EP0348156A2

EP0348156A2 - Improvements relating to microwave heating

Info

Publication number: EP0348156A2
Application number: EP89306233A
Authority: EP
Inventors: Bryan Cobley Hewitt; Melville Douglas Ball
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1988-06-22
Filing date: 1989-06-20
Publication date: 1989-12-27
Also published as: AU615755B2; JPH0264323A; DK306489A; EP0348156A3; NZ229606A; AU3665489A; FI893049A0; FI893049A; CA1306509C; DK306489D0; ZA894620B; NO892519L; NO892519D0; BR8903046A

Abstract

A container (10) for heating a substance, e.g. food, in a microwave oven has a bottom (12) transparent to microwave energy. The container is supported on a stand (22) that contains one or more electrically conductive plates (32) (or apertures in an electrically conductive sheet) that generate at least one mode of microwave energy of higher order than the fundamental modes. The stand (22) also serves to support the container (10) above the higher order mode generating plates (or apertures) so that the undersurface of the substance is spaced from them by a uniform predetermined distance. The arrangement has the known features of "bottom heating", i.e. taking advantage of the natural heat flow in the substance, while achieving more uniform heating of the substance by virtue of the higher order mode or modes. As an alternative to or in addition to the generation of higher order modes, the stand may embody elements that modify the microwave pattern in a manner that enhances the coupling of microwave energy into the undersurface of the substance to be heated.

