Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Field of the Invention
- The invention relates to the food packaging and distribution
art and more particularly to an improved microwave heating pack-
- age containing a heat absorber for converting microwave energy
to thermal energy and to a method for distributing foodstuffs.
The Prior Art
Heating a foods directly i.e. conventionally in a microwave
- oven, often gives them a soggy character or if the food is a
bread product, it sometimes takes on a leathery character quite
unlike that of the
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~09~3~5
` same product heated in a non-microwave oven. The crust of some
products such as pizza pies develop an unusual texture which is
- either soggy or leathery and is quite unappealing. Thus, while
`` sogginess and texture is a problem in some food products, the
inability of an ordinary microwave oven to brown the surface is
particularly important in heating of meat, eggs, bread or veget-
ables such as hash brown, french fried or augraten potatoes. In
. .
recent years, ceramic dishes that become hot in a microwave oven
have been sold to solve this problem. Such a dish is quite
heavy, relatively expensive and must be pre-warmed without food
on it for about 2 to 5 minutes. A number of other containers
that have been proposed for browning or searing the surface of
a food fall into three general categories. The first are those
which include an electrically resistive film usually about
.00001 cm to .00002 cm thick applied to the surface of a non-
conductor such as a ceramic dish and described, for example in
U.S. patents 3,853,612; 3,705,054; 3,922,452 and 3,783,220.
Heat is produced because of the I2R loss (resistive loss). This
system is not acceptable for use in the invention primarily
because of the bulk weight and cost of the dish and its break-
ability. Second are microwave energy absorbers formed from a
mass or bed of particles that become hot in bulk when exposed to
microwave energy. The microwave absorbing substance can be
composed of ferrites, carbon particles, etc. Examples are
described in U.S. patents 2,582,174; 2,830,162; 3,302,632;
3,773,669; 3,777,099; 3,881,027; 3,701,872 and 3,731,037 and
German patent 1,049,019. These materials are useful components
in the present invention. The third category comprises elec-
trical conductors such as parallel rods, cups or strips which
function to produce an intense fringing electric field pattern
that causes surface heating in an adjacent food. Examples are
U.S. patents 2,540,036; 3,271,552; 3,591,751; 3,857,009;
.,
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```` 10913QS
3,946,187 and 3,946,188. This system of heating is not used
:
in the present invention.
In the development of the present invention, microwave
energy absorbers when used alone were found unsatisfactory for
most purposes particularly in conjunction with heating farin-
aceous foods such as bread products, fruit pies or pizza pies
primarily because the microwave energy received directly by the
food product from the magnetron or other microwave generator
caused the internal temperature of the food product to rise
quite rapidly whereas the heat conducted from the microwave
absorber was applied more slowly so that by the time the exterior
became brown or was seared, the interior was burned, dried, or
otherwise overdone. U.S. Patent 3,941,967 describes a microwave
cooking vessel or utensil having a body formed from glass, --
porcelain, and ceramic or synthetic resin such as fluorine-
containing resin, polyprop~lene, or the like. In the vessel is
a metal plate beneath which is provided a heating element such
as the ferrite ceramic, silicon carbide ceramic or a resistive
film. A shield cover formed from a metal or mesh is placed over
the food to isolate the microwave radiation from the food and
the internal heating of the material to be cooked is set at a
suitable level by properly adjusting the leakage of the micro-
; wave radiation through the shield cover. While the system
described in the patent can be used to provide a balance between
internal and external heating, the vessel is expensive costing
$20 or more and heavy. Much of the weight and cost of the
patented vessel results from the inherent bulk and weight of the
heat absorber. It is therefore used as a permanent utensil by
the homemaker and is totally unsuited as a container for vending
a food product. Moreover the relatively large bulk and mass ofthe heat absorber causes it to stay very hot, say 500-600F for
quite a time after removal from the oven which makes it possible
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- 1091305
for the fingers to be burned.
By contrast with the prior art, one major goal of the
present invention is to find a way to provide an inexpensive and
disposable microwave food heating container or package u,seful
for shipping, heating and when desired to hold the food as it is
being eaten as well as to provide an improved method of distri-
buting and heating foods with microwave energy. Another heater
is described in U.S. patent 3,777,099. Similarly massive, the
heat absorber is placed inside an insulator such as sand or
concrete with cardboard-or ceramic around it. All forms of the
invention utilize a heavy slab or plate on which the food is
placed. The food is not shielded or enclosed. U.S. Patent
3,731,037 describes a microwave kiln for food having heat
insulating walls preferably of a material capable of withstand-
ing refractory temperatures lined with a material such as glass
or ceramic which is made lossy. The patent also discloses a
- disposable kiln containing an aluminum food dish, polyurethane~
foam walls and a lossy floor lining which consists of water.
It has been previously proposed to provide a paper box with
a metal foil layex which partially shields a food contained in
the package from microwave radiation when heated in a microwave
oven. Examples are U.S. Patents 2,714,070; 3,865,301 and
3,219,460. When food are heated in packages of this kind, the
aforementioned problems of sogginess or leatheriness and absence
of surface scorching occur rendering the container unsuited for
the purpose to which the present invention is directed.
OBJECTS OF THE INVENTION
.
