DK162492B - PACKAGING FOR USE BY HEATING FOOD PRODUCTS IN A MICROWAVE OVEN - Google Patents

Description

DK 162492 B

This invention relates to packaging for food to be heated or prepared by microwave energy.

It has become commonplace to heat 5 foods using microwave energy. It is highly desirable to be able to heat the food in a cheap combined disposable packaging for transport, heating and cooking. The most suitable packaging is traditionally made of metal foil such as aluminum foil, since aluminum foil in this connection has many advantages including economic, manufacturing, strength and hygiene.

However, there are also some serious disadvantages associated with the use of aluminum foil containers, such as pans 15 for microwave heating, because aluminum is an electrical conductor that acts as a shield and tends to reflect the microwave radiation. This reflection results in uneven heating of the food in the container. Furthermore, the reflected radiation can damage the furnace, including the magnetron, and interfere with the furnace's alignment, resulting in radiation emissions.

It has been proposed to pack food in boxes or containers made partly of a microwave reflective material such as aluminum foil which is provided with holes in 25 specific areas. The idea was that the microwave radiation should pass through the holes and be reflected within the packaging by the aluminum foil, thereby promoting the heating of the product. The microwave energy actually acting on the food was moderated or weakened in the hope of improving its distribution in the food and thus heating it uniformly. Not only did this technique weaken the packaging and increase its manufacturing costs, but the use of perforated aluminum foil as part of the packaging itself was found unsatisfactory. In contrast, the present invention focuses or enhances the microwave energy that 2

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acts on the foods, thereby improving the heating efficiency.

U.S. Patent No. 4,190,757 discloses a combined disposable package for both transportation, heating, and food preparation and consisting of a cardboard board and a loss-making microwave energy absorber that becomes hot when exposed to microwave radiation. By convection, the absorber heats the nearby food surface to a sufficiently high temperature to brown or shake it, while microwave radiation controlled by a shield heats the food's interior. Compared to a metal foil pan, this is a very expensive build and the energy absorber wastes energy. This known arrangement does not intensify or increase the microwave energy acting on the food.

U.S. Patent No. 4,230,924 discloses a food packaging having a flexible wrapping sheet of a dielectric material capable of adapting to the shape of the food. The dielectric sheet has a flexible metallic coating such as aluminum in film or foil form, and the coating is subdivided into a number of individual metallic islands separated by non-metallic spaces. With this arrangement, part of the microwave energy is converted to heat by means of the metallic coating and browns or shakes the surface of the nearby food. The metallic coating preferably faces the food and the heat generated is passed directly into it without first having to pass a gap. Also, this arrangement does not intensify or increase the microwave energy acting on the food, as the present invention does.

The invention aims to provide a very inexpensive modification, according to which standard aluminum foil containers, for example pans already used in the food industry, can also be used for heating in a microwave oven.

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In accordance with the invention, it has now been found that standard metal foil packages, e.g. aluminum, can be used in microwave ovens provided they are used in conjunction with a special lid located at some distance above the surface of the food in the metal foil container.

According to one embodiment, a package for food which can be heated in a microwave oven, which has a bowl for the food with a bottom, 10 side walls and an open top and a cover for the dish, according to the invention is characterized in that the cover is of a substantially uniform dielectric material having a dielectric constant greater than 10 and having reflectance and transmission characteristics depending on the thickness, the cover having a distance from the food surface of between 1/15 and 1/6 of the wavelength of the microwave energy, whereby the dielectric constant of the cover and the distance over the food allow microwave radiation to pass through the cover and into the package, where it interferes with reflected radiation within the package, whereby the microwave energy is retained and concentrated in the packaging.

According to another embodiment, a microwave-heated food package in a microwave oven has a food bowl with a bottom, side walls and an open top and a cover for the dish, according to the invention. the cover comprises a dielectric substrate in which or on which metal powder or flakes 30 are distributed to provide an effective dielectric constant greater than 10, and that the cover has a distance from the surface of the food (18) between 1/15 and 1/6 of the wavelength of the microwave energy, whereby the dielectric constant of the cover and the distance over the food allow microwave radiation to pass through the cover and into the package where it interferes within the package.

