GB1591424A - Process of heating food in a package of a polymeric laminate film - Google Patents

Description

PATENT SPECIFICATION

( 21) Application No 11048/80 ( 22) Filed 6 October 1977 ( 62) Divided out of No 1591423 ( 31) Convention Application No 730874 ( 32) Filed 8 October 1976 in ( 33) United States of America (US) ( 44) Complete Specification published 24 June 1981 ( 51) INT CL 3 B 32 B 27/04 27/08 27/30 27/32 27/34 ( 52) Index at acceptance B 5 N 2704 2708 2730 2732 2734 B 8 C 16 A 17 A 17 B 24 A 24 B 6 B 25 B G 16 ( 72) Inventor: HENRY BRYAN THOMPSON ( 11) 1 591 424 ( 19) ( 54) PROCESS OF HEATING FOOD IN A PACKAGE OF POLYMERIC LAMINATE FILM ( 71) We, W R GRACE & Co, a Corporation organised and existing under the laws of the State of Connecticut, United States of America, of Grace Plaza, 1114 Avenue of the Americas, New York, New York 10036, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in

and by the following statement:

This invention relates to a process of heating, especially cooking, food in a package of a polymeric laminate.

For packaging most foods, the film must provide an "oxygen barrier", i e have a low permeability to oxygen Film made of a copolymer of 65 to 95 % by weight of vinylidene chloride and 5 to 35 % by weight of a vinyl comonomer such as vinyl chloride, acrylonitrile or methyl acrylate, known in the United States of America generically as "Saran" and herein referred to as "VDC copolymer" (since "Saran" is a Registered Trade Mark in many other countries including the U K) is known to give a good oxygen barrier.

In order to sell packaged cooked foods, the package should appear attractive to the eye of the customer This requirement poses a problem For economy and food hygiene the food is best packaged before cooking, cooked in the package and sold in the same package During the cooking process many food articles expand, which may cause the packaging film to become stretched After the cooking is complete and the package is cooled, the food article shrinks, whereas the stretched packaging film maintains its stretched dimensions This results in a packaged article having an unaesthetic "prune-skin" appearance to the customer.

Also, many food packages must have a regular geometrical shape across the entire package.

Chicken rolls or bologna rolls, for example, having a generally cylindrical shape are often sliced such that each slice is ideally of a uniform thickness and weight Such uniformity can only be achieved if the cylindrical shape is highly precise and of constant diameter throughout the length of the cylindrical article This shape cannot be maintained when the packaging film stretches during cooking.

VDC copolymer has commonly been used for the above process of packaging food and cooking it in the package, herein referred to as a cook-in-package process, since it resists stretching However, it possesses many disadvantages which make it a difficult material to use in food packaging If a single film of VDC polymer is used, the problem arises that the polymer is difficult to heat-seal Even if a seamless tube of VDC polymer is used, the ends of the piece of tubing have to be heat-sealed around the article in order to complete the package It is necessary to weld the VDC copolymer using radio frequency energy This operation does not always give predictable and satisfactory results For some unknown reason, particular batches of VDC copolymer will weld with no problems, while others are virtually unweldable These problems can be overcome by using a laminate of polymeric films in which a heat-sealable layer of another type of polymer is laminated to a VDC copolymer.

Such laminates are, in principle, most conveniently made by co-extrusion from the melt, but difficulty arises because VDC copolymer is readily degraded at temperatures most suitable for extrusion It is difficult to extrude it for long periods of time, even as a single film, without the extrusion dies becoming clogged by solidified particles of carbon.

Another problem with the use of VDC co polymer is that of maintaining large rolls in an unwarped state after the film has been printed with a label in its central region The printing process produces a thickness variation across the film, the centre being thicker than the edges This permits natural shrinkage to occur at the edges, with the result that when the roll of film is used for packaging, the film becomes "baggy" (wrinkles) at the centre.

In 1 591 424 While oxygen barrier films of other materials are available, their use gives problems associated with the heating of food products within the film.

