CH648804A5 - Process for packaging a foodstuff, multi-layer film therefor and production process for such a film - Google Patents

Description

The invention relates to a method for packaging a food, a multilayer film for carrying out the method and a method for producing such.

When packaging most foods, the packaging film must form an oxygen barrier, i.e.

have a low permeability to oxygen. A good oxygen barrier layer results in a film which is produced from 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 methacrylate. This material is known in the USA under the generic term “Saran” (registered as a trademark in a large number of countries) and is referred to below as “VDC copolymer”.

In order to sell packaged cooked food, the packaging must have an attractive appearance. However, this creates a problem. For reasons of economy and food hygiene, food is best packaged before cooking, cooked in the packaging, and then sold in the same packaging. Many foods expand during cooking, which can cause the packaging film to stretch. When the package has finished cooking and cooling, the food article shrinks while the stretched packaging film remains in the stretched state. This results in a packaged food item with an unaesthetic appearance because the packaging resembles the surface of a prune.

In addition, many food packs must have a regular geometric shape over their entire extent. Chicken rolls or sausage rolls e.g. with a generally cylindrical shape are often cut so that each disc is of the same thickness and weight. Such uniformity can only be achieved if the cylindrical shape is adhered to very precisely and the diameter is constant over the entire length. However, if the packaging film stretches during cooking, such a shape cannot be maintained.

VDC copolymers have been commonly used for the food packaging process described above and cooking in packaging, hereinafter referred to as the cook-in packaging process, because they resist stretching. However, this material has many drawbacks, which makes it difficult to use in food packaging. If only a film made of VDC polymer is used, the problem arises that the polymer is difficult to heat seal. Even if a seamless tube made of VDC polymer is used, the ends of the tube piece must be heat sealed around the food item to close the package. It is necessary to weld the VDC copolymer using radio frequency energy. This approach does not always produce predictable and satisfactory results. For reasons not known, some batches of VDC copolymers are easily weldable, while others are practically non-weldable. These problems can be eliminated by using a laminate of polymeric films in which a heat-sealable layer of a different type of polymer is applied to the VDC copolymer. In principle, such laminates are most advantageously produced by co-extrusion from the melt. Difficulties arise, however, from the fact that the VDC copolymer easily decomposes at the temperatures most suitable for extrusion. It is therefore difficult to extrude it as a single film over a long period of time without the extrusion nozzle being blocked by solid carbon particles.

Another problem with the use of VDC copolymers is the storage of large rolls of untrimmed film after the film has been printed in the middle of the label. Printing leads to a change in thickness in the film, the center of which is thicker than the edge areas. This enables natural shrinkage at the edge areas, so that when the film roll is used for packaging, the film bulges in the middle or forms folds. If an attempt is made to wind up the film printed in this way into rolls of large diameter, it is no longer possible to process the printed film into bags because of the shrinkage in the unprinted areas.

There are oxygen barrier films made from other 55 materials, but their use in connection with cooking food in the film causes difficulties.

The production of a packaging film suitable for a cook-in packaging process, in which a VDC-Copo-60 lymeres is not an essential component, is therefore a problem. That is, the problem is to develop a heat-resistant packaging film that resists stretching during cooking and maintains package uniformity. Such a packaging material is also intended to avoid the disadvantages of VDC copolymers described above.

The method according to the invention for packaging a food is characterized by the

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Proverb 1 specified combination of process steps. Advantageous developments of the method result from the dependent claims, the multilayer film is characterized in claims 19 and 20 and a manufacturing method for such in claims 21 to 25.

It has been found that a multilayer film which has at least one heat-shrinkable layer which confers this property of the multilayer film and which adheres to a layer which is not heat-shrinkable by itself, with none of the required layers necessarily consisting of a VDC copolymer, has a high degree of stretch resistance and largely maintains the dimensions of the package. The heat shrinkable layer usually gives the film some resistance to stretching and maintains the dimensions of the package during cooking. As the food article shrinks as it cools, there is still a gap between the food article and the packaging film, but this lack of appearance can be remedied. Surprisingly, it has been found that a multilayer film which contains a heat-shrinkable layer and a non-heat-shrinkable layer or layers has heat-shrinkability properties. It has been found that a single heat-shrinkable layer causes the otherwise non-heat-shrinkable layers to shrink without causing distortion or wrinkling of the multilayer overall film. Accordingly, the film can then be shrunk hot onto the surface of the food article to eliminate the distance between the food article and the packaging film. Surprisingly and contrary to expectations, the multilayer film does not become wrinkled and unsightly despite the presence of a non-heat-shrinkable layer.