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Reflected radiation, with which microwave energy is retained and concentrated in the packaging.

According to yet another embodiment, a microwave-heated food package in a microwave oven has a bowl for the food having a bottom, side walls and an open top and a cover for the dish, according to the invention. said cover having a plurality of conductors on or embedded in said dielectric substrate to provide a 10 dielectric constant greater than 10, said cover having a distance from the surface of the food between 1/15 and 1/6 of the wavelength of the microwave energy, wherein the dielectric constant of the cover and the distance over the food allow microwave radiation to pass through the lid 15 and into the package where it interferes with reflected radiation within the package, thus retaining and concentrating the microwave energy in the package.

In such packages, the cover will function in the same way as non-reflective coatings in optics and will allow the microwave radiation to enter the bowl containing the food and substantially prevent the release of microwave energy reflected from the surface of the food and from the bottom of the bowl. In this way, the microwave energy can be retained and concentrated in the bowl, thus ensuring a more efficient and more uniform heating of the food.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention is explained in more detail with reference to the drawing, in which: FIG. 1 shows an idealized sketch for illustrating the operation of the invention; FIG. 2 is a perspective view of an embodiment of the invention for use in connection with a largely rectangular pan; FIG. 3 is a perspective view of an embodiment 35 of the invention for use in connection with a largely circular pan, and 5

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FIG. 4 is a perspective view of a multi-compartment bucket utilizing the invention.

The new lid, which is impervious to reflected radiation, which is hereafter referred to as the non-reflective lid or simply the lid, has a high effective dielectric constant and promotes destructive interference with microwave radiation reflected from the food surface and the container bottom. It is known that a high dielectric constant interface provides energy reflection at this surface. The invention combines the use of a high dielectric constant separation interface with destructive interference so that most of the microwave energy enters the container and most of the energy remains in the container and is absorbed by the food. The lid may consist of substantially homogeneous dielectric materials with dielectric properties as described below and whose reflection and transmission characteristics depend on thickness. The non-reflective lid may also be in the form of an artificial dielectric comprising metal powders or flakes distributed in or on a dielectric substrate having reflection and transmission characteristics at least equal to those obtained with the above-mentioned homogeneous dielectrics. Alternatively, the non-reflective lid may comprise groups of electrical conductors, for example, metal or metal foil pieces disposed on or incorporated in a dielectric substrate with reflection and transmission characteristics at least equal to those obtained with the above homogeneous dielectrics.

The lid must have a certain distance over the surface of the food in the container, which distance is determined by the lid's properties and structure, as well as by the conductivity and dielectric constant of the food. Generally, it can be said that the optimum distance between the lid and the food 35 decreases as the conductivity of the food increases. The distance is usually in the area approx. 0.8-2 cm with a 6

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optimum distance of approx. 1.2-1.5 cm at a microwave radiation of 2450 MHz.

Also, when the surface of the food is flat, the lid is preferably flat and arranged substantially parallel to this surface, although the lid may have other contours to improve uniform absorption of microwave energy in the food. If the food surface ripples, the lid may have a corresponding shape to achieve constant distance from the surface.

The largely homogeneous dielectric materials used for the non-reflective lid of the invention are dielectrics having dielectric constants greater than 10. Examples thereof are porous materials containing labile water and whose dielectric constants are due to the presence of water since its dielectric constant can approach values of approx. 80th

Lids of these substantially homogeneous dielectric materials must be quite thick, for example 0.4-1 cm at a working frequency of 2450 MHz, and must be located at a relatively small distance from the food to effectively block reflected energy. Due to the relatively small distance between the lid and the surface of the food, the efficiency of this lid is very sensitive to irregularities in the food surface.

Therefore, there is a need for a non-reflective cover material which can be used for a thin cover with a high effective dielectric constant, for example greater than 100. It has been found that this is possible using either metal powder or flakes distributed in or 30 on a dielectric substrate, or using groups of metal or metal foil pieces placed on or incorporated into a dielectric substrate.

