Note: Descriptions are shown in the official language in which they were submitted.
1339540
Methods and devices used in the microwave heating of foods
and other materials
This invention relates to methods and devices for
modifying microwave energy fields, having utility in the
microwave heating of bodies of material exemplified by
(but not limited to) foodstuffs.
It is well known that the conventional microwave
cooking or heating of a food load does not provide
effective browning or crispening of the food surfaces.
Those food products that have a surface composed of a
material different from that of the main portion of the
food article, such as a crust or a layer of batter or
breading, for example a pie or a breaded fish fillet,
require this separate surface layer to reach a higher
temperature than the bulk of the food, in order that
such surface layer be browned or crispened. For this
reason, a conventional convection oven set at a relatively
high temperature has been the traditional method of cooking
such food products.
There are also other types of food articles in which
the nature of the surface layer is essentially the same as
that of the main portion of the article, but it
nevertheless requires to be browned and/or crispened.
Examples in this category are the undersurface of a pizza,
13 39 a 40
-- 2 --
the two surfaces of a pancake, hash brown potatoes or
french fried potatoes.
The conventional method of trying to meet this need in
microwave cooking has been by means of devices known as
susceptors. A susceptor is a device that incorporates
lossy material, i.e. material that absorbs the microwave
energy to become heated. This device is then placed close
to the surface layer to be browned or crispened so that
the heat in the susceptor is transferred by conduction and
radiation to this surface layer. This process necessarily
requires the temperature of the susceptor to be higher
than that of the surface layer in order for the heat to
flow into such layer. It has been found that there are
practical disadvantages in heating the susceptor to the
necessary high temperatures. There is always the risk of
overheating and of breakdown of the material of the
susceptor, and even the generation of toxic products.
An example of one method of attempting to achieve this
type of surface heating effect has been suggested by W.A.
Brastad et al in United States Patent No. 4,230,924 issued
October 28, 1980. This proposal involves wrapping the
food product in a flexible sheet of dielectric material
that functions as a substrate to carry a thin metallic
coating that is subdivided into a number of individual
metal islands separated by non-metallic gaps formed by
- exposed strips of the dielectric material of the substrate.
When a food load wrapped in such a flexible sheet is
exposed to microwave energy in a microwave oven, some of
the energy passes through the sheet to heat the food load
dielectrically in the usual manner, while some of the
energy is converted in the metallic islands into thermal
energy, i.e. the islands act as susceptors, so that,
provided the islands are closely adjacent to a surface of
the food load, the heat generated in the islands will be
transferred directly by conduction to the food surface to
1339~40
elevate its temperature and thus achieve a browning or
crispening effect. The mentioned patent discloses that
the microwave-transparency of the wrapping can be varied
in order to adapt to the requirements of a particular food
article by modifying the ratio between the dielectric
(bulk) heating and the thermal heating generated in the
wrapping and transferred therefrom to the food surface.
In other words, the wrapping described in this prior
~- patent simultaneously acts as a microwave-transparent
covering for part of the energy, and as a susceptor, i.e.
a structure that absorbs microwave energy and hence
becomes heated, for the remainder of the applied energy.
Another known type of susceptor is that embedded in a
cooking utensil, such as a frying pan or baking dish. The
utensil can be placed in a microwave oven, initially with
or without food in it. The susceptor in the utensil
absorbs microwave energy, so that the cooking surface of
the utensil becomes heated to a high temperature. When
the food is added, or if it has been present from the
outset, its bulk is heated dielectrically in the usual
manner in a microwave oven and its surface is browned or
crispened by the cooking surface of the utensil.
In contrast to the concept of using a susceptor to
heat a surface layer of a food article, either directly by
a wrapping, or indirectly through a preheated dish, the
present invention provides an arrangement in which the
surface layer of the article to be heated as well as its
main portion beneath the surface layer continue to be
heated dielectrically, i.e. by the microwave energy,
without first converting such energy into heat in a
susceptor. According to the invention, the microwave
energy field is so altered that the dielectric heating
effect within the surface layer is enhanced relative to
the dielectric heating effect in the main portion of the
article. As a result, the surface layer reaches a higher
1339 ;~0
-- 4 --
-temperature. In the case of a food article, this non-
uniformity of 'neating results in browning and/or
~crispening of the surface layer.
The surface layer of the food article may be a top
layer (for example, a pie crust), or a bottom layer
tfor example, a pizza base), or both top and bottom layers
(for example, a breaded f ish fillet).
According to the invention the desired enhanced heating
effect within the surface layer is achieved by means of a
so-called mode-filtering structure that causes the micro-
wave energy to enter the absorber (foodstuff or other body
to be heated) in the form of cut-off propagation (herein
sometimes also referred to as "evanescent propagationn),
thus causing the heating effect to be concentrated at the
absorber surface adjacent the mode-filtering structure.
The term "mode-filtering" is employed to refer to
accentuation of the transmission of higher order modes
while reflecting fundamental modes.
~ icrowaves that are in cut-off are referred to as
propagating evanescently because they decay exponentially.
Due to this strong decay of evanescent microwaves, the
ratio of surface to bulk field intensities (or heating) is
increased. Analogously with the skin effect observed at
high frequencies in conductors, more energy is deposited
on the surface layer than in the bulk from the modes of
microwave energy that propagate through the apertures in
cut-off in the surface layer.
The device~employed for this purpose, in accordance
with the invention, consists of a sheet of microwave
transparent material provided with substantially non-
absorptive conductive material defining either an island-
aperture array or an array of annuli defining apertures,
the dimensions of the annuli or of the gaps between the
islands and the apertures being such as to achieve the
desired absorption profile.
The apertures will be of such dimensions that, at the
frequency of the microwave energy and with the sheet
located adjacent the surface layer of the article to
1339~0
be heated, the modes of energy that propagate through the
apertures will be in cut-off in the surface layer.
Such modes may or ~ay not be in cut-off in the main
portion (bulK) of the article lying beneath the surface
layer.
A device according to the invention can be located on
a separate sheet of microwave-transparent material, or it
can be embodied in a container for the article, e.g. as a
bottom wall or lid of such container. In the latter case,
small holes for venting steam and/or for draining liquids
such as fat can be provided in the container structure.
The present arrangement readily lends itself to the
provision of such venting and draining holes, whereas it
would be difficult to incorporate this feature into a
standard susceptor or the devices described by the
aforemented U.S. patent.
The term "container" as used herein embraces all
manner of elements or devices (including, but not limited
to, flat sheets, laminar members, pouches, pans, lidded
containers, etc.) that at least partially enclose,
contain, hold, support, or are supported by, the foodstuff
or other material during heating in a microwave oven.
The invention also relates to a method of enhancing
the heating of a surface layer of an article being heated
by microwave energy by utilizing the foregoing concept of
arranging the energy to be in cut-off in at least the
surface layer.
In those instances where the surface layer is formed
of the same substance as the main portion of the article,
the modes of micr~wave energy that propagate through the
apertures will be in cut off for both the surface layer
and the main portion. As a result, attenuation will be
higher per unit distance into the article, but there will
still be a greater heating effect in tne surface layer due
to evanescent propagation. The fact that the propagation
13 39~40
- 6 -
into the main portion is also evanescent will result in
less depth of heating in such main portion, but this
feature may well be acceptable in practice if the product
is a thin one, e.g. pizza, pancake, sliced potato etc.
It is known that the generation or enhancement of
~odes of the microwave energy of higher order than the
fundamental modes propagating in the article to be heated
can enhance the uniformity of heating of the article in
its lateral dimension. See for example U.S. patent No.
4,866,234 of R.M. Keefer issued September 12, 1989
(European patent application 86304880 filed June 24, 1986
and published December 30, 198~ under No. 206,811).
See also European applications of R.M. Reefer published
November 19, 1987 under No. 246041; published May 24, 1989
under No. 317203 and published June 22, 1988 under No.
271981.
It should be ex~lained that the term "mode" is used in
this specification and claims in its art-recognized sense,
as meaning one of several states of electromagnetic wave
oscillation that may be sustained in a given resonant
system at a fixed frequency, each such state or type of
vibration (i.e. each mode) being characterised by its own
particular electric and magnetic field configurations or
patterns. The fundamental modes, i.e. normally the 1-0
mode and the 0-1 mode in a rectangular system, of a body
of material to be heated, or of such body and a container
in which it is located, are c'naracterised by an electric
field pattern (power distribution) typically concentrated
around the edge (as viewed in a horizontal plane) of the
body of the substance to be heated, or around the periphery
of its container when the substance is enclosed by and
fills a container, these fundamental modes predominating
in a system that does not include any higher order mode
generating means. The fundamental modes are thus defined
by the geometry of the container and the contained body of
material to be heated, or alternatively by such body itself
133~540
-- 7
when it constitutes a separate article that is not placed
in a container.
A mode of a higher order than that of the fundamental
- modes is a mode for which the electric field pattern
(again, for convenience of description, considered as
viewed in a horizontal plane) corresponds to each of a
repeating series of areas smaller than that circumscribed
by the electric field pattern of the fundamental modes.
Each such electric field pattern may be visualized, with
some simplification but nevertheless usefully, as
corresponding to a closed loop in the horizontal plane.
The preferred embodiments of the present invention
combine the uniformity of heating of the load in the
lateral dimensions that can be achieved with the generation
of higher order modes, with the desired disuniformity of
heating in the direction perpendicular to the lateral
dimensions of the load, i.e. in the direction perpendicular
to the surface of the load, as is required for the surface
to be browned or crispened.
In the accompanying drawings:
Fig. 1 is a top plan view of an example of a mode-
filtering structure embodying the present invention in a
particular form;
Fig. 2 is a fragmentary sectional view taken along the
line 2-2 of Fig. l;
- Fig. 3 is a fragmentary top plan view, similar to Fig.
1, of another embodiment of mode-filtering structure of
the invention;
Fig. 4 is a fragmentary sectional view taken along the
line 4-4 of Fig. 3;
Fig. 5 is a perspective view of a microwave heating
container, for holding a body of foodstuff, incorporating
an embodiment of mode-filtering structure of the invention
generally similar to that of Fig. l;
Fig. 6 is a fragmentary elevational sectional view of
the same container, taken along the line 6-6 of Fig. 5;
- 8 - 1 3 3 9 5 q O
Fig. 7 is a view similar to Fig. 6 of a modified
container incorporating two of the mode-filtering
structures of the invention;
Figs. 8, 9 and 10 are fragmentary elevational
sectional views of the container lids and mode filters of
three additional embodiments of the invention;
Fig. 11 is a top plan view of a container of circular
plan, embodying the invention;
Fig. 12 is a top plan view of a further embodiment of
the mode-filtering structure of the invention, having
~tility, for example, with a container as shown in Fig. 5;
Fig. 13 is a perspective view of another container
incorporating an emboaiment of the invention;
Figs. 14 through 22 are fragmentary plan views of
further configurations of mode filters in accordance with
the invention;
Fig. 23 is a fragmentary perspective view of yet
anocher container em~odying the invention;
Fig. 24 is a fragmentary perspective view of another
embodiment of the invention;
Fig. 25 is a fragmentary elevational sectional view of
a curved microwave heating container lid incorporating a
mode filter in accordance with the invention;
Fig. 26 is a similar view oE a curved container lid
incorporating another mode filter in accordance with the
invention;
Figs. 27 and 28 are fragmentary sectional elevational
views of mode~filters in accordance with the invention,
formed in container bottoms;
Fig. 29 is a top plan view of the container tray of
still another em~odiment of the invention;
Fig. 30 is a sectional elevational view of the tray of
Fig. 29;
1~39540
Figs. 31 and 32 are fragmentary perspective views of
two additional embodiments of the mode-filtering
structures of the invention;
Fig. 33 is a view, similar to Figs. 14-22, of yet
another mode filter in accordance with the invention;
Figs. 34-36 are graphs illustrating the relationship
between absorption profile and cut-off of an induced mode;
Fig. 37 is a perspective view of a device according to
one embodiment of the invention, illustrating a manner of
using such device;
Figs. 38-41 are fragmentary plan views of alternative
embodiments;
Fig. 42 is an illustrative diagram;
Fig. 43 is a diagram explaining an experiment;
Fig. 44A and 44B are graphs showing the results of
this experiment; and
Figs. 45-48 show different arrays that were used in
Experiment 3 described below.
