NZ236962A - Coated slotted electroconductive material for microwave browning - Google Patents

Description

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No: Date: NEW ZEALAND PATENTS ACT, 1953 COMPLETE SPECIFICATION CONTROLLED HEATING OF.' FOODSTUFFS BY MICROWAVE ENERGY BECKETT INDUSTRIES INC, a company organised and existing under thelaws of the Province of Ontario, Canada of 610 South Service Road, Oakville, Ontario, Canada L6K 2H4. hereby declare the invention for whiclvf/we pray that a patent may be granted to tygfus, and the method by which it is to be performed, to be particularly described in and by the following statement:- (foil owed by page la) — la-' ?36962 "AMAZE-MET" 1679-152 MIS 367 1991 01 25 D3 TITLE OF INVENTION CONTROLLED HEATING OF FOODSTUFFS BY MICROWAVE ENERGY FIELD OF INVENTION The present invention relates to the cooking of foodstuffs by microwave energy.

BACKGROUND TO THE INVENTION The use of microwave energy to cook a variety of 10 foodstuffs to an edible condition is quick and convenient. However, some foodstuffs require crispening or browning to be acceptable for consumption, which is not possible with conventional microwave cooking.

It is known from U.S. Patent No. 4,641,005 15 (Seiferth), assigned to James River Corporation, that it is possible to generate thermal energy from a thin metallic film (microwave susceptor) upon exposure thereof to microwave radiation and this effect has been used in a variety of packaging structures to achieve cooking of 20 foodstuffs with microwave energy, including achieving crispening and browning, for example, of pizza crust.

Some food products which are to be cooked by microwave energy are in the form of an outer pastry dough shell and an inner filling. An example is an apple 25 turnover. One problem which has arisen when packages employing thin metal films to generate thermal energy to obtain crispening and browning of such products, is that there is a considerable moisture loss from the filling and sometime a spilling of filling as the shell splits 30 open, leading to an unsatisfactory product.

In addition, certain foodstuffs are difficult to brown and crispen satisfactorily. For example, while it is possible to improve the cooking of pot pies when compared to conventional oven-cooked pot pies, by the 35 employment of microwave energy and multiple thin films of electroconductive material in the bottom of the dish, nevertheless the resulting product does not/^Mfca^^n ideal degree of browning. ff? II 'Y " Z 12 JUL 1993 ° \ $ ? 3 6 9 6 2 Attempts have been made to improve the overall uniformity of heating which results when thin metal film microwave susceptors are exposed to microwave radiation. One such proposal is contained in U.S. Patent No. 4,927,991 (Wendt), assigned to The Pillsbury Company, which describes the employment of a microwave-reflective grid in combination with a thin metal film microwave susceptor. The structure is stated to achieve a more uniformly heated foodstuff by controlling surface heating 10 and microwave transmittance.

Another approach to the microwave cooking of foodstuffs is described in U.S. Patent No. 3,845,266 (Derby), assigned to Raytheon Company. This patent describes a utensil for microwave cooking, which is 15 intended to be reusable in a microwave oven and is illustrated, in one embodiment, as taking the form of a slotted rigid stainless steel plate. The slotted nature of the stainless steel plate is said to achieve browning and searing of foodstuff in contact with it in a 20 microwave oven. The stainless steel plate sits on a member of microwave transparent material, such as glass, in the cavity of a microwave oven to effect such heating.

E « / , I* « °'l ;\\ 12 JUL 1993 " J ;' *o-/ 236962 3 It also has been previously suggested from U.S. Patent No. 4,23 0,924 (Brastad et al) to provide microwave energy generated browning of a foodstuff from a food package which includes a flexible wrapping sheet 5 of polymeric film having a flexible metal coating, which either may be relatively thin film or relatively thick foil and which, in either case, is subdivided into a number of individual metallic islands in the form of squares. It has been found that, while some thermal 10 energy generation is achieved by such structures, both with the relatively thin film and the relatively thick foil, little or no shielding of microwave energy is achieved using the described relatively thick foil structure. In this latter prior art, the metal is 15 provided in the form of discrete islands which are separated one from another, and hence the metallized portion of the substrate is discontinuous in character.