Description

This invention relates to improvements in the heating of substances in a microwave oven. While the substances most commonly heated will be foodstuffs, and the examples below will therefore relate to foodstuffs, the present invention is not limited in this respect and can be used for heating other substances.
In normal microwave transparent containers, the microwave energy can enter through the top, bottom and sides of the container. This is similar to the situation encountered during conventional oven cooking (or heating). With normal microwave foil containers the microwave energy can only enter through the top (food surface).
Prepared foods are commonly reheated in a cooking utensil on a stove top. One characteristic of this type of reheating is that the heat enters the food through the bottom of the container/utensil.
Heating the food from the bottom offers some advantages, as a result of the heat transfer mechanisms that take place. The food in contact with the base heats and becomes less dense. This provides a driving force for convective transport, the warm food rising and being replaced by cooler food from nearer the surface. The extent of this convection depends on the viscosity of the food. At a later stage of heating, bubbles of steam nucleate at or near the base and rise through the food. This transfers heat throughout the food as well as agitating the product.
With a view to simulating this type of "bottom heating" in a microwave oven, there has been proposed in R. Nakanaga U.S. pagent 4,661,672 issued April 28, 1987, a rectangular container having a microwave energy shielding layer extending over the top and down at least the upper portions of the side walls, while the remainder, and particularly the bottom of the container, was made of a microwave transparent material. In this way, the microwave energy is caused to enter the container through its bottom, and possibly to some extent through the lower parts of the side walls. This prior patent also discloses the feature of elevating the container so that its bottom is spaced above the floor of the microwave oven. A similar arrangement is disclosed in K. Sugisawa et al European patent application 0,185,488 published June 25, 1986, although in this case the top of the container is only shielded at its edges, whereby to avoid excessive heating of the upper surface of the material located at the sides of the container.
For food loads that have low viscosity and hence allow substantial heat transfer by convection, conventional containers will often perform satisfactorily. However, in many cases the characteristic non-uniform heating that results from the dominant fundamental energy distribution will not be sufficiently equalised by convective heat transfer, and an unsatisfactory product will result. In particular, there will tend to be excessive heating at the edges and insufficient heating in the middle of the body of food. Viscous products such as meat stews or casseroles, lasagne, macaroni cheese, thick soups and chowders are particularly difficult in this respect.
The present invention seeks to minimise this difficulty, and in particular, to provide an arrangement in which the food product (or other substance) is not only heated at its undersurface (although not necessarily only at its undersurface), but is also heated more rapidly and/or more uniformly across its lateral dimensions.
To this end, the present invention provides a stand for use with a container having at least one portion transparent to microwave energy including a bottom on which there is supported an undersurface of a substance to be heated. The stand comprises means for modifying the microwave field pattern to which the container is exposed, and means for supporting the container spaced above said field modifying means so that the undersurface of the substance is maintained at a predetermined distance from said field modifying means.
In the preferred form of the invention, the modifying means takes the form of means for generating a modified microwave field pattern having at least one mode of microwave energy of higher order than the fundamental modes of such energy.
Higher order mode generating means are known per se. See, for example, R. Keefer Canadian patent application Serial No. 485,142 filed June 25, 1985 (U.S. patent application Serial No. 878171 filed June 25, 1986 and European patent application No. 86304880 filed June 24, 1986 and published December 30, 1986). Such higher order mode generating means may take the form of one or more electrically conductive plates (or apertures in an electrically conductive sheet) arranged in a symmetrical planar array. Examples of such structures are discussed below.
The term "mode" is used in the specification and claims in its art-recognised sense, as meaning one of several states of electromagnetic wave oscillation that may be sustained in a given resonant system, 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 of the container and body are characterised by an electric field pattern (power distribution) confined or concentrated around the edge of the container (as viewed in a horizontal plane), these fundamental modes predominating in a system that does not include any higher order mode-generating means. The fundamental modes are defined by the geometry of the container and the contained body of material to be heated.