The embodiment of the invention illustrated in Figs.
1 and 2 is a mode-filtering structure including a flat
sheet 20 (shown as rectangular) of microwave-transparent
material such as a suitable plastic, which may, in one
illustrative example, be the flat top portion of a
microwave heating container lid. In this embodiment,
a single mode filter 22 is mounted on a flat surface of
- the sheet 20. Specifically, an electrically conductiveplate 24 (e.g., a plate of household-gauge aluminum foil,
or of so-called converter gauge foil, the thickness of
which would typically range between 6 and 7 microns) is
bonded to the sheet top surface, the outer dimensions of
this plate being about equal to the latter surface so
that the plate 24 extends substantially over the entire
sheet in a horizontal plane. This plate is formed
with a plurality of apertures 26 each with a
13395~0
- lC -
-
clased perlphery of generally rectangular confiquration;
a 5 x 4 array of t~enty apert~res is shown, with all a~
the apertures bein5 identical in size and spaced apart
from each other and from the auter periphery o~ the
plate ~y strip or ~ullion portions Z8 a~ the plate.
The ape~ ~~L e3 are e~uidistantly spaced in an arran~-
ment that is symmetrical with respect to the plate Z4
and shest 20.
The mode filter 22 also includes a plurality a~
electrically conductive islands 30, which in this par-
ticular ~mhqA iment are identical to each other in shape
and dl -~cions and are aqain conveniently fa~ricated af
hausehald-gauge all~in~ foil, bonded to the same sheet
surface as the plate 24. These islands 30 are equal i~
num~er to the apertures 26, and have closed generally
r2ctanqu1ar peripheries substanlially confor~ing in
shape to the aperture peripheries ~ut are smaller in
area than the apeL~u~3; the islands 30 are respectively
dispased within (and in register with) the apertures 26,
sa that the periphery of each island 3~ is substantially
unifor~ly spaced from the surrounding aperture periph-
ery, and defines therewith a rectang~lar annular gap 32
(which, in thLs embadinent, is of su~stantially uni~or~
width) closed or spanned ~y the microwave-trans3issive
dielectric material of the sheet 20. Thus a 5 x 4 array
of twenty spaced, unifor~ly and sy~metrically distri~ut-
ed rectan5ular annular gaps 32 is provided in the made-
filterin~ structure. It will be seen that these gaps
constitute essentially the only microwave-transmissive
arezs or windows in the entire structure, since the
s~.eet 20 is other~ise covered by the conductive plate
22.
In a specific dimensional exanple of the embodiment
of Figs. 1 and 2, each o~ the apertures 26 is a 2.20 x
1.8 cm rectangle, the strips Z8 between apertu.es (on
133~540
11 -- -
bcth the long and short sides of the apertures) being
a . s C3 in witth. Each of the conductive islands 3 0 is a
1.70 x 1.3 cm ractangle, and is centered in its assaci-
ated aperture so as ta def ine therewith a rectang~lar
annular gap 12 having a unifo~ width of ~.25 c~ on alL
sides. ThQ outer end columns o~ ~our apertures aree
spaced 1.0 c~ from the short side edges 24a of the plate
24, and the outer s~de rows of fi~e apeL~u~e5 are spaced
1.5 cm ~rc~ the long side edges 24b of the plate 24,
which has 2.0 cm-radiused corners.
~ single mode filter as exemplLfied ~y the a~ove-
desc~i~ed emkadiment of Figs. 1-2 may be used (by way o~
non-limiting example) with metallic, co~p~site, or
microwave-transparent containers, includins those de-
scr~bed in one or more c~ the aforementioned cop~n~in~
applications. In some inst~c~s, the use of metallic
cont~ s is pre~erred, because radiation entering the
cont~l n~r is then forced to inter7ct with the mode
filter. By contrast, when a mcde ~ilter is use~ with a
microwa~e-transparent container, it exerts little in-
fluenc on radiaticn entering the container through the
other s~rfacas not ad~acent to it.
To constituto a mcde filter as in the abcve-de-
scribed ~ho~i~ent cf the invention, an ar~ay of one or
more metallic islands ("island array") is superimposed
on an array af one or more correspandinq apertures
("ape-ture,array") o~ a metallic area or plate. Bcth the
island and aperture arrays may be constructed (as by
die-cutting) from aluminum foil, for example. When used
with a metallic container, the mode filter is pre~erably
pos ;ioned over the container, in electrical isolation
from it, as in the em~adl~ent of Figs. 5-6 described
below. ~owe~er, the made filter may als~ be in close
mechanical and electrical contact with the container,
or may be inte~ral with it (as in a pouc~ type o~ con-
1339540
struction, also described belcw, with reference to Fig.23). When the aperture array is fabricated from rigid
foil, a container or pan for holding the food to be
heated may be constructed ~rom the same foil, using
similar techniques to thase e~ployed in the manu~acture
of "wrinkle-wall" or "smacthwall'' pans.
Dielectric material is used to maintain the spatial
relationship between the island and aperture arrays.
Suitable dielectric material (typified by such plastics
as polypropylene, polyester, or polycar~onate resins)
shcws g~od resistanc2 to dielectric breakdown, has low
dielectric losses, and ~aintains its strength properties
at the service temperatures impased by the heating o~
the focd.
Each island is generally centered on each aper-
ture, but may be either coplanar with the aperture, cr
(as hereinafter further explained, for example with
reference to Figs. 3-4 and 8-10) displaced vertically,
s~ as to be approximately plane-parallel with the aper-
ture. When the island array is coplanar with the aper-
ture array, the area cf each island is constrained to be
less than that of the corresponding aperture. However,
when the island and aperture arrays are vertically
displaced tas, for example, in Figs. 3-4), the islands
may be of greater or lesser area than the corresponding
apertures.
One example of an arrangement for use of the mode-
~iltering structure of Figs. 1 and 2 in conjunction with
a microwave heating container is illustrated in Figs. 5
and 6, which show a microwave heating container having a
gene~ally rect~n~ular, upwardly opening tray 10, with a
bcttom 11 and side walls 12, fabricated of metal (e.g.
stiff, formed aluminum foil), for receiving and holding
a body of fo~dstuff 1~ to be heated. A molded plastic
(dielectric material) lid 16, transparent to mic.owave
1339510
- 13 -
energy and havin~ a downwardly ext~n~ing portion 18 and
a flat top or sheet pcrtion 20, ccvers the upward
opening of the tray, the downwardly extending portion
seatin~ on the tray rim. Typically, the upper surface
of the contained foodstuff is spaced below the top of
the lid.
The mote filter 22, as described with reference to
Figs. 1 and 2 (but here shown as having a 4 x 4 array of
apertures and islands) is mounted on the upwardly facing
flat surface of the lid top 20. Thus, the electrically
conductive plate 24 (e.g., of hcuseh~ld-gauge aluminum
foil) is bonded to the lid top surface, extP~ g sub-
stantially cver the entire lid in a hcrizontal plane,
though the plate is electrically isclated from the
metallic tray 10 ~y the downwardly eXt~n~i n~ portion 18
of the dielectric material lid. The islands 30 are
liXewise bonded to the lid top surface. The arrangement
of apertures and islands is sy~metrical with respect to
the rectangular lid top and thus with respe~t to the
container as viewed in a horizontal plane. Thus a 4 x 4
array ~f sixteen spaced, uniforcly and symmetrically
distri~uted rectangular annular gaps 32 is provided in
the lid top, constituting essentially the only micro-
wave-transmissive areas or windows in the entire lid
top, which is otherwise covered by the conductive plate
22.
Fig. 7 ,illustrates a madified for~ of container in
which the tray lOa, liXe the lid, is for~ed of micro-
wave-transparent material rather than metal, and in
which a second mode-filterins structure 122a (which may
be identical to, and in register with, the abave-de-
scribed mode filter 22) is mounted on the downwardly-
facing flat bottom surface of the microwave-transparent
tray lOa. Both the plate lZ~a and islands 130a of the
structure 122a may be constituted of household-gauge
13395~
- 14 -
al~min~l~ foil bonded to the tray battom surface. The
plate 12~a defines an array af apertures 126a, the
per~pheries of which in cooperation with the per~pher-
ies a~ the islands 13aa define an array of gaps 132a
e~ual in size and nu~ber to, and respectiYely in'regis-
ter with, the gaps 32 of the upper ~ode filter 22.
Also, in Fig. 7, the con~in~ bady of ~oadst~
14a is sel~-sustaining in shape and smaller than the
internal di~ensions of the tray, so that it is spac~
inwardly away frc~ the side walls of the tray. Sinca
the cantainer tray is microwave-transparent, the body
~4a acts as a dielectric resanator, in determini ng t~e
f~n~ tal mades a~ the systeu. Stated mare precisely,
in the system of Fig. 7 the overall resonant boundaries
are deter~ined both by the bcdy of foodstuff and by the
mode-filtering structures, but pawer absarpticn ~s
ccnfined to the focd crass-section. If the bady 14a
~illed the c~ntain~r out to the side walls of the tray,
sa that the tray side walls defined the gecmetry cf the
bcdy, still the ef~ect of the microwa~e-transparent tray
side walls in detP~ini~ the resonant boundaries would
he simply the conse~uence of the effect of the tray side
walls Ln defining the food bcdy seometry, in contrast to
the situation that obtains when the tray is electrically
ccnductive (microwave-reflective) and constitutes a
ca~ity resonator.
~ efer~ing now to Figs. 3, 4 and 8-10, vertical
dis~lac~ment of the island and aperture arrays may he
abtained by locating each array at opposite faces of a
dielectric sheet or, alte-nati~ely, by locating the
islands on dielectric protrusions (which may be filled
cr unfilled). Dielectric protrusions may be obtained in
the ther~oformin5 of plastic film, for example.
Figs. 3 and 4 show a rectangular flat plastic
micr~wave-transparent sheet 20 Oe the type desc~i~ed
- 15 - 13 39 ~i ~ 0
a~ave with re~erenc~ to Figs. l and 2 (e.g. the flat top
of a microwave heating cont~ r lid 16 as shawn in Fi~.