Further, there have been a variety of proposals to moderate the proportion of incident microwave energy 2 0 reaching a foodstuff by using perforated aluminum foil.

For example, U.S. Patents Nos. 4,144,438, 4,196,331, 4,2 04,105 and 4,268,738, all assigned to The Procter & Gamble Company, disclose a microwave cooking bag formed from a laminate of two outer thermoplastic films 25 sandwiching a perforated aluminum foil having a series of large circular apertures therethrough. While this arrangement may be useful in moderating the microwave energy entering the foodstuff, these openings are not of a size or shape which permits the generation of thermal 3 0 energy, so that no surface browning can result.

Similarly, U.S. Patents Nos. 3,219,460 (Brown), 3,615,713 (Stevenson), 3,985,992, 4,013,798 and 4,081,646 (Goltsos) describe T.V. dinner trays intended for use for microwave cooking of such foods, in which 35 the lid is provided with apertures of varying dimension through microwave opaque materials incorporated into the 236962 4 lid structure to control the flow of microwave energy to the different food products in the tray. Again, the apertures are not of a size or shape to permit the generation of thermal energy.

In the present invention, the manner of thermal energy generation described in the aforementioned U.S. Patent No. 3,845,266 is utilized, but in a unique manner in an article of manufacture from which packaging structures may be formed. This arrangement enables me to overcome the prior art problems that I have referred to above in the microwave heating of a variety of foodstuffs, particularly those requiring crispening and browning.

It has now been found that flexible electroconductive materials, normally opaque to microwave energy, can be modified so that thermal energy can be generated from the flexible electroconductive material in a selected and controlled manner.

Such normally microwave-opaque electroconductive materials, for example, aluminum foil, have been commonly-used to achieve shielding of foodstuffs from microwave radiation during microwave cooking, but are not known as being capable of converting any portion of the incident microwave radiation to thermal energy, in contrast to the very thin electroconductive material layers described in U.S. Patent No. 4,641,005 mentioned above.

I have found that, if a plurality of elongate apertures of appropriate dimensions is formed in the flexible electroconductive material, then thermal energy is generated in the region of the apertures upon exposure of the flexible electroconductive material to microwave radiation. For the generation of thermal energy, it is essential for each of the plurality of the apertures to be elongate and to T3e~~-separate and .. -•».

SUMMARY OF INVENTION 236962 discrete.

For the layer of flexible apertured electroconductive material to be usefully incorporated into a packaging structure, it is essential that the 5 layer of flexible electroconductive material be supported on and be in adhered structural relationship with a substrate layer of microwave energy transparent material. provided a multiple layer article of manufacture adapted to be formed into a packaging structure in which a foodstuff may be heated by microwave energy to an edible condition. By providing an article of manufacture which is able to be formed into a packaging structure, in 15 accordance with the present invention, a food product may be maintained in the same structure through the multiple steps of filling, freezing, storing, shipping, retailing and then microwave reconstitution for consumption, before discard. 2 0 The article of manufacture of the invention comprises a layer of flexible electroconductive material supported on a substrate layer. The layer of flexible electroconductive material has a thickness which is normally substantially opaque to microwave radiation 25 and has a plurality of elongate apertures extending wholly through the thickness of the electroconductive material layer and effective to generate thermal energy in the plurality of apertures when the article of manufacture is exposed to microwave energy and the 30 foodstuff is in contact with or proximate to the plurality of apertures.

The plurality of apertures is sized and arranged in the layer of flexible electroconductive material to generate sufficient thermal energy to effect a desired 35 surface browning of the foodstuff while permitting sufficient microwave energy to s the layer of In accordance with the present invention, there is 1 236962 flexible electroconductive material through the plurality of apertures into the foodstuff to effect a desired degree of dielectric heating of the foodstuff, whereby the foodstuff may be provided in an edible 5 condition.

The substrate layer is formed of microwave energy transparent material and is in adhered structural supporting relationship with the flexible layer of electroconductive material so that a packaging structure 10 may be formed from the article in which the foodstuff may be positioned.

