Abstract:
A tray package for food has a stiff tray having a rim and formed inward of the rim with at least one throughgoing hole and a cover film fixed to the rim. A lining made of a flexible film is fixed to the rim between the tray and the cover film and forms with the cover film a closed compartment adapted to hold the food and with the tray an outer compartment that is open to the exterior through the hole.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a food-packaging tray. More particularly this invention concerns such a tray and a method of making it. 
       BACKGROUND OF THE INVENTION 
       [0002]    A standard food package comprises a tray produced by thermoshaping and a cover film attached to the rim of the tray. The tray package should, in particular, allow the food to undergo high-pressure treatment to improve the shelf-life of the food. Such high-pressure treatment enables the food to be safely stored for a longer period of time. 
         [0003]    The standard method for such treatment entails putting the packaged food in a pressure chamber and subjecting it to a treatment pressure for a time period that can vary from a few minutes to approximately 30 minutes with a treatment pressure that can vary between approximately 1000 bar and 10,000 bar depending on the type of food. As an option, the high-pressure treatment can be combined with a heat treatment of the food. 
         [0004]    One particular possible approach used is to preheat the food before the high-pressure treatment up to the point that the subsequent high-pressure treatment causes a temperature to be reached that effects pasteurization. The pressure for the high-pressure treatment is generated in the pressure chamber by a liquid pressure-transfer medium, usually water. Since the high-pressure treatment of the food is effected in the final package, subsequent re-contamination of the packaged food is impossible and a high level of product safety is thus ensured. The described method has been disclosed, for example, in U.S. Pat. No. 6,440,484. 
         [0005]    Food packaging must meet special requirements due to the pressure of a high-pressure treatment. Compressible package contents, for example, allow the package to be compressed and kinks to be formed in the packaging film. As a result, application of high-pressure technology has therefore up to now been restricted to vacuum packaging with shrink-wrap film and for so-called slice products, for example raw ham. Packages that have a tray created by thermoshaping a multilayer film and are sealed by a cover film entail the problem that the packaging does not adhere to the packaged food with a contour-true fit and the tray is irreversibly deformed under external pressure. The deformed package is not marketable. Another factor is that the stiff film from which the tray is produced is irreversibly damaged by kinks, and the physical properties of the film, in particular, the barrier effect against oxygen is modified disadvantageously. 
         [0006]    US 2008/0260916 relates to a tray package for food that includes a prefabricated tray and is sealed by a cover film. The cover film is composed of an elastically stretchable film that contacts the packaged food during the subsequent pressure treatment and essentially returns again into its original shape after the pressure treatment. However, the pressure treatment is effected at low pressures in a range less than 10 bar. The package is furthermore suitable only for soft products and requires that the space filled by the food inside the package to be relatively small. This design with elastic cover film does not give satisfactory results for packages with enclose a headspace of less than 15% and lumpy contents. 
       OBJECTS OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to provide an improved food-packaging tray and method of making same. 
         [0008]    Another object is the provision of such an improved food-packaging tray and method of making same that overcomes the above-given disadvantages, in particular that does not exhibit any changes in its physical properties and appearance after high-pressure treatment. 
         [0009]    Furthermore, the package must have an unchanged functional barrier effect against oxygen, and must not be deformed or damaged when subjected to high-pressure treatment. 
       SUMMARY OF THE INVENTION 
       [0010]    A tray package for food has according to the invention a stiff tray having a rim and formed inward of the rim with at least one throughgoing hole and a cover film fixed to the rim. A lining made of a flexible film is fixed to the rim between the tray and the cover film and forms with the cover film a closed compartment adapted to hold the food and with the tray an outer compartment that is open to the exterior through the hole. 
         [0011]    Thus the invention starts with a tray package comprising a tray produced by thermoshaping and a cover film attached to the rim of the tray. According to the invention, the tray includes an outer tray composed of a tray film that can be formed by thermoshaping and an inner lining composed of a flexible film, the outer tray and the inner lining being strongly attached only at the rim of the tray and/or within a region adjacent to the rim. The outer tray here includes an orifice to allow for the exchange of gas and liquid, the orifice opening into the unattached intermediate space between the outer tray and the inner lining. The inner lining is flexible and contacts the tray surface of the outer tray. 
         [0012]    During a high-pressure treatment of the food packaged in the tray package, a pressure-transfer fluid, for example water, flows through the orifice of the outer tray and compresses the volume of space enclosed by the inner lining and the tray film until the package contents come into complete contact with the cover film and the barrier film forming the inner lining. The shape of the outer tray is not modified thereby since the pressure within the intermediate space between the outer tray and the inner lining matches the treatment pressure, and the inner lining is not subject to any pressure differential. After the pressure is released, the inner lining is forced back into its original shape by the trapped and re-expanding gases. The pressure transfer fluid that has accumulated between the barrier film and the outer tray is again virtually completely pressed out of the package. Optionally, the package can then be dried after the high-pressure treatment. 