5), but bearin~ a mcde filter constituted af a conduc-
tive plata 34 (defining a 5 x 4 array of twenty rect-
angular apertures 36 separated by strips 38) maunted an
the dawnwardly facing horizontal planar majar surface
of the sheet 20, and a 5 x 4 array of twenty rectanqu-
lar conductive islands 40 maunted on the oppasite (i.e.
upwardly facing) horiz~ntal planar major sur~ace cf the
sheet 20 in register, respectively, with the apertures
36. In this mcde ~ilter, the apertures 36 and islands
40 are spaced apart vertically ~y the thicXness of the
sheet 20, and the rectangular annular ~ap 42 defined
between each island 40 and the periphery af its assaci-
ated aperture 36 is provided by virtue af the vertical
spacing, since (as shawn) the islands 40 are larger than
the apertures 36, though confor~ing thereto in peripher-
al configuration.
Alternatively, as Fig. 8 illustrates, the top 20a
of a plastic lid 16a (atherwise similar to lid 16 of
Fig. S) may be molded with a multiplicity of hollow
vertical protrusions 43 (one for each island 40) to
increase the vertical spacing between the islands 40
and the apertures 36 of the plates 34. Each protrusian
43 pro~ects upwardly from the upper horizontal surface
of the lid top 20a, and itself has a generally rectangu-
lar, horizontal flat top surface, on which is mounted
one of the islands 40. As in Figs. 3 and 4, the aper-
ture-defining plate 34 is mounted on the lower (dawn-
wardly facing~ hcrizontal surface of the lid top, and
the apertures i6 are d~sp~sed in reqister with the
protrusicns 43, bein~ thus a!so in register with the
islands 4a. Again as in Fig. 4, the gaps (here desig-
nated ~2a) ~etween the islands and the peripheries of
their associated apertures are prav~ded by the vertic~l
- 16 - 1339540
.
spacing between the islands 40 and plate 34, since the
islands 40 are larger than the apertures 36. In top
plan view, the structure of Fig. 8 (liXe that of Fig. 4)
is as shown in Fiq. 3.
~ igs. 9 and lO illustrate further embadiments a~
mcde ~ilters having vertical spacing between apertures
and islands. In the structures of these latter figures,
the conductive plate 54 defines an array of apertures
56 which are larger in area than the conductive islands
60, so that the arransement of apertures and islands, in
plan view, correspands to Fi~. 1. Each aperture and its
associated island define an annular gap 62, which sap
results both from the vertical spacing ~etween apertures
and islands and from the fact that the islands are
smaller in area than (though conforming in shape and
orientation to) the apertures.
~ ore particularly, Fig. 9 shows a plastic lid 16b
havin~ a top 20b for~ed with a multiplicity of solid
(rather than hollow) molded protrusions 63, i.e., one
for each island 60, projectin~ upwardly from its upper
horizontal surface, but otherwise similar to the above-
described lid 16. Each protrusion has a flat horizontal
top surfaca cn which one of the islands 60 is mounted.
The plate 54 is mounted on the main upper horizontal
surface of the lid top 20b, in such position that the
protrusions 63 respectively project upwardly through the
apertures 56.
Fig. 10 shows a plastic lid 16c with a top 20c
havinq a planar horizontal upper surface and a multi-
plicity of hollow protrusicns 6S (one for each island
60) p.ojectin~ dawnwa~dly frcm its lower s~rSace. Each
protrusion 65 has a flat, downwardly facing horizontal
lower surface on which is maunted one of the islands 60,
while the aperture-defining plate 54 is mounted on the
upper surface of the top 2~c with the apertu_es 56 in
- 17 - 1 3 3 9 5q o
.
re~ister with the protrusions 65. The latter protru-
sions may be so dLmensioned that, when the lid 16c is
placed on a tray 10, the islands 60 are substantially in
contact with the top surface of the contained body 14 o~
foadstuff.
It will be seen that in all the embodiments of
Figs. 3, 4 and 8-10, the aperture-defining conductive
plate (34 or 54) and the conductive islands (40 or 60)
are respectively disposed in parallel, but vertically
spaced, horizontal planes. In all of these embadiments,
and (except where otherwise n~ted) in those that follow,
bath the plate and the island or islands may convenient-
ly be ~abricated of aluminum foil and mounted on a lid
or other container wall of microwave-transparent dielec-
tric material such as one of the plastics mentioned
a~ove.
In the mcde ~ilters of the invention, the islands
and apertures may assume a number of geometries, among
~hich are the following:
(a) palygonal (including polyganal with rounded
apices3, e.g., triangular, rectangular, pentagonal,
hexagonal, etc. Fig. 14 shows a conductive plate 74
defining a hexagonal aperture 76 in which is dispased a
smaller hexaganal conductive island 80 providing a
hexagonal annular gap 82.
(b) round or elliptical (including epitrochoidal,
multifoil, ,and siuilar variants). Fig. 15 shows a
conductive plate 84 defining a circular aperture 86 in
which is disposed a smaller circular conductive island
9o, ccncentric with t~e periphery of aperture 86, such
that a circular annular gap 92 is defined t~erebetween:
Fig. 16 shcws a conductive plate 94 defining an array of
generally elliptical apertures 96 within each of which
is disposed a smaller but conf~rmal conductive island
100 to define, with the aperture periphery, a generally
1339~40
- 18 -
elliptical gap 102.
(c) confor~al (nat n~cPss~ily de~inable in ter~s
of simple geometric shapes), having a geometric resem-
blance to the shape of the food and/or container, and
being intPn~e~ to promote the propasation of higher
mcdes ~ithin the food. Fig. 13 shows a multi-co~part-
ment container 140 in which each co~rtment is treated
separately. The container has a series of metallic
walls (not shown) which for~ compart~ents directly under
reqians 150, 152, 154 and 156 in a lid 158. The lid is
made o~ a microwave dielectric material and is basically
transparent to microwave energy. Each compa,rtment has a
corrPcF~in~ top surface area in lid lsa and each top
surface area has an approxi~ately confornal plate o~
metallic foil. Such conformal plates are shown in Fig.
13 at 160, 162, 164 and 166. The area of each confor~al
plate is substantially equal to the lid top surface area
on which it is mounted. Each confor~al plate has a
confor~al csntral aperture (170, 172, 174, and 176,
respectively) within which is disposed a smaller but
conformal metal foil island (180, 182, 184, and 186,
respectively) definins, with the associated aperture
periphery, an annular gap (190, 192, 194, and 196,
respectively). Each gap is dimensioned so as to provide
the proper cocking energy and distribution to the food-
stuff located in the compart~ent in questian. For
example, ga~ 190 is large with respect to region 150.
an the other hand, the foodstuff in region 156 requires
a d~fferent distribution of heating and so gap 196 is
appropriately di~ensioned.
(d) nonconformal and/or nonun form in gap width.
In the abovedescribed e~bodiments, a substantially
unifor~ gap width has been shown. ~owever, the gap
width may be nonuniform, as illustrated for ex2mple in
Fig. 17, where a rectanqular aperture 26~ and a rectan-
1339~40
,, - 19 -
gular island 3~a dispcsed therein have different aspect
ratias so that the width of the short sides 32a' o~ the
rectangular gap 32a between t~eu is greater than the
width of the long sides 32a~ of the gap. Again, in Fig.
18, elliptical aperture 96a and elliptical island lO~a
pasitioned therewithin differ in configuration so that
the gap 102a between them is of variable width. The
geometry of the island may be nQnco~ormal to the aper-
ture periphery, as in Fis. 19, where a multilobed island
30b is dispased within a rectangular aperture Z6~ to
define a variable-width gap 32b; in Fig. 2a, where ~oth
the island 30c and the aperture 26c are nonconfor~al but
once more define a variable-width gap 32c; and in Fig.
21, where a trifoliate island 9Oa is disposed within
circular aperture 86a to define a variable-width gap
92a.
(e) with apertured islands, as shown in Fiq.:22,
where a ~ultilobed island 3Od is p~ositioned in a rectan-
gular aperture 26d to define therewith a gap 32d af
varia~le width (similar to the arrangement shown in Fig.
1~), but the island itself also has a central aperture
27d.
The ~arious configurations illustrated in Figs. 13-
22, as will be understood, are merel~ exemplary of the
di~erse arransements (with unifor~ or nonunifor~ gaps,
and geometrically conformal or nonconformal island-
aperture pairs) that may be employed in the structures
of the present invention. Also, while for convenience
only a single island-aperture pair is shown in most of
these figures, it will be understoad that an array
co~pris~..g a ~ultipl city of such pairs may be provided
ir. a complete mode-filtering structure, and that the
pairs of such an array may (as hereinafter further
discussed) be identical or nonidentical to each other
deFendent on the heatins eCfect desired and the particu-
1339540
-- ZO . -
lar conditions of use.
Far a focd article and~or ccntainer of rectangularcross-section, the island and aperture geometry wilL
typically or commonly also be rectangular. F~r an
article and/or container of cylindrical shape, the
preferred island and aperture geometry will typically zr
co~m~nly be based on a cylindrical coardinate system,
i.e., divided into ~calls" whase position is defined by
radial and angular (harmonic) nodes. A system of the
latter type is shawn in Fig. 11, which is a top plan
view of a cylindrical cont~;n~r havinq a plastic lid
16d with a planar circular tap surfaca on which are
mcunted a con~ctive plate 204 defining five or six
identical segment-shaped apeL~Les 206 distributed in a
radially symmetrical arrangement, and five or six con-
for~ally shaped but smaller conductive islands 210
respectively pcsitisned in register with the apertures
to define, with the aperture peripheries, five or six
annular gaps 212, together with a central circular
aperture 205 and island 207 defining a circular sap 209.
In the mode filters o~ the described embs~ nts of
the invention, the mini~u~ separation between apertures
is dictated by the heating distributicn desired in the
foad, by the mechanical ruggedness required by the
application, and the amcunt of ohmic heating occurring
in the metal defining the aperture array. In aperture
a.rays constructed from foil of household gauge and
havins r~ctangular apertures, the width of foil between
the apertures (e.g., the width of the strips 23 in Figs.
1-2) will tvpically be 5 mm or sreater.
~ eating distributions cver the plane of a mode
f.lter comprised of a multiplicity of islands and aper-
tures may be ~odified by varying differentially, over
the cross-section of the structure, the island and/o~
aperture si~e, and/or the vertical displacement of the
1339~40
- 21 -
islands in relation to the apertures. For example, in
the 3 x 3 array of apertures ~6 and associated islands
3a sho~n in Fig. 12, in a mode-~iltering structure
other~ise of the general type shown in Figs. 1-2 and 5-
6, the ~e.~ l one o~ these apertures 26e, to~ether with
its ass~ciated island 30e, is made larger than the
others, to control in a desired manner the heatins in
the central region of an adjacent ~ody of foodstuff.
The size of this central aperture and island correspand
to a fa~orable higher mode in the foadstuf~.
