Food package for microwave ovens

Abstract: A food package formed from a high temperature resistant container (10) having a side wall (16) extending upwardly from said base to a top edge (18) which defines an opening and a substantially microwave transparent cover (24) positioned over substantially all of either the opening or the surface of the base characterised in that the cover has aluminium laminated depending side edges (19) adapted to be secured to a major or total part of the side wall (16).

Description: This invention relates to a food package for controlled heating or cooking of prepared food in hot air, convection, household and microwave ovens. The rapid increase of the microwave oven sale to private households and the development of catering have changed the conditions for the prepared food manufacturers implying great advantages as well as problems. The advantages are evident and connected with the rapid heating in microwaves which make frozen prepared food even more convenient. There are two main problems: The traditional metallic tray is opaque to microwave radiation and is not suitable in microwave ovens as arcing may occur inside the oven cavity resulting in a very bad perception of this package by the consumer. Since metals are not transparent to microwaves and heating is obtained only from the top, when compared to heating in a plastic container, cooking is not only longer but also uneven especially with frozen products for which the bottom layer is still frozen while the top layer is overcooked and unacceptable (dry or burnt). Plastic trays are transparent to microwave radiation but owing to the limited penetration of microwaves and to the different absorptions of microwaves by the components of the products e.g. water and ice, the cooking of frozen products is uneven. Hot spots currently appear in the corners and along the walls of the tray while the centre is still frozen. Development work in the package industry in relation to microwave cooking has dealt with the problem of selective cooking of multicomponent meals in which the individual food components generally require different quantities of microwave energy exposure. For example, U.S.P. No 3'865'301 describes a shielded container for a plurality of ingredients of a sandwich-type food product that are to be heated or cooked to a different extent and which is opaque to microwave radiation except for radiation-transparent windows. U.S.P. No 4'081'646 illustrates a tray in a material transparent to microwave energy with a plurality of compartments, a cover formed of a material that is transparent to microwave radiation and adapted to the tray and a shielding box caving walls opaque to microwave energy and bearing apertures at predetermined locations for inserting the tray with the cover therein, in order to control the amount of radiation received by each of the individual components of a meal. In the prior developments very little attention has been paid to uneven cooking of a prepared dish within a single compartment or tray in a microwave oven. In US-A 4351997, there is described a new design for a dual ovenable tray to avoid this uneven cooking in microwave ovens. This design is related to a tray including a bottom wall and a peripheral wall ending with a horizontally extending rim. The rim is coated partly or totally with a material reflecting or opaque to microwave radiation such as an aluminium foil. This provides a package in a tray form which gives an excellent temperature distribution in microwave ovens and is possible to use in hot air, convection and household ovens at temperatures up to 250°C. The present invention relates to technical and economical new solutions to achieve such a package. US-A-4661672, GB-A-1593523 and DE-A-3242402 describe containers for microwave heating using an aluminium laminate shield which are designed so that, when heating takes place, the shield covers at least part of the top or the base of the container. The present invention provides a food package formed from a high temperature resistant container having a base and a side wall extending upwardly from said base to a top edge which defines an opening, and a substantially microwave transparent cover positioned over substantially all of either the opening or the surface of the base characterised that the cover has aluminium laminated depending side edges adapted to be secured to a major or total part of the side wall. The high temperature resistant container is conveniently made of a cardboard or paperboard or a plastic material with good thermal resistance, e.g. crystalline polyethylene terephthalate (CPET), polyether sulfone, polyether imide, polymethylpentene or suitable combinations of such materials. The container may be polygonal and may contain, for example, from 3 to 10 sides. Preferably, the corners have angles rather than curves. The aluminium laminate forms a shield and may be a simple laminate with a layer of aluminium foil coated