EP0116394B1

EP0116394B1 - Packaging of fresh meat

Info

Publication number: EP0116394B1
Application number: EP19840300067
Authority: EP
Inventors: Andrew Nicholas Ferrar; Arthur Neville Jones
Original assignee: Bunzl Flexpack Ltd
Current assignee: Bunzl Flexpack Ltd
Priority date: 1983-01-14
Filing date: 1984-01-05
Publication date: 1987-03-25
Also published as: DK13284A; DE3462775D1; EP0116394A1; DK13284D0

Description

This invention relates to a method of packaging fresh meat by forming a first web of material into a receptacle to receive fresh meat, placing the meat in the receptacle and sealing the receptacle with a second web of material, wherein the meat, placed in the receptacle, is subjected to a vacuum treatment. The term "fresh meat" as used in this specification includes frozen fresh meat, meat offals and meat products all uncooked.
Such a method is known from DE-A-2364565.
Most retail packs of fresh meat which has been cut and packed for retail self service display have a critically short shelf life imposed by the loss of red oxymyoglobin colour (DE-A-2240234). This turns brown by autoxidation, the ion on the Haem ring being oxidised from the ferrous to the ferric state. The rate of this autoxidation is governed by several factors including:-

1. the partial pressure of oxygen at the surface of the meat;
2. the ambient temperature;
3. the pH of the meat;
4. microbial activity.

