U.S. patent application number 10/955954 was filed with the patent office on 2005-02-24 for uniform microwave heating of food in a container.
Invention is credited to Hayert-Bonneveau, Laurence, Helstern, Gary, Loizeau, Gerard, Yout, William, Zhang, Hua.
Application Number | 20050040162 10/955954 |
Document ID | / |
Family ID | 28674337 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050040162 |
Kind Code |
A1 |
Zhang, Hua ; et al. |
February 24, 2005 |
Uniform microwave heating of food in a container
Abstract
The invention relates to a device and method for ensuring more
uniform heating of food by microwaves. The method includes
providing food in a portion having a predetermined size and shape;
and providing a container adapted for receiving and reheating the
portion of food in a microwave oven. The container forms a
supporting cavity having peripheral sides and a bottom side, with
the portion of food placed within the supporting cavity. The
peripheral sides of the container are circumferentially shielded by
a microwave reflective material that forms a circumference having
axial and transverse distances that are determined so as to change
the wavelength of resonant modes inside the food thereby resulting
in a more uniform heating food pattern.
Inventors: |
Zhang, Hua; (New Milford,
CT) ; Hayert-Bonneveau, Laurence; (Auffay, FR)
; Yout, William; (Gouchaupre, FR) ; Helstern,
Gary; (Newtown, CT) ; Loizeau, Gerard; (New
Milford, CT) |
Correspondence
Address: |
WINSTON & STRAWN
PATENT DEPARTMENT
1400 L STREET, N.W.
WASHINGTON
DC
20005-3502
US
|
Family ID: |
28674337 |
Appl. No.: |
10/955954 |
Filed: |
September 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10955954 |
Sep 30, 2004 |
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PCT/EP03/03712 |
Apr 9, 2003 |
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PCT/EP03/03712 |
Apr 9, 2003 |
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10118033 |
Apr 9, 2002 |
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6777655 |
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Current U.S.
Class: |
219/725 |
Current CPC
Class: |
B65D 81/3453 20130101;
A23L 7/11 20160801; B65D 2581/3441 20130101; B65D 2581/3404
20130101; B65D 2581/3462 20130101; B65D 2581/3491 20130101; A23L
5/15 20160801 |
Class at
Publication: |
219/725 |
International
Class: |
H05B 006/80 |
Claims
What is claimed is:
1. A method for ensuring a more uniform heating of food by
microwaves comprising: providing food in a portion having a
predetermined size and shape; providing a container adapted for
receiving and reheating the portion of food in a microwave oven;
wherein the container includes a supporting cavity having
peripheral sides and a bottom side, with the peripheral sides of
the container being circumferentially shielded by a microwave
reflective material such that the microwave reflective material on
the peripheral sides forms a circumference having axial and
transverse distances that are determined so as to change the
wavelength of resonant modes of microwave energy in the supporting
cavity; placing the portion of food in the supporting cavity; and
heating the food and container with microwaves so as to expose the
food to the changed wavelengths of resonant modes of microwave
energy in the cavity thereby resulting in a more uniform heating
food pattern and a more uniform heating of the food.
2. The method of claim 1, wherein the microwave reflective material
forms a resonating layer having a transverse distance at the
surface of the food of about 12 cm or less.
3. The method of claim 1, wherein the resonating layer has a closed
rounded contour.
4. The method of claim 1, wherein the microwave reflective material
forms a resonating layer having an oval contour with an axial
distance of about 20 cm or less.
5. The method of claim 1, wherein the microwave reflective material
forms a resonating layer having a height of about 10 to 60 mm.
6. The method of claim 1, wherein the microwave reflective material
forms a resonating layer having a transverse distance at the
surface of the food within a range of from 15 to 20 cm and an axial
distance at the surface of the food within a range of from 20 to 26
cm.
7. The method of claim 1, wherein the microwave reflective material
forms a resonating layer having a height of 25 to 45 mm.
8. The method of claim 1, wherein the cavity predominantly supports
TE.sub.n,m,l modes where n, m, l are such that 0.ltoreq.n.ltoreq.4,
0.ltoreq.m.ltoreq.2 and 0.ltoreq.l.ltoreq.1.
9. The method of claim 1, wherein the bottom side of the container
is substantially free of shielding material.
10. The method of claim 1, wherein the reflective material forms at
least a part of a supporting stand that substantially nests the
container in a removable manner.
11. The method of claim 10, wherein the stand has a body portion
with a wedge-shaped receiving microwave reflective surface to
snuggly engage the complementary surface of the container and
leverage means pivotally associated to the body portion to act on
the bottom surface of the container to disengage the container from
the wedge-shaped receiving surface.
12. The method of claim 1, wherein the microwave reflective
material has a thickness of at least 3.2 microns and is coated upon
or attached to the sidewalls of the container.
13. The method of claim 1, wherein the container is made of
ceramic, glass, plastic, cardboard or combinations thereof.
14. The method of claim 1, wherein the reflective material is in
the form of a shielding aid member that is configured to be placed
substantially around the peripheral sides of the container and is
disposable after a finite number of uses.
15. The method of claim 14, wherein the shielding aid member is a
single use piece adapted to surround the peripheral sides of a
non-disposable container.
16. The method of claim 15, wherein the shielding aid member
comprises a layer of cardboard or plastic which is associated with
a microwave reflective layer.
17. The method of claim 1, which further comprises removing the
food portion in a frozen state from a package of a defined shape
and size prior to placing the food portion in the container,
wherein the shape and size of the packaged food portion are
predetermined to substantially match the shape and size of the
cavity of the container.
18. The method of claim 17, wherein the package is disposable and
removed for microwave heating of the food and wherein the food is
placed in the cavity of the container in a manner to snuggly fit
into the cavity of the container.
19. The method of claim 17, wherein the portion of food is filled
in a thermoformed package cell that precisely determines the size
and shape of the food portion.
20. A container assembly adapted for receiving and reheating of a
food portion with microwaves; comprising a container forming a
cavity having peripheral sides and a bottom side for the portioned
food to be placed within the cavity, with the peripheral sides of
the container being shielded by a microwave reflective material and
the microwave reflective material of the peripheral sides defining
a circumference having axial and transverse distances that are
determined so as to promote propagation of certain resonant modes
of microwave energy inside the cavity and in a food portion that is
placed into the cavity, thus resulting in a more uniform heating
pattern for a more uniform heating of the food portion.
21. The container assembly of claim 20, wherein the microwave
material forms a resonating layer having a transverse distance of
about 13 cm or less, an axial distance of about 20 cm or less, and
a height of about 10 to 60 mm.
22. The container assembly of claim 20, wherein the microwave
material forms a resonating layer having a transverse distance of
from 18 to 22 cm and an axial distance of from 20 to 26 cm.
23. The container assembly of claim 20, wherein the bottom side is
substantially free of reflective material.
24. The container assembly of claim 20, wherein the resonating
layer has a circular or oval contour.
