Note: Descriptions are shown in the official language in which they were submitted.
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B~CKGROUND O~`'l'IIL INVENri'ION
Field of the I;lventioll
The present invention relates to foocl packac~es in
gelleral, and in particular, to food receptac]es for use in
the mierowave cooking of foods which incorporate therein
provisions for the brownincJ of the cxterior of the food in
the receptacle.
Deseription of the Prior ~rt
It is of general concern in the art of microwave cookinc3
that the exterior of the food being cooked is browned so as
to more elosely resemble food eooked by conventional methods.
If a speeial provision is no-t made for brownincJ the exterior
of the food, the exterior of the food eooked in a mierowave
oven will remain undereooked beeause of the surfaee eooliny
lS effect of the food as it is heated by mierowave radiation.
Therefore it has been a c~eneral objee-t within the art of
mierowave eooking of foods to perfeet an economical and
efficient method for browning the exterior of the foods
beinc~ eooked in a mierowave oven.
l`he prior art is generally eogllizant, therefore, of
various attempts to ineorporate layers into reeeptaeles usecl
~or the mierowave eookinc~ of foocls, whiell layers are designed
to be partieularly suseeptible to being heated by mierowave
radiation. Sueh layers are eonventionally made of seMi- ¦
eonduetive materials sueh as tin oxide. ~.xamples of N.S.
patents showillg the use of sucl- layers can be seen in [l.S.
Patents No. 3,853,612, and No. 3,965,323. Other examples of
methods used to obtain browning heat from microwave radiation
are shown in ll.S. Patellts No. 2,582,17~, No. 3,701,872, No .
3,773,669, all(l r~O. ~, 003,8~0.
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In acl~lition at leas~ one exclmple is known of a mcthod
of heatincJ foods in a microwave oven which incorporates
therein the use of ferro-magnetic me-tals. Such example is
shown inlU.S. Patent No. 2,~30,162. In addition at least
¦ one U.S. patellt leaches the incorporat:ion of an electrically
conductive film in a ceramic dish or other heavy vessel for
use in -the browning of foods coo]ced in a microwave oven.
llowevcr, the exalllples shown in that patent, which is U.S.
Patent No. 3,7~3,220, are directed to a Eilm of tin oxide,
with alternative substances disclosed beincJ cellulose fiber
and si-licon carbide. No example of such a container for use
in tho browniny of foods in a microwave oven is known in the
prior art which incorpora-tes therein -the use of an elemental
metal, nor the use of an extremely thin conductive Eilm on
a low cost, disposable substrate.
SUMIIARY OF THE INVENTION
he present invention is summarized in =ha-L in a food
preparation receptacle for use in microwave cooking of foods
there is an improvement including a layer of electrically
conductive elemental metal being incorporated into the
receptacle, the layer of metal being sufficiently thin so as
to be rapidly heated upon expose to microwave radiation and
cause surface browning of the food in the receptacle.
¦ It is a primary object of the present invention to
25 ¦ provi~e a means for the browning of foods cooked in a micro-
wave oven which can be ineorpora-ted in-to disposable packages
or reeeptacles for pre-packayed or frozen food.
It is another object of the present invcntion to
construct a suitable food pac]caye whi ch incorporates therein
such a provision for browning the exterior of the food to be
coolced in a microwave OVell.
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It is yet another object of the present invention to
incorporate a provision for sueh browning of foods during
mierowave cooking which is economical and efficient in its
operation.
Other ob~ects, advantages, and features oE the present
invention will beeome apparent from the following speci- ¦
fication when taken in conjunction with the accompanying'
drawings.
BRIEF DESCRIPTION OF 'l`HE DR~INGS
Fig. l'is a perspective view of a Eood package con-
structed in accorclance with the present inven-tion.
Fig. 2 is an enlarged cross-sectiollal view of the
material of the package of Fig. 1.
Fig. 3 is a perspective view of an alternative emLocli-
ment of a food package constructed in accordance with -the
present invention.