onto the inside or the outside of a layer of plastics or cardboard material i.e. an aluminium plastics laminate or an aluminium cardboard laminate or it may be a complex laminate having at least one layer of aluminium foil, plastics, cardboard or paperboard. The shield may act as an insulating label so that the consumer can handle the tray after reheating without burning his fingers. The aluminium foil may have a thickness of from 4 to 80 µm and preferably from 5 to 15 µm. The container is preferably provided with a peelable CPET film lid or any other system for easy opening. The lid could also be a simple heat sealable film. In both cases it is preferably pilfer proof. In a first embodiment, the substantially microwave transpartent cover may be a cardboard or plastics cover. The cover has depending side walls partially laminated along a major or total part of the depending side walls with an aluminium foil. A suitable design permits a complete shield of the side walls of the tray after folding by means of fold lines. The advantages of this alternative is that the consumer does not have to remove the top of the cover for heating in a microwave oven. The container of the invention may be locked by a conventional plastics cap or a cardboard cap. In a second embodiment of the invention there is provided a microwave transparent high temperature resistant container suitable for holding a food product provided with an outer cardboard packaging which is partially laminated along two of the side walls with an aluminium foil. Preferably, the two aluminium shielded side walls of the outer cardboard packaging are long enough to insure a complete shield of the side walls of the container after folding. In this second embodiment, the outer cardboard packaging preferably covers only the bottom and side wall of the container. The container is advantageously provided with a peelable plastics film lid and a plastics cover. The container is advantageously made of a conventional plastics material such as CPET. Preferably, the outer cardboard packaging is secured to the side wall of the container by means of an adhesive or suitable seals. The container is provided with a peelable lid made of plastic or cardboard. A plastic over cover may also be provided. The shield may be secured to the walls of the container by using glue, by sealing or by mechanical locking. The mechanical locking enables easy removal of the shield should it be desired to recycle the container. In a third embodiment of the invention there is provided a high temperature resistant cardboard or paperboard container having a major portion of the side wall laminated with an aluminium foil. The cardboard or paperboard container may be a conventional high temperature resistant container and is usually laminated with a plastics material such as polyester or other temperature resistant polymers. The plastics material may also be applied by extrusion coating. The aluminium foil may be coated on the outside of the container, but it is preferably applied on the inside. The container may be formed from a blank having corner flaps which is folded to form the walls and glued in the corner flaps. The food packages of the present invention are very cheap and convenient because they may be formed by a simple adaptation of conventional containers commercially available at the present time. The different designs allow to achieve a shield very close to the product and this is very important. The present invention will now be further illustrated by way of example with reference to the accompanying drawings in which Figure 1 is a cross-sectional view of a package, Figure 2 is a top plan view of the package of Figure 1, Figure 3 is a plan view of the package of Figure 1, Figure 4 is an exploded perspective view of a package, These packages do not represent the invention claimed. Figure 5 is a plan view of the cover of a package of the first embodiment of the invention, Figure 6 is an exploded perspective view of a package according to the second embodiment of the invention, Figure 7 is a plan view of the partially laminated cardboard of the second embodiment of the invention, Figure 8 is a plan view of a cardboard blank for forming the package according to the third embodiment of the invention, and Figure 9 is a plan view of a laminated aluminium/cardboard. Referring to Figures 1 to 4 of the drawings the package comprises a high temperature resistant CPET tray 10 having a peelable film lid 11 and is provided with an aluminium laminate cover 12 with depending edges 13 and provided with precut lines of weakening 14, 15 to enable the top of the cover to be removed by tearing off. Fold lines 17 are provided to conform to the shape of the container.The depending edges 13 are glued or sealed to the side walls 16. The top of the laminate cover 12 may be sealed together with the lid 11 and the flange of the container 18. Figure 5 illustrates an alternative cover with two depending walls of