         [0013]    The food can be packaged under protective gas so as to allow the package content space enclosed by the inner lining and the cover film to contain a protective gas. 
         [0014]    The tray film for the outer tray is composed of a thermoshapable film that can be sealed against the barrier film. In particular, the tray film can be composed of a multilayer film that preferably includes a substrate of polyester, polypropylene, polystyrene, or polycarbonate, and a cover layer of a weldable polymer, for example a polyolefin, on the inside of the tray. The outer tray composed of the tray film gives the package its external shape and functions to stabilize the flexible inner lining. Inexpensive materials can be used for the outer tray since the oxygen barrier function has been transferred to the inner lining by using a suitable flexible film. Any number of thermoshapable materials are appropriate that can be strongly attached by sealing to the barrier film or the inner lining. Amorphous and tough materials are preferred, for example the material combination APET/PE. The tray film is preferably between 150 μm and 2000 μm thick. 
         [0015]    The film that forms the inner lining is composed of a tough, flexible film that contacts the packaged food during the high-pressure treatment without damaging it or creating creases, and after the pressure is released again creates an essentially crease-free, and especially undamaged, inner lining of the tray. The flexible film is preferably composed of a multilayer barrier film that has at least one barrier layer for oxygen. The barrier film protects the food from oxygen and preferably includes polyolefin outer layers, and at least one intermediate layer composed of EVOH as the oxygen barrier layer. An ethylene fraction of 24 to 48 mol % is well-suited. The EVOH layer can be combined with at least one polyamide layer. The EVOH layer is preferably embedded between two polyamide layers. Embedding the EVOH between PA layers enables a good adhesive attachment and a very efficient oxygen barrier effect to be achieved. The barrier film can furthermore include one or more additional barrier layers for liquids, gases, aromas, or the like, and be composed of a coextruded film or a film laminate. 
         [0016]    The barrier film preferably is between 50 μm and 350 μm thick, and can be produced as a blown film or cast film. In particular, the barrier film can be produced by blocking two multilayer films that each have the above-described layer structure. The multilayer layer structure contributes to making the film tough and flexible, and furthermore simultaneously constitutes a very effective oxygen barrier. 
         [0017]    Either an inelastic or an elastic film having a sufficient oxygen barrier effect can be used as the cover film. The cover layer includes at least one oxygen barrier layer and a sealing layer composed of weldable polymer on the inside of the packaging. In particular, a conventional composite OPET/PE film can be used as the cover film. The thickness of the cover film is a function of the size of the package and, in particular, can range between 30 and 60 μm. 
         [0018]    The method of the invention involves first incorporating orifices in a tray film that can be formed by thermoshaping. A flexible film is then placed on the perforated tray film. The perforated tray film and the flexible film are then brought into gas-tight contact in surface areas that surround a forming region to undergo thermoshaping. The forming region is then formed to create a two-layer tray in a thermoshaping die by applying a subatmospheric pressure that acts on the film arrangement, which tray comprises a dimensionally stable outer tray composed of the tray film and includes at least one orifice penetrating the outer tray and an inner lining composed of the flexible film. One advantageous aspect here is that the orifices can already be incorporated in the tray film before all other processing steps. The gas-tight attachment with the flexible inner lining in a surface area surrounding the forming region enables the arrangement of the two film layers to nevertheless be formed by vacuum. 
         [0019]    The perforated tray film and the flexible film are preferably attached by at least one gas-tight sealing seam before thermoshaping, which seal surrounds the forming region. A sealing seam of this type enables the two film layers to be attached to each other in a durable and gas-tight manner. Any subsequent slippage between the two layers is prevented. This allows the two-layer composite film to be effectively guided as a material web during the subsequent production process. 
         [0020]    In another embodiment, the tray film and the flexible film are brought into gas-tight contact during thermoshaping by a clamping die that surrounds the forming region, then strongly attached after thermoshaping in the rim region of the tray or in a rim-adjacent region of the tray. 
         [0021]    The outer tray and the inner lining can, in particular, be thermally bonded after thermoshaping by a at least one sealing seam that surrounds the forming region. It is furthermore also possible in a single sealing step to attach by sealing the tray film and the flexible film simultaneously to a cover film that is applied in a subsequent packaging step. 
         [0022]    In another alternative method, the tray is formed by thermoshaping a two-layer composite material that is prefabricated from the tray film and the flexible film. Here the tray film forms a first layer of the composite material and the flexible film forms the second layer of the composite material. The two layers are attached face to face outside the forming region to be formed by thermoshaping. There is no strong adhesive attachment between the tray film and the flexible film within the forming region. The tray film includes at least one orifice in the forming region. A laminate of this type can be produced as a preliminary product in advance, thereby simplifying and accelerating the production process for the tray package. 