It has therefore been a problem to provide a packaging film suitable for a heat-in-package process in which a VDC copolymer is not an essential component That is, the problem is to devise a packaging film with heat strength so that it resists stretching during the cooking process and maintains the dimensional uniformity of the package and also enables the disadvantages, referred to above, of VDC copolymer, to be avoided.

The present invention provides a solution to the above problem According to this invention we provide a process of preparing a package of food, which process comprises:

(a) enclosing said food tightly in a laminate film comprising at least one heat-shrinkable layer adherent to at least one non-heat-shrinkable layer, said laminate film containing at least one oxygen barrier layer having a permeability to dry oxygen of less than 70 c c /m 2 atm at 23 C; (b) heating the food to a temperature at which it expands; steps (a) and (b) being carried out in either order; (c) thereafter cooling the package to a temperature at which the food shrinks, leaving gaps between the food and the packaging film; and (d) thereafter heating the packaging film to shrink it onto the food and thereby eliminate said gaps.

The above-defined invention is based on our finding that a laminate film comprising at least one heat-shrinkable layer adherent to a nonheat-shrinkable layer, and containing an oxygenbarrier layer, none of which essential layers is necessarily of a VDC copolymer, provides a high degree of resistance to stretching and maintenance of dimensions of the package The heat-shrinkable layer usually imparts some resistance to stretching and maintains the dimensional stability of the package during cooking.

Since the food article shrinks on cooling, there will still be a gap between the food article and the packaging film, but this defect in appearance can be remedied Surprisingly, we have discovered that a laminate which comprises a heat-shrinkable layer and a non-heat-shrinkable layer or layers, has suitable heat-shrinkable properties for use in the heat-in package process It has been found that only a single heat-shrinkable layer causes the otherwise non-heat-shrinkable layers to shrink without producing a distortion or wrinkling of the overall laminate film in the heat-in-package process Thus, to remove the gap between the food article and the packaging film, the film is subsequently heat-shrunk onto the surface of the food article Surprisingly, and contrary to expectations, the laminate does not wrinkle and become distorted despite the presence of a non-heat-shrinkable layer.

According to a preferred feature of the invention, the overall laminate has heat-sealable surface layers The compulsory heat-shrinkable layer can serve the dual purpose of being an optional heat-sealable surface layer, as well as being the required heat-shrinkable layer.

Another preferred feature is that one of the layers is an oxygen barrier layer, preferably a non-heat-shrinkable layer Thus, a laminate film for use in this invention can comprise a first surface layer of material which is both heat-shrinkable and heat sealable, such as oriented polyethylene, an adjacent layer of an oxygen barrier material which is non-heat-shrinkable and a second surface layer of a material which is heat-sealable to the first surface layer Such a laminate can of course be produced as a flat film and wrapped around the food article with the first surface layer innermost A longitudinal seam is then formed by heat-sealing the two surface layers at the overlap In a particularly preferred form of this invention the essential heat-shrinkable layer is sandwiched between layers which comprise a non-shrinkable oxygen barrier layer and a heat-sealable surface layer, the essential heat-shrinkable layer providing shrink properties to the entire laminate film.

There is no need for the layers other than the compulsory heat-shrinkable layer to possess heat-shrinkable properties.

In considering the heat-in-package process of the invention, it will be appreciated that the food is preferably heated, usually cooked, at a temperature below that at which the packaging film would, when free to shrink, exhibit the necessary heat-shrinking effect.

Conversely, it is preferred in the process of the invention that the packaging film employed be heat-shrinkable at a temperature above the heating temperature These are not essential requirements: since the food is usually incompressible, the film will be usable to shrink because it is wrapped so tightly around the food that it has no room within which to shrink.

However, in that case, tension is imparted to the film and excessive tension is best avoided by suitable choice of cooking temperature and/or film materials Preferably the heatshrinkable layer or layers are heat-shrinkable at a temperature above 1740 F ( 790 C).