According to a preferred embodiment of the invention, a multi-layer overall film with heat-sealable surface layers is used. The required heat-shrinkable layer can therefore serve two purposes at the same time. It can be an optionally desired heat-sealable surface layer as well as the required heat-shrinkable layer. In a further embodiment of the invention, one of the layers is designed and used as an oxygen barrier layer and preferably as a non-heat-shrinkable layer. Accordingly, a multilayer film used in the invention can have a first surface layer made of a material that is both heat shrinkable and imparts this property to the multilayer film and is heat sealable, e.g. oriented polyethylene, an adjoining layer of an oxygen barrier material that is not heat-shrinkable, and a second surface layer of a material that is heat-sealable with the first surface layer. Such a multilayer film can of course be produced as a flat film and wrapped around the food article with the first surface layer inwards. Where the two surface layers overlap, a longitudinal seam can be formed by heat sealing. In a particularly preferred embodiment of the invention, the required heat-shrinkable layer is sandwiched between layers which have a non-heat-shrinkable oxygen barrier layer and a heat-sealable surface layer, the required heat-shrinkable layer imparting shrink properties to the entire multilayer film. There is no need for the other layers other than the required heat-shrinkable layer to have heat-shrinkable properties.

With regard to the multilayer film used in the invention, it should be noted that the foodstuffs are preferably cooked at a temperature which is below the temperature at which the required heat shrinkage of the packaging film would occur if it could shrink freely. Conversely, it is preferred in the cook-in packaging process that the packaging film used can be heat-shrunk at a temperature above the cooking temperature. This is not absolutely necessary, as foods are usually not compressible; the film cannot shrink because it is firmly wrapped around the food and there is no room for shrinking. In this case, however, tension is applied to the film and excessive tension is best avoided by appropriate selection of the cooking temperature and / or the film material. The heat-shrinkable layer or 20 the heat-shrinkable layers are preferably heat-shrinkable at a temperature above 79 ° C. The method has the sequence of method steps specified in claim 1.

A 25 multilayer film of the above type suitable for use in this process contains, apart from possible adhesive layers, 5 layers in the following order:

1. With heat-sealable surface layer (heat-shrinkable or not);

30 2. Oxygen barrier layer which cannot be heat-shrunk on its own;

3. heat-shrinkable middle layer imparting this property to the multilayer film;

4. Oxygen barrier layer 35 that cannot be heat-shrunk on its own, preferably made of the same material as

Layer 2;

5. Surface layer which is at least heat sealable with layer 1 and preferably made of the same material as layer 1.

40 This multilayer film can be symmetrical around the middle layer, i.e. layers 4 and 5 can be mirror images of layers 1 and 2. The middle layer made of heat-shrinkable material gives the multilayer film its strength and its shrinkability properties. 45 If desired, the multilayer film can contain adhesives or tackifiers.

In the following description, various polymeric materials are designated with the following abbreviations:

Vinylidene chloride copolymer as indicated above, polyethylene

Low Density Polyethylene Medium Density Polymer of ethylene and vinyl acetate with adhesive properties hydrolyzed (saponified) copolymer of ethylene and vinyl acetate as described below.

Unless the context indicates otherwise, the term “polymer” includes homopolymers, copolymers and interpolymers, regardless of whether they are block copolymers or statistical copolymers, as well as ionomers or graft copolymers.

