Microwave heating package and method

A novel cover arrangement is described for use with foodstuff holding pans to be heated in a microwave oven. The cover is one which in terms of microwave energy, does not transmit reflected energy. Thus, the cover acts in a manner analogous with non-reflecting coatings in optics and permits passage of the microwave radiation into the container holding the foodstuff, while substantially preventing escape of microwave radiation reflected from the foodstuff surface and container bottom. In this manner the microwave energy is retained and concentrated within the container, resulting in more efficient and uniform heating of the foodstuff. The novel cover is particularly valuable when used with aluminum foil pans, which without the cover of this invention seriously reflect microwave radiation.

The present invention relates to microwave energy cooking and more 
particularly to an improved package for foodstuffs to be heated or cooked 
with microwave energy. 
The heating of foodstuffs with microwave energy has now become commonplace. 
It is, of course, highly desirable to be able to heat foodstuffs in an 
inexpensive disposable shipping, heating and serving container or package. 
The most desirable such container or package for foodstuffs has 
traditionally been made from a metal foil, such as aluminum foil. The use 
of aluminum foil for this purpose has many advantages including economy, 
ease of manufacture, container strength, sanitation, etc. 
However, there have remained some very serious drawbacks in the use of 
aluminum foil containers, e.g. pans, as microwave heating containers in 
that the aluminum is a conductor which acts as a shield and tends to 
reflect the microwave radiation. The reflective qualities of the aluminum 
foil results in uneven heating of the foodstuff in the container. 
Moreover, the reflected radiation may damage the oven, including the 
magnetron, and it may also upset the tuning of the oven, resulting in 
radiation leakage. 
There have been proposals to package food products in boxes or containers 
formed in part of a microwave reflective material such as aluminum foil 
having holes in selected areas. This was based on the idea that the 
microwave radiation would enter the holes and be reflected about within 
the package by the aluminum foil, thereby facilitating the heating of the 
product. The microwave energy actually acting on the food was moderated or 
attenuated in the hope of improving its distribution within the food 
thereby uniformly heating the food. This technique not only weakened and 
increased the cost of the package, but the use of perforated aluminum foil 
as a part of the package itself was found to be unsatisfactory. On the 
other hand, the present invention focuses or increases the microwave 
energy acting on the food thereby improving the efficiency of heating. 
U.S. Pat. No. 4,190,757 describes a disposable microwave shipping, heating 
and serving package for food composed of a paperboard carton and a lossy 
microwave energy absorber which becomes hot when exposed to microwave 
radiation. The absorber heats the adjacent surface of the food by 
conduction to a sufficiently high temperature to provide searing or 
browning while microwave exposure controlled by a shield heats the inside. 
This is a very expensive structure compared with a metal foil pan and the 
energy absorber is wasteful of energy. This prior art arrangement does not 
focus or increase the microwave energy acting on the food. 
In U.S. Pat. No. 4,230,924 there is described a food package which includes 
a flexible wrapping sheet of dielectric material capable of conforming to 
the shape of the food. The dielectric wrapping sheet has a flexible 
metallic coating, such as aluminum, in the form of a film or foil, the 
coating being subdivided into a number of individual metallic islands 
separated by non-metallic gaps. With this arrangement, a part of the 
microwave energy is converted into heat by the metallic coating so as to 
brown or crispen the adjacent food. The metallic coating is preferably 
contiguous to the food and the heat that develops is conducted directly 
into the surface of the food without having to be radiated through any 
intervening space. Once again, this arrangement does not focus or increase 
the microwave energy acting on the food as does the present invention. 
It is the object of the present invention to develop a very inexpensive 
modification whereby the standard aluminum foil containers, e.g. pans, now 
used in the food industry may be used for heating within a microwave oven. 
In accordance with this invention, it has now been discovered that the 
standard metal, e.g. aluminum, foil packaging containers can be used in 
microwave ovens provided they are used in association with a special cover 
which is spaced a distance from the surface of the foodstuff in the metal 
foil container. 
More particularly, the present invention relates to a cover for metal 
containers which in terms of microwave energy, does not transmit reflected 
energy. Thus, the cover acts in a manner analogous with non-reflecting 
coatings in optics and permits passage of the microwave radiation into the 
container holding the foodstuff, while substantially preventing escape of 
microwave radiation reflected from the foodstuff surface and container 
bottom. In this manner the microwave energy is retained and concentrated 
within the container, resulting in more efficient and uniform heating of 
the foodstuff.