An especially preferred laminate film of the above type comprises 5 layers, excluding possible adhesive layers, in order as follows:

1 Surface layer which is heat-sealable, preferably to itself (can be heat-shrinkable or not).

2 Non-heat-shrinkable oxygen barrier layer.

3 Heat-shrinkable central layer.

4 Non-heat-shrinkable oxygen barrier layer, preferably of the same material as layer 2.

Surface layer which is heat-sealable, preferably to layer 1 and is preferably of the same material as layer 1.

This laminate film may be symmetrical about 130 1 591 424 the central layer, i e so that layers 4 and 5 are "mirror images" of 1 and 2 The central layer of heat-shrinkable material gives the laminate film its strength and shrink properties The laminate film may include adhesives or tackifiers where desired.

In the following description, various polymeric materials will be referred to by the following abbreviations:VDC copolymer = vinylidene chloride copolymer as defined above.

PE = polyethylene LDPE = polyethylene of low density.

MDPE = polyethylene of medium density.

EVA copolymer = polymer of ethylene and vinyl acetate having adhesive properties.

HEVA copolymer = hydrolysed (saponified) copolymer of ethylene and vinyl acetate, described hereinafter.

Where the context permits, the term "polymer" includes homopolymers, copolymers and interpolymers, whether block or random, ionomers and graft copolymers.

The heat-shrinkable layer is composed of a normally solid plastics material which provides satisfactory structural strength during a cooking process, e g a polymer of any of the following monomers: the mono-olefins and conjugated diolefins, e g ethylene, propylene, butene-1, isobutene, 1,3-butadiene, isoprene and other aliphatic mono and diolefins; the halogen substituted olefins, e g vinyl chloride, vinylidene chloride (although, of course, use of VDC copolymer is not preferred); the monovinylidene aromatic compounds, e g styrene, alphamethylstyrene and chlorostyrene, other aromatic olefins; and other ethylenically unsaturated monomers such as acrylonitrile or acrylamide The preferred class of heat-shrinkable films utilized in the laminate films is crosslinked and oriented olefin polymers, especially ethylene polymers and polypropylene Polyethylene which has been crosslinked by irradiation, with e g high energy electrons, and oriented, usually biaxially, is the preferred heatshrinkable material for the central layer of the laminate film Preferably the PE has a density of 0 910 to 0 970 g /cm 3, most preferably about 0 93 g /cm 3 It may be crosslinked by any suitable technique, e g that described in United States Patent No 3,144,399 The layer of polyethylene is usually from 0 75 to 1 50 mils ( 19 to 38 microns) thick, preferably about 1 mil ( 25 microns), and is desirably biaxially oriented to provide a shrinkage of at least 5 per cent when heated to a temperature of about 205 F ( 96 C) and a shrinkage of about 70 per cent at a temperature of 325 F ( 163 C) in both the axial and transverse directions.

The operability of the invention is not dependent on the relative thicknesses of the heat-shrinkable and non-heat-shrinkable layers, although the amount of shrinkage which the lamine will undergo may depend on this factor In general, the thickness of the heat-shrinkable layer (or total thickness if there is more than one) is best made at least 10 % of the thickness of the non-heat-shrinkable layer (or total thickness if, as is preferred, there is more than one) 70 The heat-shrinkable layer may be printed by conventional techniques which are well known to the art Polyethylene, which is the preferred heat-shrinkable material, is readily printable Printed polyethylene as well as the 75 overall laminate of this invention may be wound into large rolls without the bagging problem associated with conventional VDC copolymer film.

Adhesives for adhering the various layers to 80 one another can be any conventionally utilized to adhere the materials of the layers It is preferably an ionomer such as a "Surlyn" (U K.

Registered Trade Mark) of E I Du Pont de Nemours and Co ("Ionomers" are usually co 85 polymers of ethylene with from 5 to 20 weight percent of methacrylic acid, partly neutralised by an inorganic cation such as zinc) Such adhesive layers are generally on the order of 0 1 mil ( 2.5 microns) thick Frequently an adhesive 90 layer is not required, because of the inherent natural adhesion of the two layers to be laminated together Generally, when it is necessary to adhere a layer of heat-shrinkable ethylene polymer to an amide polymer, an adhesive is 95 necessary No adhesive is required, however, when adhering such a heat-shrinkable layer to an EVA copolymer.