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) is concentrated around the periphery of an area smaller than that circumscribed by the electric field pattern of the fundamental modes. Each such electric field pattern may be visualised, with some simplification but nevertheless usefully, as corresponding to a closed loop in the horizontal plane.
Alternatively, or additionally, the modifying means may take the form of means for enhancing the coupling of microwave energy into the undersurface of the substance to be heated. Such coupling enhancement means are more fully described below.
In addition to its microwave transparent portion or portions, the container may have some portions that are reflective of microwave energy. When bottom heating is to be the dominant mode of heating, the transparent portions will be principally constituted by the bottom of the container. Thus, in one embodiment, the lid and side walls of the container can be reflective of the microwave energy, while the bottom is transparent to such energy, so that all the energy enters the food by its undersurface. There may, however, be instances where it will be convenient to allow some of the microwave energy to enter the food through areas other than the undersurface, and such an arrangement is not excluded by the present invention. For example, some foods such as baked goods, or those having a surface layer needing to be melted, e.g. a cheese layer on lasagne, or a potato layer on shepherd's pie, are ideally heated at the top and the bottom simultaneously. In this case the container lid can be microwave transparent, or can simply be removed during the heating process. Nevertheless, in the preferred embodiments of the invention, the majority of the microwave energy will enter through the undersurface of the container to maximise the bottom heating effect, the advantages of which have been discussed above.
In addition, the invention does not exclude the possibility that may be desirable in some instances, namely that parts of the bottom of the container, for example the peripheral edge of such bottom, can be shielded, so as to concentrate the microwave energy in the central portion of the undersurface of the substance being heated.
Finally, it should be mentioned that, in those examples where it is desired to avoid microwave energy entering the food at its lateral edges, the stand can include upwardly projecting metallic parts that shield these lateral edges, thus avoiding the need for the container itself to have reflective side walls.
In one preferred form of the invention, the container will have a flat bottom, the field modifying means will be planar, and the supporting means will be so dimensioned as to support this flat bottom in a plane parallel to the field modifying means. As a result a predetermined spacing between the container bottom and the field modifying means is maintained uniform throughout the lateral dimensions of the container.
The invention also consists of the assembly of a stand as described above and a container for holding the substance to be heated. This assembly can consist of two separate elements that are brought together for use, with the stand being available for re-use with the same or another container. Alternatively, these two elements can be joined together and sold as a single assembly, either for single or multiple use. For multiple use the assembly will constitute a permanent cooking vessel.
The shape of the container may be that of a conventional tray in which frozen food is commonly sold, i.e. a relatively shallow, rectangular or round tray with a flat bottom, side walls and a flat removable lid. However, one of the advantages of bottom heating, is that the normal limitations on product depth are much less important. Since other heat transfer mechanisms (convection, steam bubbles) are being encouraged, deeper loads (similar to those which would be used in a stove top saucepan) can be satisfactorily dealt with. This represents a real advantage. It is also worth noting that microwave heating from the bottom will be better than normal stove top heating, because the penetration of the microwave heating obviates the need to stir the product. In normal stove top cooking, the heat energy is transferred to the food through the base by conduction. Rapid heating of the food normally requires the temperature of the base of the utensil to be raised to a high temperature. To avoid burning the food in contact with the base, low power settings can be used (which extend the heating time) or, alternatively, the food must be stirred frequently (for viscous foods).
According to a still further aspect, there is provided a method of heating a substance by microwave energy, said method comprising the steps of confining such substance in a partially shielded container that is of such a nature that at least some and preferably the majority of the energy enters the substance through its undersurface, while modifying the microwave field pattern by means located a predetermined distance below such undersurface to enhance the coupling of microwave energy into such undersurface and/or to improve the uniformity of heating in the lateral dimensions.
In order that the invention may be better understood, several embodiments thereof will now be described by way of example only, and with reference to the accompanying drawings in which:-

