GB2300791A

GB2300791A - Heat-shrink material container for use in a microwave oven

Info

Publication number: GB2300791A
Application number: GB9609856A
Authority: GB
Inventors: Arthur Neville Jones; Joseph Peter Singleton; Roger William Lukehurst
Original assignee: United Biscuits Ltd
Current assignee: United Biscuits Ltd
Priority date: 1995-05-12
Filing date: 1996-05-10
Publication date: 1996-11-13
Also published as: GB9609856D0; WO1996035626A1

Abstract

A method of heating food comprising the steps of providing the food in a container (2) which is shrinkable, placing the container (2) with the food therein in a microwave oven and exposing the container to microwaves to heat the food, an effect of the heating being to shrink the container (2) and thereby reduce the internal volume of the container. The food container (2) is housed within an outer container (1).

Description

Improvements in and relating to Dackaaina food products The invention relates to packages for food products, and more especially to a food package to be placed in a microwave oven and to a method of heating food in a microwave oven.

References in the specification to "microwave(s)" are to be understood as referring to electromagnetic radiation of a wavelength such that it is suitable for use in heating food and able to penetrate into the food so that the direct heating is not confined to the exterior of the food. A microwave oven is hereby defined as an oven in which heating is carried out at least partly by microwaves.

When certain food products, notably those containing a substantial proportion of vegetable matter, are cooked, the volume of the products reduces significantly. Thus, a product, such as stir-fry vegetables, for heating in a microwave oven may fill its container prior to heating but after heating may leave vacant a significant part of the volume of the container. In such a case, a consumer inspecting the heated product may consider (wrongly) that the container was inadequately filled initially, and, because the top of the product is at a lower level within the container, the appearance of the product in the container may be impaired and access to the product may also be difficult.

It is an object of the invention to provide a method of heating food, and a food package suitable for use in such a method, which overcomes or mitigates the disadvantages referred to above.

The present invention provides a method of heating food, comprising the steps of providing the food in a container which is shrinkable, placing the container with the food therein in a microwave oven and exposing the container to microwaves to heat the food, an effect of the microwave heating being to shrink the container and thereby reduce the internal volume of the container.

Usually, care would be taken when designing a food container to avoid or minimize any changes in the dimensions of the container during heating and changes would be confined to slight expansion during heating. In contrast, the present invention specifically sets out to ensure that the internal volume of the food container is reduced as a result of the heating, so that any reduction in the level of the food in the container as a result of the shrinkage of the food is reduced, or eliminated, or converted into an increase in level.

The invention is principally concerned with the case where an effect of the heating is to reduce the volume occupied by the food, but it is possible also to apply the invention to a case where the food does not shrink significantly, in which case the level of the food in the container will be raised as a result of the reduction in the internal volume of the food container.

A further advantage of the shrinkage of the container may be, especially in the case where the food is not fluid, to draw the food together thereby promoting conduction of heat from one region of the food to another leading to a more even temperature throughout the food.

Preferably, the internal volume of the container after heating of the food is less than 75 per cent of the internal volume of the container before heating.

With a reduction in internal volume of at least that amount, the level of food in the container is affected to a significant extent. A substantially greater reduction in internal volume may be preferred and can be obtained; in a specific example of the invention described below there is a reduction in internal volume of about 50 per cent.

A variety of mechanisms for obtaining the shrinkage of the container can be used, including, for example, an arrangement for moving a boundary wall of the interior of the container inwards as a result of expansion of an element outside and adjacent to the boundary wall.

Whilst a mechanism such as that just described is to be regarded as within the scope of the invention and the reference to shrinking of the container should be understood accordingly, it is preferred that shrinking of the container is obtained as a result of at least a portion of a wall of the container reducing in area.

It is possible to arrange for approximately uniform shrinkage of the container to provide the reduction in volume, but in general it is preferable to provide for shrinkage to be confined mainly to selected parts only of the container and to be in a selected direction only. In a case where the container has a base and one or more side walls it is preferred that the reduction in the internal volume of the container is caused principally by a reduction in the height of said one or more side walls.

