rectangular apertures 36 separated by strips 38) mounted on the downwardly facing horizontal planar major surface of the sheet 20, and a 5 x 4 array of twenty rectangular conductive islands 40 mounted on the opposite (i.e. upwardly facing) horizontal planar major surface of the sheet 20 in register, respectively, with the apertures 36.
  • a mode filter constituted of a conductive plate 34 (defining a 5 x 4 array of twenty rectangular apertures 36 separated by strips 38) mounted on the downwardly facing horizontal planar major surface of the sheet 20, and a 5 x 4 array of twenty rectangular conductive islands 40 mounted on the opposite (i.e. upwardly facing) horizontal planar major surface of the sheet 20 in register, respectively, with the apertures 36.
  • the apertures 36 and islands 40 are spaced apart vertically by the thickness of the sheet 20, and the rectangular annular gap 42 defined between each island 40 and the periphery of its associated aperture 36 is provided by virtue of the vertical spacing, since (as shown) the islands 40 are larger than the apertures 36, though conforming thereto in peripheral configuration.
  • the top 20a of a plastic lid 16a may be molded with a multiplicity of hollow vertical protrusions 43 (one for each island 40) to increase the vertical spacing between the islands 40 and the apertures 36 of the plates 34.
  • Each protrusion 43 projects upwardly from the upper horizontal surface of the lid top 20a, and itself has a generally rectangular, horizontal flat top surface, on which is mounted one of the islands 40.
  • the aperture-defining plate 34 is mounted on the lower (downwardly facing) horizontal surface of the lid top, and the apertures 36 are disposed in register with the protrusions 43, being thus also in register with the islands 40.
  • Figs. 9 and 10 illustrate further embodiments of mode filters having vertical spacing between apertures and islands.
  • the conductive plate 54 defines an array of apertures 56 which are larger in area than the conductive islands 60, so that the arrangement of apertures and islands, in plan view, corresponds to Fig. 1.
  • Each aperture and its associated island define an annular gap 62, which gap results both from the vertical spacing between apertures and islands and from the fact that the islands are smaller in area than (though conforming in shape and orientation to) the apertures.
  • Fig. 9 shows a plastic lid 16b having a top 20b formed with a multiplicity of solid (rather than hollow) molded protrusions 63, i.e., one for each island 60, projecting upwardly from its upper horizontal surface, but otherwise similar to the above-described lid 16.
  • Each protrusion has a flat horizontal top surface on which one of the islands 60 is mounted.
  • the plate 54 is mounted on the main upper horizontal surface of the lid top 20b, in such position that the protrusions 63 respectively project upwardly through the apertures 56.
  • Fig. 10 shows a plastic lid 16c with a top 20c having a planar horizontal upper surface and a multiplicity of hollow protrusions 65 (one for each island 60) projecting downwardly from its lower surface.
  • Each protrusion 65 has a flat, downwardly facing horizontal lower surface on which is mounted one of the islands 60, while the aperture-defining plate 54 is mounted on the upper surface of the top 20c with the apertures 56 in register with the protrusions 65.
  • the latter protrusions may be so dimensioned that, when the lid 16c is placed on a tray 10, the islands 60 are substantially in contact with the top surface of the contained body 14 of foodstuff.
  • the aperture-defining conductive plate (34 or 54) and the conductive islands (40 or 60) are respectively disposed in parallel, but vertically spaced, horizontal planes.
  • both the plate and the island or islands may conveniently be fabricated of aluminum foil and mounted on a lid or other container wall of microwave-transparent dielectric material such as one of the plastics mentioned above.
  • the islands and apertures may assume a number of geometries, among which are the following:
  • Figs. 13-22 are merely exemplary of the diverse arrangements (with uniform or nonuniform gaps, and geometrically conformal or nonconformal island-aperture pairs) that may be employed in the structures of the present invention. Also, while for convenience only a single island-aperture pair is shown in most of these figures, it will be understood that an array comprising a multiplicity of such pairs may be provided in a complete mode-filtering structure, and that the pairs of such an array may (as hereinafter further discussed) be identical or nonidentical to each other dependent on the heating effect desired and the particular conditions of use.
  • the island and aperture geometry will typically or commonly also be rectangular.
  • the preferred island and aperture geometry will typically or commonly be based on a cylindrical coordinate system, i.e., divided into "cells" whose position is defined by radial and angular (harmonic) nodes. A system of the latter type is shown in Fig.
  • FIG. 11 is a top plan view of a cylindrical container having a plastic lid 16d with a planar circular top surface on which are mounted a conductive plate 204 defining five or six identical segment-shaped apertures 206 distributed in a radially symmetrical arrangement, and five or six conformally shaped but smaller conductive islands 210 respectively positioned in register with the apertures to define, with the aperture peripheries, five or six annular gaps 212, together with a central circular aperture 205 and island 207 defining a circular gap 209.
  • the minimum separation between apertures is dictated by the heating distribution desired in the food, by the mechanical ruggedness required by the application, and the amount of ohmic heating occurring in the metal defining the aperture array.
  • the width of foil between the apertures e.g., the width of the strips 28 in Figs. 1-2
  • the width of foil between the apertures will typically be 5 mm or greater.
  • Heating distributions over the plane of a mode filter comprised of a multiplicity of islands and apertures may be modified by varying differentially, over the cross-section of the structure, the island and/or aperture size, and/or the vertical displacement of the islands in relation to the apertures.
  • the central one of these apertures 26e, together with its associated island 30e is made larger than the others, to control in a desired manner the heating in the central region of an adjacent body of foodstuff.