Figure 1 shows a vertical central section of an assembly of a container and a stand therefor, according to an embodiment of the invention;
Figure 2 is a section on II-II in Figure 1;
Figure 2A is a modification of Figure 2;
Figure 3 is a vertical central section of a modified stand according to a further embodiment of the invention;
Figure 4 is a view on IV-IV in Figure 3;
Figure 4A is a modification of Figure 4;
Figure 5 shows a view similar to Figure 1 of an alternative embodiment;
Figure 6 is a similar view of yet another embodiment;
Figure 7 is a side view of an alternative form of stand;
Figure 8 is a side view of a still further alternative form of stand;
Figures 9A and 9B are diagrams illustrating the positions of temperature sensors used in the tests illustrated in Figures 10 and 11;
Figures 10(a) and (b) and 11(a) and (b) are comparative graphs comparing the performance of different arrangements;
Figure 12 is a vertical central section illustrating an application of the invention to a multi-compartment container;
Figures 13 to 15 each show alternative stand constructions;
Figure 15A is a graph related to Figure 15; and
Figure 16 to 18 each show still further alternative stand constructions.

Referring to the drawings, Figures 1 and 2 show a container 10 having a bottom 12 of a suitable microwave transparent material, e.g. fibreboard or a plastic material, side walls 14 of metal foil or of a laminate containing metal foil 15, and a lid 16 also of metal foil or of a fibreboard laminate including metal foil 17, the lid being held in place by a fold down rim 18. The design of the lid and rim is such that there is no possibility of arcing. A food load 20 is supported in the container with its undersurface 21 on the bottom 12. This container 10 can be circular, rectangular, or any other convenient shape in plan view. In Figures 1 and 2, it has been assumed that the container 10 is circular. Figure 2A shows a rectangular stand for a rectangular container.
The cooking assembly includes a stand 22 on which the container 10 is designed to be seated, such stand 22 consisting of a base 24, side walls 26 and a rim 28 with an inwardly sloping portion 30, all made of a microwave transparent material. The base 24 is formed with either a continuous peripheral depression or a series of such depressions forming feet 25 that serve to elevate the base 24. Centrally of the base 24, there is a plate 32 of conducting material, e.g. aluminium, that will serve to modify the microwave field pattern and generate the higher order modes. In Figure 2, the plate 32 is circular; in Figure 2A it is rectangular. The dimensions of the stand 22 are such that the spacing S between the undersurface 21 of the food load 20 and the upper surface of the plate 32 is set at an optimum value for the conditions. The choice of the value for this spacing S is discussed below. Since the undersurface of the food into which the microwave energy is being propagated lies in continuous contact with the bottom 12 of the container, this spacing S is uniform across the lateral dimensions X and Y of the container.
The stand 22 may be a re-usable kitchen appliance that is constructed of a sturdy plastic or glass, or it may be a more cheaply made disposable element that is sold with the container 10 either as a separate item to be assembled in the oven or as a fixture secured to the bottom of the container 10.
The size and arrangement of the plate 32 centrally of the base 24 in Figures 1 and 2, is similar to arrangements of conducting plates shown in the Keefer patent application referred to above. If it is preferred to generate still higher order modes of microwave energy at the bottom 12 of the container, an array of a larger number of smaller plates 34 can be provided on the base 24′ of a modified stand 22′ shown in Figures 3 and 4 and designed for use with a rectangular container, this array of plates 34 being generally similar to that shown mounted on a container lid in Figure 10B of said Keefer patent. This latter arrangement is well suited to the heating of relatively shallow food loads, since the higher order modes may not penetrate as far into the food load as the fundamental modes. On the other hand, they achieve enhanced uniformity of heating across the lateral dimensions of the container.