VDC copolymer =

PE = LDPE

55 MDPE =

EVA copolymer -

HEVA copolymer

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The heat shrinkable layer is made of a normally rigid plastic material which provides satisfactory structural strength during a cooking process, e.g. a polymer of one of the following monomers: monoolefins and conjugated diolefins, e.g. Ethylene, propylene, butene-1, isobutene, 1,3-butadiene, isoprene and other aliphatic mono- and diolefins; halogen substituted olefins, e.g. Vinyl chloride, vinylidene chloride (although the use of VDC copolymers is of course not preferred); aromatic monovinylidene compounds, e.g. Styrene, a-methylstyrene, chlorostyrene and other aromatic olefins; and other ethylenically unsaturated monomers such as acrylonitrile, acrylamide and the like. The preferred class of heat shrinkable films are cross-linked and oriented olefin polymers, especially ethylene polymers and polypropylene. Cross-linked polyethylene which can be obtained by irradiation, e.g. with high energy electrons, which has been crosslinked and is usually biaxially oriented, is the preferred heat-shrinkable material for the middle layer of the multilayer film according to the invention. The PE preferably has a density of 910 to 970 kg / m3 and in particular of approximately 930 kg / m3. It can be networked in any suitable way, e.g. by the method described in U.S. Patent 3,144,399. The layer of polyethylene is usually 19 to 38 and preferably about 25 [in thickness. It is desirable that it be biaxially oriented to have at least 5% shrinkage in both the axial and transverse directions when heated to a temperature of about 96 ° C and a shrinkage of about 70% at one temperature of 163 ° C.

The usefulness of the multilayer film does not depend on the relative thickness of the heat-shrinkable and non-heat-shrinkable layers, although the extent of the shrinkage to which the film is subjected may depend on this factor. In general, the thickness of the heat-shrinkable layer (or the total thickness if there is more than one heat-shrinkable layer) is most preferably at least 10% of the thickness of the non-heat-shrinkable layer (or the total thickness if, as is preferred, more than one layer is present) ).

The heat-shrinkable layer can be printed using known conventional methods. The preferred heat shrinkable material, polyethylene, is easily printable. Both printed polyethylene and the multi-layer overall film can be wound up into large rolls without the bagging problems of the conventional VDC copolymer film.

Any conventional adhesive suitable for joining the layer materials can be used as the adhesive for connecting the different layers to one another. Preferably, the adhesive is an ionomer such as a "Surlyn" (registered trademark in many countries) from E.I. DuPont de Nemours and Co. ("Ionomere"

are usually copolymers of ethylene with 5 to 20 wt .-% methacrylic acid, partially neutralized by an inorganic cation such as zinc). Such adhesive layers are generally about 2.5 µm thick. An adhesive layer is often not necessary due to the natural adhesion of the layers to be laminated. When a layer of heat shrinkable ethylene polymer is to be bonded to an amide polymer, an adhesive is generally required. However, no adhesive is required if such a heat-shrinkable layer is laminated with an EVA copolymer.

To produce the multilayer film, the required heat-shrinkable layer can be embedded in the middle of a double-wound film, as described in more detail below. The double wound

Foil preferably has nylon-containing opposing layers that are easily separated symmetrically by the heat-shrinkable layer, which in turn is preferably made of cross-linked! There is polyethylene. At least to connect the polyethylene layer with the nylon layers, an adhesive is required and can advantageously be present in the double-wound film on the contact surface of these nylon layers. The adhesive must be such that it enables the double-wound film to be separated along the contact surface of the two opposite mirror-image layers. It is also preferably self-adhesive with regard to the heat-shrinkable layer. Such sticky self-adhesive resins are generally known. Resins of this type which are particularly effective include unsaturated ester polymers such as ethylene / unsaturated ester copolymers, the unsaturated ester e.g. Vinyl acetate, 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 for example acrylic acid, methacrylic acid, maleic acid, fumaric acid or itaconic acid, and low molecular weight olefin polymers, in particular polyethylene or polypropylene. A particularly preferred resin for the preferred case of joining polyethylene to nylon is an EVA copolymer, preferably with a vinyl acetate content of 10 to 30% by weight, to provide the required tackiness. The layer of EVA copolymer can generally have a thickness of 2.5 to 12.5 μm.

In the preferred embodiment of the method, the heat-shrinkable layer is glued to opposing oxygen barrier layers. The oxygen barrier layer is made of a material of low oxygen permeability, i.e. a permeability to dry oxygen at 23 ° C and 0.1 MPa of less than 6.9 X 10 -4 m3 / m2 / MPa / 24 h, for example measured according to ASTM D-1434 and ASTM D-3985-81. The oxygen barrier layer may contain an amide polymer, preferably nylon-6. The thickness of the amide polymer layer is not critical. Dik-kere layers provide better oxygen barrier properties. Preferably, the or each oxygen barrier layer in the multilayer film is a double layer (two-layer layer), and therefore four barrier layers are present in the most preferred 5-layer embodiment described above. Each individual barrier layer is preferably 7.6 to 15.2 (t thick.