The metal powder or flakes associated with a dielectric substrate create an artificial dielectric that meets the requirements of the invention. The metal powder or flakes can be applied in the form of coatings of 7

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or ink containing dispersed aluminum or bronze flakes. The minimum thickness of the metallic islands is determined by the magnitude of the current circulating in each of the metal islands and by the associated ohmic heating of that stream. When dimensioning the size of the islands, it has been found that metallized islands as thin as 600 Angstrim can be used. On the other hand, thicknesses of the metallic islands of ca. 2.5 · 10 ”5 m also found suitable.

The groups of electrical conductors on or in a di-electric substrate are exemplified by groups of metal or metal foil squares - or other geometric shapes - on a dielectric substrate, the dimensions and spacings of the pieces being substantially less than the microwave length. According to the invention, the best results are obtained when the area of the metal foil pieces is between 50 and 80% of the total area of the lid. The foil pieces are preferably arranged as islands, ie. that each piece is surrounded by a strip of dielectric. The lateral lengths of the pieces 20 can vary a great deal, although it is desirable that each lid consists of several foil islands.

The dielectric substrates should be materials with relatively low dielectric loss factors and resistant to degradation during microwave radiation.

Typically, these are sheets or films of cellulosic or plastic resin-containing materials, such as, for example, low dielectric loss factor paper, polyolefin film such as polyethylene, and polyester film such as poly (ethylene terephthalate).

30 The microwave radiation enters the container through the new non-reflective lid. The very high effective dielectric constant of the lid, combined with the lid's distance above the food surface, creates destructive interference with microwave radiation reflected from the surface of the food and from the bottom of the container. As this results in the microwave energy being retained 8

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and concentrated in the container, the energy is conserved, understood by the fact that virtually all microwave energy is used for direct heating of the food.

With the lid according to the invention, fields of more than 80 times the field within the food are created in the gap between the food surface and the lid. The result is not only a more uniform heating of the food, but also an appealing browning and / or crispness of the food surface. It will be appreciated that the lid may also be used in conjunction with a transparent container and still provide a brown and / or brittle food surface.

The intense field in microwave ovens excludes most conventional in situ measurements of both these fields and of local food temperatures. Thus, desolate 15 is easily deposited by shielded probes and thermocouples, and even those that are not destroyed provide misleading values.

Apart from newer IR scanning devices for detecting temperatures on the food surface, methods for measuring both food testing and oven design are coarse and generally based on temperature rise measurements of water or of a current food. By changing the location of a small amount of water in a furnace, the homogeneity of the fields can be determined, while a larger quantity of water can be used to determine the assumed maximum performance. The exerted effect on a quantity of water is found by converting the observed heat absorption to Watt [Δt (° C) x Weight (g) x 4,184 / h (sec)]. The determination of the energy absorbed by food is not so simple due to observed generally large fluctuations in temperature rise. Furthermore, the use of calometry to circumvent this problem is very flawed due to the wide variation in the heat capacity of the food which depends on the temperature. In addition, IR methods only provide surface temperatures which do not necessarily indicate the rest of the food mass temperature distribution.

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The energy absorption of the food is governed by three conditions, namely (1) the dielectric constant which influences absorption but does not in itself contribute to this, (2) dielectric losses resulting from, for example, release processes and provides most of the food absorption in food with low electrolytic content, and 10 (3) electrical conductivity caused by the absence of free ion passage through water and electrolytic dissociation and cause of ohmic or almost ohmic losses.

When assessing energy absorption, conductivity and dielectric losses are combined into a single loss factor ("conductivity"). For many foods, it is found that both the conductivity and the dielectric properties are primarily determined by the presence of water, with water being a major constituent and the values of the conductivity and dielectric constant of the water being substantially greater than that of the other constituents.

However, taking into account the differences between the properties of the food and the water, measurements of energy absorption in water are nevertheless a simple way to test and simulate the behavior of food in microwave ovens.

In the following, various embodiments of the invention are described by way of example.

Example 1 Water absorption results: Comparison of film containers with non-reflective lids according to the invention and unmodified containers.

Because of their simple design, this series of foil containers, catalog no.

35 441-3 from Alcan. This container size is typical of many of the containers and packages used for 10

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frozen finished goods (for example, the so-called "entree dish"). In order to best simulate food, these containers were filled with 310 g of tap water, as it was thought that within 5 certain limits this water's electrolyte concentration would behave like that of a number of foods.