         [0023]    The tray film and the flexible film of the composite material can be attached by an adhesive layer that is interrupted in the forming region of the composite material. Additionally or alternatively, a release layer can be applied in the forming region to a film surface of the composite material adjacent to the adhesive layer, which release layer does not adhere to the adhesive layer. An adhesive attachment of the two film layers specifically allows laminate of the tray film and the flexible lining inner film to be prefabricated since this enables the drying times for the adhesive to be decoupled during the production run from the timing of the remaining production process. 
         [0024]    The release layer can be composed of a release coating, silicone treatment, or similar approach. The flexible film of the composite material can include a polyolefin layer on its outer surface, the release layer creating releasable attachment with the sealed-on cover film. The outer layer of the flexible film can be provided, in particular, in the form of a peelable polyethylene layer. The invention also comprises providing the contact surface of the cover film that faces the tray with a release layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0025]    The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which: 
           [0026]      FIG. 1  is a vertical section through a tray package for food; 
           [0027]      FIGS. 2A and 2B  show the package of  FIG. 1  at two successive steps during high-pressure treatment for preservation of the packaged food; 
           [0028]      FIG. 3  is a section showing the layer structure of the tray forming the tray package and a cover film sealing the tray; 
           [0029]      FIG. 4  shows an alternative layer structure of a barrier film forming the inner lining of the tray; 
           [0030]      FIG. 5  is a diagram illustrating the method for producing a thermoshaped tray for the tray package; and 
           [0031]      FIG. 6  is an exploded view of a composite material from which a thermoshaped tray can be produced for a tray package. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    As seen in  FIG. 1  a tray package  1  for food includes a stiff tray  4  made of a thermoshapable plastic sheet  5  ( FIG. 5 ) as well as a cover film  3  attached to a rim  2  of the tray  4 . More particularly, according to the invention the tray package  1  also has a tray-shaped inner lining  6  made of a flexible film. A hermetically completely closed inner packaging compartment space  7  defined between the cover film  3  and the lining film  6  can contain a protective gas and is intended to also hold pieces  10  of normally perishable food. The lining film  6  also forms with the tray  4  a vented outer compartment  9  normally positioned underneath the compartment  7 . 
         [0033]    The flexible film of the lining  6  is a multilayer barrier film that forms a barrier layer for oxygen. The outer is tray  4  and inner lining  6  are strongly attached to each other only at the rim  2  of the tray  1 , the rim  2  here also including a surface adjacent the rim  2 . In addition, the outer tray  4  is formed with at least one vent orifice  8  for the exchange of gas and liquid between the compartment  9  and the exterior. In this embodiment and according to a preferred embodiment of the invention, the base of outer tray  4  has a large number of such vent orifices  8  in the form of punched holes. 
         [0034]    The packaged food  10  can be preserved by a high-pressure treatment. This treatment involves autoclaving the package  1  as shown in  FIG. 1  at a high pressure of up to 10,000 bar, typically for a few minutes at most, order to destroy germs in the food  10 . 
         [0035]      FIGS. 2A and 2B  show how the tray package  1  responds when undergoing such high-pressure treatment. During the high-pressure treatment, a pressurized fluid  11 , typically water, flows through the orifices  8  in the base of outer tray  4  into the intermediate space  9  between the outer tray  4  and the inner lining  6 , and compresses the volume in the space  7  enclosed by the inner lining  6  and cover film  3  until the food  10  comes into contact with the cover film  3  and the barrier lining film  6  as shown in  FIG. 2A . This action does not change the shape of the outer tray  4  since the same pressure on its inside face in the space  9  and on its outside face exposed outside the package  1 . 
         [0036]    As shown in  FIG. 2B , after the high-pressure treatment and a corresponding pressure drop, the barrier film  6  returns to its original tray shape by the expansion of the gas trapped in the package space  7  between the inner lining  6  and the cover film  3 , until the lining  6  again comes into contact with the inner face of the outer tray  4 . At the same time, the water that has collected in the space  9  between the inner lining  6  and the outer tray  4  is forced back almost completely out of the package  1  through the vent holes  8 . Following this high-pressure treatment, the package  1  can optionally also be dried. 
         [0037]    The outer tray  4  created by thermoshaping the tray film  5  has a shape stability that is sufficient for use in the tray package  1  and reinforces and protects the significantly more flexible barrier film that forms the inner lining  6 . The film  5  for the outer tray  4  can be produced from virtually any material capable of undergoing thermoshaping that can be strongly attached by sealing to the inner lining  6 . Preferred materials are amorphous and a tough composite material or laminate as shown in  FIG. 3  that include a substrate  12  composed of polyester, polypropylene, polystyrene, or polycarbonate, and a cover layer  13  of weldable polymer on the inside of the tray. One particularly well-suited composite material is composed of APET/PE. The tray film  5  is advantageously between 150 μm and 2000 μm thick. 