In a preferred mode of making the lar-Ainate film for use in the invention, the essential heat 100 shrinkable layer is provided by interposing a self-supporting film thereof into the centre of a double-wound film, as hereinafter described.

The double-wound film preferably has facing layers comprising nylon ready to be split sym 105 metrically by the heat-shrinkable film, which is preferably of crosslinked polyethylene An adhesive is usually required, for adhering the polyethylene to nylon layers and may conveniently be provided in the double-wound film 110 at the interface of these nylon layers The adhesive must be one which enables the doublewound film to be split along the interface of the two mirror image facing layers Preferably also it will self-adhere to the heat-shrinkable 115 layer Such tacky self-adhering resins are well known to the art Particularly effective such resins include unsaturated ester polymers such as ethylene/unsaturated ester copolymers, the unsaturated ester being e g vinyl acetate, 120 vinyl propionate, methyl methacrylate, ethyl methacrylate, ethyl acrylate, or isobutyl acrylate; unsaturated carboxylic acid polymers, e.g ethylene/unsaturated carboxylic acid copolymers, the carboxylic acid being e g acrylic, 125 methacrylic, maleic, fumaric, or itaconic acid; low molecular weight olefin polymers, especially polyethylene or polypropylene An especially preferred resin for the preferred case of adhering polyethylene to nylon is an EVA co 130 1 591 424 polymer, preferably having a vinyl acetate content of 10 to 30 weight percent, to provide the requisite tackiness The layer of EVA copolymer can have a thickness generally of 0 1 to 0 5 mil ( 2 5 to 12 5 microns).

In the preferred embodiment, the heatshrinkable layer is adhered to facing oxygen barrier layers by the above described techniques The oxygen barrier layer is a material of low oxygen permeability, i e a dry oxygen permeability of less than 70 c c /m 2 atm at one atmosphere and 730 F ( 230 C) The oxygen barrier layer can comprise an amide polymer, preferably nylon-6 The thickness of the amide polymer layer is not critical, thicker layers providing better oxygen barrier properties Preferably the or each oxygen barrier layer within the laminate film is a "double layer" (two-ply layer) and therefore, in the most preferred 5layer construction described above there are 4 barrier plies Each individual barrier ply is preferably 0 3 to 0 6 mil ( 7 6 to 15 2 microns) thick.

While single or double plies consisting of amide polymer alone are perfectly satisfactory for many purposes, and a layer consisting of HEVA copolymer alone could be used, it is preferred to use a blend of a nylon and HEVA copolymer.

HEVA copolymers are described e g in United States Patents Nos 2,386,347 and 3,595,740 The degree of saponification or hydrolysis may range from 85 to 99 5 percent.

The ethylene-derived content of the copolymer may be within the range of 15 to 65 mole per cent Copolymers of lower than 15 mole per cent of ethylene tend to be difficult to extrude while those above 65 mole percent are deficient in their performance.

The preferred oxygen barrier layer is thus a blend of nylon-6 and HEVA copolymer The blend can be composed of 60 to 80 mole per cent nylon with the remaining 20 to 40 mole percent being HEVA copolymer, preferably 70 mole percent nylon, with the balance being HEVA copolymer having an ethylene content of about 30 mole percent The oxygen barrier layer in this instance comprises a double layer (two plies) of nylon/HEVA copolymer blend, each ply having a thickness of about 0 35 mil ( 9 microns) Such a blend of oxygen barrier materials confers an oxygen permeability of less than about 17 c c /m 2 atm at one atmosphere at 73 F ( 230 C) on the laminate film for use in this invention.

The oxygen barrier layer could consist of separate nylon and HEVA copolymer layers.

Each of such layers can be on the order of 0 30 mil ( 7 6 microns) thick to provide the requisite low oxygen permeability It is possible, although not preferred, to use a VDC copolymer layer or both VDC copolymer and nylon layers as an oxygen barrier film.