The novel reflected energy impenetrable cover, referred to hereinafter as 
the "non-reflecting energy cover" or "cover" has a high effective 
dielectric constant and precipitates destructive interference with 
microwave radiation reflected from the foodstuff surface and container 
bottom. It is known that a high dielectric constant interface provides a 
reflection of energy at the interface. However, the present invention 
combines the use of a high dielectric constant interface with destructive 
interference so that the majority of microwave energy enters the container 
and the majority of microwave energy stays within the container and is 
absorbed by the foodstuff. The cover may be comprised of substantially 
uniform dielectric materials having dielectric properties as described 
hereinafter, and for which the characteristics of reflectance and 
transmittance are functions of thickness. The non-reflecting energy cover 
may also be in the form of an artificial dielectric comprised of metal 
powder or flakes dispersed in or on a dielectric substrate, for which the 
characteristics of reflectance and transmittance are at least equivalent 
to those obtained from the above uniform dielectric material. 
Alternatively, the non-reflecting energy cover may be comprised of arrays 
of conductors, e.g. metal or metal foil shapes, on or embedded in a 
dielectric substrate, the reflectance and transmittance characteristics 
thereof being at least equivalent to those which are obtained from the 
above uniform dielectric material. 
The non-reflecting energy cover must be spaced from the surface of the 
foodstuff in the container and the distance between the cover and the 
surface of the foodstuff is determined by the properties and structure of 
the cover and also by the conductivity and dielectric constant of the 
foodstuff. In general, as the conductivity of the foodstuff increases, the 
optimum distance between the cover and foodstuff decreases. The distance 
between the cover and the surface of the foodstuff is usually in the range 
of about 0.8 to 2 cm., with the optimum being about 1.2 to 1.5 cm. at a 
microwave frequency of 2450 MHz. 
For a flat foodstuff surface, the non-reflecting energy cover is preferably 
also flat and disposed substantially parallel to the foodstuff surface, 
although it may be contoured to improve uniformity of absorption of 
microwave energy by the foodstuff. If the surface of the foodstuff is 
curved, then the cover may also be provided with a similar curvature, so 
as to maintain a constant spacing from the foodstuff surface. 
The substantially uniform dielectric materials used for the non-reflecting 
energy cover of this invention are dielectrics having dielectric constants 
greater than 10. These are exemplified by porous media containing labile 
water, the dielectric constants thereof being attributable to the presence 
of water, whose dielectric constant can approach 80. 
Covers made of these substantially uniform dielectric materials must be 
quite thick, e.g. 0.4 to 1 cm. at an operating frequency of 2450 MHz., and 
also must be spaced from the foodstuff by a relatively small distance to 
be effective in blocking reflected energy. Because of the relatively small 
distance between the cover and the foodstuff surface, the effectiveness of 
this cover is very sensitive to unevenness in the foodstuff surface. 
There was, therefore, a need for a non-reflecting energy cover material 
which could provide a thin cover having a high effective dielectric 
constant, e.g. more than 100. It has been found that a thin cover meeting 
these requirements can be obtained by using either metal powders or flakes 
dispersed in or on a dielectric substrate or arrays of metal or metal foil 
shapes on or embedded in a dielectric substrate. 
The metal powder or flakes dispersed in or on a dielectric substrate create 
an artificial dielectric meeting the required characteristics of the 
invention. The metal powder or flakes may be applied in the form of paint 
or ink coatings having aluminum or bronze flakes dispersed therein. The 
minimum thickness of the metallic islands is determined by the size of the 
current circulating in each of the metal islands and that current's 
associated ohmic heating. By dimensioning the size of the islands it has 
been found that metallized islands as thin as 600 Angstroms have been 
operable. On the other hand, thicknesses for the metallic islands in the 
neighborhood of 0.001" have been found to be convenient. 
The arrays of conductors on or in a dielectric substrate are exemplified by 
arrays of metal or metal foil squares or other geometrical shapes on a 
dielectric substrate, the dimensions of such squares or other shapes and 
the spacings therebetween being substantially less than one wavelength of 
the microwave energy. For best effects according to the invention, the 
area of the metal foil shapes should be 50 to 80% of the total area of the 
non-reflecting energy cover. The foil shapes are preferably arranged as 
islands, in that each shape is surrounded by a strip of dielectric. These 
shapes can vary quite widely in side dimensions, although it is desirable 
that each cover consist of a plurality of foil islands. 