To confor~ better with the shape of food articles,
the overall shape o~ a mode filter may ~e curved or
corrugated, for example, rather than planar. Figs. 2S
and 26 show curved plastic container lids having mode
filters in accor~ance with the invention. In Fiq. 25,
the upper surface of the top 20e of lid 16e has a smooth
continucus c~nvex curvature. An aperture-defining
conductive plate 214 (generally similar to the plate 24
o~ Fi~. 1) is mounted thereon, with the conductive
islands 220 (generally sLmilar to islands 30 of Fig. 1)
also disposed on the sa~e li~ top surface. This curved
mode filter c~rresponds to an arrangement (as shown,
e.g., in Figs . 1-2 ) wherein the conductive plate and
islands lie in a co~mon plane, and is embraced within
the definition of a mcde filter having coplanar plate
and islands. In Fig. 26, the top 20f of the lid 16f has
an overall ~pwardly convex curvature and is formed with
a plurality of radially extending upward protrusions
223 each having an upper surface curved concentrically
with the overall top cur~ature. A conductive plate 224,
mcunted on the lower surface of the lid top 20f, defines
ar. ar.ay of apertures 226 respecti~ely in register with
the protrusions 223; a correspandinq array of conductive
islands 230 are mounted on the upper surfaces of the
protrusions. The curved mcde filter of Fiq. 22 thus
- 22 1339590 "'
.
.
correspands to arrangements (as shown, e.g., in Figs. 3-
4 and 8-lO) wherein the plate and islands are respec-
tively dispased in spaced parallel planes, and is em-
bracsd within the definition of parallel-plane plat~
arrangements.
Further in accordance with the invention, and as
discussed above with referenc~ to Fig. 7, a plurality
of mode filters may be provided at different walls or
surfaces of the same microwave heating cont~iner, e.g.
for si~ultaneaus treatment o~ multiple f~cd surfaces.
When used at the upper and lower sur~aces of a body o~
~o,odstuff in a container, the mcde-filtering structures
employed may be two distinct, electrically isclated mcde
filters, or two mode filters ha~ing aperture arrays
constructed from the same metallic sheet. When two
electrically isolated mcde filters are used, the remain-
der of the package or container is formed from dielec-
tric material, so that the overall package may be con-
sidered by the consumer a "composite" rather than "foil"
container. Electrically isolated mode filters m2y als~
be used at the upper and lower surfaces of a container
havin~ metallic sidewalls.
Fig. Z4 shows a microwave heating container embody-
in~ the invention and including both top and bottom mode
filters, between which is disposed the body of focdstuff
to be heated. This container may be of the familiar
"clamshell" type, viz. a typically thermoforned foamed
plastic package having an upper portion 231 and a lower
portion 233 joined by an integral hinge or folding
reg~on (not shcwn) formed along one side, and arranged
to close positively or latch (by suitable and e.g.
conventional means, also not shown, formed along their
edge portions), the walls of the package being somewhat
deformable. A mode-filtering structure of the type
shown in Figs. 1-2 (including aperture-defininq plate 2~
13~9~0
- 23 -
and islands 30) i ~aunted on the flat top of the pack-
age upper partion 231. Another similar made-filterin~
structure Z34, maunted on the flat b~ttom of the lower
partion 233, defines an array of apertures 236 (which,
in this embadiment, are identical in shape, size, an~
arrangement to, and are in register with, the apertures
26 of the top mode filter 22) and also includes a like
plurality of aluminum foil islands 240, each defining
with the periphery of its associated aperture a rectan-
gular annular gap 242. These gaps 242 are equal in size
and nu~ber to, and respectively in register with, ~he
gaps 3Z of the top made filter 22.
As an alternative, an electrically isolated mode
filter in or on a container lid may be used with a
metallic container tray which incorporates the aperture
array of a se~ond mode filter. As a further alterna-
tive, one mode filter ~ay be in close me~h~ica} and
electrical contact with a container incorporating an
aperture array af a second made filter in its base, cr
may be integral wit~ it (as in a pauch type of construc-
tisn). When twa ~ode filters having aperture arrays
ccnstructed from the same sheet are used, this sheet may
be folded in a U-shape to enclose the foad article to be
heated, as shown in frag~entary view in Fig. 23, which
illustrates a U-bent aluminum foil/plastic laminate
sheet 243 ha~ing a plurality of rectangular apertures
246 for~ed in the foil and a liXe plurality of smaller
but conformal foil is~ands 250 supported on the plastic
of the laminate within the apertures to define therewith
rectangular annular gaps 252 of uniform width. By
~irtue of the U-bend 253 of the sheet 243, first and
second arrays o~ the gap-defining apertures 246 and
islands 250 are respectively disposed on opposite sides
of a contained bady (not shown) of foodstuff, so that
these two arrays in effect constitute two ~ode filters
- 24 - 13 39 5 ~ o
acting at opposite surfaces of the foodstuff body. The
edges of the sheet 243 may be suitably sealed together to
form a pouch within which the body of foodstuff to be
heated is enclosed.
Because microwave oven heating characteristics tend to
be uneven in the vertical direction (owing to coupling
effects caused by the presence of a glass tray or ceramic
oven floor), different upper and lower mode-filter designs
may be incorporated in the same container, i.e. to
compensate for such vertical unevenness of heating
characteristics. Compensation may be obtained by variation
of relative island and aperture areas and/or of the
vertical displacements of the island and aperture arrays.
While the foregoing embodiments have been chosen as
illustrative of constructions which may be used with two
mode filters, mode-filtering structures may also be used
three-dimensionally, with mode filters located at all of
the surfaces of a food.
The nature and principles of operation of the mode-
filter containers of the invention may now be explained.
In the described mode filter structures, as in the higher-
order-mode-generating means of the aforementioned U.S.
patent No. 4,866,234, the boundaries of each island or
aperture define a set of modes with corresponding cross-
sections. However, while an island array permits the
entry of "lower" (or more fundamental) modes through strip-
lines and slot-lines defined by the combination of islands,
the entry of t~e "lower" modes is impeded by an aperture
array. The aperture array may thus be perceived as
analogous to a series of waveguide (or cavity) openings,
each of which would effectively cut off or attenuate the
lower modes.
It is useful to explain features of operation or
function of a mode filter as herein contemplated, by
-25- 13~9540
analogy to a complete container with the boundaries of each mode filter aperture
considered analogous to metallic container walls, and each associated mode filter
island considered analogous to a higher-mode-generating metallic plate. In a mode
filter wherein the islands are vertically displaced over the aperture array (relative to a
5 body of food to be heated), the mode filter islands "feed" the aperture array so as to
increase the amount of power (at the corresponding higher mode) available to the food
over that which would enter the unmodified aperture array. It should be noted that in
the absence of an island array, the use of apertures is regarded in the art as providing
moderation (or reducing the amount of power available), such that an aperture array
10 with small openings would be regarded essentially as a "shield".
A distinction exists between the vertical displacements (between the island
and aperture arrays) which are possible with the mode filters of the present invention,
over those obtaining between a higher-order-mode generating plate and metallic
container walls of the containers of the last-mentioned patent. In the latter containers,
15 there will seldom be occasion for the metallic plates to be positioned below the plane
determined by the rim of the metallic container, and it would be clearly impractical
for the plate to be immersed beneath the surface of the food. Contrastingly, since the
ap~l lw e array of a mode filter as herein contemplated can be separated by an air gap
from the food, the lower bound of island array vertical displacement relative to the
20 aperture array is determined by the food surface.
1339~5~0
- 26 -
Nevertheless, pursuing the aforementioned analogy, it
is apparent that by differentially varying relative island
and aperture size and/or island vertical displacement,
food heating distributions may be varied over the mode
filter cross-section.
Control of vertical heating gradients stems from the
following considerations:
(a) Absorption/attenuation becomes particularly
pronounced when the induced mode is cut off
(i.e. when the condition of evanescent
propagation obtains), this being the principal
feature of the present invention.
(b) When a food contains layers with distinct
dielectric properties, control of the mode
structure can give rise to free propagation in a
layer with a relatively high dielectric constant
and cut off (steep attenuation/absor~tion) in a
low dielectric constant layer.
(c) Thus, when a layer Of pastry or batter/breading
with a relatively low dielectric constant
overlies a food of higher dielectric constant,
intense heating may be selectively obtained in
the surEace layer. Thus, mode-filtering is a
valuable tool in promoting browning or crispening
effects, while minimizing undesirable overcoo~ing
of a good bulk.
(d) When the mode structure is fixed, as by metal wall
boundary conditions, in the horizontal plane, the
absorption coefficient of the food is determined
by the mode cross-sectional dimensions.
te) That is, for fixed food dielectric constant, and
conductivity and dielectric losses, the absorption
coefficient in the vertical axis increases with
decreasing mode dimensions.
(f) By determining the mode dimensions, control is
thus exerted over vertical heating gradients.
133~540
- Z7 -
Figs. 34-36 illustrate graphically the vertical
absorption profile of nicrowave energy, in a bady a~
~oadstuff adjacent a made filter gap for conditicns
ranging from abave cut off (Fig. 34) to below cut of~
(Fig. 36), where the bady is sufficiently thic~ and/or
absarptive so that the effects of reflection and/or
propagation at oppasite surfaces of the bady can be
ignored for purpases of the present analysis. Stated in
general, it is the dimensi~n of the individual mcde
~ilter gap (or open gap segment, in the bridged-gap
structures described below with reference to Fig. 36~
that determines whether a made is in cut off for a food
bady o~ a particular dielectric constant, at a given
wa~elen~th (typic~lly 2.45 G~z) of microwave energy.
Figs. 34-36 may be explained by reference to a body
of foadstuff pasitioned ad3acent a planar harizontal
gap- or aperture-defining electrically conductive
plate, wherein the z axis is the axis of propagation
into the food bady (i.e., z=0 is the surface of the bady
adjacent the plate, and the distance along z in these
graphs is penetration depth vertically into the bady).
In the graphs, ¦ E¦ 2 ( the squared magnitude of the elec-
tric ~ield intensities, in the vectorial sansa) is
plotted against z, t~e intercept of the curves with the
¦E¦2 axis b(,eing the squared magnitude,of a reference cr
surfacs intensity. Owinq to the dependence of pawer
abscrption (heating intensity) on ¦E¦t, the curves in
these graphs indicate the steepness of a~sorp-
tion/at~enuation (in the vertical direction into the
focd) for the various conditions represented, showing
that the steepness of the absorption/attenuation profile
is much greater below cut of~ than above cut off.
- 28 - 13 39 ~ 40
.. -, ..
¦E¦ Z is propartLonal to e ~Z, where ~ is de~lned
~y t~e relation
a ~ ~ 2~ k2) ~ ~ t (~2~ _ kZ) 2 ~, 2 Z 2
and
2~ being frequency;
magnetic per~eability (typically,
~r ~ 1 for nonmagnetic ~aterials);
4~ x 10 7 joule (s~c)2/tc~ulomb)2-meter;
'r'o ~ dielectric constant, ~r bein~ the
relati~e dielectric co~ ant and ~c ~eing
the free-space (elec~ric) per~it~ivity,
cO - 8.8541878 x 10-12 (coulo~b)2j~aule-meter;
~ - conductiYity.
E~ propagatian is gcverned by an ~Yp~n~ntial law
whcse argument includes bath the real component (+/-)
and a complex c~mpanent (+/-) ~, de~ined by the relation
~2 ~ 2~ 2 ) ~ ~ 2J~ _ JC2 ) 2 + ~2~2
which gives rise to a nearly periodic variation
energy absarption wit~ depth Or penetration. The cur~es
shawn in the graphs ignare ~. For foads which are thick
- or highly absorpti~e, ~ may be neglected. Thus, the
cur~es should be understoad as s~cothed representations
af the actual values involved.