The multiple layer article of manufacture of this invention, while superficially similar to the structure disclosed in aforementioned U.S. Patent No. 3,845,266 15 (Derby) , in that both employ a slotted structure to generate thermal energy from microwave radiation in the cooking of foodstuffs. However, significantly structural differences exist: (a) The article described in '266 is a utensil for 20 a microwave oven in the form of a slotted rigid stainless steel plate, whereas the article with which the present invention is concerned is suitable for formation of packaging material for foodstuffs, which 25 enables the advantage of employing the same structure for the foodstuff through its multiple stages of processing from filling of the packaging structure to microwave reconstitution of the foodstuff to be 30 realized. The structure shown in '266 is not capable of utilization or adoption as a packaging structure and is employed solely during microwave reconstitution of the foodstuff; (b) In the present invention, the layer of apertured electroconductive material is a 236962 7 flexible material, such as aluminum foil, whereas the element that produces thermal energy in '266 is described as load supporting and hence must be capable of supporting the 5 load of the foodstuff to be heated. As noted above, the element takes the form of a rigid stainless steel plate. The flexible electroconductive material layer used in the present invention is not itself load 10 supporting, in the sense the term is used in '266; (c) In the present invention, the flexible electroconductive material layer is supported on and adhered to a substrate layer of 15 microwave transparent material. By providing this multiple-layer structure, packaging structures may be provided in which the foodstuff may be packaged from filling to consumption. It should be noted that the 20 article resulting from the combination of the flexible electroconductive material layer with the substrate layer may itself take a variety of physical forms, depending upon the packaging structures to be formed therefrom 25 and the foodstuff to be packaged therein, including flexible, stiff or semi-stiff or rigid. The '2 66 patent discloses a second element, which is a member of microwave transparent material, which also is rigid and 30 underlies the slotted stainless steel plate in the microwave oven cavity.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a perspective view of a pot pie dish provided in accordance with one embodiment of the 35 invention; Figure 2 is a sectiona-1. view of the pot pie dish of 236962 8 Figure 1; Figure 3 is a plan view of a blank from which the pot pie dish of Figure 1 is formed; Figures 4 to 11 show alternative forms of the blank 5 of Figure 3; and Figure 12 is a perspective view of a microwave bag provided in accordance with an additional embodiment of the invention.

GENERAL DESCRIPTION OF INVENTION 10 It is generally known that electroconductive metals having a thickness above that at which a portion of the microwave radiation is converted into thermal energy become largely opaque to microwave radiation, such as aluminum of foil thickness, and this effect has been 15 employed to achieve shielding of foodstuffs from microwave energy, in a variety of structures, such as is described above.

In the present invention, a plurality of elongate apertures is formed through the electroconductive metal 20 layer. In this structure, the metal or other electroconductive material shields the foodstuff from the passage of microwave energy therethrough while microwave energy is permitted to pass through the elongate apertures into the foodstuff. At the same 25 time, a portion of the microwave energy passes through the apertures, producing an intense field at the periphery and access the whole dimension of each aperture, which, in turn, causes surface browning of the food. 3 0 In this way, the intensity of microwave energy reaching the foodstuff filling is considerably decreased by the shielding effect of the metal, while permitting browning and crispening of the exterior, so as to produce a cooked food product with much decreased 35 moisture loss.

The proportion of incidertti mi ^36962 9 through the apertures into the foodstuff may be increased by making the gap wider, while making the gap longer and narrower increases the intensity of the surface heating. By appropriate choice of individual 5 aperture size and number of apertures, heating of the foodstuff by microwave energy is controllable to a considerable degree.

As noted above, several structures have been described which employ circulator or similarly 10 geometrically-shaped openings in shielding structures. However, the different geometry of opening employed in the present invention produces a dramatically-different result, namely that the present invention enables thermal energy to be produced for surface browning and 15 crispening while achieving shielding of the foodstuff from exposure to the full effect of the microwave energy.

This result enables a much greater degree of control to be achieved over the microwave cooking of 20 food products which are comprised of component parts which require different degrees of cooking, and, in particular, those that require outer crispening or browning and yet may suffer from moisture loss, which may lead to some sogginess of the product, if over-25 exposed to microwave energy. Examples of foodstuffs which may be cooked or reheated for consumption with advantage by microwave energy, using the structure of the present invention, are french fries, pot pies, pizzas, burritos and apple turnovers.