The dielectric substrates should be relatively low dielectric loss factor 
materials which are resistant to breakdown under microwave conditions. 
They are typically sheets or films of cellulosic or plastic resinous 
materials, and may, for example, include low dielectric loss papers, 
polyolefin film, such as polyethylene, polyester film, such as 
poly(ethylene terephthalate). 
The microwave radiation enters the container through the novel 
non-reflecting energy cover. However, the very high effective dielectric 
constant of the cover, combined with the spacing of the cover from the 
surface of the foodstuff, creates a destructive interference with 
micro-wave radiation reflected from the foodstuff surface and container 
bottom. Since this results in the microwave energy being retained and 
concentrated within the container, energy is conserved in that the 
microwave energy is substantially all used to directly heat the foodstuff. 
With the non-reflecting energy cover of this invention, fields have been 
created in the space between the foodstuff surface and the cover which may 
be as much as 80 times the field within the foodstuff. The result of this 
very high field is not only more uniform heating of the foodstuff, but 
also a highly desirable browning and/or crisping of the surface of the 
foodstuff. It will, of course, be appreciated that the cover may also be 
used together with a microwave transparent container to obtain the benefit 
of its ability to brown and/or crisp the foodstuff surface. 
METHODS OF MEASUREMENT 
The intense fields of microwave oven cavities preclude most conventional in 
situ measurements either of these fields or of local food temperatures. 
Thus, shielded probes or thermocouples are easily destroyed, with spurious 
readings being obtained from those remaining intact. 
With the exception of recent IR scanning devices for sensing food surface 
temperatures, methods of measurement used both in the testing of foods and 
in oven design have remained crude, being generally based on 
temperature-rise measurements for water or actual food loads. Varying the 
position of a small water load in an oven might be used to determine 
constancy of the fields, while a large water sample is used to determine 
presumed maximum output. Power output for a water load is found by 
converting the heat absorption so determined into Watt units 
[.DELTA.T(.degree.C.)Xwt(gm.).times.4.18400/t(sec)]. Determination of the 
power absorbed by foods is less straightforward, owing to the generally 
wide fluctuations of temperature-rise observed. Moreover, the use of 
calorimetry to circumvent this problem is prone to error because of wide 
variations of food heat capacity with temperature. Furthermore, IR methods 
only provide surface temperatures, which are not necessarily indicative of 
bulk temperature distributions. 
Power absorption by foods is governed by three quantities, as follows: 
(1) dielectric constant, affecting the distribution of absorption, but not 
in itself contributing to absorption, 
(2) dielectric losses, resulting from relaxation processes, for example, 
and providing the major portion of food absorption, for foods with low 
electrolyte content, and, 
(3) electrical conductivity, caused by the presence of free ions through 
water and electrolyte dissociation, and giving rise to ohmic or near-ohmic 
losses. 
In evaluating power absorption, conductivity and dielectric losses are 
grouped as a single loss term ("conductivity"). For many foods, it is 
found that both conductivity and dielectric properties are determined 
primarily by the presence of water, water being a major constitutent, and 
water conductivity and dielectric constant values being far greater than 
those of the other components present. Taking into account deviations of 
food properties from those of water, water power absorption measurements 
nevertheless provide a simple means of testing and simulating food 
performance in microwave ovens. 
Various embodiments of the invention will now be illustrated by the 
following examples: 
EXAMPLE 1 
Water Absorption Results: Comparison of Foil Containers With Non-Reflecting 
Energy Covers Against Unmodified Containers 
Because of their simplicity of design, Alcan (trade mark) Catalogue No. 
441-3 foil containers were used in this series of tests. This size of 
container is typical of many of the foil containers used in consumer 
frozen food applications (i.e.--the so-called "entree dish"). To best 
simulate performance with foods, these containers were filled with 310 gm 
of tap-water, it being felt that the electrolyte concentration of this 
water would give acceptably similar performance to that of a range of 
foods. In all cases, a Litton (trade mark) 80-08, 700 W commercial oven 
was used, this oven having similar wattage and a similar cavity size to a 
large portion of the consumer microwave oven market, with a microwave 
frequency of 2450 MHz. 