In all cases, an oven labeled Litton 80-08, 700 w was used, as this oven had about the same power and oven room size as most of the microwave ovens on the frequency market around 2450 MHz.

10 When using this type of furnace, it was found that the pyro-ceramic floor exhibited fluctuating temperatures during use, causing problems with errors during the experiments. Therefore, styrofoam spacers having a thickness of approx. 3 mm to obtain thermal insulation from the oven floor, the low thickness being chosen to minimize disruption to the oven's normal operation. Taking electrical wiring presumably from the floor, the results with the spacers showed good agreement with the average of ordinary test results. However, the standard deviation was reduced to approx.

3.5% versus almost 10%. To exclude errors with the oven clock, etc. the stove was set to "HI". Each of the test series was started only after an appropriate oven heating time.

(I) Results for the unmodified container:

Based on six experiments of each 1 minute duration, a temperature rise of 16.5 ° C was indicated, giving an absorbed energy level of approx. 357 watts.

(ii) Non-reflective lids with groups of foil 30 squares on paper.

The foil pieces were carefully cut and glued to dry paper. The side lengths of the squares increased in leaps of 2 mm from 1 cm to 2.4 cm and placed at mutual distances increasing in leaps of 1 mm from 2 mm to 10 mm.

35 Styrofoam spacers were cut with leaps of approx. 6 mm from approx. 6 mm to approx. 25 mm and with such 11

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circumferential cross section, the width of the resulting frame was approx. 6 mm to minimize any effects arising from the presence of this styrofoam. Control tests with water and the frame alone indicated no change in the energy absorption of the water. The non-reflective lids described above were applied to the styrofoam supports by means of adhesive tape, and tests were conducted with 310 g of water and for 1 minute duration. The results were as follows: 10 (a) In all cases, the best energy absorption usually occurred at support thicknesses of 6 mm and 12 mm.

(b) Typical maximum temperature increases were as follows (square pieces): 15 (sidelgd.mm) dt (C) +% Am. P (W) 10 21.0 27.3 454 12 21.0 27.3 454 14 20.5 24.2 443 16 22.5 36.4 486 20 18 23.0 39.4 497 20 22.0 33 0 476 22 23.5 42.4 508 24 24.0 45.5 519 25 In each of these tests, a significant improvement in energy absorption results from the use of the non-reflective lids, the greatest improvement being generally equivalent to a foil area. of from 50-80% of the total lid area, where the non-reflective lids typically had 30 dimensions of 14.1 by 11.3 cm. It is assumed that the energy absorption is limited by the dielectric properties of the paper and by the lack of precision during fabrication and mounting of the foil squares.

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Example 2

Square foil pieces on other substrates: (a) Using the foregoing procedure and non-reflective lids with foil squares with a side length of 5 22 mm and mounted on a approx. 0.1 mm thick Mylar and an approx. 0.25 mm thick oriented polystyrene sheet and with approx. 13 mm distance from a water portion of 310 g, temperature increases of 22, 0 ° C and 23.5 ° C were measured, respectively, representing improvements of 33.3% and 42.4% and energy levels of 476 Watts and 508 watts.

The greater temperature stability of the mylar substrate allowed extended tests. In 2-minute tests, a temperature increase of 24.0 ° C was obtained without modifications, while a Mylar lid with foil squares with a side length of 22 mm gave a temperature rise of 43.5 ° C or an improvement of approx. 81.3%. The respective energy levels were 259 watts and 470 watts. Comparative experiments with thawing ice from -20 ° C were also performed.

(B) Using the same non-reflective lid, the thawing - measured by fluid weight as a function of time - was approx. 20% faster, and the melting was qualitatively more uniform than in the unmodified container.

Example 3

Use of metal particle compositions in dielectric aluminum paint:

Non-reflective lids were made using stationary paper as before, applying ordinary aluminum spray paint compositions. In the effort to obtain as uniform coverage as possible, paint thicknesses of approx. 0.025 mm. The finished lids were mounted on a 13 mm styrofoam support as previously mentioned and the energy absorption of 310 g of water samples 35 was compared with the previous results for the unmodified container. Typically a temperature 13 was obtained

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increase of 20/0 ° C / which represented an absorption increase of 21.2% and an energy absorption of 432 Watt.