  • the size of this central aperture and island correspond to a favorable higher mode in the foodstuff.
  • a mode filter may be curved or corrugated, for example, rather than planar.
  • Figs. 25 and 26 show curved plastic container lids having mode filters in accordance with the invention.
  • the upper surface of the top 20e of lid 16e has a smooth continuous convex curvature.
  • An aperture-defining conductive plate 214 (generally similar to the plate 24 of Fig. 1) is mounted thereon, with the conductive islands 220 (generally similar to islands 30 of Fig. 1) also disposed on the same lid top surface.
  • This curved mode filter corresponds to an arrangement (as shown, e.g., in Figs.
  • the top 20f of the lid 16f has an overall upwardly convex curvature and is formed with a plurality of radially extending upward protrusions 223 each having an upper surface curved concentrically with the overall top curvature.
  • a conductive plate 224, mounted on the lower surface of the lid top 20f, defines an array of apertures 226 respectively in register with the protrusions 223; a corresponding array of conductive islands 230 are mounted on the upper surfaces of the protrusions.
  • the curved mode filter of Fig. 22 thus corresponds to arrangements (as shown, e.g., in Figs. 3-4 and 8-10) wherein the plate and islands are respectively disposed in spaced parallel planes, and is embraced within the definition of parallel-plane plate arrangements.
  • a plurality of mode filters may be provided at different walls or surfaces of the same microwave heating container, e.g. for simultaneous treatment of multiple food surfaces.
  • the mode-filtering structures employed may be two distinct, electrically isolated mode filters, or two mode filters having aperture arrays constructed from the same metallic sheet.
  • the remainder of the package or container is formed from dielectric material, so that the overall package may be considered by the consumer a "composite” rather than "foil” container.
  • Electrically isolated mode filters may also be used at the upper and lower surfaces of a container having metallic sidewalls.
  • Fig. 24 shows a microwave heating container embodying the invention and including both top and bottom mode filters, between which is disposed the body of foodstuff to be heated.
  • This container may be of the familiar "clamshell” type, viz. a typically thermoformed foamed plastic package having an upper portion 231 and a lower portion 233 joined by an integral hinge or folding region (not shown) formed along one side, and arranged to close positively or latch (by suitable and e.g. conventional means, also not shown, formed along their edge portions), the walls of the package being somewhat deformable.
  • a mode-filtering structure of the type shown in Figs. 1-2 (including aperture-defining plate 24 and islands 30) is mounted on the flat top of the package upper portion 231.
  • Another similar mode-filtering structure 234, mounted on the flat bottom of the lower portion 233, defines an array of apertures 236 (which, in this embodiment, are identical in shape, size, and arrangement to, and are in register with, the apertures 26 of the top mode filter 22) and also includes a like plurality of aluminum foil islands 240, each defining with the periphery of its associated aperture a rectangular annular gap 242. These gaps 242 are equal in size and number to, and respectively in register with, the gaps 32 of the top mode filter 22.
  • an electrically isolated mode filter in or on a container lid may be used with a metallic container tray which incorporates the aperture array of a second mode filter.
  • one mode filter may be in close mechanical and electrical contact with a container incorporating an aperture array of a second mode filter in its base, or may be integral with it (as in a pouch type of construction).
  • this sheet may be folded in a U-shape to enclose the food article to be heated, as shown in fragmentary view in Fig.
  • FIG. 23 which illustrates a U-bent aluminum foil/plastic laminate sheet 243 having a plurality of rectangular apertures 246 formed in the foil and a like plurality of smaller but conformal foil islands 250 supported on the plastic of the laminate within the apertures to define therewith rectangular annular gaps 252 of uniform width.
  • first and second arrays the gap-defining apertures 246 and islands 250 are respectively disposed on opposite sides of a contained body (not shown) of foodstuff, so that these two arrays in effect constitute two mode filters acting at opposite surfaces of the foodstuff body.
  • the edges of the sheet 243 may be suitably sealed together to form a pouch within which the body of foodstuff to be heated is enclosed.
  • microwave oven heating characteristics tend to be uneven in the vertical direction (owing to coupling effects caused by the presence of a glass tray or ceramic oven floor), different upper and lower mode-filter designs may be incorporated in the same container, i.e. to compensate for such vertical unevenness of heating characteristics. Compensation may be obtained by variation of relative island and aperture areas and/or of the vertical displacements of the island and aperture arrays.
  • mode-filtering structures may also be used three-dimensionally, with mode filters located at all of the surfaces of a food.
  • each island or aperture defines a set of modes with corresponding cross-sections.
  • an island array permits the entry of "lower” (or more fundamental) modes through strip-lines and slot-lines defined by the combination of islands, the entry of the "lower” modes is impeded by an aperture array.
  • the aperture array may thus be perceived as analogous to a series of waveguide (or cavity) openings, each of which would effectively cut off or attenuate the lower modes.
  • Figs. 34-36 illustrate graphically the vertical absorption profile of microwave energy in a body of foodstuff adjacent a mode filter gap for conditions ranging from above cut off (Fig. 34) to below cut off (Fig. 36), where the body is sufficiently thick and/or absorptive so that the effects of reflection and/or propagation at opposite surfaces of the body can be ignored for purposes of the present analysis. Stated in general, it is the dimension of the individual mode filter gap (or open gap segment, in the bridged-gap structures described below with reference to Fig. 36) that determines whether a mode is in cut off for a food body of a particular dielectric constant, at a given wavelength (typically 2.45 GHz) of microwave energy.