The major object is to provide a microwave heating package
and distribution method for foodstuffs having the following
characteristics and advantages among others: a) the package can
be considered inexpensive and disposable, b) can be used for both
shipping and heating a food and will sear or brown its surface,
1305
c) can be used as a serving plate or tray, d) can be constructed
.~
primarily of known packaging materials which are readily obtain-
able and inexpensive, e) provision is made for locating a heat
absorber in position to receive microwave energy at a point in
the oven where the energy is coupled efficiently to the absorber,
f) the food can in some forms of the invention be heated simul-
taneously by the dual application of microwave energy directly
and by conduction heating from a heat absorber to the surface of
the food product to thereby brown, dry or scorch the surface in
contact with the heat absorber, g) the package is safe to use
without danger of sparks, arcing or burning during heating, h)
` provision is made if desired for totally shielding the food
product from direct exposure to microwave energy while heating
is accomplished solely through conduction from a heat absorber,
i) there is a provision for allowing the heat absorber to very
quickly cool after it is turned off to prevent burning the
fingers, j) the package has enough strength to adequately protect
the food during shipping and will not break or contaminate the
food, k) the package is light weight and specifically, a package
for a single 3 3/4 x 3 3/4 inch 66 gm. slice of pizza will weigh
about 30 gm. or less and contain a microwave absorptive heating
material in a layer weighing about 15 gms. or less.
Thus, in accordance with the present teachings, a package
adapted for containing food for heating the food in a microwave
oven is provided. The package comprises a heating body in sheet
form and includes a lossy microwave energy absorber for convert-
ing microwave energy to heat energy. The body is in a conductive
heat transfer relationship with the food when the food is placed
in the package to thereby heat the food as a result of heat trans-
fer from the heating body to the food. The thickness of the heat-
ing body is substantially within the range when the heating body
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1()91305
thickness and temperature response are positively corelated.
Microwave shielding means is provided at least partially surround-
ing the food product to control the direct passage of microwave
energy into the food product whereby the food receives a lower
- amount of direct microwave energy transmission than would be
received thereby without the shield.
In accordance with a further embodiment, a method is provided
of heating a food by the application of microwave energy to a
package that will function as a serving dish in which the food
can be safely eaten. The method comprises providing a heating
package which has a heating member in sheet form, the heating
member having a thickness substantially within the range wherein
the heating member and the temperature response are positively
corelated. A microwave shield member is provided and the thick-
ness of the heating member is small enough so that it cools sub-
stantially to the temperature of the food before the food is
eaten. The package is placed in a microwave oven with the food
product therein in heat conductive relationship with the heating
member and at least partially shielded from microwave energy by
the shield. The package is exposed to microwaves to heat the
heating member to at least a temperature that will crisp, brown
or sear the surface of the food in heat conductive relationship
with the heating member, the amount of direct microwave radiation
reaching the food being reduced by the shield, the heating member
i~ is flexible and resistant to breakage.
The Figures
Figure 1 is a perspective view of a microwave oven containing
a package embodying the invention.
Figure 2 is a perspective view of the package of Figure 1
on an enlarged scale shown with the top open.
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Figure 3 is a vertical transverse sectional view taken
. on line 3-3 of Figure 1 with the package in a closed condition.
`~ Figure 4 is a greatly magnified partial sectional view
taken on line 4-4 of Figure 2.
Figure 5 is a partial vertical sectional view similar to
. Figure 3 of a modified form of the invention. ~-
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` ~ 1091305
Figure 6 is a vertical sectional view of another form of
the invention.
Figure 7 is a vertical sectional view of another modified
form of the invention.
Figure 8 is another form of the invention in vertical cross
section.
Figure 9 is a graph illustrating the relationship between
the composite absorber thickness and the resulting surface tem-
perature after heating for one minute.
.j
Figure 10 is a graph showing the time/temperature response
for absorbers of different thicknesses.
Summary of the Invention
The invention provides an inexpensive disposable microwave
food shipping, heating and serving container or package composed
of lossy microwave energy absorber or heating body which becomes
hot when exposed to microwave radiation associated in conductive
heat transfer relationship with a food product when the food is
placed in the package. The expression heat conductive relation-
ship herein means thermal conduction through a solid as well as
- 20 the transmission of radiant heat by electromagnetic waves and
the convection of heat through the air. Thus although the food
usually touches the absorber or is in contact with a layer
adjacent to it, contact is not always essential. The food while --
usually refrigerated can be frozen or at room temperature. The
absorber or heating body is usually but not necessarily a layer
or sheet of lossy material bonded to a structural supporting
sheet such as metal foil. The package preferably includes a
shield which is usually an electrical conductor to reduce by a
controlled amount the direct transmission of microwave energy
into the food product. The shield can comprise a metal screen
or a metal foil cover having holes adjusted in size to provide
a predetermined and controlled amount of direct microwave energy
` ` 1091305
transmission into the food product or when required a single
nonperforated sheet. In some embodiments of the invention parts
. .
of the package are enclosed and supported in an outer container
body formed from microwave transparent semi-rigid dielectric
sheet material such as a paperboard carton which forms a part of
the package. The absorber heats the adjacent surface of the
food by conduction to a sufficiently high temperature to crisp
~i
~ or scorch the surface while direct microwave exposure of the
`` food when provided heats the inside. It is preferred that the
thickness of the heating body be substantially in the range
wherein absorber thickness and temperature response are posi-
tively correlated. In one preferred form of the invention the
microwave absorber layer is of the minimum thickness that will
reach without exceeding a preselected equilibrium operating
temperature.
The invention also provides an improved method of distri-
buting and heating foodstuffs by packing them in a disposable
container having a shield and absorber for converting microwave
energy to thermal energy then transporting and heating them in
the container to provide surface scorching and reduced direct
microwave transmission to the food as will be described more
fully below.