25. The container assembly of claim 20, wherein the resonating
layer is arranged to predominantly support in the cavity
TE.sub.n,m,l modes where n, m, l are such that 0.ltoreq.n.ltoreq.4,
0.ltoreq.m.ltoreq.2 and 0.ltoreq.l.ltoreq.1.
26. A stand-like device for improving the heating of food in a
container which comprises a support body of a shape adapted for
receiving a container in a removable manner and a circumferential
shielding surface of microwave reflective material adapted to be
positioned substantially adjacent to peripheral sidewalls of the
container.
27. The stand-like device of claim 26, wherein the microwave
reflective material of the circumferential shielding surface is
defined by axial and transverse distances that are determined so as
to change the wavelength of resonant modes inside the cavity and
inside of a food portion placed within the cavity thereby resulting
in a more uniform heating food pattern and a more uniform heating
of the food portion.
28. The stand-like device of claim 26, wherein the support body is
made of a metal and includes a wedge-shaped receiving surface to
snuggly engage the complementary surface of the container, and
leverage means pivotally associated to the metal body to act on the
bottom surface of the container to disengage the container from the
wedge-shaped receiving surface.
29. A combination comprising a food container and a stand-like
device for improving the heating of food in the container, wherein
the stand-like device comprises a support body of a shape adapted
for receiving the container in a removable manner and a
circumferential shielding surface of microwave reflective material
adapted to be positioned substantially adjacent to peripheral
sidewalls of the container.
30. The combination of claim 29 further comprising a food in the
container.
31. A method for more evenly heating a meal served in a reusable
dish by microwaves which comprises placing a food portion in the
stand-like device of claim 26 and subjecting the device and food
portion to microwaves to obtain a more uniform heating of the food
portion.
32. The method of claim 31, wherein the dish is a "plat sabot", a
"china plate" or another ceramic dish or a plastic dish.
33. A shielding aid for improving the microwave heating of food in
a container which comprises a band of microwave reflective material
of a shape configured for surrounding the peripheral sides of the
container to provide a circumferential shielding surface of
microwave reflective material, wherein the band is made of a
destroyable material adapted for a finite number of uses, and
wherein the band forms a microwave reflective circumference having
axial and transverse distances that are determined so as to change
the wavelength of resonant modes of microwave energy in the
supporting cavity to provide more uniform heating of the food in
the container.
34. The shielding aid according to claim 33, wherein the band is a
thin band of cardboard or plastic coated with a reflective material
configured to connect or attach to the sides of the container.
35. The shielding aid according to claims 33, wherein the band has
substantially the form of a crown.
36. The shielding aid according to claim 33, wherein the band is
destroyed by the effect of separating the band from the container
after heating.
37. The shielding aid according to claim 36, wherein the band
comprises an adhesive layer at its inner surface to adhesively
attach to the sides of the container and wherein the effect of
separating the band from the container after heating destroys the
adhesive layer.
38. The shielding aid according to claim 36, wherein the heating
creates a tearing of the band.
39. The shielding aid according to claim 36, wherein the band
comprises a shrinkable polymeric material that shrinks when exposed
to microwave heating.
40. A method for more evenly heating a meal served in a dish by
microwave heating which comprises placing the food in the dish,
placing the shielding aid of claim 33 about the sides of the dish
to shield it, and subjecting the shielded dish to microwave heating
to obtain a more uniform heating of the food in the dish.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
application PCT/EP03/03712 filed Apr. 9, 2003, which is a
continuation of application Ser. No. 10/118,033 filed Apr. 9, 2002,
the entire content of each of which is expressly incorporated
herein by reference thereto.
BACKGROUND ART
[0002] The present invention relates to a device and method for
improving heating of food in a microwave oven. The invention
addresses the problems of cold spot, uneven heating and splattering
that traditionally occur when foods are heated in a microwave oven.
In particular, the present invention proposes a method for handling
and evenly heating frozen food that can be economically,
conveniently and rapidly served in a foodservice location to the
consumer. The invention also relates to a stand-like device adapted
to receive a food container for improving the reheating of the food
in a microwave oven.
[0003] Microwave reheating of frozen meals provides convenience for
people seeking quick meal solutions because of the rapid thermal
energy transfer into the food materials by the microwaves. However,
microwave heating of frozen food is, in general, perceived as
difficult and has many problems associated with it, such as
overcooking of some portions of meals, cold spots, burnt edges and
sauce splatter. In many instances, the meal experiences serious
burnt spots yet some places are still very cold, even often below
O.degree. C. Furthermore, the concentration of the microwave energy
on spots tends to create local boiling of the water contained in
the food and/or sauce and therefore splattering off the dish.
[0004] Many attempts have been made in the past to solve this issue
of re-heating frozen food in a more effective way.
[0005] It has previously been proposed in U.S. Pat. No. 4,351,997
to provide a modified form of tray structure to attempt to provide
a more even heating of foodstuff in the tray when exposed to
microwave radiation. This prior art has a bottom wall of
microwave-transparent material and an upwardly extending peripheral
wall that is outwardly curved at its upper end to define a
horizontal peripheral rim. The rim is partly or completely coated
with a material that is reflective and opaque to microwave
radiation, such as aluminum foil. The peripheral wall also is
partly or completely coated with the foil material. The effect of
this aluminum foil coating on the wall is to provide reflection of
microwave energy toward the center of the tray.
[0006] U.S. Pat. No. 4,626,641 describes an embodiment in which a
similar structure is provided. In addition to the provision of
aluminum foil in the side wall of a tray, the foil also extends
into the base of the container but leaves a rectangular open area
in the bottom wall.
[0007] WO 92/19511 relates to a tray useful for the microwave
cooking of prepared foodstuff that comprises an outer layer formed
of paperboard or molded plastic to which is laminated an inner
polymeric film layer. A layer of microwave-reflective material,
usually aluminum foil, is positioned between the outer and inner
layers in the location of the peripheral wall of the tray and in a
pattern in a portion of the bottom wall.
[0008] JP 09-369450 relates to a container for a microwave oven
that comprises a first microwave reflecting plate placed along a
circumferential lateral parts and a second microwave reflecting
plates in the base section of the container.
[0009] It is apparent from the prior art that attempts have
essentially been made to provide energy transmission structures
with reflective material placed in locations that enable energy
transfer from the edges and corners of the plate to a more central
area of the plate. However, experimental trials have shown that
these structures are, by themselves, insufficient to overcome the
problems of uneven heating. In particular, cold areas are still
present in the food despite the presence of these structures. Thus,
improvements in these devices are needed and are provided by the
present invention.
SUMMARY OF THE INVENTION
[0010] The present invention aims at providing a satisfactory
solution for evenly heating a frozen food in a plate by adopting a
different approach where not only energy transfer is carried out
but more importantly a modification of the overall heating pattern
inside the food block is achieved by changing the wavelength inside
the food. Thus, the present invention aims at providing a
convenient and easy way for improving microwave reheating of food
while enabling the use of standard dishes such as ceramic and
ceramic-like plates that may commonly be found in restaurants,
cafeterias, hotels, or other foodservice locations.