For example, in the case of a cylindrical container of circular cross-section, confining the shrinkage substantially to the side wall and to longitudinal, rather than circumferential, shrinkage enables the reduction in volume of the container to be achieved without distortion of the container walls and by simply a reduction in the axial length of the container.

The invention also provides a food package especially suitable for use in the method described above.

According to the invention there is also provided a food package comprising an outer container and an inner food container housed within the outer container, the inner food container being such that when it is placed in a microwave oven with food therein and exposed to microwaves for heating the food, the container shrinks and the internal volume of the inner food container is thereby reduced.

The provision of an outer container housing the inner food container has several advantages. A particular advantage is that the provision of an outer container impedes the conduction of heat from the food to the hand of a person holding the package; that is especially advantageous when it is intended that the food be eaten directly from the package by a person holding the package. Preferably, the outer container is made of a thermally insulating material; it is also preferable that there be a space formed around the inner container between the inner and outer containers; such a space further inhibits the conduction of heat from the food to the hand of a person holding the package and also reduces temperature differences between different regions of the food in the inner food container by facilitating the transfer of heat around the inner container.

Preferably the upper end of the inner food container is supported by the outer container and the inner food container is suspended within the outer container. By suspending the inner food container at its upper end, shrinkage of the container can be accommodated simply by the remaining portion of the inner container adjusting its shape and/or size within the outer container and, for example, rising up within the outer container.

The inner food container may be of various shapes known oer se including, for example, the shape of a rectangular tray. Preferably, the inner container is of generally frusto-conical form, the axis of the conical section being upright. The inner food container may be of generally cylindrical form and of circular crosssection, the axis of the cylinder being upright, or the inner food container may be tapered with the smaller cross-section towards the bottom. Such axially symmetric shapes are especially suitable for allowing shrinkage of selected parts of the container.

The outer container may also be of various shapes, not necessarily being the same as the shape of the inner container. A first preferred arrangement is for the outer container to have the general outline of a sixfaced solid object with opposite, substantially horizontal, top and bottom faces and four side faces; the four side faces may be upright or inclined. A second preferred arrangement is for the outer container to be of generally frusto-conical form, the axis of the conical section being upright; more particularly the outer container may be of generally cylindrical form and of circular cross-section, the axis of the cylinder being upright, or the outer container may be tapered with the smaller cross-section towards the bottom.

Preferably there is an opening in the top of the outer container, in which opening the inner food container is received. Such an arrangement is especially suitable for facilitating suspension of the inner food container within the outer container.

A cover is preferably provided, which in a closed condition of the outer container covers over the opening but which can be moved or removed to expose the opening.

Preferably, the cover is provided by a flap which is pivotable away from a position overlying the opening.

The inner food container is preferably secured to the outer container in the region of the opening.

Preferably a peripheral flange formed on the inner food container is supported by portions of the outer container that surround the opening. The underside of the flange may be fixed to upper surfaces of the portions of the outer container that surround the opening.

The opening may be defined by openings or recesses in a plurality of separate flaps forming a top face of the outer container, each of the flaps being connected to respective side faces of the outer container.

The outer container is preferably formed from folding a sheet material. In such a case the arrangement described immediately above, in which the opening is defined by openings or recesses in a plurality of separate flaps forming a top face of the outer container, is especially advantageous. The sheet material is preferably cardboard.

The inner food container is preferably such that when it is placed in a microwave oven with food therein and exposed to microwaves for heating the food, at least a portion of the wall of the container is caused to shrink as a result of the microwave heating.

The inner food container is preferably made of a plastics material especially a thermoplastic material.