In the present invention, there is employed a layer of flexible electroconductive material which is of a thickness which is normally opaque to microwave energy, and which is supported by and adhered to a layer of microwave transparent material. The minimum thickness 35 varies with the material chosen. Generally, the layer has a minimum thickness of abotTt>i_micron. The flexible 236962 electroconductive material layer conveniently may be provided by aluminum foil having a thickness of about 1 to about 15 microns in thickness, preferably about 3 to about 10 microns, typically about 7 to about 8 microns.

Other suitable electroconductive materials include stainless steel, copper and carbon.

The layer of electroconductive material is provided with a plurality of elongate thermal energy-generating apertures therethrough. The number, size and 10 relative location of the elongate apertures depends on the size of the foodstuff and the degrees of internal cooking and of surface browning desired.

Each aperture is elongate and may comprise a single opening formed into a spiral or other pattern so 15 as to have the physical appearance of a plurality of apertures. Each aperture of the plurality of apertures generally is no shorter than about 1.75 cm and may extend for any desirable length. An aperture generally varies in width from about 1 mm to about 2 cm, provided 20 that the length is greater than the width. In general, more surface heating of the foodstuff is achieved as the apertures become longer and narrower. As the apertures become wider, more microwave energy is able to pass through into the interior of the foodstuff, so that less 25 intense heat generation and less shielding of the microwave energy from penetration to the foodstuff result.

Where a plurality of individual apertures is employed or the plurality of apertures is provided by a 30 single aperture arranged in a pattern, a metal spacing of at least about 0.5 mm is maintained between individual apertures or between portions of the same aperture.

Where a plurality of individual apertures is 35 employed, the apertures may be equally dimensioned and equally spaced apart, which produces an even degree of 236962 ii heating over the expanse of the continuous layer of electroconductive material containing such plurality of individual ones or groups of the plurality of apertures 5 may be varied and may be located only in selected portions of the expanse of the continuous layer of electroconductive material, so as to achieve differential degrees of heating, differential ratios of internal and surface heating and shielding only, as 10 desired, in various locations of the expanse of the layer of electroconductive material. The number, location and size of the apertures may be such as to achieve any desirable combination of microwave energy reflected, transmitted and converted into thermal energy 15 for the packaging structure, both in the overall structure and locally within the structure.

Another alternative which may be used, depending on the result which is desired, is to provide, in each aperture, an electroconductive material of sufficient 20 thinness that a portion of microwave energy incident thereon is converted to thermal energy, as described in U.S. Patent No. 4,641,005 (Seiferth), referred to above, so as to augment the browning effect which results from the aperture itself.

Using the guidelines above, it should be possible for a person skilled in the art to manipulate the apertures in the layer of flexible normally microwave-opaque electroconductive material to provide the required degree and type of heating for any given 3 0 foodstuff to achieve the optimum cooked condition for consumption.

The elongate apertures may be formed in the continuous flexible electroconductive material layer in any convenient manner, depending on the nature of the electroconductive material and the physical form of the electroconductive material. apertures. However, the dimensions and spacing of U 236962 12 For example, with the electroconductive material being a self-supporting aluminum foil layer, the apertures may be stamped out using suitable stamping equipment, and then adhered to the substrate layer.

Alternatively and more preferably, with the electroconductive material being aluminum foil or other etchable metal supported on a polymeric film, such as by laminating adhesive, the apertures may be formed by selective demetallization of metal from the polymeric 10 film using, for example, the procedures described in U.S. Patents Nos. 4,398,994 and 4,552,614, the disclosures of which is incorporated herein by reference, wherein an aqueous etchant is employed to remove aluminum from areas unprotected by a pattern of 15 etchant-resistant material. Another possible procedure involves the use of ultrasonic sound to effect such selective demetallization.

Following such selective demetallization, a polymeric lacquer or other detackifying material may be 20 applied over the exposed surfaces of laminating adhesive in the selectively demetallized electroconductive layer to inhibit adjacent layers from adhering to one another as a result of exposed adhesive in the apertures, when a web of such selectively demetallized material is rolled 25 up, as is often the case prior to formation of the desired packaging material.