As explained in the Keefer patent-application, an array of plates, such as the plates 34, can be replaced by an array of apertures in a metallic sheet that otherwise covers the surface. Figure 4A shows a suitable array of apertures 36 in a conductive plate 38 on the base of a stand 22˝, or the whole stand may be conductive, e.g. made of aluminium.
Figure 5 shows a further modification in which a stand 22 made of aluminium has upwardly extended sloping end and side walls 27, and a base 39 containing apertures 36. A container 11 with a food load 20 has end walls 13 that nest snugly within the walls 27 to support the container with its bottom 12 and hence the undersurface 21 of the food load a predetermined distance S above the base 39. The container 11 has a lid 16. In this arrangement the metallic walls 27 of the stand provide lateral shielding for the food load, so that the container 11 can be made entirely of a microwave transparent material. The lid 16 may be metallic, if top shielding is required, or microwave transparent, if such shielding is not required, or some combination thereof, if partial shielding is required.
Figure 6 shows an application of a somewhat similar construction, as applied to a re-usable cooking vessel 41 made of glass with a metallised outer surface layer 43 having apertures 45 in the portion 47 thereof that extends across the bottom surface of the bottom portion 49. This bottom portion 49 of the utensil 41 is relatively thick compared to its sides whereby to provide the necessary spacing S′ between its upper bottom surface that supports the undersurface of the food load (not shown) and its bottom surface 47.
Figure 7 shows an alternative arrangement in which a stand 40 consists of a flat base 42 supporting four posts 44 on which the container 10 is placed. Conductive plates 46 are located on the upper surface of the base 42 for generating the higher order modes.
Figure 8 shows another construction of stand 50 made of a solid plastic or glass slab 52 on the upper surface of which the container 10 will be placed. Legs 54 hold the slab 52 above the oven floor, and conductive plates 56 are secured to the underside of the slab 52.
Figures 9A and 9B show how the tests reproduced in Figures 10 and 11 were conducted. As shown in Figure 9A, four temperature probes A, B, C, D were inserted into the food load 20, approximately centrally of both lateral dimensions of the container, and at varying depths, probe A being nearest the undersurface of the food and probe D nearest the top surface. Figure 9B shows the locations of four temperature probes C, E, F, G inserted into the food load, all at the same depth, i.e. at approximately one quarter depth, and respectively located at approximately the centre, the left end, the right end and the side (located at the back when placed in the microwave oven) of the container.
Figure 10(a) shows the temperatures measured by probes A-D when heating a load of about 680 grams of canned beef and vegetable stew for 15 minutes in a 700 watt microwave oven in a conventional circular foil container, i.e. one having the following dimensions: outside top diameter 181 mm; inside top diameter 171 mm; bottom diameter 140 mm; slant depth 38 mm; and capacity 796 ml.
Figure 10(b) shows the same experiment when conducted in a similar container modified to make the lid and sides microwave reflective and the bottom microwave transparent, and mounted on a stand as shown in Figure 2 having a single circular aluminium plate 32 with a diameter of 55 mm.
The results illustrate dramatically how the more uniform heating of the invention enables all levels in the food to assume an acceptable temperature, i.e. at least 80^oC, within 6 minutes, in contrast to the 15 minutes of Figure 10(a).
Figures 11(a) and (b) respectively show the readings obtained from probes C, E, F and G in a rectangular container having the following dimensions: outside top 146 x 121 mm; inside top 130 x 105 mm; bottom 115 x 89 mm; slant depth 38 mm; and capacity 455 ml. The first test was conducted with a microwave transparent base, but no higher order mode generating stand (Figure 11(a)), and then with such stand (Figure 11(b)). The load was about 400 grams of a frozen Chili-con-Carne product. Figure 11(a) shows that the outer regions of the product had thawed and heated to an acceptable temperature (60^oC) in nine - ten minutes, while the central region was still frozen until after about eleven minutes had elapsed. Acceptable temperatures were not achieved in the central region until after about 15 minutes. It should be noted that, at this time, some regions around the edge of the container had been boiling for about five minutes, which is undesirable. The erratic temperature variations during the rapid heating part of the curves are indicative of turbulence caused by bubbles of steam rising through the product.