Fig. 4 is an enlarged cross-sectional view of the
material of -the package of Fig. 3.
DESCRIPTION OF TIIE PREI;`FRRED l~MBODIi`lI~NT
Shown in Fig. 1 is a food recep-taele for use in micro-
wav~ eooking, generally indicated at 10, constructed in
accordanee with the present invention. The reeeptaele 10,
as shown in Fig. 1, is a four-sicled generally tray shaped
paekage for reeeiving a prepared cluantity of food material
therein. The reeeptaele 10 as shown ineludes shallow sicles
and a wide bottom and may or rnay no-t include a top eovering
thereover, or may be constructed i,n any desirecl ~eometry or
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shape depending on the contents to be carried by the recep- ¦
tacle 10. The receptacle 10 is designecl especially so as to
be disposable, and is thereEore cc.nstructed of economical
commercially available components.
Shown in ~ig. 2 is a cross-sectional cliagram of the
package material from which the package 10 of ~ig. 1 is
construeted. This package material, which is indicated at
12 in Fic~. 2, includes generally three layers. The upper-
most layer as illustrated in Fig. 2, which is designed to be
the layer most near to the food procluct, is a high heat
tolerant protective layer 14. Formed unclerneath the pro-
tective layer 1~ is an ex-tremely thin layer of conductive
material, such as vacuum vapor cleposited elec-trically conductive
elemental metal 16, for e~ample a thin layer of elemental
aluminum. The combination of the protective ]ayer 1~ with
the metal layer 16 adhered thereto is moun-ted on a base
layer of structural s-tock material 18. The function of each
oE these layers will be described in turn.
"'he function oE the stock material 18 is to provide
struetural rigidity and support for the physical shape of
the paekage 10, whatever the configura-tion of the package 10
may be. The stock matericll :Layer 18 is preEerably formecl of
a low density material having a relatively high insulating,
ca~aeity and a heat stability suficient to withstand cooking
temperatures in a microwave oven. Suitable materials for
use as this stock material are papers, glassine materials, ~ ¦
plasties, eeramies and various coated papers. Preferred
materials Eor use in disposable receptacles include coated
kraft paper and other eonventional kraft paper combinations
eonventionally used for paperboard cartons and packages.
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The ~unction of the protective layer :l.4 is to ensure
that the ~oocl product inside of the receptaele 10 cloes not
contact either the metal layer 16 or the stock material
l.ayer 18. rrhe protective l.ayer 14 also sea^ves as a stock
material onto which the metal layer 16 may be deposited
during the construction of the package material 12. Suitable
materials for use in cons-tructing the protective layer 14
. inelude polyesters, polyet}lylene, nylon, cellophane, poly-
sulphone, and other relatively stable plastie substanees.
It is importan-t that the material of the proteetive layer 14
be of sufficient stability at hicJh temperatures that it will
not degrade during the operation of the microwave oven at
the termperature selected for cookiny the desired food. It
has been founcl that polyester is a particularly well suited
material for use as the protective layer 14 in view of its
stability and its surface smoothness.
Also included within the package ma-terial 12 is the
metal layer 16 bonded to the rear surface of the proteetive
layer 14 and the food contactin~ surface of the s-tock material
1~. 'rhe metal layer 1.6 is formed as a vacuum vapor deposited
ilm of electrically conductive elemental. metal, whieh, as
stated abovc, is preferably clo~?ositecl on the lower surEace
of the protective layer 14 before such layer is bonded -to
the stoek material 18. It is a primary eharacteristic of .
the metal layer 16 that it is ex-tremely thin in terms of its
mechanieal thickness. In fact, under current technology it
is vlrtually impossible to mechanically measure the exact.
thickness of the layer of the metal film 16. In the c~eneral
art of vacuum vapor deposition, i.t is therefore conventional
to measure the thickness of vacuum vapor deposited layers o
el.eetrical.].y conclucti.ve meta~ teria:L in terms of the
. surfaee resistivi.ty of the metal layer i.tself. This is
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possible inasmuch as these clepositecl metal layers are so
thin thclt they have an appreciable and easily measurable
resistance to the flow of electric current such as is not
the case in conventional thickness metal films or foils.