For the purpose of providing a packaging material, the apertured flexible electroconductive material layer is supported on and adhered to a continuous substrate of 30 suitable microwave-transparent substrate, which generally is microwave-transparent stock material which does not deform upon the generation of heat from the layer of electroconductive material during exposure of a foodstuff in the packaging material to microwave energy. 35 The flexible layer of electroconductive material may conveniently be laminated to a paper o] 236962 13 substrate as the stock material, which may be semi-stiff or stiff, with the packaging material being formed from the resulting laminate. Similarly, the layer of flexible electroconductive material may be laminated to 5 a heat-resistant polymeric material substrate as the stock material to provide the article of manufacture. The layer of flexible electroconductive material also may be laminated between two outer paper or paperboard layers, or may be laminated between a heat resistant 10 polymeric material layer, and a paper or paperboard layer. In these structures, the polymeric material layer, such as polyester or polyethylene, may be flexible or rigid.

Alternatively, the flexible layer of 15 electroconductive material may be laminated to a single or between two rigid thermoformable polymeric material layer(s), by adhesive bonding, and the laminate may be thermoformed to the desired product shape.

The multiple layer article of manufacture of the 20 present invention may be incorporated into a variety of packaging structures for housing foodstuffs where the generation of thermal energy during microwave heating is desired. The structures may include a variety of trays and dishes, such as disposable pot pie dishes and rigid 25 reusable trays or dishes, a variety of bag structures, such as french fry bags and bags for cooking crusty filled products, for example, an apple turnover, a variety of box structures, such as pizza boxes, and domestic ware, such as reusable or disposable plates and 3 0 dishes.

As noted above, one of the significant advantages of the structure of the present invention is the ability to employ the structure in manufacturing, retailing and consumption of the foodstuff packaged therein. The 35 packaging structure generally conforms to three-dimensional form of the foodstuff, whetbeJ 236962 14 form of relatively stiff or rigid dish or tray, or in the form of a flexible bag structure, to enable the desired microwave heating of the foodstuff to be achieved.

It may be desirable to provide a layer of release material on food-contacting surfaces of the structure, to inhibit sticking of food to such surfaces.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to Figures 1 to 3, there is shown 10 therein a pot pie dish 10 constructed in accordance with one embodiment of the invention. (Figure 3 shows the blank for the dish 10 prior to stamping or other suitable forming operation). As seen therein, the dish is of conventional shape, having a circular base 12, an 15 upwardly and outwardly flared side wall 14 and an outwardly-extending lip 16.

The pot pie dish 10 is formed from a laminate of an outer layer 18 of paperboard of suitable thickness to provide structural support to the dish 10, an inner 20 layer 20 of heat-resistant polymeric film and a layer 22 of aluminum foil or similar flexible microwave-opaque electroconductive material sandwiched therebetween.

The layer 22 of aluminum foil has a plurality of elongate narrow thermal energy-producing apertures 24 25 formed therethrough. The apertures 24 are provided in a star-like array in the aluminum foil layer, with arms radiating in a uniform pattern from the centre of the base 12 of the dish.

By arranging the apertures 24 in this way, it has 3 0 been found that, when a pot pie is microwave cooked in the dish 10, the microwave energy 24 is channelled by the apertures 24 towards the center of the dish 10. Heat is generated along the length and width of each of the apertures 24 but the heat is more intense in the 35 base 12 of the dish 10, resulting in improved browning of the crust on the bottom of the pot'pie, inatoMBgsLson 236962 to previously-proposed structures.

In addition, the presence of the microwave-opaque aluminum foil in the side wall 14 limits the proportion of the incident microwave energy which can pass through 5 the side wall 14 into the contents of the pot pie dish.

By controlling the proportion of microwave energy entering the pot pie filling, the pot pie may be cooked by microwave energy for consumption and an adequate degree of browning both to the side wall and bottom of 10 the pot pie achieved, without overcooking the pie filing and causing significant moisture loss.

In Figures 1 to 3, the apertures 24 are rectangular in outline shape and are of the same width, although differing in length and having a significantly 15 longer longitudinal dimension than transverse dimension. Figures 4, 5 and 6 illustrate alternative arrangements of apertures in a blank from which a pot pie dish may be formed by a suitable forming operation, in which the apertures are elongate and of regular geometric shape 20 but not rectangular. In Figure 7, the apertures 24 are formed as a plurality of series of concentric rings. In Figure 8, the apertures 24 are formed in the shape of two discontinuous spirals.