In the equivalent container used in conjunction with a higher order mode generating stand (Figure 10(b)), the heating behaviour obtained is very different. The mode generating device in this case was a single foil block as shown in Figure 2A, the block being rectangular, 55 x 30 mm. In this case it is noticeable that the centre region thawed and heated in a much shorter time than before. Furthermore, the overall heating behaviour is noticeably more uniform. Thus the fastest region to heat was only boiling for about one minute before all the measured temperatures had reached an acceptable temperature (60^oC).
In another test (not illustrated) when using a standard container, the initial weight of a load of Chinese style chicken fried rice that had been pre-cooked and frozen was 330.8 grams and its final weight was 239.5 grams, for a weight loss of 91.3 grams, i.e. 27.6%, over a ten minutes heating time. In a corresponding test when the container was placed on a stand as shown in Figure 4, the initial weight was 329.5 grams and the final weight 318.8 grams, for a weight loss of 10.7 grams, or 3.2%, over a seven minutes heating time which was all that was necessary. This reduced weight loss is a further advantage of the present invention.
Figure 12 illustrates how a multi-compartment container 60 having two different food loads 62,64 can be mounted on a common stand 66. Depending on the different natures of the two food loads and the amount of microwave energy that it is desired they should each absorb, the conditions can be adjusted appropriately. For example, the portion 68 of the stand 66 situated below the food load 62 may employ a single higher order mode generating conductive plate 70, while the portion 72 situated below the food load 64 may employ multiple plates 74. Alternatively, in an example not illustrated, one of the portions of the stand 66 may not include any means for generating higher order modes and the food load associated with such portion may be entirely shielded from the microwave energy. This latter arrangement would be especially appropriate if the fully shielded food load is required to remain cold.
As far as spacing is concerned, there will be a requirement for a certain minimum spacing between the conducting plates (or foil surround, in the case of apertures) and the metal of the oven floor, in order to avoid arcing. It is for this reason that the embodiment of Figure 1, and many of the other embodiments, are provided with feet 25. However, if the oven has a sufficiently thick glass tray on its floor, or a separate microwave transparent rack is used, such feet can be dispensed with, e.g. the vessel of Figure 6. Such arcing-avoidance spacing will typically be required to be at least 3mm. It should also be mentioned that, in a case where the stand is not provided with feet and is placed directly on a glass tray on the oven floor, i.e. with mainly glass and little air between the conducting material and the oven floor, the array of plates or apertures may require dimensional modification to take into account the dielectric constant of the glass.
The following considerations should be taken into account when selecting the preferred value for the spacing between the undersurface of the food and the field modifying means, i.e. the spacing S when in air (Figures 1 or 5) or S′ when in a plastic or glass material (Figures 6 or 8).
The optimum spacing will depend in part on the properties of the foodstuff (for example, the dielectric properties will change the phase shift which occurs on reflection). A possible range for the spacing S in air is from about 3 to 30 mm. A spacing S of 15 mm (with air separating the foil structure from the container base) has been successfully used in practice. As indicated, this spacing will depend on the dielectric constant of the material between the foil array and the bottom of the food load. The following table gives examples of modifications to the 15 mm spacing that would be appropriate if materials of different dielectric constant were present between the bottom of the food and the foil array structure.