[0011] The present invention specifically relates to a method for
ensuring a more uniform heating of frozen food by microwaves. This
method is conducted by providing food in a portion having a
predetermined size and shape; and providing a container adapted for
receiving and reheating with microwaves the food portion. The
container includes a supporting cavity having peripheral sides and
a bottom side, with the peripheral sides of the container being
circumferentially shielded by a microwave reflective material such
that the microwave reflective material on the peripheral sides
forms a circumference which has axial and transverse distances that
are determined so as to change the wavelength of resonant modes in
the supporting cavity. The method includes placing the portion of
food in the supporting cavity; and heating the food and container
with microwaves so as to expose the food to the changed wavelengths
of resonant modes in the cavity thereby resulting in a more uniform
heating food pattern and a more uniform heating of the food.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0012] FIG. 1 is a perspective view of a plate adapted to form a
resonator of shortened wavelength with a frozen food product ready
for microwave heating;
[0013] FIG. 2 is a cross section along line O of FIG. 1;
[0014] FIG. 3 is a cross section along line O' of FIG. 1;
[0015] FIG. 4 is a schematic illustration of the preferred method
for re-heating a frozen food;
[0016] FIG. 5 is a preferred embodiment of a plate assembly
comprising a supporting stand of microwave resonating material in a
configuration ready for microwave heating;
[0017] FIG. 6 is a view similar to FIG. 5 when the plate is removed
from the stand after reheating;
[0018] FIG. 7 is a bottom view of the supporting stand of FIG. 5
without the container inside;
[0019] FIGS. 8 to 20 are comparative thermograph diagrams of the
heat distribution of the food after it has been submitted to
microwave radiation in different plates and trays;
[0020] FIG. 21 is a computerized view of transverse modal field
distribution for a circuit waveguide of circular contour in a TE11
mode;
[0021] FIG. 22 is a computerized view of a transverse modal field
distribution for a circuit waveguide of circular contour in a TE21
mode;
[0022] FIG. 23 is a computerized view of transverse modal field
distribution for a circuit waveguide of circular contour in TE31
mode.
[0023] FIG. 24 is a resonant mode chart for a resonating band of
circular contour;
[0024] FIG. 25 shows a schematic view of a food container shielded
with a disposable shielding aid according to an embodiment of the
invention;
[0025] FIG. 26 shows the way the shielding aid of FIG. 25 is
removed after heating of the food product in the container.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] It has been surprisingly found that it is possible to render
the heating of the food in the container relatively even by
shortening the wavelength of the resonant modes inside the cavity
of the food, especially for the transverse electric (TE) modes.
Shortening of the wavelength and supporting of specific resonant
modes can be obtained more specifically by determining the
distances that separate the microwave reflective material when
placed circumferentially along the side of the container in a
manner to promote modes that show a more even electric field
distribution.
[0027] In one embodiment, the microwave reflective material forms a
resonating layer having a transverse dimension of less than 12.5
cm. Preferably, the transverse dimension of the resonating layer is
of less than 12 cm, even preferably of from about 10 to 12 cm.
Preferably, the axial dimension of the resonating layer is of 20 cm
or less, even preferably of less than 18 cm. The preferred ratio
axial dimension to transverse dimension is of from 1:1 to 2:1, even
more preferably of from 1.5:1 to 1.8:1.
[0028] Therefore, the resonating layer can be regarded as a
resonator or waveguide where shortened standing waves occur inside
the food. As the transverse dimension of the resonating layer is
shortened, transverse electric ("TE") modes that carry more power
than transverse magnetic ("TM") modes are propagated in the cavity
of the food. In particular, the TE modes that are permitted for the
selected geometry are those with a more concentrated heating in the
center area of the cavity. More preferably, TE modes that are
predominantly supported in the food cavity are TE.sub.n,m,l modes
wherein n may be 0, 1, 2, 3 or 4 m maybe 0 or 1 or 2 and l may be 0
or 1.
[0029] Preferably, the microwave reflective material forms a
resonating layer having a closed rounded contour. Even more
preferably, the resonating layer has an oval contour with an axial
dimension of less than 20 cm, preferably less than 18 cm, even most
preferably comprised between 15 to 18 cm. In an alternative
embodiment, the resonating layer has a circular contour. In a
preferred embodiment, the bottom side is substantially free of
shielding material.
[0030] In a second embodiment, the microwave reflective material
forms a resonating layer having a transverse distance at the
surface of the food within a range of from 15 to 20 cm and an axial
distance at the surface of the food within a range of from 18 to 26
cm. Within this second range of specific transverse and axial
dimensions of the resonating layer, resonant modes that promote a
uniform heating are also selected successfully. The frozen food
block with such a shielding arrangement, which can be seen as a
dielectric resonator with resonating boundaries, can effectively
selects modes of more even field lines distribution which creates a
micro-climate of well distributed energy inside and which is
relatively not much dependent on the specific oven factors.
[0031] Remarkable results have been obtained by using such shapes
and dimensions of a resonating layer. Comparative trials have shown
that these are the preferred configurations for evenly heating food
in the container.
[0032] The invention also relates to a method of using a container
having the resonating layer as previously defined wherein, prior to
placing the food portion in the container, the food portion is
removed frozen from a package of defined shape and size which are
predetermined to substantially match the shape and size of the
cavity of the container. Therefore, the frozen food can be produced
and packaged in the manufacturing facility according to specific
standards of size and shape that are predetermined to provide
optimal re-heating properties afterwards. Therefore, the food
product may be packaged in a package that is not the final heating
tray with its resonating layer so as to reduce the packaging costs.
Therefore, contrary to the package which may usually be disposable,
the final heating tray may be a dish made of a non-disposable
material for repeated uses in a foodservice location such as a
restaurant, a fast food or a cafeteria.
[0033] The invention also relates to a stand-like device for
improving the heating of food in a container. This device includes
a support body of a shape and size adapted for receiving the
container in a removable manner and a circumferential shielding
surface of microwave reflective material adapted to be positioned
adjacent to peripheral sides of the container. The combination of
the stand-like device and a food container is another embodiment
for improving the heating of food in the container.
[0034] In a preferred embodiment, the microwave reflective material
of the circumferential shielding surface is defined by axial and
transverse distances that are determined so as to change the
wavelength of resonant modes inside the food thereby resulting in a
more uniform heating food pattern. Still in a preferred embodiment,
the support body has a wedge-shaped receiving surface to snuggly
engage the complementary surface of the container and leverage
means pivotally associated to the body to act on the bottom surface
of the container to disengage the container from the wedge-shaped
receiving surface.
