GB2202118A - Packaging materials for use in microwave ovens - Google Patents

Abstract

A disposable package 16 containing a foodstuff 22 to be placed in a microwave oven to be heated up and surface-cooked is formed at least in part from a material having discrete susceptor/receptor regions 14 that are heated by microwave irradiation (e.g. very thin metal films). These regions 14 are desirably in the form of stripes, e.g. 4mm wide with 4mm spacings 15 of microwave-transmissive material 10. The susceptor/receptor regions 14 may not only lead to surface cooking (e.g. crisping and browning) but may also control the distribution of microwave radiation within the package. The material may be multi-layer, e.g. a laminate of paper 12 and polyester film 10 whose inner face bears the stripes 14. <IMAGE>

Description

PACKAGING MATERIALS FOR USE IN MICROWAVE OVENS The present invention relates to packaging materials for use in microwave ovens. In one aspect it relates to packages, optionally containing food products, the term "package" including bags, cartons and trays. In another aspect the invention relates to multi-layer material for producing such packaging. In a third aspect the invention relates to the preparation of a food product comprising placing the food product in a package of the above type and subjecting it to microwave energy.

As is well known, the effect of microwave cooking is normally to generate heat within an item of food, particularly when this contains water. The result tends to be that the interior of an item cooks rapidly. The surface is prone to heat loss, and may not be cooked adequately at the same time as the product interior. In any case, the cooked product is likely to have a distinctive "microwave-cooked" appearance, owing to the absence of surface browning characteristic of conventional cookery. Related surface characteristics such as crispness are also likely to be missing. To ameliorate such problems, it is known to employ special browning dishes.As disclosed in GB 2,046,060, if a receptacle has a continuous but very thin coating of metal, this rapidly heats up in a microwave field, and the hot wall of the vessel can effect superficial cooking of a food product. (It must be stressed that the metal layer has to be extremely thin otherwise it seriously disrupts the operation of the microwave oven.) However, such browning dishes are of limited utility.

The metalised walls become extremely hot, and tend to burn food in contact with them. Furthermore, many of the benefits of microwave cookery are lost, and the result tends to be a food product which is burnt without and undercooked within.

We have now found that a useful packaging material for use in microwave cookery can be provided by providing a material with discontinuous susceptor/receptor areas. Such material is useful in providing disposable packages, particularly for foodstuffs.

A susceptor/receptor is a material which heats rapidly upon exposure to microwave irradiation. It may comprise a very thin layer of a material which is electrically conductive (e.g. a metal, e.g. as disclosed in GB 2,046,060) and/or which is ferromagnetic. By providing packaging material in which susceptor/receptor areas and microwave-transmissive areas form a mutually interpenetrating pattern, we can produce packages which can offer many of the advantages of both microwave and conventional cooking. Thus it is possible to provide surface browning (without scorching), and simultaneously to provide even heating throughout the bulk of a food product.

The packaging material may be based on any convenient web material, e.g. polyester or paper. This is provided with a pattern of susceptor/receptor areas. The web may be bonded to a strong (and possibly rigid) layer, e.g. with the susceptor/ receptor areas sandwiched between the web and the strong (or "stock") material.

It is desirable for the film to be protected (since it may tend to be removed by accidental abrasion, and portions might come off on a packaged article in direct contact). Thus particularly if the film is not on an internal surface of a multi layer material, there may be a protective lacquer layer. Lacquer layers as used on the inside of cans are particularly appropriate, as they are known to be heat resistant and safe for prolonged contact with foodstuffs.

The susceptor/receptor areas may be in the form of stripes. These may for example be a few millimetres wide, with spacings of a few millimetres. A suitable width may be a few millimeters (e.g. l-lOmm, preferably 2-6mm, e.g.

4mm). The optimum value may depend on the nature of the food to be cooked (e.g. its moisture content), its spatial relationship to the packaging material, and characteristics of the microwave oven, e.g. wavelength. Similarly, the susceptor/receptor may advantageously be spaced a distance above the shelf of a microwave oven.) Many alternative patterns are possible. For example, the susceptor/receptor areas may be circles or stars, e.g. of diameter of about 5mm, with spacings of about 5mm between adjacent circles or stars. The form of the patterning can be selected to suit particular food products and types of microwave field. In most cases, the desirable effect seems to be that the susceptor/receptor areas cause scattering of the microwave radiation so that it is distributed within the package.

The areas are also rapidly heated, so that they can cause superficial cooking. A package may be made entirely of material having the intermittent susceptor/ receptor pattern, or such material may form only a portion.

The currently preferred susceptor/receptor materials are metal films, such as are described in GB 2,046,060. We measure the surface resistivities or the optical densities of films to assess coating thicknesses. Thus, the types of metal layers which are known as barrier layers or for decorative effect have optical densities of around 2.

These would not behave well in a microwave oven. The materials which we use generally have optical densities in the range 0.3 to 0.5.