Material Dielectric Constant (Relative Permittivity) Spacing S′

Air 1.0 15 mm

Silica Glass 3.78 7.72 mm

Polyethylene 2.25 10 mm

Plexiglass 2.6 9.3 mm
Tests have also been carried out to measure the effect of the invention on total power absorption. A rectangular container (with a microwave transparent base) and a stand with the 9-block foil array structure as in Figure 4 was used. Power measurements were made using water as the load.

Test 1 - Container placed directly on the oven glass plate
measured power - 271.5 watts
Test 2 - Container raised 30 mm above the glass plate (no foil array)
measured power - 268.2 watts
Test 3 - Container raised 30 mm above the glass plate (with the 9-block array as in Figure 4 located midway, i.e. 15 mm from the food undersurface)
measured power - 307.2 watts

This corresponds to an improvement in power absorption of approximately 13%. Increased power absorption is useful (reduced cooking time), in addition to the improvement in heating uniformity that many of the embodiments of the present invention provide.
In the examples described so far it has been assumed that the stand will have a flat bottom. It is, however, within the scope of the invention to employ a stand embodying higher order mode generating means incorporating a stepped structure, e.g. a stepped structure of one of the types disclosed in R. Keefer Canadian patent applications Serial Nos. 508,812 filed May 9, 1986; 536,589 filed May 7, 1987; and 544,007 filed August 7, 1987 (U.S. patent applications Serial Nos. 943,563 filed December 18, 1986 and 044,588 filed April 30, 1987 and European patent applications Nos. 87304120.6 filed May 8, 1987 and published November 19, 1987 and 87309398.3 filed October 23, 1987 and published June 22, 1988). Some of the patent applications just referred to also disclose a container having a wall (e.g. a bottom wall) having a modified portion that has a different electrical thickness from that of adjacent portions of the wall, the electrical thickness being defined as a function of the actual spatial thickness of the wall and the dielectric constant of the wall material. Such a wall structure comprising appropriately arranged contiguous wall portions of respectively different electrical thicknesses can serve to generate at least one mode of a higher order than the fundamental modes. In the present invention, higher order mode generating means located in the stand can utilise such an arrangement of various portions of differing electrical thickness instead of the foil plates or apertures described above.
Figure 13 shows a stand with such a structure based on portions 75, 76 of different physical thickness, while Figure 14 shows a structure in which portions 77, 78 have the same physical thickness, but a different electrical thickness by virtue of having different dielectric constants, respectively designated L and H for low and high.
Figure 15 shows a structure in which apertures 65 are formed in a conducting base 67 supported by non-conducting supports 69, a central aperture 65a being formed in a raised portion 67a of the base, whereby its distance S2 from the undersurface of a food load (not shown) in a container 10 is less than the distance S1 of the remainder of the base 67. Figure 15A shows the effect on the power P conveyed to the load as a function of S. Curve 61 is for larger apertures 65, while curve 63 is for smaller apertures.
A plan view of Figures 13, 14 or 15 would show the portions 75, 76 or 77, 78, or the apertures 65, forming a nine block array similar to Figure 4, although this array can be modified as required.
As a further alternative, the higher order mode generating means employed in a stand according to the present invention can take the form shown used on a container in R. Keefer U.S. patent application Serial No. 051078 filed May 15, 1987 (Canadian application filed May 12, 1988). This alternative is illustrated by the plan view of a circular stand in Figure 15 where the portion 79 is a shaped piece of foil on a microwave transparent base 80.
Higher order modes of microwave energy can also be generated by a stepwise discontinuity of lossiness between a pair of regions of a susceptor. Such a susceptor, which may constitute a separate element or may form a wall component of a container, is disclosed in R. Keefer Canadian patent application Serial No. 552,110 filed November 18, 1987. In accordance with the present invention such a susceptor structure can be used in the stand to provide higher order mode generating means, as well as to generate heat that can be conveyed to the container and the food or other material therein. Such a structure is shown in Figure 17, where the portions 81 and 82 have different lossiness. A plan view of Figure 17 could show the portions 81, 82 as a single block array, similar to Figures 2 or 2A, or the portions 81, 82 could be strips extending fully across a rectangular container.
An arrangement for retaining and concentrating microwave energy in a container, i.e. enhancing the coupling of such energy into the container, is described in R. Keefer Canadian patent No. 1,228,126 issued October 13, 1987 (U.S. patent 4,656,325 issued April 7, 1987). A similar arrangement can be embodied in a stand in accordance with the present invention, as illustrated, for example, in Figure 18 which shows a stand with a substrate 83 of a dielectric material having a relatively low dielectric loss factor, e.g. polyethylene polyester film. On this substrate 83 there is an array of conductive plates or islands 84, e.g. aluminium foil. The total surface area of the metallic islands should preferably be between 50 and 80% of the surface area of the substrate. Figure 18 shows the substrate 83 on a stand having a foot portion 85 and a rim 86 for supporting a container. The dielectric substrate 83 and the array of conductive plates should cooperatively provide a dielectric constant greater than 10, and the spacing between such array and the undersurface of the substance to be heated in the container (not shown) should be between one-fifteenth and one-sixth of the wavelength of the microwave energy, which is approximately between 8 and 20 mm in air. This arrangement may also serve at the same time to generate some higher order modes of microwave energy. However, in view of the relatively large number of plates 84 used in the 20-block array shown in Figure 18, the height of the higher order modes will be greater than that of the modes generated by the single and nine-block arrays illustrated in other views. These very high order modes will penetrate a shorter distance into the food, and hence the advantage of the Figure 18 embodiment flows more from the increased coupling of energy into the food that from higher order mode generation, although the latter phenomenon will contribute to some extent to the overall improvement in performance.

Claims

1. A stand for use with a container having at least one portion transparent to microwave energy including a bottom on which there is supported an undersurface of a substance to be heated, said stand comprising:

(a) means for modifying a microwave field pattern to which said container is exposed, and

(b) means for supporting the container spaced above said modifying means so that said undersurface is a predetermined distance from said field modifying means.

2. A stand according to claim 1, wherein the modifying means comprises means for generating a microwave field pattern having at least one mode of microwave energy of higher order than the fundamental modes of such energy.

3. A stand according to claim 2, wherein said modifying means is planar and said supporting means is dimensioned to support a flat bottom of the container lying in a plane parallel to said modifying means.

4. A stand according to claim 3, wherein said modifying means comprises at least one plate of electrically conductive material lying in a plane parallel to said bottom.

5. A stand according to claim 3, wherein said modifying means comprises an array of a plurality of plates of electrically conducting material lying in a plane parallel to said bottom.

6. A stand according to claim 3, wherein said modifying means comprises at least one aperture in a sheet of electrically conductive material lying in a plane parallel to said bottom.

7. A stand according to claim 3, wherein said modifying means comprises an array of a plurality of apertures in a sheet of electrically conductive material lying generally in a plane parallel to said bottom.