In Figure 9, the aperture 24 takes the form of a 25 single spiral, with an additional small counter-spiral formed near the center to enhance thermal energy generation at that location. Figures 10 and 11 illustrate further alternative structures for the arrangement of apertures. 3 0 Each of the arrangements of apertures shown in Figures 1 to 11 is useful for a microwave oven pot pie dish. The specific arrangement employed, or whichever alternative arrangement is employed, depends on the desired proportion of incident microwave energy to be 35 converted into thermal energy, to b^reflected by the microwave-opaque aluminum foil or to be; permitted to 236962 16 pass through into the pie filling.

In Figure 12, there is shown a bag structure 40 for heating a foodstuff by microwave energy, such as french fries or apple turnovers. In this case, the bag 5 structure is formed of a laminate of outer and inner layers of paper and a layer of aluminum foil or similar flexible microwave-opaque electroconductive material sandwiched therebetween. A plurality of elongate thermal energy-generating apertures 42 is formed through 10 the aluminum foil in a regular parallel array. In a modification of this structure, the laminate may comprise only one side of the bag structure.

With this arrangement, the desired outer crispening of the foodstuff by thermal energy produced in the 15 apertures 42 may be achieved while the shielding effect of the remainder of the continuous aluminum foil layer slows down the heating of the interior of the foodstuff, decreasing moisture loss and avoiding boil-over. Various modifications to the numbers, geometry, 20 dimensions and spacing of the apertures 42 may be effected, along the lines discussed above with respect to the pot pie dish structure of Figures 1 to 11, to achieve any desired microwave heating effect with respect to the foodstuff packaged in the bag structure. 25 EXAMPLES Example 1 A chicken pot pie was cooked for 6 minutes in a standard microwave oven packaged in a pot pie dish as illustrated in Figure 1 and also as illustrated in my 3 0 aforementioned copending U.S. patent application No. 442,153 ("Pot Pie Dish").

Both products were cooked ready for consumption. However, with the pot pie dish of Figure 1, the moisture loss from the pie was around 14% while from the pie 35 cooked using my prior invention moisture Joss was around 26%, i.e. considerably less in the case of the present 236962 17 invention.

In addition, an examination of the exterior of the pot pie in the two cases showed improved browning in the base portion of the pot pie cooked using the pot pie dish of Figure 1 when compared to the prior structure. Example 2 A vegetable pastry was cooked for 3% minutes in a microwave oven in a bag structure as illustrated in Figure 11 and in a bag structure as illustrated in my copending U.S. patent application Serial No. 421,668 filed October 16, 1989 ("Ele-Met Bag"), the disclosure of which is incorporated herein by reference. In both cases, the product was cooked, ready to eat and had a browned exterior. However, in the case of the bag of Figure 11, the moisture loss was 14%, as compared with 26% for the prior structure, i.e. considerably less in the case of the present invention.

SUMMARY OF DISCLOSURE In summary of this disclosure, the present invention provides a novel structure which is able to control the flow of microwave radiation to a foodstuff, so as to control the degree of cooking and the ratio of internal to external cooking. Modifications are possible within the scope of this invention.

Claims (30)

236 962 WHAT-3/WE CLAIM IS: 18

1. A multiple layer article of manufacture, adapted to be formed into a packaging structure in which a foodstuff may be heated by microwave energy to an edible condition, comprising a layer of flexible electroconductive material supported on a substrate layer, and wherein (a) the layer of flexible electroconductive material has a thickness which is substantially opaque to microwave radiation and having a plurality of elongate apertures extending wholly through the thickness of the electroconductive material layer and effective to generate thermal energy in the plurality of apertures when the article is exposed to microwave energy and the foodstuff is in contact with or proximate to the plurality of apertures, (b) the plurality of apertures is sized and arranged in the layer of flexible electroconductive material to generate sufficient thermal energy to effect a surface browning of the foodstuff while permitting sufficient microwave energy to penetrate the layer of flexible electroconductive material through the plurality of apertures into the foodstuff to effect heating of the foodstuff, and (c) the substrate layer is formed of microwave energy transparent material and is in adhered structural supporting relationship with the flexible layer of electroconductive material so that a packaging structure can be formed from the article in which the foodstuff can be positioned.

