Abstract:
A package for storing and shipping food products incorporates a multilayer film lid that includes a gas-impermeable exterior layer and a gas-permeable interior layer. The multilayer film lid is heat sealed to a flange of a tray upon which the food product is supported. The gas-impermeable layer may be delaminated from the gas-permeable layer when it is desired to expose the food to atmospheric gases. Separation of the gas-impermeable layer from the gas-permeable layer is facilitated by a film fracture feature included in the heat seal pattern adjacent a pull tab of the lid. The film fracture feature concentrates the separation forces exerted on the tab in a small area so that the gas-permeable layer of the lid readily breaks at the film fracture feature.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to packaging for products, such as fresh red meat or other food products, that are enclosed between a tray or other support member and a flexible film lid sealed thereto, wherein a layer of the lid can be peelably removed from another layer of the lid that remains sealed to the tray.  
         BACKGROUND OF THE INVENTION  
         [0002]    Within the meat packaging industry, it is preferable to package red meats in a container without oxygen to extend the shelf life of the meat. Oxygen allows bacteria to grow which spoils the meat; therefore, depriving the packaged meat of oxygen retards the growth of bacteria. However, by denying the meat of oxygen, the myoglobin in the meat cannot generate a fresh, naturally appearing color.  
           [0003]    A device for packaging meat is disclosed in U.S. Pat. No. 5,779,050 to Kocher et al. and assigned to the present assignee, wherein a multilayer film lid is provided for a meat packing container. The multilayer film lid consists of an exterior gas-impermeable layer and an interior gas-permeable layer. The lid is heat sealed to the oxygen-depleted package containing the meat product. This system allows the meat to be initially packaged and shipped in a low-oxygen environment. Before placing the package out for retail sale, the gas-impermeable layer of the lid is peeled off so that oxygen reaches the meat through the gas-permeable layer. This exposure to oxygen allows the meat to “bloom” so that fresh, naturally appearing colors are generated.  
           [0004]    The multilayer film lid is generally affixed to the container by heat sealing. In packages of this type, difficulties have been encountered in delaminating the gas-impermeable layer from the gas-permeable layer of the lid. In particular, the gas-impermeable layer sometimes can not be completely removed in one piece from the gas-permeable layer. If the gas-impermeable layer fails to properly delaminate from the gas-permeable layer, it is possible for the heat seal between the lid and the tray to be damaged in the attempt to remove the gas-impermeable layer. Therefore, a need exists for a multilayer film lid with an exterior layer that can be reliably and consistently delaminated from an interior layer.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The invention addresses the above needs and achieves other advantages by providing a package for storing a food product in a modified atmosphere wherein a heat seal between a lid and a tray defines a film fracture feature for focusing forces exerted on a pull tab portion of the lid so as to initiate fracture between the layers of the lid in a more-reliable and more-consistent manner. In one embodiment of the invention, the package comprises a tray for supporting the food product, and a multilayer film lid attached to an upper surface of the tray by a heat seal that extends about a perimeter surrounding a food product. An outside edge of the heat seal has two substantially straight portions that are approximately perpendicular to each other and are joined by a corner portion of the heat seal. The lid has upper and lower layers joined together in such a manner that the upper layer can be peeled from the lower layer, the lower layer being permeable to oxygen and the upper layer being substantially impermeable to oxygen. The lid includes a pull tab portion that extends out beyond the corner portion of the heat seal. In accordance with the invention, a film fracture feature is defined in the corner portion of the heat seal proximate the pull tab portion of the lid. The film fracture feature comprises a portion of the outside edge of the heat seal formed as a straight-line segment oriented at a non-perpendicular angle to each of the two substantially straight portions of the heat seal. Pulling upward on the pull tab portion focuses the forces on this straight-line segment to break the lower layer of the film along the straight-line segment and thereby initiate fracture between the upper and lower layers of the lid.  
           [0006]    In another embodiment of the invention, the package comprises a tray for supporting the food product and includes a bottom wall, a side wall extending upwards from the bottom wall, and a flange connected to a top surface of the side wall. The flange extends generally perpendicular to the side wall. The side walls of the tray form a quadrilateral having four side walls and four corners at the intersections of the side walls, and the flange includes four straight portions corresponding to the side walls and four corner portions corresponding to the corners of the tray.  