Packages in accordance with the present invention can be
, used for shipping and vending foods both through retail grocery
outlets and vending machines. They can be used for a single
serving or for several foods in a single container in the manner
of a T.V. dinner.
The container body can comprise any microwave permeable
nonlossy material and is usually a dielectric such as paperboard
~ 30 or other cellulosic material or plastic resin such as a polyamide
; or polyester resin having the requisite heat resistance. The
container body, e.g., a paperboard box usually includes side,
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lO9i31)5
.
top and bottom walls to enclose and protect the food product.
me lossy microwave energy absorber preferably has the form of a thin~-
sheet or layer that serves as a heatin~body and is usually Fart of a composite
sheet of heating body composed~of a structural supporting sheet that-can;-bé
either microwave transparent or microwave opaque~such as a ce~amic or metal
sheet to which the active microwave absorber is applied as a relatively thin
palnt like-layer. me expression paint like layer-herein;m~s a coa~mg
applied as a layer having a small finite thickness up to on the
order of about 1/32 of an inch bonded directly to the structural
support layer and having a sufficient flexibility to remain
adhered to the layer when the latter is bent or deformed. When
this laminate is used to support the food product, the energy
absorbing layer is normally placed on the opposite side of the
structural support sheet from the food thus the food is ad~acent
to and usually contacts the structural support sheet or foil.
The geometry and especially the thickness of the microwave
absorber is preferably maintained within a specified range to
control the saturation i.e., equilibrium temperature reached by
the heater after a specified period of heating or indefinite
heating. It was discovered that the thickness should be main-
tained substantially within the range wherein the temperature
is positively correlated with the changes in thickness i.e.,
the temperature response rises with an increase in thickness.
The shield which reduces by a controlled amount the quantity of
; direct microwave transmission to the food product is conveniently
applied as a layer or lamination to the inner surface of the
; container body. It is preferably, but not invariably, formed
from an electrically conductive material such as metal foil,
e.g., aluminum foil.
One or more microwave absorbent heaters can be employed.
For example, if two are employed, it is convenient to place one
on the top and one on the bottom of the food product to sear or
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` ` 1091305
brown both top and bottom surfaces. The invention also contem-
plates completely surrounding the food product with a microwave
absorbent heater. This form of the invention is particularly
useful in connection with fruit pies.
In the accomplishment of the foregoing and related advan-
tages and objectives, this invention then comprises the features
hereinafter fully described and particularly pointed out in the
claims, the following description setting forth in detail
certain illustrative embodiments of the invention these being
indicative, however, of but a few of the various ways in which
the principles of the invention may be employed.
Detailed Description
Refer particularly to figures 1, 2 and 3 which illustrates
a typical application of the invention for use in shipping,
heating and serving a single portion of a food such as a slice
of pizza pie.
Figure 1 illustrates a package embodying the invention in
a microwave oven 5 of suitable known construction including the
usual controls 6 microwave generator 7 producing microwaves
under present regulations at 2450 megahertz. It is to be under-
stood, however, that the present invention is applicable to all
wavelengths at which microwaves can be used for heating. Micro-
waves are usually understood to be in the range of 1000 to
30,000 MHZ. The waves are conducted through guide 8 to a micro-
wave oven cavity 9 into which the package 10 is placed.
The package 10 comprises an outer container body 12 formed
from a microwave transparent nonlossy material such as a diel-
ectric sheet material, e.g. paperboard or plastic including
four sidewalls 14, 16, 18 and 20 joined by centrally extending
integral corner folds 22, 24, 26 and 27 each comprising a pair
of mutually hinged flaps that are also hinged along one side
edge to an adjacent sidewall. The carton 12 also includes
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`` 1091305
integral bottom wall 29 and top wall 28 having a tab 30 that can
be secured in any suitable manner, e.g., by pasting to the side-
wall 20 when the carton is closed to hold the top in place. The
top 28 also includes a pair of side flaps 32 and 34 which fold
downwardly and lie adjacent to the outside surfaces of side walls
14 and 18. The carton when used for a single serving of pizza
pie might measure 4 x 4 x 1 inch. If paperboard is used, 14 to
18 point bleached food grade sulfate paperboard is preferred.
The package is wrapped with cellophane or other protective
flexible sheet material 36 (Fig. 3) including any of the well
known packaging films such as nylon, polyester, polystyrene,
wax paper, etc. the wrapper 36 is used to protect the package
; during storage and is removed prior to placing the package in
the microwave oven.
Bonded to the inside surface of the cover flap 28 is a
shield composed of an electrically conductive metal foil 38
comprising .00035 inch aluminum foil laminated to 25 pound
kraft paper. This laminate is bonded with any suitable adhesive
to the inside surface of covèr 28. The shield 38 in this case
does not totally shield the food product contained in the pack-
age from all microwave radiabion but instead acts as a partial
shield adapted to control the passage of microwave energy into
the food product directly. The amount reaching the food directly
; is less than the amount that would reach it without the shield.