It was found in the operation of this type of oven that the pyroceram floor 
exhibited varying temperatures during oven operation, presenting problems 
of experimental error. Accordingly, styrofoam spacers of about 1/8" 
thickness were used to provide thermal isolation from the oven floor, a 
small thickness being used to minimize perturbation of normal oven 
operation. When conductivity, presumably from the floor was considered, 
results with the spacer gave good agreement with the mean of ordinary test 
results. However, standard deviation was reduced to about 3.5% from the 
previous, nearly 10%. In all cases, to eliminate oven timer or relay 
error, oven operation was at the "HI" setting. Each series of runs was 
only commenced after an adequate oven warm-up interval. 
(i) Unmodified Container Results: 
Based on six runs of 1 minute duration, a water temperature-rise of 
16.5.degree. C. was indicated, giving an absorbed power level of roughly 
357 watts. 
(ii) Non-Reflecting Energy Cover Comprised of Foil Square Arrays on Paper 
Foil squares were carefully cut and mounted with adhesive on a dry paper. 
Squares were cut in 2 mm increments from 1 cm on a side to 2.4 cm, and 
were spaced in increments of 1 mm from 2 mm to 10 mm. Styrofoam spacers 
were cut in 1/4" increments from 1/4" to 1" in thickness, with a 
peripheral cross-section, so that the width of the resulting spacer frame 
was about 1/4" to minimize any effect from the presence of the styrofoam. 
Blank tests with water and only the frame indicated no change in power 
absorption by the water. The non-reflecting energy covers described above 
were mounted with adhesive tape on the styrofoam supports, and 
temperature-rises for runs with 310 gm of water and of 1 minute duration 
noted. Results were as follows: 
(a) in all cases, best power absorption usually occurred at support 
thicknesses of 1/4" and 1/2". 
(b) typical maximum temperature-rises were: 
Square side 
______________________________________ 
(mm) dt (C) + % Chg. 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 
18 23.0 39.4 497 
20 22.0 33.0 476 
22 23.5 42.4 508 
24 24.0 45.5 519 
______________________________________ 
In each of these tests, a substantial improvement of power absorption 
resulted from use of the non-reflecting energy covers, the largest 
improvement generally corresponding to a range of foil area of from 50 to 
80% of total cover area, the non-reflecting energy covers having typical 
dimensions of 14.1 by 11.3 cm. It is believed that power absorption was 
limited by dielectric strength of the paper and by lack of precision in 
preparation and mounting of the foil squares. 
EXAMPLE 2 
Foil Squares On Other Substrates 
(a) Using the foregoing procedure and non-reflecting energy covers using 
foil squares 22 mm on a side mounted on 0.0045" Mylar.RTM. and 0.010" 
oriented polystyrene sheet at 1/2" separation from a fill comprised of 310 
gm of water, temperature-rises of 22.0.degree. and 23.5.degree. C. were 
recorded, respectively, representing 33.3 and 42.4% improvements, and 
power levels of 476 and 508 watts. 
The greater temperature-stability of the Mylar substrate permitted extended 
runs. For 2 minute runs, the blank gave a 24.0.degree. C. temperature 
rise, while a Mylar non-reflecting energy cover using foil squares 2.2 cm 
on a side gave a 43.5.degree. C. rise, for an improvement of 81.3%, and 
respective power levels of 259 and 470 watts. Comparative experiments were 
also run for the thawing of ice at -20.degree. C. 
(b) Using the same non-reflecting energy cover, thawing, gauged by the 
weight of liquid as a function of time, was about 20% more rapid, and 
melting was qualitatively more uniform than for the unmodified container. 
EXAMPLE 3 
Use of Compositions of Metal Particles in Dielectric-Aluminum Paint 
Non-Reflecting energy covers were prepared using stationary paper, as 
before, to which was applied compositions of ordinary, domestic aluminum 
spray paint. In attempting to achieve as uniform coverage as possible, 
paint thicknesses of about 0.001" were obtained. The resulting 
non-reflecting energy covers were mounted on a 1/2" styrofoam support, as 
discussed above, and power absorption results for 310 gm water samples 
were compared with previous blank results. A typical temperature rise of 
20.0.degree. C. was obtained, representing an absorption increase of 21.2% 
and a power absorption rate of 432 watts. 