[0035] The benefit of such a supporting stand is that it allows to
more evenly heat the food in a microwave oven while reusable
standard dishes such as traditional dishes can be used. Therefore,
reusable standard dishes can become more effective for microwave
reheating of food as compared to the use of the same dishes without
the supporting stand. The supporting stand may be of a convenient
use in kitchens of restaurants, hotel chains, transportation or
other foodservice areas where paper or plastic dishes can not
decently be used but only food in traditional ceramic or
ceramic-like dishes can be served to the customer.
[0036] A possible alternative to a reusable stand-like shielding
device is a non-reusable shielding aid, made of material(s)
configured to last for a limited number of uses, preferably a
single use. The non-reusable shielding aid can be used in
combination with a durable dish such as a ceramic, plastic or glass
dish to provide the adequate shielding of its sides as
aforementioned. The benefit is that it provides an economic,
flexible and temporary solution that can be conveniently associated
to a reusable container to improve the microwave heating
performance in the container. This solution is also more
hygienic.
[0037] The shielding aid can preferably form a band of microwave
reflective material of a shape configured for surrounding the
peripheral sides of the container to provide a circumferential
shielding surface of microwave reflective material. In particular,
the band forms a microwave reflective circumference having axial
and transverse distances that are determined so as to change the
wavelength of resonant modes in the supporting cavity.
[0038] The band is preferably made of a destroyable material. For
instance, the band is a thin band of cardboard and/or plastic
coated with a microwave reflective material. The band may have
substantially the form of a crown.
[0039] The band can be destroyed by the effect of separating the
band from the container after a single use. For instance, the band
comprises an adhesive layer at its inner surface to adhesively
attach to the sides of the container and wherein the effect of
separating the band from the container after heating, destroys the
adhesive layer or creates a tearing of the band that damages the
band so that no further use of the band to heat the food container
is made possible.
[0040] Tearing of the band may be obtained by the relatively low
mechanical strength of band coming from the intrinsic properties of
the material, the humid environment and/or heat during heating in
the microwave and/or additional weakening zones provided in the
band itself.
[0041] In another possibility, the band comprises a polymeric
material that shrinks upon heating in the microwave. After
shrinking of the material, the band becomes so tightly wrapped
around the container that removal of the band from the container
has for effect to tear off the band and damage it so that no second
use can be envisaged. In another possibility, the band can be made
of a material that separates or disperses upon washing of the
container. For instance, the material can be a resin of soluble or
dispersible material containing an amount of reflective particles
such as metallic particles. The resin material can be coated on the
sides of the container in a liquid or paste form. The coating
material could also be an edible material.
[0042] With reference to FIGS. 1 to 3, the method of the invention
aims at providing a solution for re-heating a frozen food product
of predetermined size and shape in a plate that is specifically
adapted for modifying the wave pattern during a microwave heating
process in a manner that enables the transverse electric (TE) modes
of higher power and more evenly distributed pattern to propagate
inside the food product. The present invention aims at providing a
method for quickly and evenly reheating food, in particular frozen
food, with microwaves, in particular those provided by a
conventional microwave oven that is 2450 MHz.
[0043] The method comprises providing a food portion 1 that is
frozen, placing the food portion in the supporting cavity 20 of a
tray or plate 2 of specific shape and size and heating the food
portion in a microwave oven. The food may be any kind of food meal
such as lasagna, pasta, rice, fish, meat, and vegetables with or
without sauce and combinations thereof. The plate 2 includes a
bottom portion 21 and upwardly oriented side portions 22 that
together define a support member with the cavity for receiving the
food portion 1. The plate should be formed, at least partly, from a
material that is substantially transparent to microwave so as to
avoid microwave energy loss in the material but be sufficiently
rigid to support the weight of the food without significant
deformation and to maintain the side portions extending upwardly.
Therefore, the material for the plate can be chosen among the list
consisting of ceramic, porcelain, glass, cardboard, polymer and
combinations thereof.
[0044] Importantly, a circumferential surface 3 made of a material
that is reflective to the microwave is provided that is placed
adjacent the sidewall 22 of the plate. The circumferential surface
may preferably be a band or layer of microwave reflective material
that is placed adjacent the sidewall 22. The surface could also be
part of the side portion itself or replace the whole sidewall 22.
The circumferential surface forms the boundaries around the food
that makes the dielectric resonator while the rest of the plate is
substantially transparent to the microwaves.
[0045] According to the principle of the invention, the
circumferential surface of reflective material has to respect
dimensions and shapes that impart a modification of the wavelength
in the dielectric food itself. As a result, in the event the plate
and reflective surfaces are distinct parts, the shape and size of
the plate should preferably be such that the plate can insert
itself inside the available volume determined by the
circumferential surface of the band. Preferably, the plate 2 should
have a shape and size that complementary match the shape and size
of the band 3, even though this is not mandatory, provided the
plate 2 can fit between the band 3. For instance, the plate could
be smaller or of a different shape as the band 3 itself.
[0046] As illustrated in FIG. 1 to 3, the band has preferably a
shape primarily extending in an elongated manner along an axial
line 0. In such a configuration, it is possible to define an axial
distance A that corresponds to a longer distance between the
internal surfaces of the band along axial line 0.
[0047] Similarly, the band 3 forms a second dimension T that is
taken transversally along a transverse axis O' that is
substantially oriented orthogonally to axis 0 and passes that
through the centerline of axial distance A. The transverse distance
T represents the distance at this centerline between the internal
surfaces of the reflective band as measured at a level of the upper
surface of the food.
[0048] The distances A and T are preferably measured at the
circumference of the band 3 at a level of the upper surface 5 of
the food when the food is properly placed in the cavity for
re-heating. However, in certain circumstances where the container
is shallow and/or has sidewalls that form a low inclination
relative to the bottom surface; e.g., the edges of a dinner plate,
the food may usually project upwardly beyond the height of the
sidewalls. In that event, the circumference for considering
distances A and T will be regarded at the upper edge of the
resonating layer.
[0049] According to one essential aspect of the invention, the
distances A and T of the band are determined to provide a
shortening of the wavelength when the food plate, as surrounded by
the band 3, is heated in the microwave oven. Therefore, reflection
or transfer of the microwave beam is not the primary effect that is
sought. The primary effect that is sought is related to the
modification of the propagation of the waves in the dielectric food
material between two or more conductors that support a certain
number of electromagnetic waves. These waves have a uniquely
defined voltage, current and impedance. Waveguides, often
consisting of a single conductor, support transverse electric (TE)
and/or transverse magnetic (TM) waves, characterized by the
presence of longitudinal magnetic or electric, respectively, field
components. Therefore, the principle of the invention is to look at
determining the boundaries of the band that acts as a waveguide so
that high power TE modes of even density are primarily induced. In
order to promote domination of the desired TE modes, it has been
found that the transverse distance T of the resonating band should
be of less than 12.5 cm, preferably lower than 12 cm, even
preferably ranging of from 6 to 11 cm. Similarly, the axial
distance A of the band should be of 20 cm or less, preferably lower
than 18 cm, even preferably ranging of from 6 to 13 cm. More
particularly, with respect to these particular dimensions,
TE.sub.nm modes of evenly distributed density such as TE.sub.01,
TE.sub.11, TE.sub.21, are primarily supported while TE.sub.nm or
TM.sub.nm modes of less evenly distributed density such as
TE.sub.31, TE.sub.41, TE.sub.51 or TM.sub.31, are preferably
discarded.