         [0038]    The barrier film of the lining  6  of the tray package  1  is flexible and during the pressure treatment contacts the food  10  forming creases. Such films are appropriate that are not damaged by the formation of creases and then again unfold and contact the inner surface of the outer tray  4  essentially without creases after the pressure has been released. In particular, the barrier film  6  must create an effective oxygen barrier so that a significant extension of the shelf-life of the food can be achieved by the high-pressure heat treatment. As also shown in  FIG. 3 , the barrier film  6  advantageously has polyolefin outer layers  21  and at least one core layer  15  composed of EVOH as the oxygen barrier layer and sandwiched between two polyamide layers  14 . The total thickness of this five-layer barrier film  6  can range between 50 μm and 350 μm. 
         [0039]      FIG. 4  shows a barrier film  6  made by blocking two multilayer films  16  and  16 ′ that each have the layer structure described above for the film  6  and a layer thickness of between 25 and 175 μm. Blocking the two films  16  and  16 ′ enables a multilayer structure to be created. Such a barrier film  6  is tough and flexible, and is characterized by a very strong oxygen barrier effect. 
         [0040]    The cover film  3  in the illustrated embodiment is composed of a inelastic multilayer film that includes at least one oxygen barrier layer and a sealing layer composed of a weldable polymer on the inside of the package. The cover film can be composed, in particular, of a PET/PE composite. Elastic cover films can also be employed. 
         [0041]    The tray package  1  shown in  FIG. 1  can be produced by thermoshaping prefabricated molded components. The barrier film for the lining  6  is inserted into an outer tray  4  that has been preformed from the thermoshaping tray film  5 . The tray-shaped inner lining  6  can be used here to portion the food. The cover film  3 , inner lining film  6 , and outer tray  4  are thermally attached by welds to surfaces that form the rim  2  of tray  1 . 
         [0042]    More particularly  FIG. 5  shows a preferred and especially advantageous method for producing a tray package  1  according to the invention. Here the holes or orifices  8  are first punched into or formed in the thermoshapable tray film  5  as shown in the lower left of the view. The flexible lining film  6  is then placed on the perforated tray film  5 . A thermoshaping die  22  bonds together the perforated tray film  5  and the flexible film  6  at the rim  2  in gas-tight contact in an rectangularly annular region surrounding a forming region  18  to be shaped by thermoshaping. The forming region  18  is formed into the tray  1  in the thermoshaping die  22  by applying a vacuum to the bottom face of the film  5  in the region  18  and drawing it down into a die cavity, as is standard for vacuum-forming. 
         [0043]    The tray  1  thus has a dimensionally stable outer tray  4  composed of the tray film  5  formed with at least one orifice  8  and a flexible inner lining  6 . In the method illustrated, the tray film  5  and the flexible film  6  are brought into gas-tight contact during thermoshaping by a clamping die that surrounds the forming region  18 . After receiving the portion of food and subsequent application of the cover film  3 , the tray film  5  and the flexible film  6  are strongly and hermetically bonded together at the rim  2  of the tray  1  in a sealing die  23 . This forms a weld seam  24  surrounding the region  18 . At the same time, the cover film  3  is also attached to the tray  1  by the sealing seam  24 . 
         [0044]    As shown in  FIG. 6 , the tray  4  for the tray package  1  can also be formed by thermoshaping a composite two-layer material  17  that has been prefabricated out of the tray film  5  and the barrier film  6 . The tray film  5  forms a first layer of the composite material  17 , while the barrier film  6  forms the second layer of the composite material  17 . The layers formed by the films  5  and  6  are attached face to face outside the forming region  18  that is to be formed by thermoshaping, no bond being present between the tray film  5  and the barrier film  6 , or at most a weakly peelable attachment exists that releases during the high-pressure treatment and allows the pressure transfer fluid to flow in. The tray film  5  already has the orifices  8  required for a gas-liquid exchange in the forming region  18  of the composite material  17 . The tray film  5  and the barrier film  6  are attached by an adhesive layer  19  in the embodiment of  FIG. 5 , such that the adhesive layer  19  in the forming region  18  of the composite material  17  is either interrupted, and/or such that a release layer  20  is applied in this region to a film surface of the composite material adjacent to the adhesive layer  19 , which release layer  20  does not adhere to the adhesive layer  19 . The composite material illustrated in  FIG. 5  can be processed as rolled product and formed in a repeating pattern on conventional thermoshaping equipment.