Such films can be produced by vacuum vapour deposition, e.g. onto paper or a plastics film. This can be arranged to produce desired patterns. Thus stripes may be produced by using a template in a deposition zone through which a web is displaced. Alternatively, a web can be uniformly coated and subsequently patterned, e.g. by etching or abrasion. Chemical and electrochemical deposition methods, as used in electroplating, e.g. in making printed circuit boards, may also be applicable. A film can be applied indirectly, via a transfer process.

The initial carrier of the film can be selected to give it a desired surface finish, e.g. matt or highly polished.

An alternative way to apply the pattern is by a printing technique, e.g. gravure or flexographic printing using conductive "inks" (e.g. containing carbon) or ferromagnetic inks (e.g. containing particles of a ferrite material). When printing is used, it is particularly easy to create a complex pattern - e.g. in register with other printing and/or comprising a desired design such as a customer's logo.

Coatings and cutting techniques such as are used in the field of magnetic recording tapes may be useful.

An embodiment of the invention will now be described in greater detail with reference to the accompanying drawings in which: Fig. 1 is a view of a portion of packaging material embodying the invention; Fig. 2 is a schematic end view of a package embodying the invention; and Fig. 3 is a schematic section on III-III in Fig. 1.

Figs. 1 and 3 show a portion of material having an inner layer 10 of polyester (e.g. Mylar (trademark) 14m thick) adhered to an outer layer 12 of paper (e.g.

grease-resistant bleached Kraft paper of 70-78 g.s.m.).

The face of the polyester 10 which is bonded to the paper 12 is patterned with an array of stripes 14 bearing metallisation, separated by unmetallised zones 15. In this example, the metallisation is aluminium applied by chemical vapour deposition to an optical density of 0.4. The metallised and unmetallised zones are each 4mm wide.

Fig. 2 shows a bag 16 which has been formed from the material of Fig. 1. The bag is generally conventional, being formed with fold lines 17 to form a gusset so that it can be stored in flat form, then opened up to provide a flat base 18 and an internal volume 20 for holding a food product 22. The upper region of the illustrated bag has been heat-sealed or glued at 24 to seal the product 22 into the bag. In this state, it can be stored, e.g. in a deep freeze. When the product is required, the container is removed from storage, and ventilation openings 26 are cut in it. It is then placed in a microwave oven. Microwave radiation passing through the uncoated areas 15 (and possibly scattered by the stripes 14) get into the interior of the container (and may be reflected by the stripes 14).

It thus causes microwave cooking" of the food product 22.

Meanwhile, the stripes are heated by microwave radiation, and can produce surface cooking of the food product 22, with attendant browning and/or crisping if desired.

The polyester and paper layers 10,12 can be bonded together over their entire areas (e.g. by an adhesive layer 30 as shown in Fig. 3 or by heat sealing) or the paper can be "loose lined", e.g. being connected to the polyester only adjacent the mouth of the bag.

The food product 22 can be a loose fit within the bag 16, (so as to be spaced from at least part of the packaging material), or it can be in intimate contact with the packaging materials, in which case its surface may be heated by conduction rather than by radiant heat.

Whereas we have referred to providing ventilation openings 26, for food that does not produce much steam, it may be sufficient for the bag to have at least portions of substantial porosity. These may be provided by a suitable grade of paper, or by a plastics film that has been rendered porous, e.g. by microperforation.

Claims (17)

CLAIMS:

1. Packaging material for use in a microwave oven having susceptor/receptor regions ("s/r regions") in the form of stripes separated by areas transmissive of microwaves.

2. Material according to claim 1 wherein the s/r regions and transmissive areas define stripes each of width l-lOmm.

3. Material according to claim 2 wherein the stripes are of width 2-6mm.

4. Material according to any preceding claim which is a multilayer material with the s/r stripes provided on an internal surface.

5. Material according to any preceding claim wherein the s/r stripes have a protective lacquer coating.

6. Material according to any preceding claim wherein the s/r regions are metal films formed by deposition.

7. Material according to any of claims 1 to 5 wherein the s/r regions are provided by a printing process.

8. Packaging material for use in a microwave oven having susceptor/receptor regions ("s/r regions") in the form of discrete islands separated by areas transmissive of microwaves.

9. Packaging material substantially as any herein described and illustrated herein with reference to the accompanying drawings.

10. A package comprising packaging material according to any preceding claim.

11. A package according to claim 10 containing a food article for heating in a microwave oven.

12. A package according to claim 11 wherein the article is a loose fit in the package.

13. A method of manufacturing a packaging material according to any of claims 1-9 comprising applying to a microwave-transmissive material. spaced regions of material to constitute s/r regions.

14. A method according to claim 13 wherein the s/r regions are provided by a metal film formed by vapour deposition.

15. A method according to claim 14 wherein the metal film is deposited on a carrier and transferred to said microwave-transmissive material.

16. A method according to claim 13 wherein the s/r regions are applied by a printing process.

17. A method of manufacturing a packaging material substantially as any described herein with reference to the accompanying drawings.