Transmission is accomplished through openings 40 of a predeter-
mined size. As heating occurs, moisture vapor and steam is
vented through the openings 40 thereby maximizing the oppor-
} tunity for moisture to be driven out of the crust and for the
; crust to become crisp. If desired, one or more movable metal
covers (not shown) can be provided to open or close the open-
ings 40 prior to heating to any desired extent to thereby allow
the user to control the amount of internal heating. Good
,
. -- 10 --
109~305
results have been achieved with pizza pie of 66 gm. in a lO00
watt oven with four openings 40 each 1 l/4 square inches, i.e.,
totaling 5 square inches of open area while the total area of
the shield 38 (including the hole area) is about 21 square _
inches. Thus, the open area of the holes 40 is about 25% of the
shielded area, however, good results can be achieved with a much
wider range of open area for example about 10% to 75% of the
shield can be open when direct microwave heating is desired. In
some cases, as described below, no directmicrowave heating is
provided for the product, the product in that case is heated
solely by conduction from the heat absorber. In determining the
size of the openings 40, i.e., the degree of shielding, one
first decides upon the amount of conduction or surface heating
that is needed and establishes that the dimensions and compos-
ition of the composite 42. The size of the openings 40 ( or in
the case of other embodiments such as that in Figure 8 where no
holes are used the size of the shields itself) is then made
larger or smaller until the desired predetermined amount of
internal heating is accomplished by direct microwave trans-
mission without burning or drying the interior. This is bestaccomplished empirically. Thus, if the product is not warm
enough on the inside, the openings 40 are made larger but if
too warm or burned, the openings 40 are made smaller. In
general, the size of openings 40 or the size of the shield/ -
itself if no holes are used will be determined by the type of
food, its composition, the amount of water contained in the food,
whether it is frozen, cooked or uncooked, etc. It should be
understood that as the amount of direct microwave transmission
to the food is increased, for example by making the openings 40
larger, the amount of energy going to the absorber and con-
sequently the amount of conductive heating decreases. The
dimensions and opening size given are merely set forth by way
of example.
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Within the carton described is provided a spacer such as a
sheet of open faced corrugated board or other suitable micro-
wave transparent material 41 of just the proper size to fit
easily in the bottom of the carton. On this spacer rests a
heating body 42. The heating body 42 is a composite or laminate
best seen in Figures 2 and 3 consisting of an upper structural
support number 44 having a substantial degree of strength and
the heat resistance necessary to withstand the temperature
involved, e.g., aluminum, steel, copper, brass or ceramic foil
or sheet mica, portland cement, or plaster of paris being typ-- -
ical and a heating layer 46 which comprises any suitable micro-
; wave absorptive lossy substance known to the art that will reach
a temperature when exposed to microwave energy above 212F.
either alone or in combination with one or more diluents and
binders. It is important to note that the body 42 is flexible
or semi-flexible in that it can be easily formed or bent with
the fingers without fracturing into pieces although coating 46
; may crack. This flexibility gives it resistance to breakage
even though struck with a hard blow as contrasted with the
performance of a rigid sheet formed from a brittle material.
The layer 46 is relatively thin like a layer of paint.
The binder bonds or cements the absorbent particles together to
~ hold them in place and also forms the heating layer 46 into a
; solid mass-thereby preventing sparks or arcing between indivi-
dual particles. The bonding function can be provided by any
suitable adhesive or solid matrix that is resistant to the
- temperatures involved such as portland cement, plaster of paris,
sodium silicate, etc. The layer 46 may not be continuous. That
is to say, it can be provided in two or more strips or bands or
may include holes or openings. The microwave absorber should
preferably be lossy enough to achieve temperatures of over 300F.,
thé most preferred being in the range of 400F. to 800F. Any
- 12 -
109130S
known lossy microwave energy absorbing substance can be used if
it is capable of achieving a temperature of over 212F. to there-
by bring to a boil any free moisture present in the food. The
microwave absorbing material may or may not be of the type which
is variable with a temperature as described in U. S. patent
- 2,830,162.
Any suitable lossy substance that will heat in bulk (as
distinguished from a resistive film)-to more than 212F. in a
microwave oven can be used as the active heating ingredient of
the microwave energy absorbent layer 46. These materials fall
primarily into four groups: first semiconductors, examples of
which are zinc oxide, germanium oxide, barium titanate, etc.
Among the second group are ferromagnetic materials that have a
- Curie temperature higher than about 212F. including powdered
iron, some iron oxides, and ferrites such as barium ferrite,
zinc ferrite, magnesium ferrite, copper ferrite, or any of the
other commonly used ferrites and other suitable ferromagnetic
materials and alloys such as alloys of manganese, tin and
copper or mangenese, aluminum and copper and alloys of iron
and sulfur such as pyrrhotite with hexagonal crystals, etc.
Other materials include silicone carbide, iron carbide, stron- -
tium ferrite and the like. Other suitable materials include
period 8 oxides and other oxides such as cromium oxide, cobalt
oxide, manganese oxide, samarium oxide, nickel oxide, etc.
One preferred material is powdered and granular Fe304 obtained
from taconite or mixtures of powdered and granular Fe304. In a
fourth group are dielectric materials such as asbestos, some
fire brick, carbon and graphite.
With regard to ferroelectric and ferromagnetic materials
it has been found that generally the Curie point must be the
same or above the maximum temperature one wants to achieve.
Thus, if 500F. is the desired temperature, the Cùrie point
must be at least 500F. Slightly higher temperatures might be
- 13 -
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achieved if the dielectric absorption gives rise to further
temperature increases. Relatively high magnetic or dielectric
constants improve the heating ability of the material and help
to achieve thinness in the finished product by reducing the mass
of material required to achieve a given temperature. The final
temperature achieved is limited in three ways in general. First
by the Curie point of the active heating material, because below
the Curie point the material absorbs microwave energy and above
this temperature the material loses its magnetic properties and
will no longer heat. Second by the percentage of active micro-
wave absorbent material in the mixture and thlrd, by the amount
or mass of microwave absorbent material and par~icularly by the
thickness of the layer 46 that is used. Clay ceramic which
while not extremely lossy alone, if made part of the heater
layer 46 will contribute to some extent to the heat produced.