           [0007]    The package also includes a lid bonded to the flange of the tray with a heat seal that extends along the entire upper surface of the flange; thus, the heat seal includes four straight portions and four corner portions. The lid includes a pull tab portion that is not affixed to the tray and extends out beyond the heat seal at one corner of the flange. The lid is a multilayer film having a first layer and a second layer that can be peeled apart. The first layer includes a film that is substantially gas-impermeable and the second layer is a gas-permeable film and is positioned between the first layer and the tray such that the first layer may be peeled from the second layer while leaving the second layer affixed to the tray. At the corner of the flange that is proximate the pull tab portion of the lid, the heat seal pattern defines a film fracture feature on an exterior edge of the heat seal pattern. The film fracture feature includes at least one straight-line segment that intersects with at least one of the straight portions of the heat seal proximate the fourth corner at an obtuse angle. The film fracture feature acts as a stress riser so that delamination between the first and second layers of the lid is initiated at the film fracture feature.  
           [0008]    In another embodiment, the film fracture feature includes two line segments that intersect to form a vertex in the heat seal pattern for initiation of the delamination of the first layer from the second layer. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0009]    Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0010]    [0010]FIG. 1 is a perspective view of a package in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is a schematic, cross-sectional view of the package shown in FIG. 1;  
         [0012]    [0012]FIG. 3 is a schematic, partial cross-sectional view of the package illustrating the removal of the gas-impermeable layer of the lid;  
         [0013]    [0013]FIG. 4 is a schematic, partial cross-sectional of the package illustrating the removal of the gas-impermeable layer of the lid, formed as a coextruded, multilayer film;  
         [0014]    [0014]FIG. 5 is a top, partial view of the package shown in FIG. 1 illustrating a film fracture feature formed as a single straight-line segment; and  
         [0015]    [0015]FIG. 6 is a top, partial view of the package shown in FIG. 1 illustrating a film fracture feature consisting of multiple straight-line segments of the heat seal. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.  
         [0017]    [0017]FIG. 1 illustrates a peelable, modified-atmosphere package  10  in accordance with the present invention. Such packages are disclosed in U.S. Pat. Nos. 5,779,050 and 6,032,800, the disclosures of which are hereby incorporated herein by reference. Package  10  includes tray  12  having a cavity  14  formed therein and a food product  16  disposed within the cavity. Tray  12  is preferably in the form of a tray having a bottom wall  20  and side wall  18  that extends upwardly from the bottom wall  20 . Tray  12  thus defines the cavity  14 , and further includes a peripheral flange  22  extending outwardly from the top edge of side wall  18 . A lid  24  is heat sealed to flange  22  to enclose the food product  16  within cavity  14 . The lid  24  comprises a flexible film structure as further described below.  
         [0018]    Tray  12  can have any desired configuration or shape, e.g., rectangular, round, oval, etc. A preferred configuration, as illustrated in FIG. 1, is generally rectangular and includes four side walls  18 . Similarly, flange  22  may have any desired shape or design, including a simple, substantially flat design that presents a single sealing surface as shown, or a more elaborate design which presents two or more sealing surfaces. In a preferred embodiment, flange  22  includes four straight portions corresponding with the side walls  18  and four corner portions corresponding to the corners of the tray. In a preferred embodiment illustrated in FIG. 2, the flange includes an upper surface  23  that extends outward from the tray side walls  18  and a recessed or stepped surface  42  connected to and surrounding an outer perimeter of the upper surface  23 .  
         [0019]    Suitable materials from which tray  12  can be formed include, without limitation, polyvinyl chloride, polyethylene terephthalate, polystyrene, polyolefins such as high density polyethylene or polypropylene, paper pulp, nylon, polyurethane, etc. The tray may be foamed or non-foamed as desired, and preferably provides a barrier to the passage of oxygen therethrough, particularly when food product  16  is an oxygen-sensitive food product. Tray  12  may also comprise a material which itself provides a barrier to the passage of oxygen and may have a substantially gas-impermeable sealant film (not shown) laminated or otherwise bonded to the inner or outer surface thereof.  