  • a mode filter gap or open gap segment, in the bridged-gap structures described below with reference to Fig. 36
  • 2 the squared magnitude of the electric field intensities, in the vectorial sense
  • the free-space value of ⁇ r is unity.
  • ⁇ r is largely determined by food moisture content or water activity, so that the value of ⁇ r for high-moisture foods will nearly approach that of water, for which (at a frequency of 2.45 GHz) ⁇ r varies from about 80 at 0°C to about 55 at 100°C, the value for ice being approximately 4.
  • ⁇ r is about 5, varying by equilibration of their moisture contents with adjacent high-water-activity foods.
  • ⁇ eff ⁇ o / ⁇ r 1 ⁇ 2 .
  • ⁇ o 12.24 cm.
  • k2 The value of k2 is dependent on the geometry and dimensions of the gap or aperture as well as on the mode under consideration.
  • k2 ⁇ 2[(m2/L x 2) + (n2/L y 2)] and the condition for cut off is ⁇ 2[(m2/L x 2) + (n2/L y 2)] ⁇ 4 ⁇ 2/ ⁇ 2 eff .
  • the condition for cut off is that the relevant dimension L (i.e., L x or L y ) be equal to or less than ⁇ eff /2.
  • k2 is easily determined in rectangular and circular systems (tables of the zeros of Bessel functions are given in G.N. Watson, A Treatise on the Theory of Bessel Functions , Cambridge Univ. Press, 1922).
  • the analysis is more complex for other gap/aperture geometries (e.g. ellipses), but the general proposition holds that cut off of a given mode occurs when the relevant horizontal dimension of the gap or aperture is equal to or less than a value determined by the gap/aperture geometry, the mode in question, and the dielectric constant of the food body or body portion of concern.
  • slot length will be ⁇ ⁇ eff /2.
  • desired segment length will approximate to ⁇ eff /2.
  • Curved slot length should likewise approximate to ⁇ eff /2.
  • the circumference should be nearly (or less than) ⁇ eff .
  • Larger gaps will allow resonance in the radial dimension.
  • the line integral of gap length for narrow, uninterrupted rounded shapes will approach or be less than ⁇ eff .
  • the width of a gap (in the island-aperture pairs or arrays) should be at least about 1 mm, to avoid excessively high field intensities.
  • the length of the gap will usually be at least about 5 mm.
  • a closed curve with apices or cusps will be expected to have cut-off dimensions corresponding both to its circumference (one effective wavelength) and to its segments (each being ⁇ eff /2). For an odd number of equal segments, however, destructive interference may cause cancellation of the modes corresponding to them.
  • An open curve with cusps or apices will similarly have its entire length as one cut-off dimension ( ⁇ eff /2), and may also support higher order modes with cut-off dimensions defined as the distance bounded by two such apices or cusps (each segment being ⁇ eff /2).
  • Wide gaps or annuli As with their narrow counterparts, wide gaps or annuli will support resonances over their lengths. However, they will also allow two-dimensional resonances, generally characterized by larger cut-off dimensions. Thus, in decreasing critical dimensions, cut-off will first occur for two-dimensional resonances, and be followed by cut-off in resonances determined by gap or annular lengths. By selecting dimensions which support resonances determined by gap or annular lengths, while providing cut-off of two-dimensional resonances, heating of the bulk of the absorber may be balanced against heating of its surfaces.
  • the wavelength will have a slightly greater value than ⁇ eff as defined above.
  • a lower bound is provided by the bulk wavelength, taken as the free-space wavelength ⁇ o , divided by the square root of the absorber relative dielectric constant, here denoted as ⁇ r(m) . If these structures were embedded well within the absorber bulk, this lower bound would accurately determine cut-off dimensions for the gaps or annuli.
  • the coexistence of fields within the air surrounding an absorber causes wavelengths used in determining cut-off dimensions at the absorber surface (the locus of interest for browning and crispening effects) to be substantially larger than ⁇ o / ⁇ r(m) 1 ⁇ 2 .
  • a useful approximation for determining the effective dielectric constant ⁇ eff at the surface of a dielectric material is that suggested by S.B. Cohn, IEEE Trans. Microwave Theory and Techniques, MTT 17(10), 768 (1969), viz. the arithmetic average of the relative dielectric constant ⁇ r(m) of the dielectric material and the relative dielectric constant ⁇ r(s) of free space overlying its surface. Since the dielectric constant of free space assumes a value of ⁇ o , the relative dielectric constant ⁇ r(s) must be unity.
  • ⁇ eff ⁇ o /[(1 + ⁇ r(m) )/2] 1 ⁇ 2
  • the "narrow gap or annulus" considerations set forth above establish that the critical dimensions for cut off are a perimeter, or sum of gap length and width, equal to ⁇ eff , or either length or width equal to ⁇ eff /2.
  • the critical dimensions are a diameter equal to ⁇ eff / ⁇ .
  • the control of horizontal plane heating distributions and of heating gradients in the "vertical" axis is increased, when the entry of radiation through other food surfaces is suppressed.
  • This suppression may be obtained by the selection of overall mode-filter dimensions and separation, by the shape or contour of the mode-filter edges (as by introducing well known "choke” structures), and/or by the introduction of metal walls (which may be integral with the mode filter(s), and which may also incorporate mode filters).
  • Operation of the mode filters of this invention is typified by the following: Considering first a single-mode filter array, i.e. at a single surface, modification of heating distributions in the horizontal plane of a container and/or food is generally obtained by positioning the island array over the aperture array, and by positioning the resulting structure over a metallic or composite container.
  • the design principles used to obtain a particular heating pattern are similar to those used in the containers described in the aforementioned U.S. patent No. 4,866,234, except that there is less need (in the present invention) to compensate for the entry of "lower" modes.
  • the island array of a mode filter in accordance with the present invention may be vertically displaced above or below the aperture array. In this configuration, crispening may be obtained simultaneously at both the upper and lower surfaces of the food.