In the above described preferred embodiment, the oxygen barrier layers are non-heatshrinkable If desired, the oxygen barrier layers could be oriented to make them shrinkable and one of the other layers could be non-heatshrinkable However, it is an important feature of this invention that it enables other oxygen 70 barrier materials than VDC copolymer to be used and these other materials are less capable of being heat-shrunk satisfactorily.

In the preferred 5-layer construction, each oxygen barrier layer is then adhered to the 75 outermost (surface) heat-sealable layer by the use of an adhesive, which can be any of the adhesives discussed above.

The surface layers of the overall laminate are heat-sealable layers for adhering the laminate to 80 itself or to another film depending upon the particular way in which the package is to be made up Such heat-sealable layers may be of a material conventionally used for their heatsealing characteristics Alternative resins for the 85 heat-sealable layer include homopolymers and copolymers of ethylene and blends of ethylene polymers, particularly those of branched ethylene polymers of density from about 0 91 to 0 94 g /cc as well as copolymers of ethylene 90 with vinyl esters of organic acids such as vinyl acetate, vinyl propionate, and vinyl isobutyrate.

Copolymers of ethylene with lower acrylate esters such as ethyl methacrylate, methyl acrylate, ethyl acrylate, 2-ethyl hexyl acrylate and 95 with methacrylic acid (ionomeric resins) can also be used LDPE (density 0 91 to 0 94 g /cc.

is preferred Typically the heat-sealable layer of PE will have a thickness of 0 25 to 0 35 mils ( 6 to 9 microns) Preferred blends of PE for 100 this heat-sealable layer comprise MDPE and LDPE A layer of such a blend may also be oriented, as discussed above, to provide the requisite heat-shrinkable layer.

It is highly desirable to have a layer adjacent 105 the heat-sealable layer which has burn-through resistance Such a material has a higher thermal degradation range than heat-sealable layers, so that the tolerance limit on the heat-sealing operation is expanded as far as is possible Such 110 materials preferably have a melting range which extends approximately 1006 F ( 54 C range) above the softening range of the heat-sealable layer In the preferred form of the laminate film for use in this invention nylon-6 or a blend 115 of nylons is adjacent the heat-sealable layer.

Such a layer provides the desired burn-through resistance Other materials having burn-through resistance, which may be desirable to include adjacent the heat-sealable layer, are polyesters, 120 polycarbonates, polysulphone and polypropylene.

Preferably the layers of the laminate film except the adhesive layers are self-supporting films per se 125 The laminate film for use in the invention can be prepared by a process comprising interposing a self-supporting heat-shrinkable film between and in contact with two non-heatshrinkable self-supporting films and lamin 130 1 591 424 ating the-resultant assembly The assembly must contain an oxygen barrier layer or layers and preferably one or both non-heat-shrinkable film provides the required oxygen barrier layer(s) Preferably the non-heat-shrinkable films are self-adhesive with respect to the interposed heat-shrinkable film and the lamination is carried out by pressing the non-heatshrinkable film onto the heat-shrinkable film.

A particularly suitable process for preparing the outer layers, i e all the layers other than the heat-shrinkable layer, is that described in our United States Patent No 3,874,967 A tubular laminate comprising a layer of an amide polymer in contact with a radially inner layer or layers is produced by co-extrusion, using the inflated bubble technique The bubble is produced so that none of the layers is heatshrinkably oriented.

A liquid is entrapped between the walls of the bubble and the inner layer(s) are sufficiently porous to enable it to migrate to the layer of amide polymer (which may be a polyamide blend) The liquid is preferably water, which exerts a beneficial action on the amide polymer.

The tubular laminate film thus produced is collapsed and rolled into a roll of doublewound flat film The double-wound flat film may further comprise surface layers, which are heat-sealable as explained above, on the opposite face of the heat-shrinkable film to the adhesive layers, i e which in the tubular extrudate would be radially outer of the amide polymer layers In the preferred embodiment, a tubular coextruded film produced by such a process has an inner layer of EVA copolymer adjacent and adhered to a layer of nylon with an adhesive between the nylon layer and an outer heat-sealable layer of LDPE Each half of the double-wound film has a preferred thickness of 1 0 to 1 2 mil ( 25 to 30 microns).