EXAMPLE 4 
Commercial Foods Products 
1. PROCEDURE: A basic calorimeter was constructed, using a polyethylene box 
of sufficient size to accommodate a food sample, and 800 ml of water, or 
1200 ml of water alone, such that 2" thick styrofoam box enclosed the 
polyethylene box. The styrofoam box was lined with aluminum foil, as was 
its cover, and the cover was gasketed with a double bead of silicone 
rubber material. Subsequent to microwave oven heating of a food sample, 
the sample was placed in the polyethylene box with 800 ml of water and a 
thermometer, both box and thermometer being pre-equilibrated to the water 
temperature, and the polyethylene box was placed within the enclosing 
styrofoam box for a sufficient interval to give equilibration between the 
food and with the water, thermometer, and polyethylene box, this interval 
ranging from 6 to 10 minutes. It was found that for a 1200 ml water blank 
run, and a temperature difference of 4.5.degree. C. between the water (and 
polyethylene box) and room, the heat loss was only of the order of 4.5 
watts over a 10 minute measuring interval. Combined water, thermometer, 
and polyethylene box heat capacities were calculated at 893.5 cal/C. 
2. TYPICAL FOOD TEST: Using Stouffer.RTM. "Scalloped Chicken and Noodles" 
samples obtained directly from the manufacturer and nominally weighing 326 
gm, which use the Alcan Catalogue No. 445-3 foil container, comparative 
tests were run. Samples with the foil/cardboard liner removed were heated 
for 6 minutes, and then tested according to the procedure noted above. For 
the unmodified blank, a food temperature-rise of 29.0.degree. C. was 
noted, while the water (and polyethylene box) temperature-rise was 
8.0.degree. C. With a non-reflecting energy cover at an approximately 13 
mm separation from the fill and using 20 foil squares 22 mm on a side, the 
respective temperature-rises were 31.5.degree. and 10.5.degree. C. 
Assuming a food heat capacity of 0.7, the modified container showed a 
20.2% increase in absorption over the blank. 
The present invention will now be described with respect to the figures. 
FIG. 1 is an empirical representation of the effect of the present 
invention. A cover having an effectively high dielectric constant is shown 
at 10. This cover is comprised of a dielectric material lid 12 having a 
plurality of metallic islands 14 located thereon. The combination forms a 
dielectric array top. The metallic islands can be rectangular and have 
widths and lengths which are advantageously less than one-quarter 
wavelength of the microwave energy. It is preferred that they have 
dimensions which are less than one-half a wavelength in order to avoid the 
propagation of modes which yield high electric field voltages along the 
perimeters of the islands to prevent arcing. It has been found that a high 
effective dielectric constant can be achieved using many small islands 
which provide good initial transmission of the microwave energy into the 
volume defined by the pan and lid. 
A ground plane 16 is provided either by using a metallic pan having a 
metallic bottom and sides or by a non-metallic pan having a conductive 
bottom intimately associated therewith. Such a bottom could be a metallic 
foil applied to a paper or plastic pan. 
FIG. 1 does not show the pan which is basically irrelevant to the invention 
as long as a metallic ground plane is provided. It should be noted that a 
ground plane is not essential to the operation of the invention since the 
foodstuff itself can be considered to be poor ground plane. However, 
optimum results are achieved using a ground plane as will be seen from 
FIG. 1. 
A foodstuff 18 to be heated is located directly on the ground plane 16 and 
spaced below the array dielectric top 10. As was mentioned above, this 
spacing ranges from between 0.8 and 2 cm. at the currently used microwave 
frequency of 2450 MHz. It should be noted that this range of spacing will 
change if the microwave frequency is altered and is more generally 
expressed as from .lambda./15 to .lambda./6 of a wavelength of the 
microwave energy used. 
The action of the combination of array dielectric top, foodstuff and ground 
plane is very schematically shown in FIG. 1. Destructive interference in 
the plane of the high dielectric top accomplishes the desired effect. 
Incident energy 20 arrives at the top plane and the majority of the energy 
enters air space 22 and foodstuff 18. A small amount of the energy 24 is 
shown being reflected from the top plane. The energy which passes through 
the top plane enters the foodstuff 18 which, because it is lossy, absorbs 
energy and is cooked. Some of the energy passes through the foodstuff and 
is reflected from the ground plane 16 and is retransmitted through the 
foodstuff 18 where it is further absorbed. Some of the energy 26, is 
reflected directly from the surface of the foodstuff. The energy which was 
not absorbed by the foodstuff in its first reflection from the ground 
plane arrives, once again, at the top plane where the vast majority is 
reflected back into the foodstuff. This process is continued until all the 
energy is either absorbed by the foodstuff or transmitted back out into 
the general interior of the microwave oven through the top plane. The 
ratio of energy absorbed by the foodstuff to the energy escaping from the 
top plane has been found to be very high. This process results in a very 
efficient concentration of energy within the container holding the 
foodstuff and the advantageous result of an even cooking of the foodstuff 
in the horizontal plane. 