         [0020]    In the embodiment of FIGS.  1 - 3 , the lid  24  includes an upper gas-impermeable portion or layer  34  and a lower gas-permeable portion or layer  32 . The two layers  32 ,  34  are joined together in such a fashion that the upper layer  34  can be peeled from the lower layer  32 . The upper layer  34  includes a gas-impermeable film, which can comprise the entirety of the upper layer  34 ; alternatively, a gas-impermeable film can be joined to one or more other layers selected to impart other desired properties to the upper layer; for instance, in a non-illustrated embodiment the gas-impermeable film can be sandwiched between two or more other film layers to form the upper layer  34 . The gas-impermeable film of the upper layer preferably includes a material that provides a substantial barrier to the passage of gas, particularly oxygen, therethrough. Suitable materials may include, but are not limited to, vinylidene chloride copolymer (saran), nylon, polyethylene terephthalate, ethylene/vinyl alcohol copolymer, and silicon oxides (SiOx). Gas-permeable layer  32  preferably allows a certain amount of gases to pass through while generally maintaining a barrier to other substances such as dirt, dust, or moisture. Gas-permeable layer  32  may also include perforations  36 , as illustrated (not to scale) in FIG. 3, which may be formed by any suitable means, such as mechanically, chemically, and/or electrically. Examples of such devices include, but are not limited to, devices that perforate with laser energy, electrostatic discharge, ultrasonic waves, flame discharge, needles or other sharp objects, and combinations thereof. Perforations  36  may range in size from 5 to 2000 microns in diameter, most preferably 75 to 100 microns in diameter. Gas-impermeable layer  34  is joined to the top surface of gas-permeable layer  32  to form the lid  24  prior to the bonding of lid  24  to flange  22  of tray  12  by a heat seal  30 . Lid  24  is frangible in that the gas-impermeable layer  34  may be delaminated from gas-permeable layer  32 .  
         [0021]    Although food product  16  is illustrated as having a maximum height which is below the maximum height of tray  12 , i.e., the level at which flange  22  is located, the invention is not limited to such “low profile” products. Rather, “high profile” products may also be packaged in accordance with the present invention, i.e., those having a maximum height which is above the level at which flange  22  is located so that the portion of the product which extends above the level of flange  22  will be in contact with lid  24 .  
         [0022]    Referring again to FIG. 2, the lid  24  is affixed to the upper surface  23  of flange  22  via the heat seal  30 . Heat seal  30  preferably extends continuously around the upper surface  23  of flange  22  to thereby hermetically seal the food product  16  within package  10 . In this manner, lid  24  and tray  12  preferably form a substantially gas-impermeable enclosure for food product  16  which substantially completely protects the product from contact with the surrounding environment including, in particular, atmospheric oxygen, but also including dirt, dust, moisture, and microbial contaminates. When food product  16  is oxygen-sensitive, i.e., perishable, degradable, or otherwise changeable in the presence of oxygen, such as fresh red meat products (e.g., beef, veal, lamb, pork, etc.), poultry, fish, cheese, etc., it is preferred that food product  16  be packaged in a low-oxygen environment within package  10  to maximize the shelf-life of the product.  
         [0023]    When packaged as shown in FIG. 2, food product  16  can be maintained under any desired environmental conditions by virtue of the substantial gas-impermeability of lid  24  (and tray  12 ) as long as the gas-impermeable layer  34  remains bonded to the remainder of lid  24 . For example, when food product  16  is a fresh red meat product, a low-oxygen environment (e.g., less than 0.5% oxygen by volume) is preferably maintained within cavity  14  to preserve the meat product during shipping and storage. However, under such conditions, a fresh red meat product will have a purple color that is unappealing to consumers. For this reason, when the packaged product is to be displayed for consumers to purchase, the gas-impermeable layer  34  of the lid  24  is peeled off such that atmospheric oxygen can enter the package, thereby causing the fresh red meat product to re-bloom to a more-appealing red color.  
         [0024]    In the embodiment of FIGS.  1 - 3 , the lid  24  delaminates at the interface between gas-impermeable layer  34  and gas-permeable layer  32  when the pull tab of the lid  24  is pulled in the direction indicated by the arrow in FIG. 3. In this manner, gas-permeable layer  32  remains bonded by the heat seal  30  to the upper surface  23  of flange  22 . However, in another embodiment shown in FIG. 4, a lid  24 ′ is employed that includes a gas-permeable layer comprising a delaminateable film  38  that splits within itself. Delaminateable film  38  is preferably a coextruded, multilayer film. When the pull tab of the lid  24 ′ is pulled, the delaminateable film  38  essentially splits into two portions  38 ′ and  38 ″ along a separation plane  40 . The portion  38 ′ remains attached to the upper layer  34 . The portion  38 ″ remains affixed to the flange  22  and serves as a gas-permeable film. Delaminateable film  38  can comprise a gas-permeable film in which the permeability increases as the thickness decreases. In some cases, the increased oxygen permeability of the bottom portion  38 ″ may be sufficient to cause the fresh red meat to re-bloom without any perforation of the film  38 ″; if not, the entire film  38  or, alternatively, the bottom portion  38 ″ only, may be perforated as necessary.  