The most preferred 5-layer film of the type described generally above is designated "Laminate A" and is therefore as follows, in order:

Layer No Material Thickness Whether mils heat(microns) shrinkable so 1 LDPE ( 0 25 to 0 35 no ( 6 to 9) Adhesive 0 1 ( 2 5) no 2 a Blend of 0 35 ( 9) no % nylon6 with 30 % HEVA, by wt., the ethylene content of the HEVA being 30 mole % 2 b Same as 0 35 ( 9) no 2 a EVA 0 1 to O 5 no polymer ( 2 5 to 12 5) adhesive having a vinyl acetate content of 10-30 % by wt.

3 Irradi 1 0 ( 25) ated, crosslinked, oriented LDPE EVA polymer adhesive as above 4 a As 2 a 4 b As 2 a Adhesive (as between 1 and 2 a) yes As 1 85 Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates schematically preferred apparatus for producing the laminate film 90 for use in this invention, and Figure 2 illustrates schematically packages formed from the laminate film for use in this invention.

Referring to Figure 1 a double-wound 95 laminate film 1, produced by the process of United States Patent 3,874,967 is fed between pinch rolls 3 and 4 where the double-wound film is split into mirror image counterparts and fed to rollers 5 and 7 Simultaneously with 100 the splitting of film 1, film 9 of heat-shrinkable material, preferably irradiated oriented PE, is fed to rolls 11 and 12 which also receive the split halves 13 and 15 of the outer laminate layers In this manner, the heat-shrinkable layer 105 is sandwiched between the mirror image layers by the pressure of pinch rolls 11 and 12.

Pinch rolls 11 and 12 may additionally be heated to assist the adhesion of film layers 13 and 15 to the inner heat-shrinkable layer 9 110 at the interfaces thereof Generally, however, the EVA copolymer layer is tacky enough to provide sufficient adhesion at ordinary temperatures This is also true for an ionomer adhesive, particularly when pinch rolls 11 115 and 12 provide sufficient pressure to make the interface between the heat-shrinkable layer and the outer layers devoid of trapped air.

In an alternative embodiment the same layer of heat-shrinkable polyethylene as described 120 above is employed, but a layer of VDC copolymer is coated thereon by conventional techniques, usually coating from a solution or latex, thereby avoiding melt-extrusion Such a laminate may then be adhered to a surface, 125 heat-sealable, layer or heat-shrinkable polyethylene The VDC copolymer is non-heatshrinkable The outer layers of heat-shrinkable polyethylene provide the requisite shrink properties for the composite laminate of this,130 is 1 591 424 embodiment One of the outer layers may be of a material which is non-heat-shrinkable but with heat sealing properties such that the outer layers would adhere to one another, e g.

LDPE or EVA copolymer or other materials described above The VDC copolymer layer may be printed with a label before the final surface layer is laminated to it, and the production of large rolls can be avoided These laminates containing a layer of VDC copolymer are designated "laminates B" and are of course, less preferred to "laminate A" Laminates B are described and claimed per se, in the parent application No 41703/77 (Serial No 1591423).

The process of the invention may be carried out using a chub machine such as the "Kartridge Pak Model 50 " (Kartridge Pak Co, Davenport, Iowa, United States of America).

Such an apparatus is illustrated in Figure 2 of the drawing Such a chub machine comprises guide rolls 21 and 23 for feeding the film to stuffer 25 and former 27 The former 27 forms the tube which is filled with a food article by means of stuffer 25 The tube stuffed with food is sealed along its length by hot air sealer 29 The longitudinal chub is then clipped at 31 by conventional techniques such as a staple to form individual chubs 33.

The chubs formed on such a forming machine are cooked by the process of this invention to provide a package of extremely uniform dimensions Typical dimensions for such a chub are 1 5 to 6 inches ( 38 to 152 mm.) in diameter and 4 to 72 inches ( 102 mm to 183 cm) long Typical of the meat products which may be cooked in such a chub are bologna, chicken rolls, turkey rolls and sausage.