Other examples are silicates and like glasses.
The structural support layer 44 should be relatively inex-
pensive, undamaged by heat, corrosion resistant nontoxic to food
and provide a degree of structural strength. When aluminum is
employed it is preferably a foil about 1-3 mil. thick. The
;~ absorber is preferably on the outside, that lS to say, on the
opposite side of the supporting sheet 44 from the food product.
Aluminum foil when inside serves two purposes. It is a struc-
tural support for the absorber and also acts as a clean cooRing
surface to prevent contamination of the food product by the
absorber. While m~tal is preferred, layer 44 can also comprise
a nonmetal such as a nonmetalic mineral or a thin.-~laze of
ceramic fused to the upper surface of the heat absorbing layer
46 but because the heating body 42 must withstand temperatures
of 500F. to 600F. such a structure does not have the strength
of a composite using a metal layer and is expensive in addition
to being more breakable. If the structural support 44 is non-
metalic it is preferred to use a temperature resistant mineral
.
` -- lU91305
or ceramic which is fused to form a homogeneous sheet either
with or without reinforcement such as a metal screen, metal or
mineral fibers, glass fibers, etc. for structural strength.
Metals are greatly preferred to ceramics and glass because of
their relative toughness, flexibility or bendability and résis-
.~,
tance to breakage. Accordingly, less material is re~uired than
in the case when a nonmetal is used for the structural support
44. A fourth group comprised formulated combinations of the
above materials, or the above materials mixed with nonlossy
microwave permeable materials such as minerals including perlite,
sand, alumina, magnesia or the like which function as inert
fillers to slow down the heating rate and help make the layer
stronger.
The best lossy material to use depends upon a number of
factors, the most important of which are its heating efficiency,
the final temperature to be achieved, the heat stability or
; resistance to cracking or other destructive factors, the lack ~-
of sparks, arcing, etc. When Fe304 is used as the primary lossy
heat absorber, one suitable formula is 37 grams Fe304 obtained
from taconite, 37 grams sand and 11.5 mil. of a 2.5 part sodium
; silicate to 1 part water solution. The sand and powdered
Fe304 are blended together and the sodium silicate solution is
added and uniformly mixed. This wet mixture is applied by ~
brushing, rolling, etc. onto a sheet of 3 mil. aluminum to a
`~ thickness of .030 inches. The laminate comprising the layers
44 and 46 is then heated with the edges held to prevent warpage
to about 200F. for about 2 hours or until dry. The resulting
laminate is very light in weight, flexible in the sense that it
can be easily bent with the fingers, stable and strong enough
to withstand shipment and storage. It is nontoxic to food
substances and will heat the surface of the food in contact
with the upper surface of the aluminum foil to 600F. or hotter.
1091305
During the drying of the coating layer 46, most of the water is
lost so that the final dry composition comprises about 37 grams
Fe304, 37 grams sand and about 5 grams sodium silicate.
The spacer 41 can be formed from many microwave transparent
articles of which open face corrugated board is merely an
example. Other suitable materials are one or more pieces of
perlite, magnesia alumina, glass, fiberglass, etc. If perlite
is used, it can be formed from powdered perlite bonded together
with sodium silicate in a manner known to those skilled in the
art. The spacer 41 preferably holds the absorber 42 about a
quarter of an inch or more from the lower surface of the oven
cavity to promote efficient coupling of the microwave energy
to the heat absorber. -
Resting upon the heating body 42 is a food product 43 such
as a square slice of pizza pie or any of a variety of other
foods including french fries, hash brcwn potatoes, onion rings,
cheese sandwich to be toasted, a slice of fruit pie, meat, etc.
While convenient to make contact between the food and the
laminate 44-46, it is not essential since heat can be trans-
ferred from the composite sheet to the food by radiation orconvection rather than conduction.
The food is placed in the package 10 at the factory and
shipped at any temperature either frozen or non frozen and can
be placed in the oven 5 in either a froz~n or non frozen con-
dition. When the food is to be heated, the wrapper 36 is
removed thereby uncovering the openings 40. The package is
then placed in the microwave oven and as microwave energy
passes into the chamber 9 through guide 8 a predetermined
controlled amount of the microwave energy enters the package
through openings 40 and passes directly into the food product
43 heating it throughout. The remainder heats the absorber
46 and is transmitted by conduction through aluminum foil
layer 44 to the bottom of the food product thereby crisping or
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- 10~1305
browning the bottom of the crust. This action has proved to be
highly effective in removing the soggy or leathery character
found when the same food product is heated alone in a micro-
wave oven. Heating in a 1000 watt oven will take about 105
seconds for a 66 gm. pizza and 180 seconds for a 264 gm. pizza.
The direct controlled microwave transmission through the open-
ings 40 allows the interior of the food product to be heated
without being burned or dried. The heat absorber reaches a
temperature typically of about 500F. to 700F., and preferably
in the range of 600F. Because of the relatively small amount
of material in the heat absorbing layer 46 and the low cost
of component parts, the container is very inexpensive and can
be considered disposable. In addition, the low mass of the
heater allows it to very ~uickly cool to the same temperature
as the food product 43 when the power is turned off thereby
minimizing the risk of burning the fingers. The microwave
absorptive heating surface is characterized by providing
sufficient heat to roast, sear or toast the surface of the
food article without burning either other parts o the package
or the hands when the package is opened.