  • a particularly efficacious configuration is that in which the islands are in contact with the food, but are displaced beneath the aperture array.
  • a mode-filter When used for browning or crispening, a mode-filter will generally use island and aperture dimensions on the order of less than 2 cm on a side.
  • a dielectric-material (e.g., molded plastic) container bottom 301 is formed with one or more upward protrusions 303 having a planar upper horizontal surface spaced above the horizontal upper surface of the bottom.
  • An aperture-defining conductive plate 304 is mounted on the latter surface, defining at least one aperture 306, through which protrusion 303 projects;
  • a conductive island 310 is mounted on the upper surface of the protrusion, to define (with the aperture) an annular gap of uniform width, thus constituting a mode filter in accordance with the present invention.
  • a dielectric-material container bottom 311 is formed with one or more downward protrusions 313 having a planar lower horizontal surface spaced below the horizontal lower surface of the bottom; aperture-defining conductive plate 314 is mounted on the latter surface, defining an aperture 316 through which protrusion 313 projects downwardly, while a conductive island 320 is mounted on the lower surface of the protrusion 313, again so as to define with the aperture periphery a uniform-width annular gap.
  • Each of these mode filters of Figs. 27 and 28, as will be understood may include an array of apertures, islands and protrusions, only one being shown in each case for simplicity of illustration.
  • Mode filter structures arranged for simultaneous treatment of multiple surfaces are of considerable interest for the browning or crispening of battered and breaded foods such as fish sticks, fried chicken, etc.
  • one or more food articles are placed between two mode-filtering structures.
  • the island and aperture dimensions are chosen so as to intensify heating at the food coating. Two types of browning or crispening may be obtained:
  • the mode filters described above are a subset of a much broader set, which is conceptually linked also to the conductive indented structures of the last-mentioned European application.
  • This much broader set may be characterized in the following features:
  • mode filters were prepared from foil sheets which effectively incorporated a mode filter as herein contemplated in a structure as described in the aforementioned EP-A-0 246 041. These mode filters were intended for the crispening of breaded and coated fish fillets.
  • the foil areas contacting the fillets were of the same size and disposed in the same positions as the "islands" of mode filters previously used for the same purpose.
  • the areas of contact were electrically integral with the foil sheets, such that two opposite sides of these rectangular areas were connected with the sheet, and upwardly displaced from it (i.e., towards the fillets). The other opposite sides were not connected, so that air gaps or slots existed at these sides.
  • these structures may also be viewed as patterns of slots, such that the slots define tabs or other shapes, and may even define structures resembling slot/strip meander lines. Since a single slot produces a field maximum at its middle (and thus, localized heating in the same region), it is desirable that the slots be configured so as either to give a desired pattern of heating, or even heating. While the structures defined by the slots may have apices or be angular in nature, rounded or convoluted shapes may also be used.
  • a metallic plate 350 is formed with a plurality of spaced-apart rectangular projections 352 each having a flat top 354 lying in a plane spaced from and parallel to the major surfaces of the plate.
  • each conductive "island" is the flat top 354 of a projection, its periphery consisting of bends 360 and gap top edges 362.
  • Each "aperture” has a periphery defined by bends 364 and gap bottom edges 366.
  • Walls 356 constitute conductive bridges spanning the gap between aperture and island.
  • the open gap portions 358 provide dielectric isolation between aperture and island while the vertical displacement between top 354 and plate 350 due to phasing or electrical distance effects.
  • the structure of Fig. 31 is formed from a single sheet of metal by slitting and drawing to form the projections 352.
  • the modified structure of Fig. 32 also formed from a single metallic plate 350a, the plate portions 368 intermediate adjacent projections 352a are bent out of the plate major surface planes to an extent equal and opposite to the bending of the projections, so that drawing of the metal is not required.
  • Fig. 33 illustrates a planar mode filtering structure in which a metallic (conductive) plate 370 defining a rectangular aperture 372, and a metallic (conductive) island 374 of rectangular configuration, smaller than and disposed within the aperture, are connected by conductive bridge portions 376 spanning the gap 378 defined between the island and aperture peripheries.
  • the plate, island and bridges are formed integrally from a single metal sheet (e.g. an aluminum foil sheet of suitable gauge) by cutting out from the sheet opposed C-shaped portions 380 of the gap 378. These portions 380 are open (microwave-transparent) portions or segments of the gap.
  • a mode filter thus constituted provides effects comparable to those of mode filters in which there is complete isolation between island and aperture periphery, as in the structures of Figs. 1-30 described above.
  • a mode-filtering structure in accordance with the invention may have one such mode filter, or an array of these bridge-type mode filters, arranged for example in the same manner as the rectangular mode filters of Fig. 1.
  • Fig. 33 is merely exemplary of bridging arrangements by which islands are made integral with their associated aperture-defining conductive plates by spaced-apart conductive bridges spanning the annular gaps between aperture peripheries and islands.
  • Such arrangements afford important advantages from a manufacturing standpoint, in that a complete mode-filtering assembly of apertures and islands can be formed integrally in a single sheet of aluminum foil or the like and mounted as a unit on a microwave-transparent container lid or other supporting surface.
  • the open gap portions or segments (380 in Fig. 33) are dimensioned to provide sharp attenuation in the vertical direction so as to achieve browning or crispening of the surface of the body of foodstuff being heated.