Example 4 5 Common food products.

Procedure: A calorimeter was constructed using a polyethylene box large enough to contain a food sample and 800 ml of water or 1200 water alone. The polyethylene box was placed in a ca. 50 mm 10 thick styrofoam box, which was also lined with aluminum foil and the lid was sealed with a double bead of silicone rubber. After heating a food sample in a microwave oven, the sample was placed in the polyethylene box along with 800 ml of water and a thermometer, both the box and the thermometer having the water temperature beforehand. The polyethylene box was placed inside the styrofoam box long enough to allow the food to stabilize in equilibrium with the water, thermometer and polyethylene box, which took from 6 to 10 minutes. It was found that with a "pure" 1200 ml water sample and a temperature difference of 24.5 ° C between the water (and the polyethylene box) and the room, there was only a heat loss of the order of 4.5 Watts over a 10 minute measuring interval. The combined water, thermometer and polyethylene box heat capacity was calculated at 893.5 cal / C.

2. Typical food test: With samples of Stouffer's "Scalloped Chicken and Noodles" using Alcan catalog No. 445-3 foil containers and supplied by the manufacturer with a nominal weight of 326 g, comparative tests were performed. Samples in which the foil / cardboard lining was removed were heated for 6 minutes and then tested as above. For the unmodified vessel, a temperature rise of 29.0 ° C was recorded, while the temperature rise of the water (and polyethylene box) was 8.0 ° C. With a non-reflective lid placed approx. 13 mm above the contents and with 20 foil squares with a side length of 22 mm was 14

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the respective temperature increases 31.5 ° C and 10.5 ° C. With a putative food heat capacity of 0.7, the modified container showed a 20.2% increase in absorption over the unmodified container.

In the following, the invention is explained with reference to the figures.

Pig. 1 shows an empirical representation of the effect obtained by the invention. A lid with a high effective dielectric constant is designated 10. This lid 10 consists of a lid portion 12 of a dielectric and with several metallic islands disposed thereon 14. The combination forms a top surface of grouped dielectrics. The metallic islands may be rectangular and have lengths and widths that are advantageously less than a quarter of the micro-wavelength. It is preferred that the islands have dimensions less than half a wavelength to avoid oscillation, which gives high electrical voltage fields along the edges of the islands, and thus to avoid. electric arc formation. It has been found that a highly efficient dielectric constant can be formed by means of many small islands which provide a good initial transmission of the microwave energy into the volume defined by the pan and the lid portion.

A base surface 16 is provided either by using a metallic pan with resulting metallic sides and bottoms or by using a non-metallic pan in close association with an electrically conductive bottom.

Such a bottom may be metal foil attached to a paper or plastic pan.

30 Pig. 1 does not show the pan, since it is not essential to the invention as long as there is a metallic base surface. It should be noted that the base surface is not essential to the practice of the invention, because one can regard the food itself as a poor base surface. On the other hand, the best results are obtained by using a base surface as shown in fig. First

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A food 18 to be heated is placed directly on the base 16 and at a distance below the grouped dielectric top plane 10. As mentioned earlier, the distance is between 0.8 and 2 cm at the 2450 MHz microwave wave frequency used. It should be noted that this distance will change if the microwave frequency changes, the distance being more generally expressed as being between 1/15 and a 1/6 of the microwave length used.

The effect of the combination of the grouped dielectric peak, the food and the base surface is very schematically indicated in FIG. 1. Destructive interference in the plane of the dielectric peak gives the desired effect. The energy 20 reaches the surface and the majority penetrates into the air space 22 and the food 18. A small amount of energy 15 24 is reflected by the top surface. The energy passing through the top surface penetrates into the food 18 which absorbs the energy and thus is cooked. Some of the energy penetrates through the food, is reflected by the base surface 16 and transmitted back through the food 18, where it is further absorbed, and some of the energy 26 is directly thrown back from the surface of the food. The energy that is not absorbed by the food after its first reflection from the ground surface reaches the top plane once again, where the vast majority of the energy is again reflected back into the food. This process continues until all the energy is either absorbed by the food or transmitted back through the top plane and into the interior of the microwave. The ratio of the energy absorbed by the food to the energy escaping through the top-30 plane is found to be very high. This process results in a very efficient energy concentration in the food container as well as the advantage that the food is prepared uniformly in the horizontal plane.