Although single or double layers of an amide polymer alone are quite sufficient for many purposes and a layer of HEVA copolymer can be used alone, it is preferred to use a mixture of a nylon and a HEVA copolymer.

HEVA copolymers are described in detail in U.S. Patent 2,386,347 and U.S. Patent 3,595,740. The degree of saponification or hydrolysis can be 85 to 99.5%. The proportion of the copolymer derived from ethylene can be 15 to 65 mol%. Copolymers with less than 15 mole percent ethylene tend to be difficult to extrude, while those with more than 65 mole percent are deficient.

The preferred oxygen barrier is a blend of nylon-6 and a HEVA copolymer. The mixture can be composed of 60 to 80 mol% nylon and 20 to 40 mol% HEVA copolymers, preferably 70 mol% nylon and the rest of HEVA copolymers with an ethylene content of approximately 30 mol%. In this case, the oxygen barrier layer has a double layer (two layers) of the nylon / HEVA copolymer mixture.

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each layer has a thickness of approximately 9 μm. Such a mixture of oxygen barrier materials gives the multilayer film according to the invention an oxygen permeability at 23 ° C. and 0.1 MPa of less than 1.7 × 10 -4 m3 / m2 / MPa.

The oxygen barrier layer can consist of separate nylon and HEVA copolymer layers. Each of these layers can be about 7.6 µm thick to provide the required low oxygen permeability. It is also possible, although not preferred, to use a VDC copolymer layer or both a VDC copolymer layer and a nylon layer as an oxygen barrier.

In the film described above, the oxygen barrier layers are not heat shrinkable. If desired, the oxygen barrier layers can be oriented to make them shrinkable and one of the other layers cannot be heat shrinkable. However, oxygen barrier materials other than VDC copolymers can be used: however, these other materials are less able to be satisfactorily heat shrinkable.

In the 5-layer multilayer film, each oxygen barrier layer is bonded to the outermost (surface) heat sealable layer using an adhesive, which can be any of the adhesives described above.

The surface layers of the overall film are heat-sealable layers for gluing the multilayer film to itself or to another film, depending on how the packaging is opened. Such heat-sealable layers can consist of a material that is conventionally used due to its heat-sealing properties. Alternative resins for the heat sealable layer include Homopolymers and copolymers of ethylene and mixtures of ethylene polymers, in particular those of branched ethylene polymers with a density of about 910 to 940 kg / m3, and copolymers of ethylene 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, methacrylate, ethyl acrylate, 2-ethylhexyl acrylate and with methacrylic acid (ionomeric resins) can also be used. LDPE (density 910 to 940 kg / m3) is preferred. Typically, the heat sealable layer of PE has a thickness of 6 to 9 µm. Preferred blends of PE for this heat sealable layer contain MDPE and LDPE. A layer of such a blend can also be oriented as described above to provide the required heat shrinkable layer.

It is very desirable to have a layer adjacent to the heat sealable layer that is resistant to burnout. Such a material has a higher thermal decomposition range than heat-sealable layers, so that the tolerance limit for heat sealing is increased as much as possible. Such materials preferably have a melting range which is approximately 54 ° C. above the softening range of the heat-sealable layer. The preferred multilayer film is adjacent to the heat-sealable layer nylon-6 or another mixture of different types of nylon. Such a layer provides the desired burn-through resistance. Other materials with burn-through resistance that can optionally be used adjacent to the heat-sealable layer are polyesters, polycarbonates, polysulfones and polypropylene.

A particularly suitable method for producing the outer layers, i.e. all layers except the heat shrinkable layer is described in US Pat. No. 3,874,967. A tubular multilayer film that contains a layer of an amide polymer in contact with a radially inner layer or layers is produced 5 by coextrusion using the "inflated bubble" technique. The bubble is produced in such a way that none of the layers is oriented to be heat shrinkable. A liquid is trapped between the walls of the bladder and the inner layer or layers are sufficiently porous that the liquid can migrate to the layer of amide polymer (possibly a polyamide blend). The liquid is preferably water, which has a positive effect on the amide polymer. The tubular multilayer film thus produced is collapsed and rolled up into a roll of double-wound flat film. The double-wrapped flat film may also have surface layers on the opposite sides of the heat-shrinkable film which, as explained above, are heat-sealable, i.e. Layers 20, which are located in the tubular extradate in the radial direction outside of the amide polymer layer. In the preferred embodiment, a coextruded tubular film made by such a method has an inner layer of EVA copolymers adjacent 25 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 25 to 30 µm.