I-~aving measured the electrical resisti.vity oE a metal layer,
it is possible with a reasonable degree of certainty to
appro~imate the mechanical thickness of such a layer, based
on the amount of metal material applied to a qiven square
area during the application of the metal layer onto the
substrate, which in this case is the protec-tive layer 1~.
It is to be understood however, -that the film oE the metal
layer 16 is of such a thinness that direct measurement of
its dimensions are impossible, and such measurements must be
made inclirectly. Such .Ellms are conventionally so thin that
when deposited on transparent ma-terials they may readily be
seen through by the human eye.
~ film of the type of the me~tal layer 16 in the package
of the present invention serves to provide a browning function
in a pàckac~e for food when cooked in a microwave oven. In
conventional microwave cooking, i.t is norma:Lly believe(.~ that
it is bad practice to include utensils or packages i.ncorpo-
ratincJ therein anv significant amounts of elemental conductive
metals. ~lally microwave oven manufacturers inelude instruc-
tions with their ovens warning against the use of aluminum `
or similar metal utensils withln -the microwave oven. Such
warnings are included to avoid a refl.ection problem inasmuch
as microwaves are unable to pene-trate such metal utensils
and are reflected backwardly :therefrom. Such refleetions
can result in areas of food within the oven not being cooked,
and ean potenti.ally, in some si.tuations, eause damacJe to the
. klystron of the mi.erowave oven. It is also readily observable
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53069
that conventional foil products such as aluminum foil clo not
heat up when subjecte~ to microwave radiation, thus making
sucIl Material inoperative as a heat source for surface
browniIlg oL Eoocls in a microwave OVCIl .
~pplicaIlt has found, however, that when a metal layer
16 is of sufficient thinness, it surprisingly does not
detract from the operation of the microwave oven ancl, apparently
because of its incomplete reElection of the microwaves
incident thereoIl is, in fact, heatecI very efficiently by
the microwave oven. Thus, when the receptacle 10 of the j;
present invention with food therein is placed within a
microwave OVetl allCl subjected to microwave radiation, the
metal layer 16 rapidly heats -to a relatively high tempera- !
ture. Tlle heat generated by the metal layer 16 acts to
brown the surface of the Eood contained within the recep- ¦
tacle 10. Such browning eEfect is a si~nificant advantage
in receptacles for food designed to be cooked in microwave
ov~lls.
In conducting experimentation upon the use of -the me-tal
layer 16 and receptacles for use in cooking foods in a
microwave, a variety of different thickness of the me-tal
layer 16 have been used with success. It has been generally
found that metal layers 16 having a surface resistance which
varies between about .4 and 8 ohms per s~uare inch offer
satisfactory results in a receptacle according to the present
invention. ~s stated, the thickIless of this material is not
directly mechanically measurable, but appropriate calcula-
tions indicate tha-t a film oE aluminum in the metal layer 16
would have a thickness of between 200 and 300 angstroms if
its resistance was 1.5 ohms per square inch. Using such
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numbers as a basis for calculakion, it is believed tha-t for
the metal aluminum, thicknesses having a surface resistance
of between .4 and ~ ohms per square inch vary in thickness
between approximately 700 and 40 angs-troms.
Such films as the metal layer 16 may be cleposited on
substrates, in this case -the protective layer 14, most
efficiently through the use of vacuum vapor cleposition.
Such technique involves the melting of elemental metal
material in a vacuum charnber and drawing the subs-trate
through the vacuum chamber in close proximity to the mel-ted
metal material. The molten metal ma-terial emits a metallic
vapor which is deposite~l on the substra-te as it moves through
the chamber, and the amount of material deposited on the
substrate may be adjusted by the rate at which the substrate
moves through the vacuum chamber. It is believed that such
vacuum vapor deposition technique is the most efficient
manner for making the metal layer 16 usakle within the
present invention. It is an advantage of such materials
that many types of such materials, such as aluminum coated
Oll polyester, usable in the present invention, are readily
commercially available a-t the present time.