It wlll ~e noted that the expressions for ~2 and ~2
allow 2 and ~ to assume pcsitive or ne~atlve signs. The
chaice af a ne~ative sign of ~ in the propcrtionality
deter~ining ¦E¦ reflects the absorption of energy and
concomitant decrease of ¦E¦ with penetration. The
existence of a companent with a positive 3 ~er~ indi-
- 29 - 1339S40
cates either ~eflectlan at, ar the entry of power from,
an opposing surfaca. The ~uasi-periodic variation due
to +~ follows from Euler's formula
z
- e~ = cos ~z ~ ~ sin ~z
where j = (~ . Also to be noted is the relationship
of ~ a~d ~ to the combined propagation constant p in the
vertical axis, viz. p = 3 ~
- The f~c ~yaca value ~f 'r is unity. In foads, ~r
is largely deter~ined by food moisture content or water
actlvity, so that the value of 'r for high-moisture
foods will nearly approach that of water, for which (at
a frequency of 2.45 GHz) ~r varles from abaut 80 at O C
to abaut 55 at lOO C, the value for ice being approxi-
mately 4. For low-density, low-moisture-content foad
components s~ch as partially precooked batters or coat-
ings, the value ~f ~r is a~out 5, varying by equilibra-
tion of their m~isture contents with adjacent high-
water-activity foads.
- For any given mode of propaqation of microwave
energy, the condition for cut off is that k2 (defined
as the separation constant for the equations gaverning
the comronents directed in the plane of the plate) be
equal to or greater than ~2~c. From the well-known
relatians c = (~c~) ~ and c = ~Of, where c is the speed
of light and~O is the free-space wavelength, it follows
that ~2~ = 4~Zcr/~O2 when ~ = 1. Thus, the condition
for cut off (k2 2 ~2~c) can be expressed as k2
4~2/~Zeff~ where the effective wavelength ~eff = ~J'r~
For f = 2.45 GXz, ~O = 12.24 c~. When cut off occurs,
the magnitude of the ter~ Q (governing the penetration
- of m~crowave energy into the food) increases substan-
tially.
1~39540
- 30 -
The valuQ of k2 is dep~A~nt on the geometry and
di~ensians of the gap or apertur~ as well as on the mode
under consideration. In the simple case of a rectangu-
lar aperture havins horizontal dimensions ~x~ Ly~ for
the t~,n~ mode,
~2 = ~2~(m2/ ~2) ~ (n2/ ~ 2
and the condition for cut off is
~ ~ tm /I~C ) + (n2/r,y2) 1 ~ 4~r2/~2e~-
Thus, for example, for the ~0,1] or ~1,01 mode, the
condition for cut off is that the relevant dimension
(i.e., ~ or ~) be e~ual to or less than ~eff/2~
Again, in the case of a circular geometry, in which for
the tm~nl uade
~2 = j2m n/ro2
where jm n is the nth zero of the mth order Bessel
function and rO is the radius of the aperture opening,
the condition for c~tof~ in the ~0,1~ mcde is that rO be
e~ual to-or less than 0.3827~eff. In summary, critical
dimensions for cut off for exemplary aperture ~e~metries
znd modes are as follows:
Geometry , Mode Critical Dimension
Square (L= ~=Ly) ~0,1], tl,O] L-= ~eff/2
.. [1,1] L ~eff/2
Rectansular (~=~ ~) [1,1] ~ = ~eff(l+q2)~/2q
Circular [0,1] rO = 0 3827~eff
.- [ 1,1] rO = ~ ~ 6098~eff
The ter~ k2 is easily deter~ined in rectan~ular and
circular systems (tables of the zeros of Bessel func-
- 31 - 1339~40
....
tions are given in G.N. Watson, A Treatise on the TheorY
of Bessel Functions, Cam~ridge Univ. Press, 1~22). The
analysis is more complex for other gap/aperture geome-
tries (e.g. ellipses), but the general yru~aition
holds that cut of~ of a given mode occurs when the
relevant horizontal dimension of the gap or aperture is
e~ual to or less than a value deter~ined by the
gap/aperture geometry, the mode in question, and the
dielectric constant of the food body or body portion o~
concern. Thus, where the o~jective is to achieve brcwn-
ing and crispening by provision ~f a condition below cut
off, with resultant steepness of food heating profile,
it is feasible to do so by appropriate dimensioning o~
the mode filter gaps or apertures, viz. by keeping such
dimensioning below the ~ r value for cut off.
Applying these considerations to the design of mode
filters to achieve browning and crispening, it is ob-
se~ved that open, wide apertures are ineffective for
this purpose, since large square or circular apertures
will have low field intensities across them and should
therefore fail to produce desired browning and crispen-
ing. By introducin5 islands in such apertures, higher
har~onics result, and more intense fields may be expect-
ed across the narrowed gaps.
Evidently, narrow slots give the desired heatin~
effects. Slot length will be < ~eff/2~ For segmented
slots the désired seqment length will approximate to
~ef~/2~ Curved slot length should likewise approximate
to ~efc~2 In the case of a round aperture with a small
q2p hetween island and plate, the circumference should
be nearly (or less than) ~eff. Larger gaps will allow
resonance in the radial dimension. Similarly, it is ex-
pected that the line inteqral of gap lenqth for narrow,
uninterrupted rounded shapes will approach or be less
th~n ~e~
1339S40
- 32 -
In general, the width of a gap (in the island-
aperture pairs or arrays) should be at least about 1 mm,
to avoid excessively high field intensities. The length
of the gap wil~ usually be at least abcut 5 mm.
Stated with reference both to embo~iments of the
invention employing island-aperture (gap) pairs or
arrays and to those employing annuli (further described
below), the following general principles governing
critical (cut-off) dimensions may ~e set forth:
~ imensions of narrow gaps ar annuli: for gaps or
annuli defined by a smooth curve lacking prono~lnced
cusps or apicec and encircling a closed area, cl~t off of
the lawest order mode will occur when the line integral
of the curve is less than one effective wavelength
(~ef~) If the ends of a smooth, open curve are not
closely spaced, cut-~ff will correspond to a curve line
inte~ral of ~e~/2~ A closed curve with apices or cusps
will be expected to have cut-off dimensions correspond-
ing both ~o its circumference (one ef~fective wavelength)
and to its segments (each being ~eff/2)~ For an odd
num~er of equal segments, however, destructive interfer-
ence may cause cancellation of the modes correspondin~
to them. An open cur~e with cusps or ~pices will simi-
larly have its entire length as one cut-off dimension
(~eff/2)/ and may also support higher order modes with
cut-off dimensions defined as the distance bounded by
t-~o such apices or cusps (each segment being ~eff/2)~
Wide gaps or annuli: As with their narrow counter-
parts, wide gaps or annuli will support resonances over
their lengths. However, they will also allow two-dimen-
sional resonances, generally characterized by larger
cut-off dimensions. ~hus, in decreasing critical dimen-
sions, cut-off will first occur for two-dimensional
resonances, and be followed by cut-off in resonances
determined by gap or annul~- lensths. By selecting
_ 33 _ 1339S~O
dimensions which suppart rPs~n~nCPs deter~ined by gap or
annular lengths, while providing cut-of~ of two-dimen-
sional resonances, heatins cf the bul~ of the absar~er
may be balanced against heating of its surfaces.
In przctice, since at and adjacent the foot sur~aco
some af the field will be in air, the wavelength will
have a slightly greater value than ~eff as defined
abave. In deter~ini~ cut-off dimensians for island-
aperture ga~s or annuli, a lcwer ~ound is provided ~y
the ~ul~ wavelength, taXen as the free-space wavelength
~0, ~Lvided by the square roat of the absor~er relati~e
dielectric constant, here denoted as ~r(m) If these
structures were embedded well within the absorber bulk,
this lower bound would accurately determine cut-off
dimensions for the gaps or annuli. However, the coexistence
of fields within the air surrounding an absorber causes
wavelengths used in determining cut-off dimensions at the
absor~er surface ~the locus of interest fQr browning and
crispenins effects) to be su~stantially larger than
~J 'r(m) -
A useful approximation for deter~inin~ the effec-
tive dielectric constant 'eff at the surface of a di-
electric material is that suggestPd by S.B. Cohn, IEEE
Trans. Microwave Theor~ and Techni~ues, MTT 17(10), 76~
(1969), viz. the arithmetic average of the relative
dielec~ric constant ~r(m) of the dielectric material and
the relative dielectric constant ~r(s) cf free space
overlvins its su--ace. Since the dielectric constant of
free s~ace assumes a value of cO, the relative dielec-
tric constant 'r(s) ~ust be unity. ~hus, the approximat-
ed effect ~e wavelength ~eff for purpcses of determining
cut-off dimensions for gaps or annuli at the surface of
a focd or other load to ~e heated Is given by
~eff AJ~ r(l~) )/2] ~
r 1 3 3 !~ ~ i O
- 34 -
Referring, then, by way of exa~ple, to the d~mensions
given above for the structure shown in Fig. 1 (a 5 x 4
array of rectangular apertures each 2.2 x 1.9 c~ and
.enclosing an island 1.8 x 1.4 cm, such that the maximum
gap width is 2.5 mm), the "narrow gap or annulus" con-
siderations set forth above establish that the critical
ncions for cut cff are a peri~eter, or sum of gap
length and width, equal to ~ef~ or either length or
width e~ual to ~eff/2~ ~ith the ~arious gap lengths
based on aperture (rather than island) dimensions, we
obtain:
~eff (1 ~ ~r(~))/2 ~r(m)
(c~) cut-of~ _ lower bound at cut-aff
3.8 10.4 19.8
4.1 8.9 16.8
4.4 7.7 14.5
8.2 2.2 3.5
For a focd product ha~ing a batter or breaded coating
with a dielectric constant of less than about 14, propa-
gation in the coatin5 will be in cut-off for all but the
hypathetical mode correspanding to the perimeter dimen-
sion of the gap; propagation will not be in cut off in
the underlying food bul~, however, because of its sub-
stantially greater dielectric constant.
For circu~ar gaps or annuli, the critical dimen-
sions are a diameter equal to ~eff/~
The control of horizontal plane heating distribu-
tions ar.d of heating gradients in the '~ve-ticall~ axis is
inc~eased, w~.en the entry cf radiation through ather
food surfaces is suppressed. This suppression may be
obtained by the selection of overall mode-filter dimen-
sions and separation, by the shape or contour of the
mode-filter edges (as by introducing well known "choke"
structures), and/or by the introduction o~ netal walls
133g54~
- 35 -
(which may be integral with the mode filter(s), and which
may also incorporate mode filters).
Operation of the mode filters of this invention is
typified by the following:
Considering first a single-mode filter array, i.e. at
a single surface, modification of heating distributions in
the horizontal plane of a container and/or food is
generally obtained by positioning the island array over
the aperture array, and by positioning the resulting
structure over a metallic or composite container. The
design principles used to obtain a particular heating
pattern are similar to those used in the containers
described in the aforementioned U.S. patent No. 4,866,234,
except that there is less need (in the present invention)
to compensate for the entry of "lower" modes.