30 The most preferred 5-layer film of the type described above is called "Laminate A" and contains in the following order:

Layer material thickness pim hot

35 no shrink bar

6 to 9 no

2.5 no

9 no

9 no

2.5 to 12.5 no

3. irradiated cross-linked 25 yes oriented LDPE

- EVA polymer adhesive

60 fabric as above

4a. like 2a.

4b. like 2a.

65 - adhesive (as between 1. and 2a.)

5.As 1.

1. LDPE

40 - adhesive

2a. Mixture of 70

% By weight

Nylon-6 with 30% by weight 45 HEVA, the

Ethylene content of the HEVA is 30 mol%

50 2b. like 2a.

- EVA polymer adhesive

substance with a vinyl acetate content of 10 to 30% by weight

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Implementations of the invention will be described below with reference to the drawings; show it:

Fig. 1 is a schematic representation of an apparatus for producing a multilayer film, and

Fig. 2 is a schematic representation of the production of packaging from such a multilayer film.

1, a double-wound multilayer film 1, which is produced by the process described in US Pat. No. 3,874,967, is passed through the nip rolls 3 and 4, where the double-wound film is separated into mirror-image counterparts and to the rolls 5 and 7 is performed. At the same time, the film 9 made of heat-shrinkable material, preferably irradiated, oriented PE, is guided to the rollers 11 and 12, which likewise take up the separated halves 13 and 15 of the outer laminate layers. In this way, the heat-shrinkable layer is sandwiched between the mirror-image layers by the pressure of the pinch rollers 11 and 12. The pinch rollers 11 and 12 can also be heated in order to promote the adhesion of the film layers 13 and 15 to the inner heat-shrinkable layer 9. In general, however, the EVA copolymer layer is sticky enough to provide adequate adhesion at ordinary temperatures. This also applies to an ionomer adhesive, in particular if the pinch rollers 11 and 12 supply sufficient pressure so that no air is trapped between the contact surfaces of the heat-shrinkable layer and the outer layers.

In another embodiment, the same layer of heat-shrinkable polyethylene as described above is used, but a layer of a VDC copolymer is applied thereon by conventional methods; usually by coating from a solution or latex while avoiding melt extrusion. Such a laminate can then be glued to a heat-sealable layer of heat-shrinkable polyethylene. The VDC copolymer is not heat shrinkable. The outer layers of heat-shrinkable polyethylene provide the necessary shrink properties for this embodiment of the film. One of the outer layers can be made of a material that is not heat shrinkable but heat sealable so that the outer layers adhere to each other, e.g. LDPE or EVA copolymer or other materials described above. The layer of VDC copolymers can be printed like a label before the last surface layer is laminated on, and the production of large rolls can be avoided. These multilayer films, which contain a layer of VDC copolymer, are referred to as "Laminate B" and are less preferred than "Laminate A".

The multilayer film can be filled with a food product on a “chub” machine such as “Kartridge Pak Model 50” (Kartridge Pak Co., Davenport, Iowa, USA). Such a device is shown in FIG. 2. Such a “chub” machine has guide rollers 21 and 23 for guiding the film to the filling device 25 and to the shaping device 27. The shaping device 27 forms the tube, which is filled with a food item by means of the filling device 27. The tube filled with the food is sealed on its long side by a hot air sealing device 29. The filled and sealed tube is then removed at 31 using conventional methods such as e.g. constricted by a bow (staple) to individual pieces 33 (chubs).