  • Fig. 37 shows a device in the form of a thin sheet 410 of microwave-transparent material on which there is located an array of rectangular annuli 411 of aluminum or other metallic foil. Each annulus 411 defines an aperture 412 that remains microwave-transparent, as do the spaces 413 and 414 between the annuli.
  • the sheet 410 can be used in association with a standard food container 415, and may be placed therein beneath a food article (not shown) or above such article, depending upon which surface of the food article is to be subjected to an increased temperature for browning and/or crispening. Alternatively, if both the top and bottom surfaces are to be subjected to an increased temperature, two such sheets can be employed in the container 415, one below and one above the food article.
  • Each sheet 410 may be flexible so as to be able to conform to an irregularly shaped food article.
  • it may be made of polypropylene, polyester, polycarbonate or other low loss material that will be substantially transparent to microwave energy.
  • the sheet 410 can be more rigid, i.e. made of a low loss plastic foam or cardboard-like material.
  • it may be made of a ceramic or glass, provided that such material is substantially transparent to the microwave energy.
  • a sheet 410 can be embodied in the container 415 as a part thereof, e.g. as the bottom or as a lid, or as both.
  • the sheet 410 can be a separate element that is employed by the user in conjunction with a container.
  • the user can place a standard food container (with a microwave-transparent bottom) on top of a sheet 410 in a microwave oven, or can place a sheet 410 on top of the food article after removing the conventional container lid.
  • a sheet 410 can be used directly with a food article without need for a container at all.
  • a pizza can be heated by simply placing it on a sheet 410 in the microwave oven, provided the sheet 410 is sufficiently spaced above the oven floor to avoid arcing.
  • the thickness of the metal film forming the metallic annuli 411 will be sufficient to prevent it functioning as a susceptor, such metal film being virtually entirely reflective of the microwave energy and absorbing negligible amounts of such energy.
  • its thickness will preferably be about 6 or 7 microns, since this is a convenient rolling thickness for aluminum.
  • a thickness of as little as about 0.2 microns might be used. This is in contrast to a thickness of about 0.01 microns which would absorb microwave energy and become heated.
  • Fig. 38 shows square annuli 420; Fig. 38A shows square annuli 420a interrupted at 420b; Fig. 39 shows circular annuli 421; Fig. 40 shows triangular annuli 422 and Fig. 41 shows hexagonal annuli 423. Any of these latter shapes can also have interruptions in the annuli, analagous to those of Fig. 38A, and such interruptions need not necessarily be two in number, but may be a single interruption or more than two interruptions. Moreover, the shape of the aperture defined by the annulus need not necessarily conform to the outer shape of the annulus For example, a circular aperture in a square annulus can be used.
  • the function of the annuli is to set up a condition in which the aperture in each annulus causes the modes of microwave energy that propagate through the apertures to be in cut-off for air and for substances containing substantial quantities of air, e.g. batter, bread crumbs, pastry, etc., of which the surface layer of the food article will likely be composed, but preferably not to be in cut-off for the main portion of the food article inwardly of its surface layer.
  • substances containing substantial quantities of air e.g. batter, bread crumbs, pastry, etc.
  • the wavelength in air of the microwave energy at the standard frequency of 2.45 GHz is approximately 12.24 cm
  • the wavelength in the food bulk (which will normally be composed mainly of water which has a relative dielectric constant of the order of 80)
  • the wavelength will be in a range from about 1.3 cm (pure water) to about 2.0 cm, depending on the proportion of water in the food. It is to be understood that these values and those given below are necessarily approximate and can vary quite widely with the nature of the food or other article being heated.
  • the wavelength in such surface layer will normally be somewhere in between that of air and that of the main portion of the article.
  • ⁇ r for such a layer will vary (owing, as mentioned above, to equilibration of a relatively low initial surface layer water activity with that of the underlying food); an exemplary low-end value of ⁇ r for coatings (such as batters and the like) subject to these considerations is 5. More broadly, an illustrative (but non-limiting) range of ⁇ r for a wide range of surface layers is 1.5 - 16, for which the corresponding range of wavelengths (at 2.45 GHz) is 10.0 - 3.0 cm. For example, a crumb coating or puff pastry crust, which includes a large number of air pockets, can typically have an overall relative dielectric constant that will result in a wavelength of the order of 8.0 - 10.0 cm.
  • a more dense coating e.g. a batter
  • the dimensions of the annuli can be tailored to specific foods and coatings (surface layers) once their approximate relative dielectric constants are known, or by trial and error, in order to arrange that the apertures, i.e. the width dimension Di, should be such that some of the modes of microwave energy that propagate through the apertures will be below cut-off, i.e. commonly referred to as "in” cut-off, in the surface layer (and in air), but above (“not in”) cut-off in the main portion of the food itself.
  • the dimension Di in the case of a rectangular structure must be smaller than half the wavelength in the substance concerned.
  • the half wavelengths will be bulk food 0.65 - 1.0 cm food surface coating 1.0 - 3.0 cm air 6.0 cm
  • Di a good choice for the value of Di will be in the range of 10 - 16 mm, preferably about 12 - 14 mm, because this value should achieve a situation where the dominant modes of microwave energy that propagate through the apertures are in cut-off for the surface coating (and air) while not in cut-off for the bulk of the food.
  • the lower end of this range can be extended down, e.g. to 5 or 6 mm.
  • the upper end of this range can be extended up, e.g. to about 20 - 25 mm.