Such rolls are generally cooked to an internal temperature of about 1450 to 165 'F ( 63 to 740) in hot water having a temperature of about 1550 to 175 'F ( 68 to 79 C) this is a typical temperature for most purposes During such cooking process the meat article within the chub swells However, the strength and dimensional stability provided by the central heat-shrinkable layer of the laminate of this invention maintains the dimensional stability and resists the stretching force exerted by the meat The dimensional stability provided by the laminate of this invention has been found to be surprisingly superior, in that the circularcross-section of the cylindrical chub is preserved during the cook-in-package process rather than slumping to an elliptical shape cross-section.

At the completion of the cooking process, the laminate film containing the food article is rapidly chilled, e g in a cold water bath The chilling causes the food article, but not the laminate, to shrink The shrinkage of the food article produces a gap between it and the laminate film, giving the package an unsightly appearance The process of this invention, however, enables the unsightly appearance to be remedied At the completion of the chilling step the laminate film is heated to the shrink temperature of the heat-shrinkable film to shrink the entire package around the food article, thus producing an attractively packaged cooked article within a laminate film package 70 The shrink process is preferably carried out in a hot water or shower bath, using water at about 200 to 212 'F ( 93 to 100 C) A temperature of about 20501 F ( 96 TC) is preferred when utilizing irradiated oriented PE Higher 75 temperatures may be used for greater shrinkage if desired Surprisingly, it has been found that not just the oriented, crosslinked PE central layer, but the entire laminate, shrinks.

Another advantageous process of this 80 invention is in the packaging of hot food directly out of a pasteurizer In this process the step (b) of heating is for pasteurisation purposes and is carried out before the step (a) of enclosing the food in the laminate film, 85 and step (a) is carried out at substantially the pasteurisation temperature Typically, the food is packaged by a chub machine at a temperature of about 1550 F ( 680 C) and chilled as described above to produce gaps between 90 the shrunken meat article and the laminate film packaging The unsightly gaps are in a similar manner removed by heat shrinkage to produce an attractive package.

Claims (1)

1. WHAT WE CLAIM IS: 95

   1 A process of preparing a package of food, which process comprises:

   (a) enclosing said food tightly in a laminate film comprising at least one heat-shrinkable layer adherent to at least one non-heat-shrink 100 able layer, said laminate film containing at least one oxygen barrier layer having a permeability to dry oxygen of less than c c /m 2 atm at 230 C; (b) heating the food to a temperature at 105 which it expands; steps (a) and (b) being carried out in either order; (c) thereafter cooling the package to a temperature at which the food shrinks, leaving gaps between the food and the packaging 110 film; and (d) thereafter heating the packaging film to shrink it onto the food and thereby eliminate said gaps.

   2 A process according to claim 1 wherein 115 the heat-shrinkable layer or layers are heatshrinkable at a temperature above the heating temperature in step (b).

   3 A process according to claim 1 or 2 wherein the heating step (b) is effected to cook 120 the food.

   4 A process according to claim 1 or 2 wherein the heating step (b) is for pasteurization and is carried out before the step (a) of enclosing the food in the laminate film, and 125 step (a) is carried out at substantially a pasteurization temperature.

   A process according to any preceding claim wherein the oxygen barrier layer or layers are non-heat-shrinkable 130 1 591 424 6 A process according to any preceding claim wherein the heat-shrinkable layer or layers are heat-shrinkable at a temperature above 79 TC.

   7 A process according to any preceding claim wherein the laminate film has a first surface layer which is heat-sealable to itself and a second surface layer which is heat-sealable to the first surface layer.

   8 A process according to claim 7, wherein the laminate film comprises the following layers, in the order set out: i ( 1) a first surface layer which is heat-sealable to itself; ( 2) an oxygen barrier layer as defined in claim 1 of one or more plies, which is nonheat-shrinkable; ( 3) a heat-shrinkable layer; ( 4) an oxygen barrier layer as defined in claim 1, of one or more plies, which is nonheat-shrinkable; and ( 5) a second surface layer which is heatsealable to the first surface layer.