[0050] In a second embodiment, larger dimensions have also been
found working to produce uniform heating. However, these larger
dimensions must also be determined precisely to promote TE mode of
even propagation while discarding the dimensions that promote TE
modes of uneven propagation. For this, it has been found that the
transverse distance T of the resonating band should be comprised
within a precise range of from 15 to 20 cm and the axial distance A
of the resonating band should be comprised within a precise range
of from 20 to 26 cm.
[0051] The number of modes increases as the size of the resonating
band increases and new modes usually adds on existing modes while
certain being more dominant than others. By selecting the correct
size of the resonating band, one promotes the dominant modes which
have a field lines distribution which is more even over the surface
food. In particular, for the larger sizes of the resonating band,
as defined, modes that are promoted are, for instance: TE.sub.01,
TE.sub.10, TE.sub.22, TE.sub.32, TE.sub.42 and others.
[0052] As a result, remarkable and surprising results on the final
temperature distribution have been obtained if those distances are
properly respected. This effectively changes the wavelength of the
resonant modes inside the dielectric cavity of the plate (inside
the food), especially for those TE modes. This shortening of the
wavelength makes the heating happens in the center of the food,
therefore promoting a more uniform heating pattern.
[0053] The band may comprise tapered surfaces as shown in FIGS. 2
and 3 that form an angle .theta. with respect to the bottom of the
plate comprised between 10 to 120 degrees, preferably, 15 to 90
degrees, and even more preferably 20 to 85 degrees. In fact,
slightly tapering down surfaces promote a better heating. However,
a problem in energy transmission in the bottom of the food may
occur if the bottom surface of the plate is reduced, e.g., becomes
less than 5 cm. Therefore, regardless of the inclination of the
sidewall, the flat bottom of the plate should be maintained at a
transverse dimension of at least 5 cm, and preferably at least 7
cm.
[0054] It has also been found that the microwave reflective
material band should preferably be a continuous peripheral band of
a height of at least 10 mm, preferably of from 10 to 60 mm, even
preferably of from 25 to 55 mm. The food should preferably be
placed so that its upper surface 10 is placed below the upper line
30 of the band 3, preferably at 0 to 15 mm below, even preferably
at 2 to 10 mm below. Similarly, the food lower surface 11
contacting the bottom of the plate should be at a level above the
lower line 31 of the band of from 0 to 10 mm, preferably 0.5 to 5
mm, preferably 2 to 3 mm.
[0055] Although an oval contour is preferred for the band, since
the best results have been found with such a configuration, it
remains possible to have a band of circular contour. In that event,
for the first preferred embodiment the transverse distance T and
the axial distance A form a diameter that should be preferentially
determined, as for the previous example, to be a distance less than
12.5 cm, preferably equal to or less than 12 cm, even preferably of
from 6 to 11 cm and, for the second embodiment, the transverse
distance should be of from 15 to 20 cm and the axial distance A
should be of from 20 to 24 cm.
[0056] The band or layer 3 may preferably comprise a metallic
material that is placed adjacent the sidewall 22 of the plate. The
band may be either attached or simply positioned in close contact
with the outer surface of the sidewall 22. In another embodiment,
the band may be positioned adjacent to the inner surface of the
sidewall 22 of the plate.
[0057] In a preferred embodiment, the band 3 may be a rigid metal
foil that forms a ring or stand forming a large central aperture of
a shape and size adapted for the plate to nest therein.
[0058] In an alternative, the band may be a coating material, for
instance, a metal coating on the surface. In order to provide a
sufficient microwave resonating effect, the coating has a thickness
that preferably is at least 3.2 microns, and even more preferably
of at least 10 microns. The coating may be carried out by any
suitable means such as by vacuum metallizing or chemical etching.
Those coating techniques are well known by the man skilled in the
art of surface treatment and, therefore, do not need to be
described further herein.
[0059] In an alternative, the band may be a metallic tape that is
adhered by an adhesive to the surface of the sidewalls of the
plate.
[0060] The inside volume and shape demarcated by the
circumferential band may be determined to match the external volume
and shape of the plate and, therefore, can take any possible cross
sectional configuration such as parallel, trunconical or a stepped
configuration.
[0061] FIG. 4 shows a preferred method for preparing a warmed food
in a plate from a frozen food product using the teaching of the
present invention. The method preferably includes the preliminary
step of portioning the food to form food portions of given size and
shape and individually packaging the food portions in a package for
convenient and hygienic transport and storage. The portioning and
packaging are preferably carried out in a food manufacturing
facility utilizing all hygienic and quality standards required in
the food manufacturing art. The food portions are preferably
packaged in a pre-formed package 4 that predetermines the final
shape and size of each individual food portion that are
subsequently re-heated. Preferably, the pre-formed package 4
comprises at least one and preferably a plurality of packaging
cells 40, 41, and 42. The cells forms a plurality of individual
cavities wherein each of them have a shape and a size adapted to
precisely fit in the cavity of the plate 2 for the microwave
re-heating while considering the positioning constraints that have
been determined earlier in the present description for obtaining
the intended result on uniformity of heating. If the plate and the
band has an oval contour, the cavity of the cell should also
preferably have an oval contour 400 of predetermined length X and
width Y. The dimensions X and Y are measured at the outer surface
of the food. The dimensions X and Y are determined to substantially
correspond to the internal dimensions of the heating plate 2 it
fits in so that no large gaps are left in the cavity between the
product and the plate. It has been noticed that large gaps provide
the edge of the food a higher than desired heating. Therefore,
gaps, if any, between the product and the inner surface of the tray
or dish should be of less than about 8 mm, preferably of less than
5 mm. Even more preferably, there should be no gap at all between
the side surface of the dish and the side surface of the food. For
example, the contour of the food may have dimensions of X of
between about 15 to 19 cm and for Y between about 8 to 12 cm.
[0062] The pre-formed package 4 may be made of a thermoformed
plastic foil or any equivalent formable material that can retain a
given shape after forming. The foil is thermoformed to form the
cavities and edges that surround the cavity. The material should
advantageously be sufficiently deformable to allow easy removal of
the food block from the cell. The preformed package can be a
foodgrade polypropylene or any other suitable plastic of from about
0.1 to 1 mm in thickness. A plurality of cells 40-42 can be formed
in a single sheet of plastic as illustrated for cost manufacturing
reasons and can be separated along cutting lines 43, if necessary,
or be kept grouped as a collective package 4, if there is a need
for distributing or selling more than one food package at a
time.
[0063] The cells are filled in by food to form the food portions.