As seen in FIG. 1, some reflection occurs in the lid's plane. However, since this reflected amount is very small, the term "non-reflective" lid is retained.

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FIG. 2 shows a generally rectangular container 30 containing a food which substantially fills the container. The container may be of a plastic material having a metallic base surface not shown attached to its bottom. A preferred embodiment - and the one shown - uses a metallic container with a bottom 32 and sides 34. A metallic lip 36 surrounds the top of the container portion of the container. The container is completed by a lid 38 of a dielectric material having a relatively low dielectric loss factor. An example of a suitable material is a polyethylene-polyester film.

The top 40 of the lid is generally flat and oriented so that it extends substantially parallel to the surface of the food. A shoulder portion 42 extends along the circumference of the lid and enters a circumferential step 44 adapted to rest on the lip of the pan 36. The shoulder portion 42 has such a height that the top surface 40 is given the distance described above over the surface of the food. In a preferred embodiment, the lid has a down and outwardly inclined skirt 46 attached to step 44. This skirt determines how tightly the metallic pan can be placed on the furnace walls, effectively eliminating the risk of electric arc formation. The skirt also tends to lock or hold the lid of the forehead by friction against its lip.

On top surface 40 are placed metallic islands 48 which - as mentioned above - form, in connection with the dielectric material of the lid, an area of effective dielectric properties over virtually the entire surface area of the lid. The surface area of the metallic islands should be between 50 and 80% of the top area of the lid 40. The islands 48 are shown in FIG. 2 are shown as rectangular islands which together form a regular rectangular group. This particular configuration is not essential to the operation of the invention, but 35 has been found to work well.

FIG. 3 shows a circular embodiment in which elements which are the same as in fig. 2, has the same reference

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view counts. The metallic islands 48 are arranged in two axially symmetrical rings. Here, too, the metallic surface area is between 50 and 80% of the top 40 surface area. The configuration shown has six islands in the inner ring and eight islands in the outer ring and provides uniform heating of the food on the horizontal plane.

FIG. 4 is a perspective view of a multi-room container for use in heating, for example, a so-called "TV dinner". By using the process described previously, a controlled heating of the various compartments in the pan 30. can be obtained. 4, the pan 30 comprises outer side walls 34 and inner partitions defining spaces 50, 51, 52 and 53. The rooms 50 and 53 contain food which requires high heating, e.g. meat and potatoes. To achieve this, a grouped dielectric consisting of a dielectric material 40 and metallic islands 48 is placed on the lid 38 immediately above these compartments or compartments, whereby, as discussed above, a high uniform heat concentration is obtained. Room 52 20 requires only moderate heating to heat, for example, a frozen dessert and is therefore only directly covered by the dielectric material and is thus heated in a conventional manner.

For example, space 51 contains vegetables 25 or the like and requires only slight heating. Therefore, a metallic shield 54 is attached above this nam. Sufficient microwave energy leaks past the shield to heat the contents of this compartment. The contents of this room are further heated in part by convection heat from the surrounding rooms.

In the embodiment shown in FIG. 4, different foods requiring different heating are heated in such a way that all the foods are prepared simultaneously.

35 It should be noted that all of the covers described can be provided with ventilation openings,

Claims (21)