As can be seen from FIG. 1 a small degree of reflection does take place in 
the plane of the cover. However, since the amount of reflection is so 
small the term "non-reflecting energy cover" is maintained throughout the 
disclosure. 
FIG. 2 shows a generally rectangular container 30 containing a foodstuff 
which fills the container to approximately the top. The container can be 
of a plastic material with a metallic ground plane (not shown) affixed to 
its bottom. A more preferable embodiment, and the embodiment shown, 
utilizes a metallic container having a bottom 32 and sides 34. A metallic 
lip 36 surrounds the top of the pan portion of the container. The 
container is completed with a lid 38. The lid is made of a dielectric 
material having a relatively low dielectric loss factor. An example of a 
suitable material is polyethylene polyester film. 
The top 40 of the lid is generally flat and is orientated so as to be 
generally parallel to the surface of the foodstuff. A side region 42 is 
provided around the perimeter of the lid and mates with a circumferential 
step 44 which is designed to rest on lip 36 of the pan. The side region 42 
has a height dimension which locates the top surface 40 within the range 
above the surface of the foodstuff described above. A preferred embodiment 
of the lid has a downwardly and outwardly sloping skirt 46 attached to the 
step 44. This skirt limits the proximity of the placement of the metallic 
pan to the microwave oven walls which effectively eliminates any 
possibility of arcing. The skirt also tends to lock or hold the lid on the 
pan by virtue of friction due to the lip of the pan. 
Metallic islands 48 are placed on the top surface 40 and, as mentioned 
above, combine with the dielectric material of the lid to provide a region 
of effective high dielectric over virtually the entire surface area of the 
lid. The surface area of the metallic islands should preferably be between 
50 and 80 percent of the surface area of the top of the lid 40. The array 
of islands 48 are shown in FIG. 2 as being rectangular islands forming a 
regular rectangular array. This particular configuration is not essential 
to the operation of the invention but has been found to function well. 
FIG. 3 is the circular embodiment. In this figure elements which are the 
same as elements in FIG. 2 bear like reference numerals. The metallic 
islands 48 are arranged in two axially symmetrical rings. Once again, the 
configuration shown provides a metallic surface area which is in the 
neighborhood of from 50 to 80 percent of the surface area of the top 40. 
In the configuration shown there are six islands in the inner ring and 
eight in the outer ring. The configuration shown provides for an even 
heating of the foodstuff in the horizontal plane. 
FIG. 4 is a perspective view of a multi-compartment container for use in 
heating, for example, a "TV" dinner (trade mark). By using the process 
described above, a controlled heating of various compartments within pan 
30 can be achieved. In FIG. 4, pan 30 includes outer side walls 34 and 
interior compartment walls which form compartments 50, 51, 52 and 53. 
Compartments 50 and 53 contain foodstuffs requiring high heating as, for 
example, meat and potatoes. In order to do this, an array dielectric 
consisting of dielectric material 40 and metallic islands 48 is located on 
the lid 38 directly over these compartments. A high heat concentration and 
uniformity of heating is achieved in these compartments as was discussed 
above. Compartment 52 requires medium heating to warm, for example, a 
frozen dessert, and therefore merely has the dielectric material directly 
over it. Compartment 52 is heated in the conventional manner. 
Compartment 51 contains, for example, a green vegetable and requires little 
heating. As a result, metallic shield 54 is affixed directly over this 
compartment. Sufficient microwave energy leaks around the shield to heat 
the contents of this compartment. In addition, the contents of the 
compartment are partially heated by conductive heating from the 
surrounding compartments. 
In the embodiment shown in FIG. 4, various foodstuffs requiring various 
heating needs are heated so that all the foodstuffs are ready for 
consumption at the same time. 
It should be noted that any of the covers described above can be fitted 
with venting apertures to allow steam generated in the cooking process to 
escape without deforming either the pan or cover. 
It should also be noted that the cover described herein could be used with 
a rigid reusable dish or permanent cooking container and that the cover 
itself could be reusable.