Various plastics materials may be employed, including olefinic polymers, cross-linked olefinic polymers, polyamides and suitable polyamide/ionomer laminates, but thermoplastic polyesters, including thermoplastic copolymers of polyesters are especially preferred. The material chosen must be suitable having regard to the food that it receives and preferably has good shrink properties in the temperature range to which it is likely to be heated during heating of the food product contained in the container. Typically the temperature range will be 95 to 105 C. PETG polymers that are used in stretch blow moulding and similar applications provide suitable materials. Such polymers are based on the reaction of terephthalic acid and one or more glycols, for example ethylene glycol and cyclohexane dimethanol.Particular examples of materials that may be used are PETG Copolyester 6763 ESTAR and PET Copolyester 9921W EASTAPAK, available from Eastman. During heating in a microwave oven, the material will usually be heated directly by the microwaves only slightly (although this will depend on the material chosen) and will be heated mainly by the food in the container which in turn is heated by the microwaves.

A preferred form of inner food container has a base and one or more side walls and is such that when the container is placed in a microwave oven with food therein and exposed to microwaves for heating the food with the result that the internal volume of the container is reduced, the reduction in volume is caused principally by a reduction in the height of said one or more side walls.

As indicated above, the internal volume of the inner container after heating of the food is preferably less than 75 per cent of the internal volume of the container before heating.

Preferably, the inner food container is such that it shrinks when it is heated to a temperature that lies in the range of 95 to 105"C.

The manner in which the inner food container is formed will affect the way in which it shrinks when heated and by suitable choice of the forming method a particular desired mode of shrinking can be achieved.

Suitable forming methods include vacuum forming and blow moulding. For example, by adopting a vacuum forming method in which there is a much greater flow of plastics material along the one or more side walls towards or away from the base and relatively little in the base, for example by using plug assistance, it is possible to arrange for the shrinkage on heating to be primarily along the side walls. Alternatively, by using a conventional multi-stage vacuum forming process or a conventional stretch blow moulding process much more even shrinkage properties are likely to be provided throughout the container.

The present invention further provides a method of making a food container for placing in a microwave oven to heat the food, the method including the steps of: forming a container of a material, which is able to shrink at a temperature that lies within the range of 95 C to 105 C, by a method which leads to such potential for shrinkage being built into the container, placing food in the container, and closing the container.

As already indicated, there are various methods by which the container can be formed to build in the required potential for shrinkage and the particular method chosen in any case will depend on various factors, including the amount of shrinkage required of different parts of the container.

The method of making the food container preferably further includes the steps of providing an outer container to form a food package as defined above.

The method may further include the step of heating the food in the container by a method of heating food as defined above.

Several food packages and methods of heating food employing them, all in accordance with the invention, will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is an exploded perspective view of a first form of package showing an outer container partly assembled and an inner food container prior to location within the outer container; Fig. 2 is a plan view of only the outer container of Fig. 1 with the assembly of the outer container more advanced than in Fig. 1 but incomplete; Fig. 3 is a plan view of the first form of package showing the inner food container housed within the outer container with a closure flap of the outer container open and horizontal;; Fig. 4 is a side sectional view of the first form of package in the same condition as in Fig. 3, showing the package prior to heating, but with the closure flap of the outer container open and vertical; Fig. 5 is a side sectional view substantially the same as Fig. 4 but showing the package after heating; Fig. 6 is a perspective view of the outer container of a second form of package; Fig. 7A is a perspective view of a third form of package; and Fig. 7B is a side sectional view of the package of Fig. 7A, the section being taken along the lines X - X in Fig. 7A.

Referring first to Fig. 1, there is shown a package generally comprising an outer container 1 and an inner food container 2. Food that would be in the inner food container 2 when it is placed in an oven is not shown in any of the drawings.

The outer container 1 is made by folding a cardboard blank into a cuboidal shape and has four vertical side walls 3A to 3D, a base 4 (not shown in Fig. 1) formed by folding and gluing various flaps and a top portion 5 comprising four flaps 5A to 5D, each of which is connected to a respective side wall 3A to 3D. The flap 5A has a circular hole 6 centrally positioned therein and is sufficiently long to extend across substantially all of the top of the container. The flaps 5B to 5D are shorter and each extend across slightly less than half of the top of the container. The ends of the flaps each have circular segments 7B and 7D cut away. The flap 5C is solid, extends across the whole of the top of the container, and has a tab ST on its free end.