The geometry and especially the thickness of the heating ` -
body 42 and layer 46 was discovered to be an important factor
in successfully utilizing the present invention. In the
development of the present invention, it was discovered that
as the thickness of the heater layer 46 was increased starting
from a small;finite thickness typically in the range of 0.1
inch to .016 inch thereby increasing the thickness of the
heating body 42, the final temperature after a given period
of heating rises at first, in other words, is positively
correlated with changes in thickness but it then falls sur-
prisingly after some critical thickness is reached and is
negatively correlated with the thickness of the heating layer.
lV91305
~, .'-
Refer to Figure 9 which clearly shows the correlation by
plotting the thickness of heating body 42, that is, of alum-
inum layer 44 and the lossy heating layer 46 against the
surface temperature after one minute of heating in a microwave
ove~. The layer 46 in both Figures 9 and lO consisted of 50~
- 325 mesh Fe30 and 50% - 30+ 325 mesh Fe304 uniformly mixed
together and bonded as a solid paint like layer to a 3 mil.
sheet of aluminum with a binder consis~-ing of a sodium silicate
solution (2.5 parts sodium silicate to 1 part water) with 11.5
~ ~1
mil. of the sodium silicate solution added for each 74 grams
of iron oxide. The heating experiments illustrated in Figures
9 and lO were carried out in a lO00 watt Litton 70/30 oven.
The particle sizes presented herein are expressed as U.S.
screen sizes. All quantities and propor~ions herein are
expressed by weight rather than volume unless so indicated.
The strongest specimens, i.e., those that withstand heating
best without cracking or other damage contain particles of
; different sizes. For that reason the materials of more than
~ one particle size are preferred.
- 20 The preferred thickness of the heating body whether a
composite sheet or a microwave absorptive heating body that is
not a composite is substantially on the rising temperature
response portion of the curve of Figure 9, in other words,
from a small finite thickness at the left to substantially the
maximum temperature response. It is in this general range that
the temperature increases as a function of increasing thickness,
i.e.f is positively correlated. The "substantially" herein
means no more than 1/3 greater than the thickness producing
the maximum temperature response. Thus, in Figure 9 for example,
the operative range extends from the low end of the curve at
the left upwardly to 3/32 inches, themaximum;. response, plus
1/3 of 3/32 inches or 1/8 of an inch. By using thicknesses
''
- 18 -
i, ~ s
1(J~1305
in this range, the following advantages are achieved. First,
the mass of the heater and its cost is kept as low as possible.
Second, the composite 42 tends to be more flexible and is more
resistant to breakage because layer 46 is better supported by
the layer 44. Third, it cools almost immediately to the tem-
perature of the food when removed from the oven thereby mini-
mizing the opportunity to burn the fingers and finally, it
heats the surface of the food at a faster rate. This can be
seen best by comparing the slopes of the curves in Figure lO
wherein heating time in the oven is plotted against the tem-
perature at four different thicknesses of composite 42.
An important feature of the invention is the discovery
that it is useful to control the final equilibrium temperature
of the heater, i.e., prevent it from exceeding a predetermined
maximum temperature by limiting the thickness of the coating
46. Thus, it can be seen that by reference to Figure 10 that ~
laminates of 1/8 and l/16 inch thickness can reach 800F. or - -
900F. However, by limiting the~thickness to l/32 of an inch,
a maximum of 600F. will be reached. In a preferred form of
the invention, the thickness of the heat absorbing sheet is
the minimum thickness that will reach, but not exceed, a
selected equilibrium temperature. However, if the temperature
of the absorber is still rising at the point where the oven
is turned off and the food is done, this preferred optional ~ -
form of the invention is not being used. While this feature
is preferred, it is not essential since turning off the oven
at exactly the correct time will prevent overheating. However,
it is not as safe and reliable.
Refer now to Figure 5 which illustrates a modified form
of the invention in which the same numbers refer to correspond-
ing parts already illustrated in Figures l to 4. As seen in
Figure 5, the spacer 41 is not used. In its place are a
-- 19 --
,
91305
plurality of supports 50 in this case four in number (only one
being shown) each of which consists of a tab or flap made by
placing a semi-circular cut in the bottom wall 29 of the box 10
near each of the corners thereof. Each of the resulting tabs is
turned up thereby supporting the corners of the heating plate 42
and the food product 43. The package of Figure 5 is less expen-
sive than Figures 1 to 4 since the corrugated material 41 is
eliminated.
Figure 6 illustrates another modified form of the invention.
A microwave food heating package 60 includes an outer container
body 62 in this case the carton formed from paperboard having
four vertically disposed rectangular sidewalls only three which
64, 66 and 68 are shown all connected together at their edges
either with or without inwardly projecting cornerfolds as des-
cribed above in connection with Figure 1 to 4. Hinged at 70 to
the upper edge of wall 64 is a top wall 69 having a tab 72 that
is glued down to hold the cover in place prior to opening. In
the package of Figure 6 are two parallel vertically spaced heat-
ing composites or laminates 42 each similar to that already
described in connection with Figure 4. If desired the upper
composite 42 can contain a more concentrated absorber in layer 46
or be thicker so as to reach about the same temperature as the
lower composite in the slightly less concentrated field found
at the top of the package. One lamLnate is placed below the food 43 with the
aluminum layer 44 facing upwardly in contact with the lower surface of the food
and the other is placed above the food and resting on top of the food product
withthealuminum layer 44 facing downwardly in contact with the upper surface
of the food. me lower laminate 42 can be supported in any suitable manner as
by means of paperboard tabs 74 which exten~ inwardly from sidewalls 64 and 68.