The exact upper limit of the metal layer 16 usable in
the present invention is not readily determinable using
currentl~ commercially available products. For example, the
thinnest commercially available film or foil of aluminum
Material that is pin-ho]e- Eree has a thickrless of approximately ¦
.00025 inches. This thickness corresponds to approximatel~ ¦
65,000 angstroms. ~,xperimentation has shown that this
thickness is too great to allow this foil to heat up upon
exposure to microwaves. The gap between the thinnest commer- ¦
cially avai ble rri ] s, 1 . e. the .00025 inch toil, and the
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vacuum vapor deposited films present:Ly markc-ted is presently
about two orders oE magnitude, and no materials are readily
available on the shelf between -these thicknesses. It is
believed by the applicant that metal films for use within
the metal layer 16 oE the present invcntion may prove func-
tional at some -thickness yreater than that described in this
application. However the exac-t limit of mechanical thiclc-
ness is unknown at present.
In order for a technology to be useful for forming the
ln metal layer 16 of -the present invention, the me-tal layer 16
Must be of such thinness as to be readily and rapidly heated
upon the exposure thereto by microwave radiation. Such
heating of such a metal layer 16 must be rapid, which as
used herein is meant to mean -that the heating must occur
1~ within a sufficient amoun-t of time and reach a sufficient
temperature so as to be capable of browning the exterior of
foods durin~ tlle normal cookin~ time of such foods in a
microwave oven. ~s for example, it has been found that a
vacuum vapor deposited metal layer 16 having a surface
resistance of approximately 2 ohms per square inch is capable
of achieving a temperature in excess of 200 within 30
scconds. Similarily, a metal laycr 16 hav~ y a surface
rcsistance appro.Y]mately equaling ~I ol-ms per square inch
will achieve a temperature exceedillg 200 in a time period
between 20 and 30 seconds. There appears to be a generally
linear relation between the surface resistivity of the metal
film used for the metal layer 16, and the amount of time
required to achieve'a certain,temperature for that metal
layer when exposed to microwave radiation. The more surface
resistance of the metal layer, the faster the metal layer 16
heats UpOIl expos-lre to microwave radiation. In any event,
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53069
it is necessary for a met~l laycr 16 which is l:o be uscd
within the receptacle 10 oE the present invention to achieve
and hold a browning temperature, which is generally in
exeess of 200 F., within a period of time short enouc~h so
as to brown the exterior of the food during the time neeessary
to coo]c the foocl received within the receptacle 10.
Shown in Fig. 3 is an alternative embodiment of a food
reeeptaele constructed in accordance with the present
invention. The reeeptacle of Fic3. 3, generally indicated at
22, is in-tended to ellcase therein a quantity of sausage 20
or similar sausage type material. The receptacle 22 is
formed as a thin wrapping of ma-terial about the exterior of
the sausac3e 20. Fic~. 4 shows an enlarcJed view of the
pac~age material of the pacJcaye 22. Similar to the reeep-
taele 10, the receptaele 22 inelucles a pro-teetive layer 28
UpOIl the back of which a vacuum vapor deposited metal layer
26 is arranged. The combination of a protective layer 28 and
the metal layer 26 is received inside a stock material 24
whieh wraps around the exterior of the receptacle 22. The
stoek material 24 of the reeeptacle 22 is selected so as to
be much more flexible than the stock material 18 of the
reeeptaele 10.