2. An article as claimed in claim 1, in which the layer of flexible electroconductive material has a thickness of at least 1 micron.

3. An article as claimed in claim 1 or 2, in which the layer of electroconductive material is aluminum foil having a thickness of from 1 to 15 microns.

4. An article as claimed in claim 3, in which the aluminum foil has a thickness of 3 to 10 microns.

5. An article as claimed in any one of claims 1 to 4, in which each aperture has a width of at lee 19 236962 a length of at least 1-75 cm.

6. An article as claimed in any one of claims 1 to 5, in which the microwave transparent material is paper or paperboard.

7. An article as claimed in claim 6, in which the microwave transparent material is provided on one side of the layer of electroconductive material and a polymeric film is provided on the other.

8. An article as claimed in claim 6, in which the microwave transparent material is provided on both sides of the layer of electroconductive material.

9. An article as claimed in any one of claims l to 5, in which the substrate layer comprises a polymeric film layer to which the layer of electroconductive material is adhered by laminating adhesive.

10. An article as claimed in claim 7 or 9, in which a layer of food release material is provided on food-contacting areas of the polymeric film layer on the opposite side thereof from that to which the electroconductive material is adhered.

11. An article as claimed in claim 9 or 10, in which the plurality of elongate apertures in the layer of electroconductive material is formed therein by selective demetallization.

12. An article as claimed in claim 11, in which the layer of electroconductive material is coated with a layer of detackifying material following the selective demetallization.

13. An article as claimed in any one of claims 1 to 12, in which a further layer of electroconductive material having a thickness sufficiently small as to effect conversion of a portion of incident microwave energy to thermal energy is positioned in engagement with the substrate layer to achieve an augmented heating effect in the plurality of apertures by exposure of the further layer of electroconductive material to microwave energy.

14. An article as claimed in any one of claims,-;Ipfeci 'is C 'I ^ 12 J (J L1993 1 r r 236962 20 formed into a bag structure adapted to enclose the foodstuff.

15. An article as claimed in claim 14 in combination with the foodstuff enclosed within the bag structure.

16. An article as claimed in claim 15, in which the foodstuff comprises a crusty filled product.

17. An article as claimed in claim 15, in which the foodstuff comprises french fries.

18. An article as claimed in any of claims 14 to 17, in which the plurality of apertures comprises a plurality of individual parallel elongate apertures closely spaced one from another.

19. An article as claimed in any one of claims 1 to 13 formed into a dish adapted to receive the foodstuff.

20. An article as claimed in claim 19, in which the dish is a pot pie dish having a bottom wall and a side wall.

21. An article as claimed in claim 20, in which the plurality of apertures is arranged extending radially from the centre of the bottom wall of the dish and into the side walls of the dish to be engaged by the pot pie when located in the dish.

22. An article as claimed in claim 20 or 21 in combination with a pot pie located in the dish.

23. An article as claimed in any one of claims 19 to 22, in which each of the apertures has the same width.

24. An article as claimed in any one of claims 19 to 22, in which the plurality of apertures comprises an elongate spiral extending from the side wall of the pot pie dish to the centre of the bottom wall.

25. An article as claimed in any one of claims 1 to 6, in which the layer of electroconductive material is laminated between outer layers of polymeric material, one of which constitutes the substrate layer.

26. An article as claimed in claim 25, in which at least one of the polymeric material layers is formed of rigid moldable material.

27. An article as claimed in claim 26 molded into ' c" ■ /n v h r \fl2JULI993 236962 21

28. An article as claimed in claim 27 in combination with the foodstuff located in the tray or dish.

29. An article as claimed in any one of claims 1 to 28 in combination with the foodstuff packaged therein with the plurality of apertures located in thermal energy-generating relationship with the foodstuff.

30. A multiple layer article of manufacture substantially as hereinbefore described with reference to, and as illustrated, in the accompanying drawings. By the authorised agents