Depending on the food recipes, freezing may be required before
filling in the cells. Freezing may be carried out on discrete food
components, for instance, pasta layers, vegetable pieces, meat
balls, etc., or on the food block itself whereas other components
such as sauce, cheese and the like, may be placed, poured or
deposited in the cells at ambient or just at chilled
temperature.
[0064] As a matter of safety, freezing of the filled cells should
always be subsequently completed until the whole food portion has
reached the frozen temperature range required. The food containing
cells are usually rapidly cooled to the required freezing
temperatures, i.e., minus 18 to 40.degree. C. Freezing may usually
be carried out in a spiral freezer or in a freezing tunnel under
liquid nitrogen jets or any other suitable freezing technology.
Then, the cells may be closed by thin plastic wrap that is sealed
onto the edges of the cells or, alternatively, the preformed
package 4 may be simply stacked and packed in a cardboard box with
a partition film to separate them in the box.
[0065] In another embodiment (not shown), the cells may also be
made of a flexible non-preformed material such as in thin plastic
wrap with the shape and size of the cell's cavity to shape the food
portion being defined by an external mould. In that event, the
plastic wrap is covered onto the mould surface, the food components
are deposited into the plastic wrap and the final block is frozen
and removed from the mould.
[0066] The advantage of the packaging cells resides in that
packaged food blocks can be mass produced in an inexpensive manner
that are properly sized to fit a shielded container of specific
size as aforementioned. The container may, for instance, be such as
a reusable ceramic dish used in restaurant or other foodservice
catering areas. The food can, therefore, be transferred from the
cell to the ceramic dish to be reheated in a microwave oven and
served directly to the consumer in its dish. After food
consumption, the dish can thus be washed and re-used as a normal
dish.
[0067] The dish is preferably shielded by a non-reusable shielding
aid that is associated to the dish by the user when preparing the
food before the microwave heating.
[0068] The shielding aid preferably forms a flat band comprising a
reflective material that can be associated to the sides of the dish
such as by folding the band to form a crown.
[0069] The aid can, for instance, be a laminate comprising a
support substrate of reinforcing material such as one or more thin
cardboard and/or plastic layers and an outer reflective layer such
as a metallic coating. The band may be attached by it two ends and
positioned around the side of the dish.
[0070] In a preferred embodiment, the band is configured to be
adhesively attached to the sides of the container by an adhesive
layer. The laminate may thus comprises an additional external layer
opposite the microwave reflective layer to adhere to the container.
The adhesive layer can be configured such that it can be used only
once. Any separation of the shielding aid from the container after
use leads the adhesive either to loose its adhesive properties
and/or to tear off the band, for example, by damaging the support
substrate. Weakening zones such as perforated lines may be added,
such as several at intervals on the band, to ensure that removal of
the band tears off along these lines and leaves the band in several
non-reusable separate pieces.
[0071] In an alternative, the band can comprise a piece of
shrinkable polymeric material that shrinks at the particular
heating conditions in the microwave. The polymeric material would
preferably be positioned to cover the food and so to be removed by
the user to access the food.
[0072] In another alternative, the band comprises a single-use
connection device configured to place the shielding aid around the
sides of the container. The single-use connection system can be a
tear line or zipper as it is illustrated in FIGS. 25 and 26. In
these figures, the shielding aid 110 may have the form of a lid
with a shielding skirt which comprises a tear line 111 that
extends, for instance, circumferentially along the sides of the
element thus permitting to separate the lid 110a from bottom part
110b after the heating.
[0073] Finally another possibility, the food could be heated in a
disposable container with reflective sides such as a box or CPET
tray in a box and then transferred to the dish for serving.
[0074] FIGS. 5 to 7 illustrate another embodiment in which a
reusable supporting stand 7 comprising a reflective material is
adapted to substantially nest the container 2 in a removable
manner. The stand may be specifically adapted to receive reusable
dish of specific size and shape. In particular, the stand 7 has a
body 70 with a wedge-shaped receiving surface of microwave
reflective material 71 arranged to snuggly engage the complementary
shaped exterior surface 220 of the sidewalls 22 of the container.
The microwave reflective surface 71 extends circumferentially to
encompass the sidewalls of the container. The body may be formed
entirely of reflective material such as metal or be only partly
coated of microwave reflective material on its inner receiving
surface 71 while the rest is made of a different material such as
plastic, ceramic, etc. The body may therefore form a central
truncated surface of a shape that substantially complements the
external side portions 22 with a central bottom aperture 72
arranged to leave the bottom portion 21 of the container
substantially uncovered by the shielding material. The stand 7 may
further comprise leverage means 8 adapted for conveniently and
safely disengaging the container from the metal body after the
heating of the food container has been performed. For instance, the
leverage means 8 may be formed of a pair of levers 80, 81 pivotally
attached to the metal body, along rotational axis 73, to act on the
bottom surface of the container to disengage the container from the
stand by upward pushing on the bottom side 21 of the container. The
levers may preferably have a hook type shape comprising a rounded
support end 810 and a connecting portion 820. The rounded end 810
is large enough to form a well distributed fulcrum on the bottom of
the container when the lever is activated in rotation so to be able
to properly lift the container without blocking. The levers are
thus positioned in a retracted position where the support end 810
is maintained inset relative to the bottom support plane I.sub.1
formed by the stand/container assembly when on rest onto the oven's
surface (FIG. 5). Then, after sufficient heating has been carried
out, the heating assembly is taken out from the oven and the levers
are manually actuated in pivot about their axis 73 to lift the
container off the body of the stand (FIG. 6).
[0075] The stand should preferably respect the dimensions T and A,
as previously defined, for promoting support of TE modes with more
even electric field distribution. To provide an effective effect,
the dimensions should be defined depending on the type of dish that
is intended to be received, e.g., whether it is a deep or shallow
dish, so that the dimensions T and A will be taken, approximately
at the surface of the food or, alternatively, at the uppermost edge
of the reflective material surface. For instance, in FIG. 5 is more
particularly illustrated a "plat sabot" type dish that is inserted
in the stand-like device of the invention. This type of dish is
sufficiently deep to accommodate a food portion that partially
fills in the cavity of the dish until a certain stepped line which
may be considered as the effective position for measuring
dimensions T and A.
[0076] The stand may be particularly adapted for receiving a
ceramic or ceramic-like dish into which is inserted the food
portion coming from the packaging cells as described earlier.
Therefore, one benefit of the supporting stand is that traditional
dishes of appropriate dimensions and shape such as "plat sabot",
"china plates" or other dishes, can be successfully used for
reheating meals in a microwave oven such as dense frozen meals,
whereas if they would be used alone, i.e., without the help of the
stand, problems of cold spot, uneven heating and splattering would
be clearly noticed.
EXAMPLES
Example 1 (Comparative)
Heating of Lasagna in Oval Plate without Metallic Side Band
[0077] A frozen lasagna product of 387 grams is placed in an oval
plate ("plat sabot") having a long axis of 17.8 cm, a short axis of
10.4 cm, height of 3.1 cm, and no metallic shielding on the side
wall. The food product is heated in Welbilt microwave oven (850
Watts) for 6 minutes. The temperature measurement showed a cold
spot below 18.degree. C. and edges boiled over.