         [0025]    In accordance with the present invention, delamination can be performed simply and easily just prior to placing the package in a display case for customer purchase as follows. The pull tab portion  28  of multilayer film lid  24  is grasped and pulled generally upwards and backwards (i.e., towards an opposite edge or corner of the package). In the embodiment of FIG. 3, delamination occurs between the substantially gas-impermeable layer  34  and the substantially gas-permeable layer  32 . In the embodiment of FIG. 4, delamination occurs along separation plane  40  and separates delaminateable film  38  into a top portion  38 ′ and bottom portion  38 ″. Once the substantially gas-impermeable layer  34  has been removed, oxygen may pass through the substantially gas-permeable layer  32  or the bottom portion  38 ″ to cause product  16  to re-bloom.  
         [0026]    Delamination of lid  24  is shown in FIG. 3 in greater detail, wherein the gas-impermeable layer  34  is shown as it begins to delaminate from gas-permeable layer  32 . Gas-permeable layer  32  is preferably bonded to the upper surface  23  of flange  22  with a heat seal  30  that has a greater cohesive strength than the intra-film cohesive bond strength between gas-impermeable layer  34  and gas-permeable layer  32 . In this manner, when gas-impermeable film  34  is subjected to a delaminating force as indicated by the arrow in FIG. 3, the lid  24  delaminates such that the gas-permeable portion  32  remains bonded to tray  12  while the gas-impermeable portion  34  is removed from the package and discarded. Thus, gas-permeable layer  32  remains bonded to tray  12  via heat seal  30 .  
         [0027]    In the embodiment of FIG. 4, gas-impermeable layer  34  is preferably bonded to delaminatable film  38  with a bond-strength that exceeds the intra-film cohesive strength within delaminatable film  38 . Similarly, heat seal  30  preferably results in a bond between delaminatable film  38  and tray  12  that is also greater than the intra-film cohesive strength within delaminatable film  38 . In this manner, when lid  24 ′ is subjected to a delaminating force as indicated by the arrow in FIG. 4, the lid delaminates within delaminatable film  38  along separation plane  40  such that the bottom gas-permeable portion  38 ″ remains bonded to tray  12 , while the gas-impermeable portion  34 , along with the top portion  38 ′ of the delaminatable film, is removed from the package and discarded.  
         [0028]    As noted, to facilitate peeling, the lid  24  preferably includes a pull tab portion  28 . This pull tab portion  28  can be manually grasped to initiate peeling, and is preferably formed as disclosed in U.S. Pat. No. 5,779,050, the disclosure of which is hereby incorporated herein by reference. Peeling is initiated by grasping and pulling pull tab portion  28  in the direction of the arrow shown in FIG. 3 and FIG. 4, thus subjecting the heat seal  30  to an upward force. This force is in a direction tending to pull apart gas-impermeable layer  34  and gas-permeable layer  32 , or the delaminateable film  38 , as illustrated in FIG. 3 and FIG. 4, respectively. The heat seal  30  holds the gas-permeable layer  32  or the portion  38 ″ intact and secured to tray  12 . If a force of sufficient magnitude is applied, the layer(s) of the lid  24  or  24 ′ below the gas-impermeable layer  34  is (are) supposed to break at the heat seal and thereby initiate delamination of the gas-impermeable film from the rest of the lid as described above.  
         [0029]    However, in packages  10  with a heat seal pattern similar to that disclosed in US. Pat. No. 5,779,050, the delamination of the lid can be problematic. The force exerted on the pull tab of the lid can cause the flange  22  to bend upward and prevent removal of gas-impermeable layer  34 . The gas-impermeable layer  34  may tear, resulting in an incomplete removal of the gas-impermeable layer. In addition, if the lower layer of the lid does not break when the tab is pulled, the force can rupture the heat seal  30  between the flange  22  and the lid. Such failures are thought to result primarily from a non-optimum distribution of the peel force along the exterior edge of the heat seal  30 . The present invention utilizes a film fracture feature on an exterior edge of the heat seal  30  to concentrate the force so that the gas-permeable layer  32  reliably breaks at the film fracture feature and thus allows the gas-impermeable layer  34  to be peeled off without adversely affecting the gas-permeable layer  32 , the flange, or the heat seal.  