  • the effective width dimension to be considered from the viewpoint of making the aperture small enough to ensure cut-off in the surface layer i.e., equivalent to the dimension Di
  • the effective width dimension to be considered from the viewpoint of making the aperture small enough to ensure cut-off in the surface layer will be the greater internal length in the case of a rectangular annulus (Fig. 37), the internal diameter in the case of a circular annulus (Fig. 39), the height of the internal triangle in the case of a triangular annulus (Fig. 40), and the distance between a pair of opposite inside faces in the case of the hexagonal annulus (Fig. 41).
  • Fig. 42 shows energy E entering the sheet 410.
  • the sizes of the waves shown are intended to represent their respective amplitudes rather than their spatial locations.
  • the energy E passes through an aperture 412 in one of the annuli 411. First it encounters an air gap 425, where there is attenuation per unit distance travelled (because the energy is in cut-off). Then the remaining energy E′ enters a surface layer 426 where it is still in cut-off. Finally the remaining energy E ⁇ enters the main portion 427 of the food article, where it is no longer in cut-off and hence there is much less attenuation per unit distance due only to absorption.
  • the air gap 425 between the structure and the food is kept as short as possible, because the field decays evanescently in air, and the objective is that the majority of the energy should be absorbed in the surface layer 426.
  • the energy E ⁇ that does remain to be absorbed by the main portion 427 of the food article will heat the same more uniformly in the depth direction of propagation, which is desirable, because the main portion of the food article will normally have a greater depth dimension than its surface layer.
  • the overall result is thus increased heating per unit volume in the surface layer 426 relative to the heating per unit volume in the main portion 427 and hence the attainment by the surface layer of a relatively high temperature (with a consequent browning or crispening effect) and the more uniform absorption of heat (at a lower temperature) by the main portion so that the inner parts of this main portion, which are relatively remote from the surface layer, are not entirely unheated.
  • a convenient value for Do would be about 20 - 25 mm, and that for Db about 4 - 5 mm, with a minimum of about 3 mm. If the value of Db is made too small, there is a danger of arcing. If Db is made too large, the microwave energy will tend to be propagated through the spaces 413 and 414 instead of through the apertures 412. Assuming that the relative dielectric constant is within the exemplary range of values (1.5 - 16) mentioned above, as long as Db is no greater than about 6 mm, these spaces will also be in cut-off for air and the surface layer.
  • Figs. 37, 38 and 40 allow more lower order modes to propagate and hence tend to achieve more bulk heating (which may be desired in some cases, depending on the nature, especially the water content, of the food article). This effect can be reduced by avoiding such open lines, as in Figs. 39 and 41, or by staggering the rows of annuli in Figs. 37, 38 or 40.
  • the embodiments of the invention so far described and illustrated have taken the form of an array of metallic structures on a microwave transparent sheet.
  • the invention can also be practiced by defining the apertures by means of shapes or configurations of a material that differs from the microwave transparent sheet in some other electromagnetic property, such as conductivity, lossiness, dielectric constant, spatial thickness, a stepwise discontinuity or a magnetic property, as explained in the published European applications referred to above.
  • a container of frozen, battered and crumb-coated fish was heated first using the hexagonal annuli and then using the square annuli. It was found that, when using the square annuli, it was sometimes best to use a different aperture dimension on the top surface from that used on the bottom.
  • a first load used was purified agar in fine granular form dissolved in hot water to provide a gel density of 1.03 g/ml (sold as "Bacto-Agar”* by Difco Laboratories, Detroit, Michigan, U.S.A.)
  • the experiment employed a LUXTRON 750* Fluoroptic Thermometry System using a pair of probes.
  • One probe was positioned 2 mm below the sample surface and the other 5 mm below the sample surface. Both were in the center of the sample in the length and width directions.
  • the first probe effectively measured the "surface” temperature. These dimensions were maintained regardless of the depth of the load, which was either 6 mm or 10 mm. Measurement of the "surface" temperature by means of a probe that was actually 2 mm below the surface was required by the finite dimensions of the probe itself and in order to minimise the surface cooling effect.
  • the array used (Fig. 45), which is basically similar in structure to Fig. 1, was separated into two parts, namely an array of islands 428 (Fig. 46) and an array of apertures 429 (Fig. 47). Three samples were tested under identical conditions, one with the island array (Fig. 46) alone, a second with the aperture array (Fig. 47) alone, and a third with these two arrays combined to provide the composite array of Fig. 45. It was found that neither of the single arrays when used individually was effective. However the combination produced a mode-filtering array that provided uniformity of heating across the surface of the pastry and an intensification of heat at the surface, i.e. a non-uniformity of heating in the vertical direction, sufficient to achieve satisfactory browning across the entire surface.
  • the surface to bulk temperature differentials were approximately 45°C and 38°C for the 6 mm and 10 mm deep samples, respectively.
  • FIG. 48 A further run was conducted using a pastry load of 10 mm depth and a mode-filtering array as shown in Fig. 48 having metal "annuli" 431 of such a shape as to define cruciform apertures 432, these annuli being arrayed on a sheet 433 of microwave-transparent material.
  • the difference between the surface and bulk temperatures was already approximately 35°C after 20 seconds heating at full power and remained at about this value as heating progressed.
  • the array of Fig. 48 was tested in order to demonstrate that the interior shape of the annulus, i.e. the aperture, in this case cruciform, need not necessarily conform to the outer shape of the annulus, in this case square.