The packaged pieces of food produced in this way are cooked and result in a pack with extremely uniform dimensions. Typical dimensions are a diameter of 38 to 152 mm and a length of 102 mm to 183 cm. Typical meat products that can be cooked in this way are bologna (mortadella), chicken rolls, turkey rolls and sausage. Such rolls are usually cooked at an internal temperature of about 63 to 74 ° C in hot water which has a temperature of about 68 to 79 ° C. The latter is a typical temperature for most applications. During this cooking the meat swells in the packaging. However, the strength and the stability with regard to the dimensions which result from the middle heat-shrinkable layer of the multilayer film according to the invention retain the original dimensions and resist the stretching force exerted by the meat. It has surprisingly been found that the stability of the multilayer film according to the invention is so great that the circular cross-section of the cylindrical packaging is preserved during cooking and does not change into an elliptical cross-section.

Upon completion of cooking, the multilayer film containing the food article is rapidly cooled, e.g. in a cold water bath. Cooling causes the food item to shrink but not the multilayer film. The shrinkage of the food article results in a space between it and the multilayer film, which gives the package an unsightly appearance. However, the multilayer film makes it possible to remove this unsightly appearance. After cooling is complete, the multilayer film can be heated to the shrink temperature of the heat-shrinkable film in order to shrink the entire packaging film onto the food article. In this way, an attractively packaged cooked food item is produced in a multilayer film package. The shrinking process is preferably carried out in a hot water bath or shower bath using water at a temperature of 93 to 100 ° C. A temperature of about 96 ° C is preferred when irradiated oriented PE is used. If desired, higher temperatures can be used for greater shrinkage. Surprisingly, it was found that not only the oriented, cross-linked middle PE layer, but the entire multilayer film shrinks.

Another advantageous use of the multilayer film is in the packaging of hot foods directly from a pasteurizer. In this method, the heating step (b) is carried out for pasteurization purposes and before step (a), in which the food is packed in the multilayer film. Step (a) is essentially carried out at the pasteurization temperature. Typically, the food is packaged using a chub machine at a temperature of about 68 ° C and, as described above, cooled so that there are gaps between the shrunk meat article and the multilayer film packaging. The unsightly gaps are eliminated by heat shrinkage, so that an attractive package is obtained.

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Claims (25)