         [0030]    In a preferred embodiment, heat seal  30  includes four straight portions  31  (FIGS. 1 and 5) respectively extending along the four straight portions of flange  22 , three smoothly curved corner portions  33  at three of the four corners of flange  22 , and a fourth corner portion  35  (FIG. 5) that includes a film fracture feature located at the fourth corner of flange  22 . The pull tab  28  of the lid extends out beyond the fourth corner portion  35  of the heat seal. In a preferred embodiment, heat seal  30  is approximately 0.25 inches wide, but the width may vary depending on the application or material properties.  
         [0031]    [0031]FIGS. 1 and 5 depict one embodiment of a film fracture feature defined by the corner portion  35  of the heat seal  30  for concentrating peel forces exerted on the tab portion of the lid so as break the gas-permeable layer  32  of the lid at the film fracture feature. In this embodiment, the film fracture feature comprises a straight-line segment  50  of the outer edge of the heat seal  30 ; by “outer edge” is meant the edge of the heat seal  30  farther from the cavity  14  of the package. The straight-line segment  50  is proximate the pull tab portion  28  of the lid. Straight-line segment  50  intersects at an obtuse angle with each of the straight portions  31  of heat seal  30  on either side of the corner portion  35  of the heat seal and thereby creates vertices  51  (FIG. 5) at these intersections. When the pull tab portion  28  is pulled up and back, the forces are focused on the straight-line segment  50  so as to initiate fracture of the lower layer  32  of the lid  24 . Additionally, the vertices  51  between the straight-line segment  50  and the straight portions  31  of the heat seal are discontinuities in the curvature of the outer edge of the heat seal  30  and serve as stress risers to concentrate the peel force during removal of the gas-impermeable layer  34  from the gas-permeable layer  32 . The concentration of the peel force on the film fracture feature  50  allows the lower layer  32  of the lid to break at the film fracture feature and thus permits gas-impermeable layer  34  to be removed in one piece without disturbing the heat seal  30  between the lower layer  32  and the upper surface  23  of flange  22 .  
         [0032]    Another embodiment of the film fracture feature is illustrated in FIG. 6, wherein the film fracture feature can be formed by multiple line segments, such as segments  52  and  54 . The line segments  52  and  54  intersect to form a vertex  56 . Line segments  52  and  54  are preferably straight, but alternatively they may have a convex or concave shape that nevertheless creates a vertex at their intersection. Furthermore, the preferred embodiment of the vertex  56  is defined as a sharp corner, but alternatively the vertex may comprise a very small radius and still serve as a stress riser in the outer edge of the heat seal.  
         [0033]    A further problem during the delamination of a package  10  in the prior art can occur when the heat seal  30  extends outwardly beyond upper surface  23  of flange  22 . As noted, the flange  22  includes upper surface  23  and recessed or stepped surface  42 . The stepped surface  42  extends outward from the outer perimeter of the upper surface  23  and defines an upward facing surface that is lower than upper surface  23 . This stepped surface  23  is advantageously utilized during manufacture of package  10  when the material of lid  24  beyond the edges of heat seal  30  is cut. Improper delamination of the lid may occur if the heat seal  30  onto the interface between the upper surface  23  and the stepped surface  42 . In that case, exerting a peel force on pull tab portion  28  can cause the flange  22  to bend upward in the direction of the force because of the orientation of the heat seal  30  on the interface between upper surface  23  and stepped surface  42 .  
         [0034]    To address this issue, packages in accordance with some embodiments of the present invention include a modified flange structure at the corner where the pull tab is located. In particular, as illustrated in FIG. 5, the flange upper surface at that corner includes a surface  58  that forms an extension of the upper surface  23  to the outer perimeter of the flange  22 . This surface  58  exists where the stepped surface  42  would otherwise be located in a non-modified flange. In effect, the top of stepped surface  42  is raised to the level of the upper surface  23  in the corner with pull tab portion  28  to create surface  58 . Because the film fracture feature could have various configurations as exemplified by the different embodiments described herein, the surface  58  provides additional surface area to ensure the film fracture feature is confined to the flat upper surface; additionally, the extended surface  58  allows for some degree of tolerance in the location of the heat seal relative to the flange.  
         [0035]    Advantageously, the provision of the film fracture feature in accordance with the invention does not require any additional manufacturing steps or materials. The film fracture feature is a unique shape in the pattern of heat seal  30  and is created when film  24  and tray  12  are joined together.  
         [0036]    Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.