When used with the crispening containers described in
the aforementioned published European application under
No. 246,041, the island array of a mode filter in
accordance with the present invention may be vertically
displaced above or below the aperture array. In tllis
configuration, crispening may be obtained simultaneously
at both the upper and lower surfaces of the food.
A particularly efficacious configuration is that in which
the islands are in contact with the food, but are displaced
beneath the aperture array. When used for browning or
crispening, a mode-filter will generally use island and
aperture dimensions on the order of less than 2 cm on a
side.
Examples of the just discussed embodiments of the
invention, provided on the floors or bottoms of microwave
heating container trays in association with stepped
structures or protrusions formed therein, are shown in
Figs. 27 and 28. In Fig. 27, a dielectric-material
(e.g., molded plastic) container ~ottom 301 is formed
1339~0
- 36 -
with one or mcre upward protrusians 3~3 having a planar
upper horizontal surface spaced above the horizontal
upper surface of the bottom. An aperture-defining
conducti~e plate 304 is mounted on the latter surface,
defining at least one aperture 306, throuqh which pro-
trusicn 303 pro~ects; a conductive island 310 is mounted
on the upper surface of the protrusion, to define (with
the aperture) an annular gap of uniforu width, thus
constituting a mode filter in accordance with the
present invention. In Fi~. 23, a dielectric-material
container bottom 311 is for~ed with one or more downward
protrusions 313 having a planar lower horizontal sur~ace
spaced below the horizontal lower surface of the bottom;
aperture-defining conductive plate 314 is mounted on the
latter surface, defining an aperture 316-through which
protrusion 313 projects downwardly, while a conductive
island 320 is mounted on the lower surface of the pro-
trusion 313, again so as to define with the aperture
periphery a uniform-width annular gap. Each of these
mcde filters of Figs. 27 and 28, as will be understood,
may include an array of apertures, islands and protru-
sions, only one beinq shown in each case for simplicity
of illustration.
~ ode filter structures arranged for simultaneous
treat~ent of multiple surfaces are of considerable
interest for the browning or crispening of battered and
breaded foods such as fish sticks, fried chicken, etc.
In these arrangements, similarly to the containers of
Fi as. 7, 23 and 24, one or more food articles are placed
between two mode-filtering structures. The island and
aperture dimensions are chosen so as to intensify heat-
ing at the f5ad coating. Two types of browning or
crispeninq may be obtained:
(A) "Uniform": The island arrays are close to, or
in cont~ct with the food surfaces, and the aperture
1339~40
arrays are either coplanar, or displaced "~ertically"
away from the food. This configuration may be used to
give nearly uniform, intensified heating of the surfac-
es. When (as shown in Figs. 29 and 30) the dielectric
b~ttom 331 supporting the aperture-defining plate 334
and islands 340 is formed with inwardly projecting
protn1sions 333, the resulting channels 335 between the
islands improve venting and drainage from the fo~d
during its microwave heatins.
(B) "Grilling": The aperture arrays are relati~e-
ly close to the food surfaces. The pattern of brown-
ing/crispening which results roughly corresponds to the
metal areas of the aperture array. When the mode-filter
islands are carried on outwardly projecting protrusions,
the wells so formed improve venting, and allow for the
collection of drainage from the food.
These and related configurations offer many advan-
tages over sa-called "susceptor" packages:
ta) Because heating is induced in the food rather
than in the packaqe itself, lower temperature
materials may be used. A benefit of l~wer
temperatures is the re~uction of pyrolytic by-
product generation.
(b) The control of heating distributions offered
over the horizontal plane of the ncde-filter-
ing s.ructures allows more uniform heating
and~or browning and crispening ef.ects to be
obtained.
(c) The relatively hiSh impedance presented by the
structures provides mcre even distribution of
heating over multiple food items. By varia-
tion of mode-filter desisn, selective heating
may also be provided. ~lso, the attainment of
desired results is less dependent on the
particular oven used.
13395~o
- 38 -
.,.
(d) Heating and/or browning and crispening effects
can be "balanced" between the upper and lower
fo~d surfaces, by variation of mode-filter
design.
(e) The ~verall shape of the mode-filtering struc-
tures can ~e modified to ~etter conform with
the product to be treated.
(~ Drainage and venting can be accommodated as an
integral feature of mcde-filter design. When
suppcr~ing protrusions are generated by ther-
moformLng, a mcre flexible pac~age results,
- which is better able to confor~ to surface irreqularities of the f~od.
(g) The heat-resisting and hea~-distributing prop-
erties of the metal surfaces which may be used
to contact the food minimize damage to the
package resulting from localized "hot" re-
gions, and reduce the hazard of contamination
of the focd by products generated through the
heating o~ the container.
(h) Because the mode-filtering structures can ~e
supported on a plastic dielectric, a variety
of c~ntainer or packase shapes can be cffered,
owing to the versatility of plastic for~-
ins/fabricating methods.
The mode filters described above are a subset of a
--- much broader se~, wi~ich is conceptually lin~ed also to
the conductive indented structures of the last-mentioned
European application. This much broader set may be
characterized in the following features:
(1) A conductive area is made electrically distinct
from surrour.ding or adjacent conductive areas. Modes
corresponding to this distinct area are induced in a
proximate foodstuff (or absorbing material). Modes not
133~S~O
- 39 -
necessarily the same as those induced by this area but
corresponding to the surrounding or adjacent conductive
area are also induced in the foodstuff. All of these
modes will be higher modes than those fundamental to the
combination of the foodstuff and container. They may
therefore be used to modify heating distributions within
the food and/or to induce browning or crispening effects.
(2) The conductive area may be made distinct
(following (1)) in several ways, which may be used singly
or in combination, and which include the following:
(a) The conductive area may be separated from the
surrounding or adjacent areas by an air-gap or
dielectric-filled gap.
(b) The area may be conductively connected to the
surrounding or adjacent areas, being raised or
lowered in relation to the plane defined by
these surrounding or adjacent areas, but with
the vertical separation providing some measure
of electrical distinctness (i.e. vertical phase
relationship). This connection may be at one
or more sides of a polygonal area, so that when
all the sides are connected, the structures
described in Canadian patent no. 1,279,902
issued February 5, 1991 result.
(c) Electrical distinctness of such a conductively
connected area may also be obtained by
establishing an impedance (in a
stripline/slotline sense) different in the
areas from that of the connecting means, as by
varying the width of this connecting means.
Different impedances may also be obtained
through proximity of the area or connecting
means to food or another dielectric substance.
1339~0
- 40 -
(3) One or a plurality of such combinations of a
conductive area and surrounding or adjacent conductive
areas may be used as described above. These combinations
need not be of similar design, but may be varied in size
or in the choice and/or dimensions of the separating gap
or conductively connecting means.
In an illustrative example, mode filters were
prepared from foil sheets which effectively incorporated
a mode filter as herein contemplated in a structure as
described in the aforementioned Canadian patent serial
no. 1,279,902. These mode filters were intended for the
crispening of breaded and coated fish fillets. The foil
areas contacting the fillets were of the same size and
disposed in the same positions as the "islands" of mode
filters previously used for the same purpose. However,
the areas of contact were electrically integral with the
foil sheets, such that two opposite sides of these
rectangular areas were connected with the sheet, and
upwardly displaced from it (i.e., towards the fillets).
The other opposite sides were not connected, so that air
gaps or slots existed at these sides. Crispening of the
fish fillets was fully comparable to that obtained with
the "island" constructions. While crispening can also be
obtained, when the contacting areas are, in effect,
folded tabs (joined to the foil sheets), caution must be
exercised in the design of these structures to prevent
arcing or localized scorching of surfaces of the food
article. When a food is placed between two of the
structures, the slots of the structures need not be in
register.
Following from the number of island/aperture shapes
possible, it is apparent that there exists an even larger
number of combinations for which one or more sides of the
polygonal "islands" are connected to the "aperture
array." It should be mentioned that these
1339~S40
- 41 -
,. .
structures may also be viewed as patterns af slots, such
that the slots define tabs or other shapes, and may even
define structures resembling slot/strip meander lines.
Sincs a single slot prc~ s a field ~Yir~lm at its
middle (and thus, localized heating in the same region),
it is desirable that the slots be configured so as
either t~ give a desired pattern of heating, or even
heating. While the structures defined by the slots may
have apices or be angular in nature, rounded or convq-
luted shapes may also be used.
Further examples of structures in accordance with
the in~ention, e~odying some of the features just
discussed, are shown in Figs. 31-33. In Fig. 31, a
metallic plate 3S0 is far~ed with a plurality of spaced-
apart rectangular projections 352 each having a flat top
354 lyin~ in a plane spaced from and parallel to the
major surfaces of the plate. ~pposed side walls 356 of
each pr~jection 3S2, integral with the projection and
plate, connect the top 354 to the plate on two sides.
On the other two sides of the top 354 there is an apen
gap pcrtion 358 between the top and the plate. In this
structure, each conducti~e "island" is the flat top 354
of a projection, its periphery consisting of bends 360
and gap top edges 362. Each "aperture" has a periphery
defined by bends 364 and gap battom edges 366. Walls
356 constitute conductive bridges spanning the gap
between aperture and island. The open gap portions 358
provide dielectric isolation between aperture and island
while the vertical displacement between top 354 and
plate 350 due to phasing or elect-ical distance effects.
The st-ucture of Fig. 31 is formed from a single
sheet af metal ~y slitting and drawing to form the
projections 352. In the modified structure of Fig. 32,
als~ formed from a single metallic plate 350a, the plate
partions 368 intermediate adjacent projections 35'~ are
133~40
- 42 -
bent out af the plate major surface planes to an extent
equal and opposite to the bending of the projections, sa
that drawing of the metal is not required.
Fig. 33 illustrates a planar mode filtering struc-
ture in which a metallic (conductive) plate 370 defining
a rectanqular aperture 312, and a metallic (conductive)
island 374 of rectangular configuration, smaller than
and disposed within the aperture, are connected by
conductive bridge portions 376 spanning the gap 378
defined between t~e island and aperture peripheries.
The plate, island and brid~es are formed integrally from
a single metal sheet (e.g. an aluminum foil sheet of
suita~le gauge) by cutting out from the sheet opposed C-
shaped portions 380 of the gap 378. These portions ~80
are open (microwave-transparent) portions or segments of
the gap. A mode filter thus constituted provides ef-
fects comparable to those of mode filters in which there
is complete isolation between island and aperture pe-
riphery, as in the structures of Figs. 1-30 descri~ed
above. A mode-filtering structure in accordance with
the invention may ha~e one such mode filter, or an array
cf these bridge-type mode filters, arranged for example
in the same manner as the rectansular mode filters of
Fis. 1.
The arrangement of Fig. 33 is merely exemplary of
bridging arrangements by which islands are made integral
with their associated aperture-defining conductive
plates by spaced-apart conductive bridges spannin~ the
anr.ular gaps bet-Jeen aperture peripheries and islands.
suc~ arran~ements afford important advantases from a
manufacturing standpoint, in that 2 co~plete mode-fil-
tering assembly of apertures and islands can be formed
integrally in a single sheet of aluminum foil or the
li~e and mounted as a unit on a microwave-transparent
ccntainer lid or other supportin~ surface. In these
1339~40
- 43 - -
. ,
structures, the open gap portions or segments (380 in
Fig. 33) are dimensioned to provide sharp attenuation in
the vertical direction so as to achieve browning or
crispenin~ of the surface of the body of foodstuff being
heated.