aluminium laminate 19, containing fold lines 20 and 21 to conform to the shape of the container and two depending walls of cardboard laminate without aluminium foil 22 containing fold lines 23 to conform to the shape of the container. The top is a cardboard laminate without aluminium foil 24. The walls 19 and 22 will be glued or sealed to the tray wall 16. Figure 6 shows a package comprising a high temperature resistant CPET tray 25 having side walls 26, a peelable lid 27 and a plastic cover 28. A cardboard cover 29 partially laminated by coating with aluminium foil 30 is wrapped around the base and side walls of the tray so that the aluminium laminate forms a shield around the total part of the side walls. Figure 7 illustrates the design of the cardboard shield of Fig 6 partially laminated. The package has two depending walls of cardboard laminate without aluminium foil 29 containing fold lines 31 and cut lines 32 to conform to the shape of the container 25 and two side walls of aluminium laminate 30 containing fold lines 33 and 34 to conform to the exact shape of the container 25. The bottom 35 is a cardboard laminate without aluminium foil. The walls will be glued or sealed to the tray walls 26. Figure 8 shows a blank made of ovenable cardboard material 36 and comprises material which is scored with fold lines so that when folded will form the base 37, side walls 38, 39, 40, 41, the side walls being provided with rims 42, 43, 44, 45 respectively to seal a cardboard cover (not shown). Corner flaps 46, 47, 48, 49 are provided with fold lines to ensure that when the blank has been folded the side walls are held together. The side walls 40, 41 and the corner flaps 46, 47, 48 and 49 are coated with aluminium foil as shown by the darker shading. The advantage of this embodiment is that the product may be packed in the carton directly without the necessity of any overpackage. Figure 9 illustrates the design of the cardboard shield of the figure 6 completely laminated with aluminium. The shield has four depending walls of cardboard/aluminium laminate (50 and 51) containing fold lines (52 and 53) to conform to the shape of the container 25. The bottom (54) is a cardboard/aluminium laminate provided with precut lines (55) and a tab (56) in order to facilitate the removal of the bottom of the shield before reheating. The walls may be glued, sealed or mechanically locked to the tray walls 26.

Claims: A food package formed from a high temperature resistant container (10,25) having a base and a side wall (16,26) extending upwardly from said base to a top edge (18) which defines an opening, and a substantially microwave transparent cover (24,29) positioned over substantially all of either opening or the surface of the base characterised in that the cover has aluminium laminated depending side edges (19,30) adapted to be secured to a major or total part of the side wall (16,26). A food package according to claim 1 wherein the container (10,25) is quadrangular in shape and at least the two opposite side walls (16,26) comprise the aluminium laminated edges (19,30). A food package according to claim 1 wherein the depending side edges (19,30) of the cover (24,29) are secured to the side walls (16,26) of the container (10,25) by an adhesive or a seal or by mechanial locking. A food package according to claim 1 which comprises a microwave transparent high temperature resistant container (10,25) and a cardboard or plastics cover (24,29) with depending side walls (19,30) partially laminated along a major or total part of the side walls (26) with an aluminium foil. A food package according to claim 4 wherein the cardboard or plastics cover (24,29) is provided with fold lines (23,31) enabling the depending edges to contact the side walls (26) of the container. A food package according to claim 2 wherein the two aluminium shielded depending edges are long enough to insure a complete shield of the sides walls (26) of the container after folding. A food package according to claim 1 which comprises a microwave transparent high temperature resistant container (10,25) provided with an outer cardboard packaging (24,19,29,30) which is partially laminated along two side walls (19,30) with an aluminium foil. A food package according to claim 7 wherein the two aluminium shielded side walls (19,30) of the outer cardboard packaging are long enough to insure a complete shield of the side walls (16,26) of the container after folding. A food package according to claim 7 wherein the outer cardboard packaging covers only the bottom and side wall of the container. A food package according to claim 1 comprising a high temperature resistant cardboard or paperboard container (10,25) having a major portion of the side wall (16,26) laminated with an aluminium foil.

Inventors: SAUNIER CHRISTIANE-MONIQUE, SAUNIER, CHRISTIANE-MONIQUE
Applicants: FRISCO FINDUS AG, FRISCO-FINDUS AG
Application year: 1992
Main classification: B65D81
IPC codes: B65D77, B65D25, B65D81