8. A stand according to claim 7, wherein at least one of said apertures is in a portion of said sheet located at a distance from said undersurface different from the distance of the remainder of said sheet from said undersurface.

9. A stand according to claim 2, wherein said modifying means comprises portions having different electrical thicknesses from one another.

10. A stand according to claim 9, wherein said portions have different physical thicknesses from one another.

11. A stand according to claim 9, wherein said portions have different dielectric constants from one another.

12. A stand according to claim 2, wherein said modifying means comprises portions having different lossinesses from one another.

13. A stand according to claim 1, wherein said container has portions reflective of microwave energy, said at least one transparent portion being principally constituted by the bottom of the container.

14. A stand according to claim 13, wherein said container has reflective side walls and lid.,

15. A stand according to claim 1, wherein the stand includes upwardly extending, conductive outer portions for shielding side walls of the container.

16. A stand according to claim 1, wherein said predetermined distance is in the range of 3 to 30 mm.

17. A stand according to claim 16, wherein the space between the modifying means and the container bottom is air and said distance is approximately 15 mm.

18. A stand according to claim 16, wherein the space between the modifying means and the container bottom is polyethylene and said distance is approximately 10 mm.

19. A stand according to claim 16, wherein the space between the modifying means and the container bottom is glass and said distance is in the range from 7 to 10 mm.

20. A stand according to claim 4, including means for spacing said electrically conductive material from an oven floor.

21. A stand according to claim 1, wherein the modifying means comprises means for enhancing the coupling of microwave energy into the undersurface of the substance to be heated.

22. A stand according to claim 21, wherein said coupling enhancement means comprises a substrate of dielectric material with an array of conductive plates covering at least the majority of the surface area of said substrate, the substrate and the plates cooperatively providing a dielectric constant greater than 10.

23. A stand according to claim 22, wherein said predetermined distance is in the range 8 to 20 mm.

24. An assembly comprising:

(a) a container having at least one portion transparent to microwave energy including a bottom on which there is to be supported an undersurface of a substance to be heated, and

(b) a stand according to any one of the preceding claims.

25. A cooking vessel for use in a microwave oven, said vessel comprising a bottom portion and side walls, at least the bottom portion being made of a microwave transparent material, and means for modifying a microwave field pattern to which the vessel is exposed, said means extending across said bottom portion spaced a predetermined distance beneath an upper surface of said bottom portion for supporting an undersurface of a substance to be heated.

26. A vessel according to claim 25, wherein said distance is no less than 3 mm.

27. A vessel according to claim 26, wherein said distance is between about 3 and 10 mm.

28. A cooking vessel for use in a microwave oven, said vessel comprising a bottom portion and side walls of microwave transparent material and a metallised layer extending at least across said bottom portion, said layer having apertures therein to provide means for generating a microwave field pattern having at least one mode of microwave energy of higher order than the fundamental modes of such energy, said means being spaced from the inside bottom surface by a predetermined distance.

29. A vessel according to claim 28, wherein said layer extends up the side walls.

30. A method of heating a substance by microwave energy, said method comprising the steps of

(a) confining said substance in a container such that at least some of the energy enters the substance through an undersurface thereof, and

(b) at a location beneath said undersurface modifying the microwave field pattern to which said container is exposed to improve the uniformity of heating.

31. The method of claim 30, wherein said field pattern is modified to have at least one mode of microwave energy of higher order than the fundamental modes of such energy at a predetermined distance below said undersurface.

32. The method of claim 31, wherein the majority of all the microwave energy enters the substance through said undersurface.

33. The method of claim 31, wherein said undersurface is flat and said predetermined distance is uniform.

34. A method of heating a substance by microwave energy, said method comprising the steps of

(b) at a location beneath said undersurface modifying the microwave field pattern to which said container is exposed to enhance the coupling of microwave energy into said undersurface.