[0078] FIG. 8 shows the corresponding thermograph at the end of the
heating stage.
Example 2
Heating of Lasagna in Oval Plate with Half Upper Metallic Side
Band
[0079] The same food product is heated in a plate of identical
dimension with a metallic shielding tape adhesively attached to the
upper half of the upward sides of the plate. The lower half of the
sides is left uncovered. The product weighed 389 grams and was
heated in the same oven as Example 1 for 6 minutes. The product
showed a cold spot below 39.degree. C. and edge slightly boiled
over.
[0080] FIG. 9 shows the corresponding thermograph at the end of the
heating stage.
Example 3
Heating of Lasagna in Oval Plate with a Full Metallic Side Band
[0081] The same food product is heated in a plate of identical
dimension with a metallic shielding tape adhesively attached onto
the full sides of the plate. The bottom of the plate is left
uncovered. The product weighed 386 grams and was heated in same
oven and for 6 minutes. It had no cold spot with a temperature of
the surface higher than 65.degree. C. and no burnt edge and
corner.
[0082] FIG. 10 shows the corresponding thermograph at the end of
the heating stage.
Example 4
Heating of Lasagna in a Rectangular Tray with No Metallic Side
Band
[0083] The same food product is heated in a rectangular tray of
12.7 cm long, 15 cm large and 2.5 cm high, with no metallic
shielding. The product weighed 360 grams and was heated in same
oven and for 6 minutes. It showed good temperature but edge heating
was excessive.
[0084] FIG. 11 shows the corresponding thermograph at the end of
the heating stage.
Example 5
Heating of Lasagna in a Rectangular Tray with Full Metallic Side
Band
[0085] The same food product is heated in a rectangular tray of
12.7 cm long, 15 cm large and 2.7 cm high, with a metallic
shielding tape adhesively attached onto the full sides of the
plate. The bottom of the plate is left uncovered. The product
weighed 386 grams and was heated in same oven and for 6 minutes. It
showed a cold spot (34.degree. C.) but edge was not boiled.
[0086] FIG. 12 shows the corresponding thermograph at the end of
the heating stage.
Example 6
Heating of Lasagna in a Large Oval Plate with No Metallic Side
Band
[0087] The same food product frozen lasagna product of 545 grams is
placed in an oval plate ("plat sabot") having a long (longitudinal)
axis of 24 cm, a short (transverse) axis of 13.1 cm, height of 2.5
cm, and full metallic shielding on the side walls. The food product
is heated in a LG1000W oven for 6.5 minutes. The center was still
cold (2.5.degree. C.) but the edge was boiled and dried.
[0088] FIG. 13 shows the corresponding thermograph at the end of
the heating stage.
Example 7
Heating of Lasagna in a Large Oval Plate with Full Metallic Side
Band
[0089] The same food product frozen lasagna product of 545 grams is
placed in an oval plate ("plat sabot") having a long (longitudinal)
axis of 24 cm, a short (transverse) axis of 13.1 cm, height of 2.3
cm, and full metallic shielding on the side walls. The food product
is heated in a LG1000W oven for 6.5 minutes. The center was still
cold (3.5.degree. C.) but the edge was not boiled and dried.
[0090] FIG. 14 shows the corresponding thermograph at the end of
the heating stage.
Example 8
Heating of Lasagna in a Rectangular Tray with No Metallic Side
Band
[0091] The same food product frozen lasagna product of 447 grams is
placed in rectangular tray having a long (longitudinal) axis of
15.8 cm, a short (transverse) axis of 12.5 cm, height of 4 cm, and
no metallic shielding on the side walls. The food product was
heated in a Delonghi oven for 5 minutes. The center was still cold
(-2.5.degree. C.) and the edge was partially dried.
[0092] FIG. 15 shows the corresponding thermograph at the end of
the heating stage.
Example 9
Heating of Lasagna in a Rectangular Tray with Metallic Side
Band
[0093] The same food product frozen lasagna product of 446 grams is
placed in rectangular tray having a long (longitudinal) axis of
15.8 cm, a short (transverse) axis of 12.5 cm, height of 4 cm, and
with metallic shielding on the side walls. The food product was
heated in a Delonghi oven for 5 minutes. The center was still
cold.
[0094] FIG. 16 shows the corresponding thermograph at the end of
the heating stage.
Example 10
Heating of Lasagna in a Rectangular Tray with No Metallic Side
Band
[0095] The same food product frozen lasagna product of 636 grams is
placed in rectangular tray having a long (longitudinal) axis of 19
cm, a short (transverse) axis of 14 cm, height of 4.2 cm, and no
metallic shielding on the side walls. The food product was heated
in a Delonghi oven for 7 minutes. The center was warm (57.degree.
C.) and the edge was partially dried.
[0096] FIG. 17 shows the corresponding thermograph at the end of
the heating stage.
Example 11
Heating of Lasagna in a Rectangular Tray with Metallic Side
Band
[0097] The same food product frozen lasagna product of 636 grams is
placed in rectangular tray having a long (longitudinal) axis of 19
cm, a short (transverse) axis of 14 cm, height of 4.2 cm, and with
metallic shielding on the side walls. The food product was heated
in a Delonghi oven for 7 minutes. The center was warm (47.degree.
C.). For this dimension, the shielding does not help to make the
product hotter in the center and more uniform across the
surface.
[0098] FIG. 18 shows the corresponding thermograph at the end of
the heating stage.
Example 12
Heating of Lasagna in a Rectangular Tray with No Metallic Side
Band
[0099] The same food product frozen lasagna product of 1220 grams
is placed in rectangular tray having a long (longitudinal) axis of
20 cm, a short (transverse) axis of 15 cm, height of 5 cm, and no
metallic shielding on the side walls. The food product was heated
in a Delonghi oven for 14 minutes. The center was still very cold
(-1.degree. C.) and the edge was partially dried.
[0100] FIG. 19 shows the corresponding thermograph at the end of
the heating stage.
Example 13
Heating of Lasagna in a Rectangular Tray with Metallic Side
Band
[0101] The same food product frozen lasagna product of 1220 grams
is placed in rectangular tray having a long (longitudinal) axis of
20 cm, a short (transverse) axis of 15 cm, height of 5.2 cm, and
with metallic shielding on the side walls. The food product was
heated in a Delonghi oven for 14 minutes. The center was warm
(40.degree. C.).
[0102] FIG. 20 shows the corresponding thermograph at the end of
the heating stage.
Example 14
Comparative Heating of Lasagna without and with a Shielding Side
Band in a 20 by 26 cm Tray
[0103] Comparative tests were conducted on lasagna heated in a 20
by 26 cm tray heated in a Panasonic oven (1100 W). Without
shielding, the average temperature is lower (75.degree. C.) but
lost 8% of its weight; while the shielded one only has 4% weight
loss yet with a higher temperature (82.degree. C.) (see attached
table). This illustrates that shielding has made a significant
difference in heating. The results are provided in table 1.