Fig. 37 shows a device in the for~ of a thin sheet
410 of microwave-transparent material on which there is
located an array of rectangular annuli 411 of al~l~;nl~
or other metallic foil. Each annulus 411 defines an
aperture 412 that remains microwave-transparent, as dc
the spaces 413 and 414 between the annuli.
The sheet 410 can be used in association with a
st~n~rd food container 415, and may be placed therein
beneath a food article (not shown) or above such arti-
cle, depending upon which surface of the food article is
ta be sub~ected to an increased temperature for browning
and/or crispening. Alternatively, if ~oth the top and
bottom surfaces are to ~e subjected to an increased
temperature, twc such sheets can be employed in the
container 415, one below and one above the food article.
Each sheet 410 may be flexible so as to be able to
conform to an irregularly shaped food article. For
example, it may be made of polypropylene, polyester,
polycar~onate or other low loss material that will be
substantially transparent to microwave energy.
Alternatively, the sheet 410 can be more rigid,
i.e. made of a low loss plastlc foam or cardboard-like
material. As a further alternative, i~ may ~e made a~ a
ceram~c or slass, provided that such material is sub-
stantially transparent to the microwave energy.
A sheet 410 can be embodied in the container 41S as
a part thereof, e.g. as the battom or as a lid, or as
both. ~lternatively, the sheet ~lO c n be ~ separate
1339~40
- 44 -
element that is employed by the user in conjunction with
a container. For example, the ~ser can place a st~n~d
foad container (with a microwave-transparent bottom) on
top of a sheet 410 in a microwave oven, or can plac~ a
sheet 410 on top of the food article after remcving the
conventional container lid.
Mcreover, a sheet 410 can be used directly with a
food article without need for a container at all. For
example, a pizza can be heated by simply placing it on a
sheet 410 in the microwave oven, provided the sheet 410
is suf~iciently spaced above the oven ~loor to avoid
arcing.
All these various possibilities are, however,
subject to the requirement that the function of the
sheet 410 (described in more detail below) is such that
it shauld normally be located close to the food sur~ace
requiring enhanced heating in order to achieve the
maYi~l~ performance, although desirahle effects can be
achieved with some gap between the food and the sheet.
The thicXness of the metal film for~ing the metàl-
lic ann~li 411 will be sufficient to prevent it func-
tionins as a susceptor, such metal film being virtually
entirely reflective of the microwave energy and absorb-
ing ne~ligible amounts of such energy. When using
aluminum foil for the annuli, its thickness will prefer-
ably be about 6 or 7 microns, since this is a convenient
rolling thickness for aluminum. However, from the
viewcoint of remaining microwave-reflective and not
acting as a susceptor, a thickness of as lit'le as abcut
0.2 microns (as obtained by sputterins) might be used.
This is in contrast to a thic~ness of about 0.01 microns
which would absorb mic-owave energy and become heated.
8efore describin5 the function of the metallic
annuli 411, it will be con~enient to refer to alterna-
tive shapes that these annuli c~n take. Fiq. t3 shows
1339540
- 45 -
square annuli 420; Fig. 39 shows circular annuli 421;
Fig. 40 shows triangular annuli 422 and Fig. 41 shows
hexagonal annuli 423. Moreover, the shape of the aperture
defined by the annulus need not necessarily conform to the
outer shape of the annulus. For example, a circular
aperture in a square annulus can be used. Mixtures of
these different shapes in a given array are possible, as
well as modifications in the arrangement of the array and
variations in the sizes of the different shapes in a given
array. For example, alternate rows of the square annuli
420 in Fig. 38 can be staggered, to cause the microwave-
transparent material between the annuli to trace out
tortuous paths and avoiding long straight paths. Moreover,
it is to be understood that the shapes of the annuli may
only approximate the geometric shapes mentioned, and that
normally the sharp corners that have been shown in the
drawings for simplicity will be avoided by rounding to
reduce the risk of arcing, and, as indicated above, the
annuli can tolerate some measure of interruption while
still effectively defining an aperture.
For discussion of the function of these annuli, it
will be convenient to begin with the simple example of the
square shape shown in Fig. 38, showing dimensions Di, Do
and Db that respectively designate the inside distance of
each annulus, i.e. the aperture width, the outside
distance or the external width of each annulus, and the
distance between adjacent annuli(assuming a symmetrically
spaced array).
The function of the annuli is to set up a condition in
which the aperture in each annulus causes the modes of
microwave energy that propagate through the apertures to
be in cut-off for air and for substances containing
substantial quantities of air, e.g. batter, bread crumbs,
pastry, etc., of which the surface layer of the food
article will likely be composed, but preferably not to be
in cut-off for the main portion of the food article
inwardly of its surface layer.
1339S40
- 46 -
For example, the wavelength in air of the microwave energy at the standard
frequency of 2.45 GHz is approximately 12.24 cm, whereas in the food bulk (which
will normally be composed mainly of water which has a relative dielectric constant of
5 the order of 80), the wavelength will be in a range from about 1.3 cm (pure water) to
about 2.0 cm, depending on the proportion of water in the food. It is to be understood
that these values and those given below are necess~rily approximate and can vary
quite widely with the nature of the food or other article being heated. If the surface
layer is of a substance different from the main portion of the food article, the
10 wavelength in such surface layer will normally be somewhere in between that of air
and that of the main portion of the article. The value of relative dielectric constant ~r
for such a layer will vary (owing, as mentioned above, to equilibration of a relatively
low initial surface layer water activity with that of the underlying food); an exemplary
low-end value of ~r for coatings (such as batters and the like) subject to these
15 considerations is 5. More broadly, an illuskative (but non-limiting) range of ~r for a
wide range of surface layers is 1.5 - 16, for which the corresponding range of
wavelengths (at 2.45 GHz) is 10.0 - 3.0 cm. For example, a crumb coating or puff
pastry crust, which includes a large number of air pockets, can typically have an
overall relative dieleckic constant that will result in a wavelength of the order of 8.0 -
20 10.0 cm. A more dense coating, e.g. a batter, on the other hand, can typically producea wavelength more of the order of about 3.0 - 5.0 cm, although the wavelength may
vary beyond this range depending on the exact nature of the coating. It follows that
the dimensions of the annuli can be tailored to specific foods and coatings (surface
layers) once their approximate relative dielectric
13395~0
- 47 -
constants are known, ar by trial and error, in order to
arranse that the apertures, i.e. the width dimension Di,
should be such that same of ~he ma~es of microwave
energy that propagate through the apertures will be
below c~t-off, i.e. co~monly referred to as "in" cut-
off, in the surface layer (and in air), but ab~ve ("not
in"~ cut-off in the main p~rtion of the food itself. It
will be appreciate~ that in order to achieve the in cut-
off condition for the ~1,0~ and ~0,1] modes the dimen-
sion Di in the case of a rectangular structure must be
smaller than half the wavelength in the substance con-
cerned. ~ence, to ~abulate these numerical consider-
atLons, the half wa~elengths will be
bulk food 0.65 - 1.0 cm
faod surface coating 1.0 - 3.0 cm
air 6.0 cm
Under these conditions, a gcod choice for the ~alue
of Di will be in the range of 10 - 16 mm, preferably
about 12 - 14 mm, because this value should achieve a
situation where the dominant ~odes of microwave energy
that propagate through the apertures are in cut-off for
the surface coati~g (and air) while not in cut-off for
the bulk of the food. On the ot~er hand, if it is not
important ln a particular situation for the fundamental
modes not to ~e in cut-off in the bulk of the focd, the
lower end of t~is range can be extended down, e.g. to 5
or 6 mm. By the sa~e token, if the surface coating of
the foad has a relatively low dielectric constant, the
uppe~ end o f this range can be extended up, e.g. to
abo~t 20 - 25 mm.
It is important to reiterate that the numerical
values given above are only examples and can be ~odifie~
as needed to suit specific conditions, and in particular
the specific nature of the food to be heated.
1~39540
- 48 -
When the annuli are not square, e.g. cne of the
shapes sho~n in Figs. 37, 39, 40, or 41, the effective
width dimension to be considered from the viewpcint o~
making the aperture small enough to ensure cut-off in
the s~rface layer (i.e., equivalent to the d;r?ncion Di)
will be the greater internal length in the case of a
rectangular annulus (Fig. 37), the internal diameter in
the- case of a circular annulus (Fig. 39), the height of
the internal triangle in the case of a triangular annu-
lus (Fig. 40), and the distance between a pair of oppc-
site inside faces in the case of the hexagonal annulus
(Fig. 41).
Moreover, the "smaller than half the wavelength"
criterion is strictly true only for s~uare or rectansu-
lar apertures. For circular apertures it becomes more
complicated (for example, for the TE01 mcde the cut-off
wavelength ~co = ~D/2.4048, where D is the diameter of
the aperture), and even more complicated for other
geometries. However, to gain a general condition for
cut-off dimensions, suffice to say that the largest
dimension of the aperture correspcnds to approximately
l~alf a wavelength, and more exact dimensions can be
determined by routine testing.
The condition of cut-off is illustrated diagrammatically
in Fig. 42, which shows energy E entering the sheet 410.
In this drawing, the sizes of tne waves shown are intended
1339~0
- 49 -
.
to represent their respecti~e amplitudes rather than
their spatial locations. The enerqy E paCse~ through
an aperture 412 in one of the annuli 411. First it
encounters an air gap 425, where there is attenuatio~
per unit distance travelled (because the energy is in
cut-off). Then the remaining energy E' enters a surface
layer 426 where it is still in cut-off. Finally the
r~ining energy E~ enters the main portion 427 cf the
~oad article, where it is no longer in cut-off and
hence there is much less attenuation per unit distance
due only to absorption.
The air gap 425 bet~een the structure and the focd
is kept as short as possible, because the ~i~eld decays
ev~n~cc~ntly in air, and the ob~ective is that the
majority of the energy should be absorbed in the surface
layer 426.
As shown in Fig. 42, the enerqy E" that does remain
to be ~hCorh~ by the main portion 427 of the focd
article will heat the same more uniformly in the depth
direction of propasation, which is desirable, because
the main portion of the food article will normally have
a greater depth dimension than its surface layer.
The overall result is thus increased heating per
unit ~olume in the surface layer 426 relative to the
heating per unit volume in the main portion 427 and
hence the attainment by the sllrface layer of a relative-
ly high temperature (with a consequent browning or
crispening effect) and the more uniform absorption o~
heat (at a lower temperature) by the main portion sa
that the inner parts of this main portion, which are
relatively remote from the surfacs layer, are not en-
tirely unheated.
1339~5~0
- so -
Taking the example of the Fig. 38 array with a
value of Di = 10 - 14 mm, a convenient value for Do
would be about 20 - 25 mm, and that for Db about 4 - 5
mm, with a m;nirl~ of about 3 mm. If the value of Db is
made too small, there is a danger of arcing. If Db is
made too large, the microwave energy will tend to be
propagated through the spaces 413 and 414 instead of
through the apertures 412. Assuming that the relative
dielectric constant is within the exemplary range o~
values (1.5 - 16) mentioned above, as long as Db is na
greater than about 6 mm, these spaces will also be in
cut-off for air and the surface layer. On the other
hand, the fact that these spaces 413, 414 are elongate
may permit some of the energy to propagate through them.