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Claims (31)

  1. Méthode d'amélioration du chauffage d'une couche superficielle (426) d'un article étant chauffé à l'énergie à micro-ondes (E), caractérisée par le fait de diriger ladite énergie à travers une pluralité d'ouvertures (412) dans la couche superficielle et de là dans une portion principale (427) de l'article de telle sorte que les modes de ladite énergie (E') sont en coupure dans la couche superficielle pour chauffer ladite couche superficielle directement par ladite énergie.
  2. Méthode selon la revendication 1, caractérisée dans le fait que la couche superficielle a une constante diélectrique plus basse que la portion principale et l'énergie (E') n'est pas en coupure dans la portion principale, de sorte à causer une absorption d'énergie dans la couche superficielle par distance d'unité dans l'article qui doit être supérieure à l'absorption d'énergie par distance d'unité dans la portion principale, et donc pour porter la couche superficielle à une température plus grande que celle de la portion principale.
  3. Méthode selon les revendications 1 et 2, caractérisée en ce que l'énergie est propagée dans la couche superficielle par une pluralité d'ouvertures (412) définies par des couronnes (411) de matériau réfléchissant à micro-ondes et substantiellement non absorbant.
  4. Méthode selon la revendication 3, caractérisée dans le fait que lesdites ouvertures ont une dimension de largeur (Di) d'environ 5 mm à environ 25 mm.
  5. Méthode selon les revendications 1 ou 2, caractérisée par l'amélioration de l'uniformité du chauffage de l'article dans des directions latérales transversales à la direction de propagation de l'énergie à travers la couche superficielle en générant ou en améliorant des modes d'ordre plus important d'une telle énergie dans l'article.
  6. Dispositif pour mettre en oeuvre la méthode de la revendication 1, comprenant une feuille, de matériau transparent aux micro-ondes (20, 410), et des moyens
    définissant une série d'ouvertures transparentes aux micro-ondes (26, 412) pour la transmission d'énergie à microondes dans ledit article, caractérisé en ce sens que lesdites ouvertures sont de dimensions telles que, à la fréquence de l'énergie à micro-ondes et avec la feuille située adjacente à une couche superficielle d'un article à chauffer, les modes de cette énergie (E') qui se propagent à travers les ouvertures seront en coupure dans ladite couche superficielle pour chauffer la couche superficielle directement par ladite énergie
  7. Dispositif selon la revendication 6, caractérisé dans le fait que les ouvertures sont de dimensions telles que l'énergie qui se propage par les ouvertures ne seront pas en coupure dans une portion principale (427)de l'article situé au-dessous de la couche superficielle et ayant une -
    constante diélectrique plus importante que celle de la couche superficielle.
  8. Dispositif selon les revendications 6 à 7, caractérisé dans le fait que lesdits moyens définissant les ouvertures comprennent une pluralité de couronnes (411) de matériau réfléchissant les micro-ondes et substantiellement non absorbant sur ladite feuille.
  9. Dispositif selon la revendication 8, caractérisé dans le fait que lesdites couronnes sont arrangées dans une rangée.
  10. Dispositif selon la revendication 9, caractérisé dans le fait que la rangée est telle que le matériau transparent aux micro-ondes entre lesdites couronnes trace des chemins tortueux.
  11. Dispositif selon la revendication 9, caractérisé dans le fait que les couronnes de ladite rangée sont interconnectées les unes avec les autres par un matériau réfléchissant de micro-ondes et substantiellement non absorbant.
  12. Dispositif selon n'importe laquelle des revendications de 6 à 11, caractérisé dans le fait que lesdites couronnes sont substantiellement rectangulaires, substantiellement carrées, substantiellement circulaires, substantiellement triangulaires, substantiellement hexagonales ou des combinaisons de ces formes.
  13. Dispositif selon n'importe laquelle des revendications de 6 à 12, pour être utilisé avec un article alimentaire à chauffer par l'énergie à micro-ondes à une fréquence de 2,45 GHz, caractérisé dans le fait que lesdites ouvertures ont une dimension transversale comprise entre environ 5 et 25 mm.
  14. Dispositif selon la revendication 8, caractérisé dans le fait que lesdites couronnes ont une largeur extérieure allant d'environ 10 mm à 16 mm, située de préférence entre 12 et 14 mm.
  15. Dispositif selon la revendication 8, pour être utilisé avec un article dont la couche superficielle a une constante diélectrique relativement basse, caractérisé dans le fait que lesdites couronnes ont une largeur qui se situe environ entre 20 et 25 mm.
  16. Dispositif selon la revendication 9, dans lequel ladite rangée est telle qu'elle offre un espacement entre les couronnes de 3 mm environ à 6 mm environ.
  17. Dispositif selon n'importe laquelle des revendications de 6 à 16 en combinaison avec un article à chauffer, caractérisé dans le fait que le dispositif est incorporé dans au moins une paroi du conteneur dans lequel est monté l'article, ladite paroi ou lesdites parois étant un fond, ou un couvercle ou les deux dudit conteneur.
  18. Dispositif selon la revendication 6, caractérisé par une plaque électroconductrice (24) située sur ladite feuille de matériau transparent aux micro-ondes, ladite plaque définissant au moins une ouverture (26, 36, 56) ayant une périphérie extérieure fermée, et au moins une île électroconductrice (30, 40, 60) disposée substantiellement par encastrement pour définir un écart transmetteur d'énergie à micro-ondes (32, 42, 62) entre la périphérie extérieure de l'île et la périphérie extérieure de l'ouverture pour une
    génération dans l'article à chauffer d'au moins un mode d'énergie micro-ondes d'un ordre plus important aux modes fondamentaux dans ledit article.
  19. Dispositif selon la revendication 18, caractérisé dans le fait que l'écart est continuellement ouvert.
  20. Dispositif selon la revendication 18, caractérisé dans le fait que ledit écart est comblé à des intervalles espacés par un matériau électroconducteur franchissant l'écart entre ladite plaque et ladite île.
  21. Dispositif selon la revendication 18, caractérisé dans le fait que ladite île dispose d'une ouverture (27d).
  22. Dispositif selon la revendication 18, caractérisé dans le fait que ladite plaque et ladite île sont disposées en relation coplanaire l'une par rapport à l'autre.
  23. Dispositif selon la revendication 18, caractérisé dans le fait que ladite plaque et ladite île sont disposées respectivement dans des plans parallèles espacés l'une de l'autre dans une direction transversale auxdits plans.
  24. Dispositif selon la revendication 23, caractérisé dans le fait que ladite île est plus petite en surface que ladite ouverture.
  25. Dispositif selon la revendication 23, caractérisé dans le fait que ladite île est au moins égale en surface à ladite ouverture.
  26. Dispositif selon la revendication 18, caractérisé dans le fait que ladite plaque définit une pluralité desdites ouvertures distribuées sur cette surface en relation espacée l'une par rapport à l'autre, avec une pluralité desdites îles disposées encastrées avec les ouvertures qui correspondent pour donner une rangée d'écarts annulaires distribués sur la surface de ladite plaque.
  27. Dispositif selon n'importe laquelle des revendications de 18 à 26, caractérisé dans le fait que le dispositif forme une portion de première paroi d'un conteneur pour ledit article.
  28. Dispositif selon la revendication 27 caractérisé par un deuxième dispositif formant une portion de paroi supplémentaire du conteneur en face de la portion de la première paroi.
  29. Dispositif selon la revendication 27, caractérisé dans le fait que ledit conteneur comprend un plateau s'ouvrant vers le haut pour tenir ledit article et un couvercle pour couvrir l'ouverture sur le haut du plateau, ledit dispositif étant disposé sur ledit couvercle avec la plaque conductrice du dispositif s'étendant substantiellement sur l'ensemble de la surface dudit couvercle.
  30. Dispositif selon n'importe laquelle des revendications de 6 à 29, dans lequel lesdits moyens définissant lesdites ouvertures comprennent des moyens positionnés sur ladite feuille de matériau transparent aux micro-ondes et ayant une propriété électromagnétique différente de celle de la feuille.
  31. Dispositif selon la revendication 30, dans lequel la propriété électromagnétique est la conductivité, la perte, la constante diélectrique, l'épaisseur spatiale, une discontinuité pas à pas ou une propriété magnétique.
EP90300997A 1989-02-09 1990-01-31 Procédés et dispositifs utilisés pour le chauffage par micro-ondes d'aliments et autres matériaux Expired - Lifetime EP0382399B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CA000590564A CA1339540C (fr) 1989-02-09 1989-02-09 Methodes de cuisson au four a micro-ondes de produits alimentaires et autres, et dispositifs connexes
CA590564 1989-02-09