Packaging for food which can be heated in a microwave oven, said packaging having a bowl (30) for the food with a bottom (16,32), side walls (34) and an open top and a cover (10,38) for the bowl, characterized in that the cover is of a substantially uniform dielectric material with a dielectric constant greater than 10, and with reflection and transmission characteristics depending on the thickness, which cover spaced from the food (18). ) a surface of between 1/15 and 1/6 of the wavelength of the microwave energy, whereby the dielectric constant of the cover and 20 distances across the food allow microwave radiation to pass through the cover and into the package where it interferes with reflected radiation within the package, thereby retaining the microwave energy and packaging. 2. Microwave energy-heatable food packaging in a microwave oven, which has a bowl (30) for the food with a bottom (16, 32), side walls (34) and an open top and cover (10, 38) to the bowl, characterized in that the cover comprises a dielectric substrate (40) wherein or upon which metal powder or flakes (48) are distributed to provide an effective dielectric constant greater than 10 and the cover has a distance from the surface of the food (18) between 1/15 and 1/6 of the wavelength of the microwave energy, whereby the dielectric constant of the cover and the distance over the food allow the microwave radiation to pass through the cover and into the package where it enters. the packaging interferes with reflected radiation, thus retaining and concentrating the microwave energy in the packaging. 3. Microwave-heatable food packaging in a microwave oven, which has a bowl (30) for the food with a bottom (16, 32), side walls (34) and an open top and cover (10, 38) to the dish, characterized in that the cover 10 has a series of conductors (48) on or inserted into the dielectric substrate (40), to provide a dielectric constant greater than 10, which cover is a distance from the food (18). surface of between 1/15 and 1/6 of the wavelength of the microwave energy, whereby the dielectric constant of the cover and distance over the food allow microwave radiation to pass through the lid and into the package where it interferes with reflected radiation within the package, thereby retaining and concentrating the microwave energy. in the packaging. Packaging according to one or more of the preceding claims, characterized in that the dielectric substrate (40) is of low loss dielectric paper. Packaging according to claim 3, characterized in that the row of conductors (48) is shaped as a plurality of spacer islands with spaced apart and side dimensions which are spaced apart by a distance less than one wavelength of the microwave energy. and that the metal islands (48) represent 50-80% of the total area of the cover 30. Packaging according to claim 5, characterized in that the dielectric substrate (40) is a film or cellulosic or plastic containing material with a low dielectric loss factor. Packaging according to claim 6, characterized in that the metal islands (48) are of aluminum. DK 162492 B Packaging according to claim 7, characterized in that the metal islands (48) are generally rectangular or square. Packaging according to claim 5, characterized in that the distance between the top surface of the food and the cover (10) is approx. 1.2-1.5 cm at a microwave energy frequency of 2450 MHz. Packaging according to claim 5, characterized in that the cover (10, 38) has a shoulder portion (42) 10 and a substantially planar top surface (40), said shoulder portion being dimensioned such that the top surface has a distance of 1/15. -1/6 of the wavelength of the microwave energy over the surface of the food (18). Packaging according to claim 10, characterized in that each of the metallic islands (48) comprises a metal film or foil attached to the dielectric substrate (40). Packaging according to claim 1, characterized in that the film or foil has a thickness of approx. 0.0254 mm. Packaging according to claim 10, characterized in that each of the metallic islands (48) comprises a metallic film or foil embedded in the dielectric substrate. Packaging according to one or more of claims 10-13, characterized in that the bowl (30) is formed with inner partitions which form a plurality of compartments (50, 51, 52, 53) and that the cover (10, 38) shoulder portion comprises an outer shoulder portion (42) and inner partition 30 shoulder portions which are generally congruent with the inner partitions to form a plurality of top faces (40), namely one above each of said compartments. Packaging according to one or more of the preceding claims, characterized in that the frequency of the microwave energy is 2450 MHz and that the cover (10, 38) has a distance from the surface of the food (18) of approx. 0.8-2 cm. DK 162492 B Packaging according to one or more of the preceding claims, characterized in that the bowl (30) is made of metal. Packaging according to claim 16, characterized in that the bowl (30) is an aluminum bowl. Packaging according to one or more of the preceding claims, characterized in that the cover material has an effective dielectric constant greater than 100. 19 · Packaging according to claims 10 and 16, characterized in that the bowl (30) and top cover (10,38) are generally rectangular with rounded corners and that the bowl has a radially protruding lip (36), while the top cover has a radially protruding step (44) connected to the shoulder portion (42) for friction co-operation with said lip. Packaging according to claims 10 and 16, characterized in that the bowl (30) and the cover (10, 38) are generally circular and the bowl has a radially extending lip (36), while the cover has a radially protruding step. (44) associated with the shoulder portion (42) for friction co-operation with said lip. Packaging according to claim 19 or 20, characterized in that a downwardly extending downwardly extending skirt (46) is attached to said step (44).