The inner food container 2 is made of a thermoplastic material and has a defined shape to which the food conforms (rather than the "container" conforming to the shape of the food as would be the case for a wrapper). The container is of substantially cylindrical shape of circular cross-section, having a cylindrical wall 8 a base 9 (not visible in Fig. 1) and an outwardly projecting flange 10 at the top of the cylindrical wall 8. The container 2 may also be provided with a suitable film (preferably removable by peeling) of plastics material (not shown) across its top to seal the container.

In Fig. 1 the outer container 1 is shown only partly assembled. To complete assembly the flaps 5B and 5D are folded over from the position shown in dotted outline in Fig. 2 to the position shown in solid outline. Then the flap 5A is folded over to a position overlying the flaps 5B to 5D with the circular segments 7B and 7D and the circular hole 6 in the flap 5A concentric.The inner food container 2 is then lowered into the container 1 through the hole 6 until the flange 10 of the container 2 rests on the portion of the flap 5A immediately surrounding the hole 6, thereby suspending the inner food container 2 within the outer container 1; the flange 10 is secured to the flap 5A by a ring of adhesive 11 (see Fig. 4). The flap 5C is then folded over to a position overlying the flap 5A and the flange 10, and the tab 5T is folded down and glued to the side wall 3A.

When food in the inner food container is to be heated, the flap 5C of the outer container is separated from the tab 5T by tearing (a tear strip, not shown, may be provided across the flap 5C adjacent to the tab 5T for this purpose) and the flap 5C is then raised. The film of plastics material sealing the inner food container 2 is then pierced or removed as may be appropriate, and the package placed in a microwave oven to heat the food.

The microwaves have little heating effect on the outer container 1 or the inner container 2 but the base 9 and the cylindrical wall 8 are heated by the food which in turn is heated as a result of absorption of the microwaves. As a result, in a typical example in which the food is stir-fry vegetables, the temperature of the base 9 and the cylindrical wall 8 rises to about 100 C.

As explained further hereinafter, that temperature causes some shrinkage of the base 9 and some reduction in the circumference of the cylindrical wall 8, but most significantly it causes a reduction in the height of the cylindrical wall 8 causing the base 9 to rise up within the outer container 1, consequently reducing the internal volume of the inner food container 2, as shown in Fig. 5.

In the particular example being described the volume of the inner container is about 300 ml before heating and about 150 ml at the completion of heating.

The stir-fry vegetables also reduce in volume as a result of the heating and typically there may be a reduction in volume from 280 ml to 150 ml. Thus it will be seen that, whereas with a conventional food container the level of vegetables in the container would drop very significantly after heating, with the example of the invention just described the level would rise slightly.

The amounts by which the various portions of the inner food container 2 shrink will be dependent upon the material from which the container is made and the way in which the container is formed. If the container is formed by a process in which all the material forming the various parts of the container is subjected to very similar conditions the shrinkage will occur approximately equally in the base 9 and the side walls 8. On the other hand, if a process is employed in which the material that forms the side walls 8 is subjected to much greater vertical stresses and strains than the radial stresses and strains to which material in the base 9 is subjected, then shrinkage may be confined mainly to the side walls 8. In the example of Figs. 1 to 5, it will be seen that there is some shrinkage of the base 9 and some circumferential shrinkage of the side wall 8, but most of the shrinkage is shrinkage of the side wall 8 in a longitudinal direction. In the particular example shown in Fig. 5 the container is formed in a single stage vacuum forming process with plug assistance (that is with a plunger being pressed into the vacuum mould to assist the vacuum effect in deforming the flat sheet of material into the shape of the mould). In that process the plastics material forming the relatively thin side wall is subjected to much greater stresses and strains in a direction parallel to the axis of the container than in a circumferential direction and the plastics material in the relatively thick base suffers relatively little stress and strain in any direction.As a result the plastics material in the side walls obtains a significant degree of monoaxial orientation.