It will be seen that the walls 64 and 68 extend downwardly slightly beyond
the lamlnate 42 thereby supporting composite 42 a predetermuned distance, e.g.,
1/4 inch
- 20 -
130S
above the floor of the oven chamber during heating. Bonded to
the outside surface of each sidewall including walls 64-68 is a
shield comprising a strip of electrically conductive material
such as an aluminum foil strip 76 which extends all the way
around the carton thereby surrounding the food product 43. Strip
~ . .
76 together with the laminates 42, totally shields the food
product from all direct microwave energy radiation so that heat-
ing in this instance is carried out solely by means of conduction
~` from the composite 42. In this case the lower composite 42
serves as the bottom of the container. The food product in this
instance comprises any kind of food which normally is cooked very ;
little on the interior or has been precooked so that only exter-
ior scorching or browning is needed. Examples are a raw egg, a
grilled cheese sandwich consisting of two layers of bread between
which is placed a layer of cheese or a~)bao~n., lettuce and tomato
sandwich, etc. If the food product comprises a raw egg, the egg
can be surrounded by a ring or strip of paper (not shown) or
other material to prevent the albumen of the egg from spreading.
When these foods are cooked in such a package, the benefits are
surprising. In the case ofa~bacon, lettuce and tomato sandwich,
the outside of the bread is toasted and hot whereas the lettuce
and tomato remains fresh and crisp and does not become cooked,
wilted or slimey as it would if placed alone in a microwave oven
and cooked. In the case of a grilled cheese sandwich, the bread
is toasted and the cheese is warmed or slightly melted whereas if
heated alone the cheese will become extremely hot and the bread
soggy. A raw egg can be fried using the package of Flgure 6 and
it has the characteristics of an ordinary fried egg whereas when
cooked in a microwave oven alone, the finished product is some-
30 what like a poached egg. If desired, the foil strip 76 can be -
omitted to permit the entry of a controlled amount of microwave
energy into the food to heat the interior in addition to the
- 21 -
` - 1091305
.
r surface heating provided by the two composite sheets 42. This
modification is useful with a variety of foods such as batter
coated precooked filet of fish and hash brown potatoes, etc.
Figure 7 illustrates another form of the invention in which
a shipping, heating and serving package 80, particularly well
suited for heating fruit pies, comprises a container body 82
having an upper wall or cover 84 and a lower wall in the form of
a tray or pan 86. Both cover 84 and pan 86 are made from any of
the materials already described concerning the composite 42
except that the structural support layer 44 must be an electric-
ally conductive metal foil or sheet. Thus, the pan portion 86
comprises a truncated conical sidewall 88 and integral bottom
wall 90 both of which are formed from 1 mil. aluminum foil to
which is bonded a 1/32 inch thick layer of a lossy microwave
absorptive heating composition 92 on its outside surface that
can be the same as any of those described above in connection
with Figures 1-4 and 9 and 10. The cover 84 is made of the same
laminate as pan 86. It includes a metal foil layer 91 and heat-
ing composition applied as a coating or layer 93. The cover 84
has a hole 94 an inch or so in diameter to allow for the intro-
duction of a predetermined controlled amount of microwave energy
for direct heating of the food. Bonded to the top of the cover
84 is a fibrous insulating layer 96 provided with an opening 98
in alignment with the opening 94 to permit the introductio~ of
microwave energy into the food product and the escape of moisture
vapor during cooking. The insulating layer 96 can comprise any
suitable insulating material known to the art such as a mineral
; insulating material including alumina, perlite, magnesia with or
without reinforcing fibers such as glass or asbestos fibers and
the like. It can be seen that the metal foil layers 90 and 91
totally shield the food from microwave energy except that which
enters through opening 94 as well as acting as a support for
- 22 -
~091305
layers 92 and 93 respectively.
-~ Thus, the package 80 is used by placing the pie such as an
appl~e, cherry or blueberry pie in the~pan 82 at the factory,
crimping the cover 84 in place and applying the insulating layer
96. The pie is then shipped in either a frozen or refrigerated
state and if frozen can be thawed either conventionally or as
the first stage of heating in microwave oven 5. Heating to
serving temperature is carried out by placing the package in a
microwave oven and turning on the oven until the pie has reached
the proper temperature. In a 1000 watt oven this will usually
take about 5 minutes from the frozen state. As contrasted with
a pie heated alone in a microwave oven, the package 80 will
effectively heat the fruit filling primarily as a result of the
direct microwave heating due to energy passing in through open-
ing 94 while the microwave absorbing pan` and cover will crisp
the crust portion of the pie to give it an appealing taste and
texture that is much more appealing than the soggy texture of a
pie heated alone in a microwave oven. Cooking in a non-microwave
oven takes about 45 minutes. The insulator 96 was found useful
in preventing the loss of heat from the top during and immediately
after cooking. As in other forms of the invention, the container
80 is very inexpensive and can be considered disposable. More~
over, it functions for transporting the food product for heating
it and lf desired for serving the food product.