The receptacle 22 c)f Fi(Js. 3 and ~ funetions in mueh
the same way as the receptacle 10 of ~igs. 1 and 2. tpon ~
exposure to microwave radiation, the receptacle 22 serves to
browl- the extcrior of the sausa~e 2n received~witllin the
reeevtaele 22. Such exterior browning is extremely helpful
in the art of sausage making inasmuch as it steriliæes and
seals the exterior of the sausage 20. ~y usinc3 a receptacle
~ ~ 22 eonstruet / in aecordsnee \~ith the present inventlon, it
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is possible for such sterLli~Aation and se~lincJ to be aceom-
plished in an e~Lieient anci ecollomical manner by the ex~osure
of finished sausages 20 encased in the receptacle 22 to
mierowave radiation at the processing plant.
It is envisionecl that many types o~ receptacles Eor use
within the presellt invention may be eonstrueted utilizinc3
the metal layer 16 therein. Thus it is envisioned that many
disposable-type receptacles for use in the microwave cooking
of eonvenienee, frozen, or other pre-packaged foods may be
eonstrueted. In addition it is also envisioned that a metal
layer 16 may be ineorpora-ted into a reeep-taele such as
serving dishes, plate`s, or casserole dishes which are not,
of eourse r of a disposable nature and are more in the
nature of housewares to be readily reused by a homemaker
durincJ the prepara-tion of foods in a microwave oven. It is
also envisioned that the receptacle including the metal
layer 16 aecording to the present invention may be any of a
wide variety of wrapvings which are used -to wrap foods, such
as is shown in the sauage package 22 of ~igs. 3 and 4.
It is a particular advantage of -the present invention,
however, in that it allows the construetion of a disposable
reeeptaele whieh ineludes a provision Eor -the browning of
food in a mierowave oven whether the layer 16 is formed of
metal or other eonduetive material. Prior art reeeptacles
for browning mierowave cooked foods have been large, massive ¦
and relatively expensive artieles entirely unsuited for food
paekaging but usable only as dishes or eooking u-tensils. ~y
eombining readily availablc and eeonomieal ma-terials in the
reeeptaele of the present invention, a food paekage ean be
eonstrueted whieh ean be used to paekage -the food for the
eonsumer and for its eooking and browning in a mierowave
ovcn, aEter W]liCh the ~ackacJc can be disearded. This resul-t
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was not possible in thc prior art. !;urtherlllore because
conductively coated polyester and paperboard materials,
hich are the preerred materials for such a packa~e, are
relatively light and of low density, they make more efficient
use of the microwave energy absorbed -than would a heavy
ceramic or glassine utensil which re~uires a substantial
amount of heating before it becomes hot to the touch. Thus
a food package, such as the receptacle 10 of Figs. 1 and 2,
constructed in accordance wi-th the presen-t invention offers
a significant advance in the art of browning devices for
foods cooked in microwave ovens by enabling a disposable
device capable of ef~ectiv~ and efEicient operation.
It is further envisioned that a number of metals other
than aluminum may be used for the metal layer 16 when that
layer is used in a recep-tacle according to the present
invention. It is believed important, however, that the
material for use in the metal layer 16 be an elemental metal
and that the metal be of an electrically conductive character.
Other suitable me-tals for use within the metal layer 16
include copper, tin, lead, silver, gold, nickel, and ZillC~
althougll some of those obviously ~ould not be chosen Eor
dis~osabLe rcceptacles because oE cost considerations. It
is also envisioned that combinations or alloys of these
metals are also usable in the present invent1on. I~hile the .
~5 exact thickness of the metal layer 16 necessary to achieve a
browning effect may vary with the metal utilized for the
metal layer 16, it is envisioned that any such metals would
be most efficiently used in constructing such a layer by
vacuum vapor deposition, and that the surface conductivity
of the metal layer 16 would be approximately equivalent for
tl~c cli.EEcl-cllt mctal matcri.al.s whctl usccl witl~ a rcceptacle
in accor~a--c w:i th the p~-CIl t inve n t Lon.
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It is to be understood th~t the Dresent invention is
not limlted to the particular construction and arrangement
of parts disclosed and illustrated herei.n, but e~braces all
such modified forms thereoE as come within the scope of the
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