CA1153069A

CA1153069A - Food receptacle for microwave cooking

Info

Publication number: CA1153069A
Application number: CA000346808A
Authority: CA
Inventors: Oscar E. Seiferth
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-03-16
Filing date: 1980-03-03
Publication date: 1983-08-30
Also published as: JPS6015548B2; FR2451182A1; JPS55126063A; DE3010189C2; GB2046060B; GB2046060A; FR2451182B1; DE3010189A1

Abstract

FOOD RECEPTACLE FOR MICROWAVE COOKING

ABSTRACT OF THE DISCLOSURE

A disposable food receptacle for use in microwave cooking is disclosed which includes a provision to brown the exterior of the food in the receptacle. A thin layer of an electrically conductive material, such as an elemental metal is incorporated into the receptacle on the food contacting surfaces thereof, so that the conductive layer will become heated by the microwave radiation and will, in turn, brown the exterior of the food in the receptacle. The conductive layer is form as an extremely thin film deposited on a substrate protective layer by a process of vacuum vapor deposition.

Description

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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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~2- 1 - ~1~i3~)~9 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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~L5;31~69 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 -'1- _ 31~69 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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~ ~15i3069 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 11~3~69 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 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 _ I ___ __ _ .

;3(~69 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 ~- ~53~6g 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, .' 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 ~ ~ 53069 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 ~`

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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~ ~153069 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 10' 1~

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILIGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a food preparation receptacle for use in microwave cooking of foods the improvement comprising:
a layer of electrically conductive elemental metal incorporated into the receptacle, the layer of metal being sufficiently thin so as to be rapidly heated upon exposure to Microwave radiation and cause surface browning of the food in the receptacle.

2. In a food preparation receptacle for use in micro-wave cooking of foods the improvement as claimed in Claim 1 wherein the metal layer has a resistivity greater than about .4 ohms per square inch.

3. In a food preparation receptacle for use in micro-wave cooking of foods the improvement as claimed in Claim 1 wherein the layer of metal is aluminum.

4. A food preparation receptacle for use in microwave cooking of foods comprising:
a stock material formed into a suitably shaped structure to carry the desired food;
a protective layer formed on the food contacting surfaces of the stock material; and a layer of aluminum arranged between the protective layer and the stock material, the aluminum layer having a thickness of less than about 700 angstroms so that the aluminum layer will be heated when exposed to microwave radiation to brown the food in the receptacle.

5. A food preparation receptacle as claimed in Claim 4 wherein the aluminum layer has a thickness of between about 30 and about 700 angstroms.

6. A food preparation receptacle as claimed in Claim 4 wherein the protective layer is formed of a thermoplastic resin having high heat stability.

7. A food preparation receptacle as claimed in Claim 6 wherein the thermoplastic resin in the protective layer is a polyester.

8. A food preparation receptacle as claimed in Claim 7 wherein the aluminum layer is deposited on the polyester layer.

9. A food preparation receptacle as claimed in Claim 8 wherein the aluminum layer is deposited by vacuum vapor deposition on the polyester layer.

10. A preparation receptacle as claimed in Claim 4 wherein the stock material is paperboard.

11. A food preparation receptacle for use in micro wave cooking of foods comprising:
a stock material formed into a suitably shaped structure so as to carry the desired food;
a protective layer formed on the food contacting surfaces of the stock material; and a layer of electrically conductive elemental metal arranged between the protective layer and the stock material, the metal layer being of a suitable thickness so as to have an electrical resistivity of at least about .4 ohms per square inch so that it will be heated when exposed to micro wave radiation to brown the food in the receptacle.

12. A food receptacle as claimed in Claim 11 wherein the metal layer is aluminum.

13. A food receptacle as claimed in Claim 11 wherein the resistivity of the metal layer is between about .4 and 8 ohms per square inch.

14. A food receptacle as claimed in Claim 11 wherein the protective layer is a sheet of high temperature stability thermoplastic material.

15. A food receptacle as claimed in Claim 12 wherein the metal layer is deposited on one surface of the thermo-plastic film.

16. A food receptacle as claimed in Claim 15 wherein the metal is deposited on the thermoplastic film by vacuum vapor deposition.

17. A food receptacle as claimed in Claim 11 wherein the stock material is paperboard.

18. A food preparation receptacle for use in microwave cooking of foods comprising:
a stock material formed in a suitably shaped structure so as to carry the desired food;
a protective layer formed on the food contacting surfaces of the stock material; and a thin layer of electrically conductive elemental metal deposited by vacuum vapor deposition on the protective layer and positioned between the protective layer and the stock material, the metal layer being heated upon exposure to microwave radiation to brown the food in the receptacle.

19. A food preparation receptacle as claimed in Claim 18 wherein the metal is aluminum.

20. A food preparation receptacle for use in microwave cooking of foods comprising:
a stock material formed into a suitably shaped structure so as to carry the desired food;
a protective layer formed on the food contacting surfaces of the stock material; and a layer of electrically conductive elemental metal arranged between the protective layer and the stock material, the metal layer being of a suitable thickness so as to have such an electrical resistivity that it will be rapidly heated when exposed to microwave radiation to brown the exterior of food in the receptacle during the normal micro-wave cooking of that food.

21. A disposable food package comprising:
a low density insulating stock material of suitable shape and structure to contain the food;
a protective layer adhered to the food contacting surfaces of the stock material; and a thin layer of electrically conductive material deposited on the protective layer and arranged between the protective layer and the stock material the layer of electrically conductive material being rapidly heated upon exposure to microwave radiation so as to brown the surface of the food in the package.

22. A food package as claimed in Claim 21 wherein the stock material is paperboard.

23. A food package as claimed in Claim 21 wherein the layer of conductive material includes a layer of conductive elemental metal.

24. A food package as claimed in Claim 23 wherein the elemental metal is aluminum.

25. A food package as claimed in Claim 23 wherein the metal layer is formed by vacuum vapor deposition on the protective layer.

26. A food package as claimed in Claim 21 wherein the protective layer is polyester.