Refer now to Figure 8 which illustrates a modified form of
- the invention to be used in shipping, heating and serving of
several foods only one of which is to be heated on the surface in
accordance with the present invention. As seen in Figure 8 a
tray 100 and cover 102 are provided each of which may be generally
rectangular in plan view with mating edges 104 and 106 that hold
the cover 102 in place before the food is served. The tray 100
is divided into three compartments containing-foods 112, 114 and
- 23 -
~091305
116 by transverse ribs 108 and 110. Food products 112 and 114
can comprise foods that should be heated uniformly throughout
such as diced carrots and mash potatoes. The food product 116
is any of the kinds mentioned above which should be heated on
the surface to a very high temperature. The heating composite
` 42 can be of any of the compositions described hereinabove. It
includes a structural suppor~ing layer 44 facing upwardly against
the food 116 and a microwave absorptive heating layer 46 bonded
to layer 44 as already described. The composite 42 is supported
upon a spacer 41 also as described above. Laminated by means of
a suitable adhesive to the inside surface of the cover is a
shield 118 formed from an electrical conductor which in this
instance comprises a sheet of woven metal screen such as aluminum
screen of a predetermined size including a horizontal top portion
and side portion 118a that extends downwardly somewhat to provide
; the requisite shielding for the food product 116. A certain
amount of microwave energy will be able to reach the food product
116 from the sides. Thus, only partial and not complete shield-
ing is provided. The amount of energy reaching the food product
116 and hence the size`of the shield 118 is determined by the
inside temperature reached when the requisite degree of surface
crisping or browning is accomplished by the heater composite 42.
The tray 100 and cover 102 can comprise any suitable dielectric
; material such as pressed paper, paper fiber or plastic resin with
the requisite heat resistance and can be either foamed or non-
foamed. The same materials can be used as described in connection
; with the carton 12. During use, the tray and cover are placed
in a microwave oven which heats the foods 112 and 114 by direct
microwave transmission and food 116 both by controlled direct
transmission and by conduction from the composite 42. The food
can be served and eaten in tray 100 after the cover 102 has been
removed.
- 24 -
" lO9i305
It can thus be seen that the invention is adapted to provid-
ing a heater composite for heating b~ conduction to one or more
surfaces of a food while the food is shielded at least in part
from microwave energy. Specifically, in one form of the inven-
tion conduction heating is provided on the bottom and the top is
shielded. In another embodiment, conduction heating is provided
on the top and bottom while the side is either shielded or not
shielded. In other cases the product is totally shielded from all
direct exposure to microwave energy as illustrated in Figure 6
with heating carried out solely by conduction. However, in any
case the conduction heating browns, crisps or sears the surface of
the product or dries it out to such an extent as to remove the
sogginess or leathery character associated with such a product
when heated alone in a microwave oven. It can also be seen that
the present invention as illustrated in all embodiments except
Figure 6 will provide simultaneously direct microwave and thermal
heating in balanced predetermined proportions.
It can also be seen that the invention provides a package
which is so inexpensive and light in weight that it can be con-
sidered disposable and will afford excellent protection for a foodproduct during shipment, storage and can even be used as a serv-
ing dish. Moreover, because of the lightness of the microwave
absorbent lossy heat composite, the composite will heat at a very
rapid rate and cool down quickly thereby making the package safe
to handle after removal from the oven.
The temperatures reached after one minute of heating of
various absorptive compositions are presented below in Table 1.
The tests were run on a 1000 watt Litton 70/30 oven. Samples
were made with the composition listed to provide a complete lam-
inate of the thickness given by applying the wet coating to a 1mil. thick sheet of aluminum measuring 4 inches by 4 inches. The
coating was then dried in an oven for an hour at 250F. The lam-
inate was then placed in an oven and heated without any food
product in contact with it during the test.
- 1~39i305
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--26--
~ ~09i305
The food can be safely eaten directly from the package with
little danger of burning the mou~h or fingers since the heat
absorbing member cools by the ti~e the food is eaten to the
temperature of the food before the food is eaten. For the
purposes of the present invention, it is assumed that the food
is eaten about 30 seconds or more from the time that the oven
is turned off.
The packages of the invention can also be sold empty for
the consumer to use in heating any food product in the home
and can be disposed of after use or used repeatedly as desired.
In such an application of the invention the packages can be
marked with the use intended, e.g., for heating pizza pie, for
steaks, hamburgers, etc., for toasting sandwiches, for fruit
pies, etc. In each case the thickness and composition of the
heat absorbing layer 46 and the size of the openings in the
shield, if any, would be the best for the particular food
marked on the label. ~-
It can also be seen that the heating body or composite 42
has the following important attributes. First, it heats quickly
to a temperature that will brown or scorch the surface of the
food. Second, in a preferred form of the invention it reaches
a maximum temperature within the safe temperature zone for the
food being heated if left too long in the oven, and third, it
cools fast so as to reach the temperature of the food product
by the time the food is eaten. In the case of a ferrous heating
layer formed from particles held together with a binder it was
found that the preferred thickness range for layer 46 is between
about .02 and .187 inches. When thinner than this range, the
absorber does not get hot enough nor heat fast enough for most
foods. When above this range, the microwave energy absorber
tends to heat too slowly, eventually reaches an unsafe temper-
ature and retains heat too long for safety.
- 27 -
` 109~305
It will also be seen that each of the packages described
has a space therein to receive a food product and the shield
whether a separate piece as 38 and 76 or laminated to the micro-
wave energy absorber as in Figure 8 at least partially encloses
the space for the food to partially or completely shield it from
microwave energy. The heating body 42 is located adjacent to
and defines one or more boundaries of the space for the food.
-28-