The plastics material from which the container 2 is made should be one with good heat shrink properties in the temperature range to which it is to be heated (95 to 105"C in the particular example described, that being typical): during and after heat shrinking the material desirably has a high modulus of elasticity (for example, a modulus of elasticity of 109 Pa); the material desirably has a reasonably high tensile strength (for example, 5x107 Pa) both before, after and during shrinking; and the material is desirably able to shrink to less than 90% of its original length as a result of heating to a temperature between 95 C and 105 C. Various polyester resins have suitable properties and in the particular example described with reference to Figs. 1 to 5, a copolyester resin, namely that known as PET Copolyester 9921W EASTAPAK is employed.

In the description above one particular form of food package has been described by way of example but it will be appreciated that many other forms may be employed, some examples of which will now be given.

Fig. 6 shows an outer container of a second form of package. The outer container is made of paper board material and is of generally cylindrical shape having a cylindrical hollow body 12 at the top of which there is an outwardly projecting flange 13 at the upper edge of which there is a peripheral upstanding wall 14. The package uses the same inner container 2 as the first form of package with the flange 10 of the container 2 resting on, and being secured by adhesive 11 to, the flange 13, thereby suspending the inner container 2 within the outer container 1. If desired, an outer cover may be fitted across the opening defined by the wall 14.As will be appreciated, the second form of package is used in substantially the same way as the first form, and the plastics material of the outer container together with the way in which the outer container is formed are chosen such that there are no significant dimensional changes in the outer container during heating of the food.

Another form that the outer container may take in a package of the same general construction as that shown in Fig. 6 is that of a cylindrical container formed from a helically wound strip of cardboard. The base of the container may be open or it may be closed by a separate base member and the flange 10 of the inner food container rests on, and is secured by adhesive 11 to , the top of the helically wound cylindrical outer container, thereby suspending the inner food container within the helically wound cylindrical outer container.

Figs. 7A and 7B show a third form of package in which the outer container 21 is a cuboidal box as in Figs. 1 to 5 but the box is much more squat having a width and length which are each greater than its height.

The box, which is made from folding cardboard, has four vertical side walls 23, a rectangular base 24 and a rectangular top 25 in which a central rectangular opening 26 is provided. An inner food container in the form of a rectangular tray 22 of a polymeric plastics material such as that used in the other forms of package is lowered into the box 21 through the opening 26. The tray 22 has a base 29, four upright side walls 28 and an outwardly projecting flange 30 which rests on portions of the top 25 of the box 21 immediately surrounding the opening 26 and is secured to the portions by adhesive 31.

The tray 22 is thereby suspended within the box 21.

As in the first form of package, the container 22 may be provided with a suitable film of plastics material across its top to seal the container and the box may also be provided with a flap for closing over the top of the container 22.

When the package shown in Figs. 7A and 7B is heated, the plastics material of the base 29 and/or side walls 28 shrink thereby reducing the internal volume of the tray. As previously described, the way in which the tray is formed may be chosen to obtain the desired shrinkage of the tray.

Claims

1. A method of heating food, comprising the steps of providing the food in a container which is shrinkable, placing the container with the food therein in a microwave oven and exposing the container to microwaves to heat the food, an effect of the heating being to shrink the container and thereby reduce the internal volume of the container.

2. A method as claimed in claim 1, in which another effect of the heating is to reduce the volume occupied by the food.

3. A method as claimed in claim 1 or 2, in which the internal volume of the container after heating of the food is less than 75 per cent of the volume of the container before heating.

4. A method as claimed in any one of claims 1 to 3, in which at least a portion of the wall of the container shrinks as a result of the microwave heating.

5. A method as claimed in claim 4, in which the container has a base and one or more side walls and the reduction of the internal volume of the container is caused principally by a reduction in the height of said one or more side walls.

6. A method of heating food in a container, the method being substantially as herein described with reference to the accompanying drawings.