If this effect is found to be disadvantageous, it can be
reduced by staggering the annuli to create more tortuous
paths between the annuli as shown in Figs. 39 and 41.
It has also been found that good results are obtained
from the device when the annuli are interconnected with
each other (as shown, for example, in broken lines at
430 in Fig. 38) by similar microwave-reflective, sub-
stantially non-absorptive material. Such a layout of
interconnected annuli enables the whole array to be
stamped out in a single operation from a sheet of alumi-
num foil and mounted as a unit on the substrate 410.
1339~0
- 51 -
Another way of viewing the effect of the arrays of
annuli is to consider them as generators of higher order
modes of microwave energy.
The embodiments with continuous, substantially straight
open lines of microwave-transparent material (Figures 37,
38 and 40) allow more lower order modes to propagate and
hence tend to achieve more bulk heating (which may be
desired in some cases, depending on the nature, especially
the water content, of the food article). This effect can be
reduced by avoiding such open lines, as in Figures 39 and
41, or by staggering the rows of annuli in Figures 37, 38
or 40.
Experiment l
A first experiment was carried out in a square
container with side walls and a lid of aluminum, and a
bottom of lO mil microwave-transparent polycarbonate, so
that all the energy would enter the container through its
bottom. The dimensions of the container were llOmm x
llOmm x 27mm. To compare the invention with the prior art,
two different square arrays "A" and "B" were used. In
array "A" (prior art) each annulus was completely filled
in, i.e. it became an island with no aperture, and in array
"B" (according to the invention) there were apertures as in
Figs. 37 and 38. In both cases Do = 20mm and Db = 5mm.
The value of Di was
Array Di in mm
A o
B 14
Each array had nine bodies (islands in the case of array
"A" and annuli in the case of array "B") arranged in a
square, non-staggered layout as in Figure 38.
Both tests were carried out under otherwise identical
conditions, namely with a uniform load of 315g of Cream of
Wheat* made by Nabisco Co. and with
* Trade Mark
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heating on ~ull pawer in a Sanyo, Cuisine-~aste~ 70~-
watt microwave oven for three minutes. As shown in Fig.
4~, temperat~re probes were p~s5ed through rigid plastic
tubing into the center cf the load L which completely
filled the container, pro~e "X" being at cr very near to
the bottom, pro~e ~yl~ at three ~uarter af the depth of
the load and probe ~Zt~ at half the depth of the lcad.
The tPmr~ratures measured are shown diagram~tical-
ly in Figs. 44A and 44B respectively, and it will be
noted that the difference after three minutes between
curve "X" (carresponding to pro~e "X") and curves "y"
and "Z" (corresponding respectively to pro~es "y- and
l~zn~ reached about 26-C in Fig. 44B in constrast to
about 5 C in Fig. 44A. Als~, the absalute value of the
temperature of cur~e "X" was significantly higher. The
S-C advantage of curve X over curve "y" in Fig. 44A is
attributable to the normal attenuation of the micr~wa~e
energy as it passes through the load, but is considered
insufficient to pr~duce the desired browning or crispen-
in~
It will be appreciated that, since the load was a
uniform mass af Cream of Wheat cereal, the l~ad had a
surface layer that did not differ significantly in
nature tand hence in relative dielectric constant) from
its main partion. ~owever, the experiment nevertheless
demcnstrated the significantly increased surface tem-
peratures that can be achieved with an array according
to the present invention, even in the absence of a differ-
ence of dielectric constant between the surface layer and
the main portion of the load. When such a difference in
relative dielectric constant is also present, as in
Experiment 2, the temperature difference between the
surface layer and the main portion of the food is expected
to be even more pronounced.
* Trade Marks
133~40
Experiment 2
A second experiment was conducted using different food
articles, each having a different type of surface layer
requiring browning or crispening, while varying the shapes
of the annuli and hence the apertures.
For example, a container of frozen, battered and crumb-
coated fish (haddock) was heated first using the hexagonal
annuli and then using the square annuli. It was found
that, when using the square annuli, it was sometimes best
to use a different aperture dimension on the top surface
from that used on the bottom.
This fish article had a flat bottom surface and rounded
top and sides, and weighed approximately 190 g. It was
placed in a microwave-transparent container, the base of
which was fitted with an array of 28 (4x7) square annuli of
dimensions Do = 20 mm and Di = 10 mm. A similar structure
was placed over the top of the article, i.e. 28 (4x7)
square annuli, but with dimensions Do = 20 mm, Di = 13 mm.
The assembly was heated for 4-1/2 minutes on full power in
the same 700-watt oven. The result was a product in which
the fish itself was uniformly heated to an appropriate
temperature for serving, but not overcooked, while the
surface appropriately crispened.
While the theory postulated above, namely that the
improved surface heating that the above experiments have
- demonstrated to be obtainable follows principally from the
fact that some of the modes of microwave energy that
propagate through the apertures are in cut-off in the
surface layer, is the best explanation currently known to
applicants, it is desired to point out that other factors
may be at work in a system as complex as that involved when
microwave energy propagates in confined spaces. For
example, the improved surface heating that has been
observed may result from a combination of effects,
including not only the size of the aperture in each annulus
but also the width of the microwave-reflective material
forming the annuli and the spacing between annuli.
1339,540
- 54 -
It is believed that the most important consideration to
bear in mind is that substantially improved practical
results have been obtained using the structures disclosed
herein, and that this fact is independent of the theory put
forward concerning the mechanism involved in achieving such
improvements.
It is to be understood that the invention is not
limited to the features and embodiments hereinabove
specifically set forth, but may be carried out in other
ways without departure from the scope of the claims.
1339~40
- 55 -
SUPPLEMENTARY DISCLOSURE
The drawings accompanying the supplementary disclosure
include:
Fig. 38A which is a fragmentary variation of Fig. 38;
and~
Figs. 45- ~ which are diagrams serving to illustrate
Experiment 3 described below.
Fig. 38A shows square annuli 420a interrupted at 420b.
Any of the shapes shown in Figs. 39, 40 and 41 can also
have interruptions in the annuli, analagous to those of
Fig. 38A, and such interruptions need not necessarily be
two in number, but may be a single interruption or more
than two interruptions.
The embodiments of the invention so far described and
illustrated have taken the form of an array of metallic
structures on a microwave transparent sheet. However,
instead of forming the apertures necessary to achieve
cut-off by means of shapes formed of thin reflective,
metallic shapes or configurations, the invention can also
be practiced by defining the apertures by means of shapes
or configurations of a material that differs from the
microwave transparent sheet in some other electromagnetic
property, such as conductivity, lossiness, dielectric
constant, spatial thickness, a stepwise discontinuity or a
magnetic property, as explained in the published European
applications referred to above.
Experiment 3
This experiment was conducted in a 750 watt Gerling
Oven model GL701(MPS 229-10)*. The container was square
with dimensions of length 88 mm, width 88 mm and height
30 mm, and was made of brass to ensure that all the
microwave energy entered the load from the top.
* Trade Marks
1339~40
-- 56 --
A first load used was purified agar in fine granular
form dissolved in hot water to provide a gel density of
1.03 g/mQ (sold as "Bacto-Agarn* by Difco Laboratories,
Detroit, Michigan, U.S.A.)
First runs on full power for 20 seconds with an
unmodified container, i.e. no lid, and with different
depths of load, showed in all instances the usual lateral
disuniformity of heating, i.e. a cold center resulting
from the dominant influence of the fundamental modes.
Corresponding runs on full power for 20 seconds with a
mode-filtering array as shown in Figs. 37 and 38 extending
across the top of the load in contact with the load surface
exhibited a more uniform heat distribution across the
surface of the load, including significantly more heating
in the central area.
The temperature measurements were taken by means of an
AGA THERMOVISION Infrared Camera Model 780* and processed
with a VIEWSCAN LTD. Scan Converter 700* and Viewsoft*
Software.
Variations in the heating patterns were observed for
different depths of load, i.e. for a depth of 6-7 mm
(the theoretically minimum absorption depth) and for a
depth of 10-11 mm (the theoretically maximum absorption
depth). The effect of depth on energy absorption is the
subject of Canadian patent application Serial No. 590,860
filed February 13, 1989 (U.S. patent application Serial No.
359,589 filed June 1, 1989~. It was found that, while
there were differences in the heating effect with different
depths of load, at all depths the tests conducted with a
mode-filtering array in accordance with the present
invention exhibited more uniform lateral uniformity of
heating effect than those tests without such an array.
Trade Marks
_ 57 _ 1339~40
In order to observe the heating d istribution in the
vertical direction, the experiment employed a LUXTRON 750*
Fluoroptic Thermometry System using a pair of probes. One
probe was positioned 2 mm below the sample surface and the
other 5 mm below the sample surface. Both were in the
center of the sample in the length and width directions.
The first probe effectively measured the "surface"
temperature. These dimensions were maintained regardless
of the depth of the load, which was either 6 mm or 10 mm.
Measurement of the "surface" temperature by means of a
probe that was actually 2 mm below the surface was required
by the finite dimensions of the probe itself and in order
to minimise the surface cooling effect.
The loads used were pastry (Gainborough Easi-dough*)
rolled to a uniform depth of either 6 mm or 10 mm.
First runs were conducted in the same brass container
with no array on top of the load. With full power the
surface to bulk temperature differential at the end of 120
seconds was approximately 20~C for a load of 6 mm depth
and approximately 10~C for a load of 10 mm depth.
Effective crispening was not achieved.
Similar runs were conducted with various arrays in
contact with the load.
The array used (Fig. 45), which is basically similar
in structure to Fig. 1, was separated into two parts,
namely an array of islands 428 (Fig. 46) and an array of
apertures 429 (Fig. 47). Three samples were tested under
identical conditions, one with the island array (Fig. 46)
alone, a second with the aperture array (Fig. 47) alone,
and a third with these two arrays combined to provide the
composite array of Fig. 45. It was found that neither of
the single arrays when used individually was effective.
However the combination produced a mode-filtering array
that provided uniformity of heating across the surface of
* Trade Mark
13395~0
the pastry and an intensification of heat at the surface,
i.e. a non-uniformity of heating in the vertical direction,
sufficient to achieve satisfactory browning across the
entire surface.
As measured by the probes the surface to bulk
temperature differentials were approximately 45~C and 38~C
for the 6 mm and 10 mm deep samples, respectively.
A further run was conducted using a pastry load of
lO mm depth and a mode-filtering array as shown in Fig. 48
having metal "annuli" 431 of such a shape as to define
cruciform apertures 432, these annuli being arrayed on a
sheet 433 of microwave-transparent material. The differ-
ence between the surface and bulk temperatures was already
approximately 35~C after 20 seconds heating at full power
and remained at about this value as heating progressed.
The array of Fig. 48 was tested in order to demonstrate
that the interior shape of the annulus, i.e. the aperture,
in this case cruciform, need not necessarily conform to
the outer shape of the annulus, in this case square.