   9 A process according to claim 7 or 8 wherein both heat-sealable surface layers are of polyethylene of density 0 91 to 0 94 g /c c.

   A process according to any preceding claim wherein the oxygen barrier layer or layers comprises an amide polymer.

   11 A process according to claim 10 wherein the amide polymer is nylon-6.

   12 A process according to claim 11 wherein each oxygen barrier layer comprises a blend of to 80 mole percent of nylon-6 and 40 to 20 mole percent of hydrolysed polymer of ethylene and vinyl acetate having an ethylenederived unit content of 15 to 65 mole percent and a degree of hydrolysis of from 88 to 99 5 mole percent.

   13 A process according to any preceding claim wherein the laminate film contains as a heat-shrinkable layer a layer of an olefin polymer crosslinked by irradiation and oriented for heat-shrinking.

   14 A process according to claim 13 wherein the olefin polymer is an ethylene polymer.

   A process according to claim 14 wherein the olefin polymer is an irradiated, biaxially oriented polyethylene.

   16 A process according to claim 8 wherein the surface layers ( 1) and ( 5) are as defined in claim 9, the oxygen barrier layers ( 2) and ( 4) as defined in claim 11 or 12 and the heat-shrinkable layer ( 3) as defined in claim 15 and there are adhesive layers between adjacent layers, the adhesive between layers ( 2) and ( 3) and ( 3) and ( 4) being of an ethylene-vinyl acetate copolymer containing 10 to 30 weight percent of vinyl acetate-derived units.

   17 A process according to any one of claims 1 to 6 or to claim 13, 14 or 15 when appendant thereto, wherein the oxygen barrier layer is of a copolymer of 65 to 95 weight percent vinylidene chloride and 35 to 5 weight percent of a vinyl comonomer.

   18 A process according to claim 17 wherein the laminate film comprises two heat-shrinkable layers of polyethylene and a layer of the vinylidene chloride copolymer between them, as a coating on one of the polyethylene layers.

   19 A process according to claim 1 wherein the laminate film is "laminate B" hereinbefore described.

   A process according to claim 1 wherein the laminate film is "laminate A" hereinbefore described.

   21 A process according to any one of claims 1 to 16 wherein each said layer of the laminate, other than layers specified as adhesive layers in claim 16, is per se a self-supporting film.

   22 A process according to any one of claims 1 to 16 wherein the laminate is prepared by a method which comprises assembling two self-supporting non-heat-shrinkable films in contact with an interposed self-supporting heatshrinkable film and the non-heat-shrinkable films are self-adhesive with respect to the interposed heat-shrinkable film, and the lamination is carried out by pressing the non-heatshrinkable films onto the heat-shrinkable film.

   23 A process according to any one of claims 1 to 16 and 22 wherein the laminate is prepared by a method which comprises laminating (a) two self-supporting non-heat-shrinkable films, each of which is a split doublewound film of a tubular laminate made by co-extrusion of at least a layer of amide polymer and a radially inner layer of an ethylene-vinyl acetate copolymer as defined in claim 16, by the inflated bubble technique, with water trapped within the bubble, with (b) an interposed self-supporting heat-shrinkable film of polyethylene cross-linked by irradiation and biaxially oriented.

   24 A process according to claim 21 wherein the method of preparation of the laminate is carried out with apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

   A process according to claim 17 or 18 wherein the laminate film is prepared by a method which comprises coating a heat-shrinkable film with a solution or latex of a vinylidene chloride copolymer defined in claim 17 and laminating it to another heat-shrinkable film.

   26 A process according to claim 1, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

   27 A package of food prepared by a process claimed in any one of claims 1 to 26.

   J.A KEMP & CO, Chartered Patent Agents, 14 South Square, Gray's Inn, London WC 2 A l AY.

   Printed for Her Majesty's Stationery Office by MULTIPLEX techniques ltd, St Mary Cray, Kent 1981 Published at the Patent Office, 25 Southampton Buildings, London WC 2 l AY, from which copies may be obtained.