648804 1. A method of packaging a food, characterized in that: (a) the food is coated with a multilayer film which has at least one layer which shrinks under the action of heat and imparts this property of the multilayer film and at least one layer which does not shrink under the action of heat alone, and at least one of which layers acts as an oxygen barrier and has a permeability for dry ones Has oxygen less than 6.9 X 10 "4 m3 / m2 / MPa / 24h at 23 ° C and 0.1 MPa; (b) heating the food before or after wrapping (a); and (c) cooling the package formed, shrinking the food and creating gaps between the food and the wrapper; and then (d) the wrapper is heated to shrink it onto the food and to remove the gaps created by the previous cooling. 2. The method according to claim 1, characterized in that the food in step (a) is wrapped in a multilayer film, the heat-shrinkable layer or layers at a temperature above the temperature to which the heating is carried out in step (b), or is heat-shrinkable or are. 2nd PATENT CLAIMS 3rd 648804 Packing film of the type defined in claim 1 under (a) for use in the packaging method according to claim 1, characterized in that a separated, double-wound film made of a tubular film is used as the film layers which are not heat-shrinkable by themselves, and that a heat-shrinkable film is used Multilayer packaging film introduces heat-shrinkable film between the film layers that are not heat-shrinkable on their own and glued to them. 3. The method according to claim 1 or 2, characterized in that the food is cooked in step (b). 4. The method according to claim 1 or 2, characterized in that one carries out the heating step (b) for pasteurization and before step (a) and step (a) is carried out at substantially the pasteurization temperature. 5 5. The method according to any one of claims 1 to 4, characterized in that the food in step (a) is coated with a multilayer film, the oxygen barrier layer or barrier layers is not or can not be heat shrunk by itself. 6. The method according to any one of claims 1 to 5, characterized in that the food in step (a) is coated with a multilayer film, the heat-shrinkable layer or layers at a temperature above 79 ° C is or are heat shrinkable. 7. The method according to any one of claims 1 to 6, characterized in that the food in step (a) is coated with a multilayer film which has a first surface layer that is heat-sealable with itself, and a second surface layer that with the first Surface layer is heat sealable. 8. The method according to claim 7, characterized in that the food in step (a) is coated with a multilayer film which, apart from any adhesive layers present, consists of the following layers in the following order: (A) a first surface layer that is heat sealable with itself, (B) an oxygen barrier layer defined in claim 1 consisting of one or more layers which is not itself heat shrinkable, (C) a heat shrinkable layer, (D) an oxygen barrier layer defined in claim 1 consisting of one or more layers which is not itself heat-shrinkable, and (E) a second surface layer which is heat sealable with the first surface layer. 9. The method according to claim 7 or 8, characterized in that the food in step (a) is coated with a multi-layer film, the two heat-sealable surface layers of polyethylene with a density of 910 to 940 kg / m3. 10th 10. The method according to any one of claims 1 to 9, characterized in that the food in step (a) is coated with a multilayer film whose oxygen barrier layer or barrier layers contains or contain an amide polymer. 11. The method according to claim 10, characterized in that the amide polymer is nylon-6. 12. The method according to claim 11, characterized in that the food in step (a) is coated with a multilayer film, wherein each oxygen barrier layer is a mixture of 60 to 80 mol% of nylon-6 and 40 to 13. The method according to any one of claims 1 to 12, characterized in that the food in step (a) is coated with a multilayer film which contains a layer of an olefin polymer as a heat-shrinkable layer, which is crosslinked and oriented for heat shrinkability. 14. The method according to claim 13, characterized in that the olefin polymer is an ethylene polymer. 15 15. The method according to claim 14, characterized in that the olefin polymer is a biaxially oriented polyethylene. 16. The method according to claim 8, characterized in that the food in step (a) is coated with a multilayer film, the surface layers (A) and (E) according to claim 9, oxygen barrier layers (B) and (D) according to claim 11 or 12 and contains a heat-shrinkable layer (C) according to claim 15, and that adhesive layers are present between adjacent layers, the adhesive between the layers (B) and (C) and (C) and (D) having an ethylene / vinyl acetate copolymer 10 to 30% by weight of units derived from vinyl acetate. 17. The method according to any one of claims 1 to 6, characterized in that the food in step (a) is coated with a multilayer film, the oxygen barrier layer of a copolymer of 65 to 95 wt .-% vinylidene chloride and 35 to 5 wt .-% Vinyl co-monomers exist. 18. The method according to claim 17, characterized in that the food in step (a) is coated with a multilayer film which has two heat-shrinkable layers made of polyethylene and a layer between them made of the vinylidene chloride copolymer as a coating on one of the two polyethylene layers. 19. Multi-layer film of the type defined in claim 1 under (a) for carrying out the method according to claim 1, characterized by the following layering: (Sj) a first heat-shrinkable layer of polyethylene which makes the multilayer film heat-shrinkable; (52) a layer which cannot be heat-shrunk on its own as an oxygen barrier, consisting of a copolymer of 65 to 95% by weight vinylidene chloride and 35 to 5% by weight vinyl comonomer, as a coating on the layer Sx or S3; (53) a second heat-shrinkable, d: e multilayer film which makes heat-shrinkable a layer of polyethylene. 20th 25th 30th 35 40 45 50 55 60 65 20. Multi-layer film according to claim 19, characterized in that at least one of the layers Sx and S3 consists of a cross-linked polyethylene. Contains 20 mol% of a polymer which can be obtained by hydrolysis of a polymer of ethylene and vinyl acetate with a content of units derived from ethylene of 15 to 65 mol% to a degree of hydrolysis of 88 to 99.5 mol%. 21. Process for the production of a multilayer process 22. The method according to claim 21, characterized in that the non-heat-shrinkable film layers comprise at least one oxygen barrier layer as defined in claim 1. 23. The method according to claim 21 or 22, characterized in that the film layers which are not themselves heat shrinkable are self-adhesive with respect to the heat-shrinkable film introduced and are pressed onto the heat-shrinkable film for bonding. 24. The method according to claim 21, 22 or 23, characterized in that one uses a separated, double-wound film made of a tubular multilayer film as a non-heat-shrinkable film layers, which by coextruding at least one oxygen barrier layer containing an amide polymer, and a radially inner layer of an ethylene / vinyl acetate copolymer with 10 to 30% by weight vinyl acetate units with water enclosed in the bladder, and that a film made of polyethylene which is crosslinked and biaxially oriented is used as the heat-shrinkable film layer. 25. The method according to claim 24, characterized in that the non-heat-shrinkable film layers have surface layers according to claim 7 or 9 on their sides facing away from the heat-shrinkable film layer.