Publications (3)

Publication Number Publication Date
EP0382399A2 EP0382399A2 (fr) 1990-08-16
EP0382399A3 EP0382399A3 (fr) 1992-02-12
EP0382399B1 true EP0382399B1 (fr) 1994-11-30

Family

ID=4139596

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Application Number Title Priority Date Filing Date
EP90300997A Expired - Lifetime EP0382399B1 (fr) 1989-02-09 1990-01-31 Procédés et dispositifs utilisés pour le chauffage par micro-ondes d'aliments et autres matériaux

Country Status (10)

Country Link
EP (1) EP0382399B1 (fr)
JP (1) JP3049071B2 (fr)
AT (1) ATE114598T1 (fr)
AU (1) AU628918B2 (fr)
BR (1) BR9000574A (fr)
CA (1) CA1339540C (fr)
DE (1) DE69014331T2 (fr)
FI (1) FI900648A0 (fr)
NO (1) NO900546L (fr)
ZA (1) ZA90446B (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
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US8461499B2 (en) 2006-06-14 2013-06-11 The Glad Products Company Microwavable bag or sheet material
US9174789B2 (en) 2013-03-15 2015-11-03 Graphic Packaging International, Inc. Container with heating features
US9254061B2 (en) 2006-06-14 2016-02-09 The Glad Products Company Microwavable bag or sheet material

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US5310980A (en) * 1988-11-28 1994-05-10 Beckett Industries, Inc. Control of microwave energy in cooking foodstuffs
US5160819A (en) * 1991-03-11 1992-11-03 Alcan International Limited Microwave tunnel oven having means for generating higher order modes in loads
CA2046836C (fr) * 1991-05-06 2000-04-04 D. Gregory Beckett Boite a pizza
DE69420120T2 (de) * 1993-09-24 1999-12-23 Unilever Nv Mikrowellenerhitzbare Mischlebensmittel
US5424517A (en) * 1993-10-27 1995-06-13 James River Paper Company, Inc. Microwave impedance matching film for microwave cooking
EP0650670B1 (fr) * 1993-11-01 1998-08-12 Unilever Plc Procédé de fixation thermique d'une pâte enrobant un produit alimentaire au moyen d'un chauffage diélectrique
US5864123A (en) * 1995-06-02 1999-01-26 Keefer; Richard M. Smart microwave packaging structures
EP0897369B1 (fr) * 1997-02-12 2004-04-14 Graphic Packaging Corporation Capteur d'energie a motifs pour four micro-ondes
GB0219856D0 (en) * 2002-08-27 2002-10-02 Wedlock Christopher P Difractive microwave matrix cooker
US8803050B2 (en) 2006-05-15 2014-08-12 Graphic Packaging International, Inc. Microwavable construct with contoured heating surface
EP2639171B1 (fr) * 2006-05-15 2023-10-18 Graphic Packaging International, LLC Plateau de chauffage par micro-ondes
US8680448B2 (en) 2006-05-15 2014-03-25 Graphic Packaging International, Inc. Microwavable construct with contoured heating surface
ATE548611T1 (de) 2007-01-22 2012-03-15 Graphic Packaging Int Inc Gleichmässig wärmender mikrowellentauglicher behälter
CA2715590C (fr) 2008-02-18 2014-10-21 Graphic Packaging International, Inc. Appareil pour cuire des articles alimentaires bruts dans un four a micro-ondes
CA2715627C (fr) 2008-02-18 2015-02-03 Graphic Packaging International, Inc. Appareil pour preparer un article alimentaire dans un four a micro-ondes
ES2636490T3 (es) 2008-07-11 2017-10-05 Graphic Packaging International, Inc. Recipiente para calentamiento por microondas
WO2010127214A2 (fr) 2009-05-01 2010-11-04 Graphic Packaging International, Inc. Construction avec élément de positionnement
CA2928099C (fr) 2013-12-16 2018-03-13 Graphic Packaging International, Inc. Construction a caracteristiques de raidissage
JP7505588B2 (ja) 2020-12-14 2024-06-25 株式会社村田製作所 加熱用電磁波制御体及び加熱用電磁波制御体付き物品

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EP0000797A1 (fr) * 1977-08-04 1979-02-21 THE PROCTER & GAMBLE COMPANY Modérateur pour énergie à micro-ondes
DK231487A (da) * 1986-05-09 1987-11-10 Alcan Int Ltd Mikroboelgeovnbeholder
CA1279902C (fr) * 1986-05-09 1991-02-05 Alcan International Limited Contenant pour la cuisson d'aliments aux micro-ondes a generation de mode d'ordre plus eleve
US4814568A (en) * 1987-05-15 1989-03-21 Alcan International Limited Container for microwave heating including means for modifying microwave heating distribution, and method of using same
US4927991A (en) * 1987-11-10 1990-05-22 The Pillsbury Company Susceptor in combination with grid for microwave oven package
AU2642588A (en) * 1987-12-01 1989-06-01 Mccain Foods (Aust) Pty. Ltd. Food product and package
DE68916798T2 (de) * 1988-05-23 1994-12-15 Pillsbury Co Suszeptor zum erhitzen eines einzelnen nahrungsmittels.

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8461499B2 (en) 2006-06-14 2013-06-11 The Glad Products Company Microwavable bag or sheet material
US9254061B2 (en) 2006-06-14 2016-02-09 The Glad Products Company Microwavable bag or sheet material
US9174789B2 (en) 2013-03-15 2015-11-03 Graphic Packaging International, Inc. Container with heating features

Also Published As

Publication number Publication date
BR9000574A (pt) 1991-01-15
AU4917190A (en) 1990-08-16
CA1339540C (fr) 1997-11-11
AU628918B2 (en) 1992-09-24
EP0382399A2 (fr) 1990-08-16
DE69014331D1 (de) 1995-01-12
FI900648A0 (fi) 1990-02-09
EP0382399A3 (fr) 1992-02-12
JPH02270294A (ja) 1990-11-05
NO900546D0 (no) 1990-02-05
JP3049071B2 (ja) 2000-06-05
DE69014331T2 (de) 1995-04-27
ZA90446B (en) 1990-10-31
NO900546L (no) 1990-08-10
ATE114598T1 (de) 1994-12-15

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