7. A food package comprising an outer container and an inner food container housed within the outer container, the inner food container being such that when it is placed in a microwave oven with food therein and exposed to microwaves for heating the food, the container shrinks and the internal volume of the inner food container is thereby reduced.

8. A food package as claimed in claim 7, in which the upper end of the inner food container is supported by the outer container and the inner food container is suspended within the outer container.

9. A food package as claimed in claim 7 or 8, in which the inner food container is of generally frusto-conical form, the axis of the conical section being upright.

10. A food package as claimed in claim 9, in which the inner food container is of generally cylindrical form and of circular cross-section, the axis of the cylinder being upright.

11. A food package as claimed in claim 9, in which the inner food container is tapered with the smaller crosssection towards the bottom.-

12. A food package as claimed in any one of claims 7 to 11, in which the outer container has the general outline of a six-faced solid object, with substantially horizontal, top and bottom faces and four side faces.

13. A food package as claimed in claim 12, in which the outer container is of substantially cuboidal shape.

14. A food package as claimed in any one of claims 7 to 11, in which the outer container is of generally frusto conical form, the axis of the conical section being upright.

15. A food package as claimed in any one of claims 7 to 14, in which there is an opening in the top of the outer container, in which opening the inner food container is received.

16. A food package as claimed in claim 15, in which the outer container further includes a cover which in a closed condition of the outer container covers over the opening but which can be moved or removed to expose the opening.

17. A food package as claimed in claim 15 or 16, in which the inner food container is secured to the outer container in the region of the opening.

18. A food package as claimed in any one of claims 15 to 17, in which a peripheral flange formed on the inner food container is supported by portions of the outer container that surround the opening.

19. A food package as claimed in any one of claims 15 to 18, in which the opening is defined by openings or recesses in a plurality of separate flaps forming a top face of the container, each of the sections being connected to respective side faces of the outer container.

20. A food package as claimed in any one of claims 7 to 19, in which the outer container is formed from folding a sheet material.

21. A food package as claimed in claim 20, in which the sheet material is cardboard.

22. A food package as claimed in any one of claims 7 to 21, in which the inner food container is such that when it is placed in a microwave oven with food therein and exposed to microwaves for heating the food, at least a portion of a wall of the container is caused to shrink as a result of the microwave heating.

23. A food package as claimed in any one of claims 7 to 22, in which the inner food container is made of a polymeric plastics material.

24. A food package as claimed in claim 23, in which the plastics material is a thermoplastic polyester.

25. A food package as claimed in any one of claims 7 to 24, in which the inner food container has a base and one or more side walls and is such that when the container is placed in a microwave oven with food therein and exposed to microwaves for heating the food with the result that the internal volume of the container is reduced, the reduction in volume is caused principally by a reduction in the height of said one or more side walls.

26. A food package as claimed in any one of claims 7 to 25, in which the inner food container is such that it shrinks when it is heated to a temperature that lies in the range of 95" to 105 C.

27. A food package as claimed in any one of claims 7 to 26, in which the inner food container is vacuum formed.

28. A food package as claimed in any one of claims 7 to 27, in which the inner food container is blow moulded.

29. A food package as claimed in any one of claims 7 to 28, further including food therein, the food being suitable for heating in a microwave oven.

30. A food package substantially as herein described with reference to Figs. 1 to 5, or with reference to Fig.

6 or with reference to Figs. 7A and 7B of the accompanying drawings.

31. A method as claimed in claim 1, in which the food container is as claimed in any one of claims 7 to 30.

32. A method of making a food container for placing in a microwave oven to heat the food, the method including the steps of: forming a container of a material, which is able to shrink at a temperature that lies within the range of 95 C to 105"C, by a method which leads to such potential for shrinkage being built into the container, placing food in the container, and closing the container.

33. A method as claimed in claim 32, further including the steps of providing an outer container and housing the inner container within the outer container to form a food package as claimed in any one of claims 7 to 30.

34. A method as claimed in claim 32 or 33, further including the step of heating the food in the container by a method as claimed in any one of claims 1 to 6.