1TABLE 1 Heating Average Shape T A Depth Shield Y/N time (min.)
Weight (g) Loss % Temperature (.degree. C.) Oven Rec. 20 26 5.2 No
25 1658 8 75 Panasonic Rec. 20 26 5.2 Yes 25 1654 4 82
Panasonic
EXAMPLE 15
Theoretical Estimation of Modes in Heating of Frozen Meal in a
Circular shielded plate according to the invention
[0104] Without being bound by theory, it is estimated that the
invention is primarily based on a proper selection of the
transverse modal fields that propagate within the food as
determined by the rounded shape and the specific dimensions of the
resonating layer associated to the support plate.
[0105] The resonating band and the cavity of the microwave oven
make the meal in a rounded container a waveguide because standing
waves occur inside the food. The modes inside the food can be
estimated based on the theoretical analysis and the solutions of
the electromagnetic fields of modes can be obtained by Maxwell's
equations that are not given here for reasons of simplification but
which can be found in "Foundation of Microwave Engineering" (R. E.
Collin IEEE Press, 1991).
[0106] In the context of a circular resonating side layer, the
following equation can be held:
(2af.sub.nml).sup.2.epsilon..sub.r=(cx.sub.nm/.pi.).sup.2+(c.1/2).sup.2(2a-
/d).sup.2,
[0107] where a is the radius of the resonating layer; d is the
height of the layer; n, m, l are related to the wavelength of the
modes in three different directions respectively and are natural
numbers starting counting from 0 to differentiate the modes; in
particular, n and m are natural numbers related to the horizontal
direction modes and l is a natural number related to the vertical
direction modes;
[0108] c is the speed of light;
[0109] f.sub.nml is the frequency of microwaves, x.sub.nm is
P'.sub.nm for TE modes and p.sub.nm for TM modes; P'.sub.nm are
zeros for first Bessel function.
[0110] For frozen meals, the dielectric constants are usually
ranging from 3 to 5, therefore, the resonating mode inside can be
calculated based on the graphed equation of FIG. 24 for various
modes, as shown in the following table considering f.sub.nml is
2450 MHz for usual microwave ovens and height d is 3.5 cm:
2 Radius a (cm) (2af.sub.nml) 2a/d TE.sub.nml modes supported 4
15.4.10.sup.8 2.3 TE.sub.111, TE.sub.010, TM.sub.110 5 24.10.sup.8
2.8 TE.sub.111, TM.sub.010, TM.sub.110 6 34.5.10.sup.8 3.4
TE.sub.111, TE.sub.211, TM.sub.010, TM.sub.110, TM.sub.011
[0111] According to the solutions of these modes, TE modes usually
carry more electrical energy than TM modes, therefore the focus is
on TE modes. Electromagnetic fields at cross sections are plotted
in FIGS. 21 and 22. When radius is small, the only TE.sub.nm mode
that is supported is TE.sub.11 that gives a fairly concentrated
heating in the center area. When the radius increases, a new TE
mode starts to become dominating, e.g., TE.sub.21 mode (FIG. 22).
For even larger containers, there will be supposedly more modes
supported in the meal. Except TE.sub.12 and TE.sub.13 modes, most
of them have a central cold spot. However, as the number of the
"cold spot" TE modes usually increase, the influence of the non-
"cold spot" TE modes usually decreases accordingly.
[0112] It can be noted that the first two numbers n and m for the
T.sub.nml propagation modes really matter with respect to the
heating distribution in the food because it corresponds to the
largest transverse distribution in the horizontal direction. The
third number 1 describes the distribution in the vertical (or
z-axis) direction. Since it distributes along a short distance,
i.e., the thickness of the meal, there is no much change in this
direction except due to the dielectric loss factor.
Example 16
Theoretical Estimation of Modes in Heating of Frozen Meal in
Container Without Shielding Layer
[0113] When the meal is in a normal container heated in a microwave
oven, the meal can be seen as a dielectric resonator. At a
frequency of f=2450 MHz, the resonating mode can be found through
the following equations:
tan(.beta.d/2)=.alpha./.beta. where .alpha.=square root
[(p'.sub.nm/a).sup.2-k.sub.0.sup.2]
and .beta.=square root
[.epsilon..sub.r.k.sub.0.sup.2-(p'.sub.nm/a).sup.2] and,
k.sub.0=2 .pi.f/c
[0114] As an example, for a radius a of 5 cm and dielectric
constant, .epsilon..sub.r, of 4, the resonating mode is TE.sub.31
which has a cold center as shown in FIG. 23. As it is shown, for
containers without the resonating layer, the appearance of the
modes with a cold center spot occurs at a smaller size.
[0115] For simplification purpose, the calculations in the examples
have been made on a circular resonating configuration. However, it
has been found by experiments that an elongated rounded resonating
mode would perform at least as well as the circular mode and would
be supposedly based on a similar theoretical analysis. Relevant
literature references for more understanding of microwave
theoretical analysis can be found in [1] R. E. Collin, Foundations
of Microwave Engineering, IEEE Press, 1991, and [2] D. M. Pozar,
Microwave Engineering, Addison-Wesley, 1993.
[0116] As used herein, the term "rounded" refers to a curved line
configuration with no significant zone or portion of intersecting
lines that would form angles equal to or less than 90 degrees. The
term "oval" refers to any closed elongated convex curve having
preferably two axis of symmetry. The term oval includes an
elliptical curve but also a non-symmetrical or deformed curve or a
curve having a few straight or concave portions but having a
general elongated rounded shape. As an example, a bean-like shape
is included. The "axial distance" refers to the longest distance
that separates two points of the circumferential microwave
reflective surface at about the level of the food upper surface
when the food surface remains below the upper edge of the plate or
container. Alternatively, the "axial distance" refers to the
longest distance that separates two points at the upper edge of the
circumferential microwave reflective surface when the food surface
is such that it projects above the upper edge of the container. The
line that links the said two points enables to define a primary
axis. The food may project above the plate or container when the
container is shallow and/or have relatively low inclined sides (for
instance, a dinner plate).
[0117] Similarly, the "transverse" distance refers to the distance
that separates two other points of the circumferential microwave
reflective surface at about the level of the upper food surface, as
taken along a secondary axis intersecting the primary axis at its
centerline, when the food surface remains below the upper edge of
the plate or container. The "transverse" distance refers to the
distance that separates two other points at the upper edge of the
circumferential microwave reflective surface, as taken along a
secondary axis intersecting the primary axis at its centerline,
particularly when the food surface is such that it projects above
the upper edge of the container.
[0118] The axial and transverse dimensions may be equal in the
context of a regular shape such as a circular shape or contour.
[0119] Where the term "substantially" is used, that term is
generally defined to mean at least about 95% of the value referred
to, to preferably at least about 100% of the value referred to.
* * * * *