Factors 1 and 2 can be controlled by the choice of packaging system and subsequent handling procedures.
Work has already been done to prolong the shelf life of meat by packing it in highly impermeable barrier materials enclosing a controlled atmosphere. Typically this atmosphere is rich in both oxygen and carbon dioxide (75% O2, 25% C0₂), the high level of oxygen encouraging deeper formation of the oxymyoglobin into the substance of the meat. This approach has many problems including high cost, extensive in-pack atmosphere, bulk, unnatural appearance of the product, difficulty in detecting leaking packs and reported "explosive" spoilage by microbiological means when the consumer removes the pack from the point of sale.
We have now discovered that in contradistinction to what has been previously proposed an effective package can be prepared from two polymeric films which have oxygen-permeability and transparency.
According to the present invention there is provided a method as described hereinabove, characterised in that the vacuum is thereafter partially released under controlled conditions and the second web of material is sealed to the receptacle to form a package containing the fresh meat, the vacuum treatment and sealing being controlled so as to ensure that the condition of critical oxygen tension does not develop, and in that at least one of the webs is made of a material which is permeable to oxygen under the influence of the partial pressure obtaining in the package and which allows carbon dioxide to leave the package faster than oxygen enters and at least one of the webs is transparent.
The oxygen which flows into the package, under the influence of partial pressure, is to some extent transformed into carbon dioxide by the meat. The carbon dioxide has a higher diffusion rate than oxygen (approximately five times) and is also soluble in the meat system. This helps to maintain the "vacuum package" skin tight appearance as the carbon dioxide leaves the pack faster than the oxygen enters.
The effect of the vacuumising cycle is to remove the gases from the meat. These may then be replaced with oxygen by controlled partial release of the vacuum with this gas in the processing cycle on the machine, which drives the oxygenation layer deeper into the meat and ensures that the condition of "Critical Oxygen Tension" (ref. Proceedings of the Royal Society of London, Series B, vol. CIX, January 1932, London; J. Brooks "The Oxidation of Haemoglobin to Methaemoglobin by Oxygen", p.35-50) does not develop during the labile phase before the meat passes into refrigerated storage and whilst handled in the warm phases of shrink and sealing. A secondary feature of this relief of vacuum is a reduction of the atmospheric pressure on the meat and minimising of pressure effects (absence of drip exudation).
Alternatively, the controlled partial release of the vacuum may be effected with nitrogen or other inert gas instead of oxygen so that the fresh meat is packaged as a vacuum-packed anaerobic package which can be stored under vacuum or in an inert gas atmosphere in a "master pack" until required. After opening the "master pack", the exposure of the individual packages to air results in ready "blooming" i.e. reoxygenation of the respiratory pigment of the meat.
Preferably one of the webs or layers is both transparent and permeable to oxygen, so that the package consists of one transparent film of oxygen-permeable polymer shrunk around the fresh meat which is supported on a more rigid and possibly contrasting polymeric layer to give a base.
The high level of oxygen permeability for the package material is dependent upon the choice of polymer and the formation of a thin film of the polymer on a vacuum packaging machine itself by a process of either hot or cold drawing to form the receptacle. This process of cold or hot drawing not only thins down the material, allowing high oxygen permeability but also builds in shrink energy so that a tight pack can be maintained by subsequent heat treatment either during heat-sealing of thermoformed web to non-thermoformed web or later by immersion of the whole pack or part thereof, in hot water (e.g., 70°C-100°C, optimum 75°C-80°C), or in water vapour at an elevated temperature in a sealing chamber or by passing the pack through a heat tunnel.
The use of a hemispherical mould for the formation of the receptacle is particularly useful in that it permits uniform biaxial orientation of a polymeric film to take place allowing a uniform thickness to be generated and hence uniform oxygen permeability and subsequent shrinkage to be obtained. In practice, small deviations from hemispherical at the periphery of the mould are necessary to ensure the absence of stress localisation when the moulded film is sealed to the other web.
lonomeric polymers are particularly suitable for use as oxygen-permeable materials for forming the present packages. These polymers are sodium or zinc salts of ethylene-acrylic acid or ethylene methacrylic acid copolymers. They have excellent transparency and the ability to seal through contamination, whilst possessing high oxygen permeability in thin sections. A single web of ionomer having a thickness of 25 to 250 micron can be used as the web from which the receptacles are formed while the other web is a composite lidding material composed of an ionomer web reinforced with a stiffer membrane to give the required rigidity. Permeable composites include board/ionomer, oriented polystyrene/ionomer and polymethyl pentene/ionomer; semipermeable composites include oriented polypropylene/ionomer and impermeable composites include unplasticised polyvinyl chloride/ ionomer. In cases where increased puncture resistance is required a board/ionomer composite may additionally comprise a layer of polymethyl pentene to give a board/polymethylpentene/ ionomer composite. The composite may be pigmented white to give an attractive background for display purposes. Oriented polystyrene/ionomer and polymethyl pentene/ionomer are the preferred lidding materials if 'all round' oxygen permeability is to be maintained.
The present method has significant advantages over the controlled atmosphere method described above. It is of lower cost as the impermeable barrier multi-layer films have been replaced by simpler structures. Also, the necessity for a special atmosphere being costly to produce and control but moreover creating a 'greenhouse' effect when the pack is on display. Incident light cause temperature in the pack to rise as heat generated by the light cannot be dissipated so easily by conduction. A vacuum packed product is much more compact for transportation and storage. The stretch/cling appearance is attractive and leaking packages are readily apparent and in any case are of limited concern because of the skin pack contour holding property. Due to the absence of an unusual atmosphere, there is only the normal spoilage flora to be found after an extended storage period.
A further potential feature of the present packaging process is its extension to products which cause package failure by puncture of the overwrap web. In particular bone, in products such as chops and offals with sharp cartilages, may cause failure of the ionomer films, especially when a hard vacuum is present. These films can draw down around sharp points and such drawing may continue until pressure on the point causes physical failure on the thinned membrane. This failure can be prevented by combining the ionomer with a substantially non-extensible film or ply which limits the progressive thinning of the ionomer. Such a combination can be produced by conventional methods of co-extruding the. ionomer with a suitable polymeric material. Clearly a ply of sufficiently high oxygen permeability must be used. The preferred material is a non-extensible grade of polymethyl pentene which allows the ply to have a thickness of up to 300 microns, preferably 100 to 200 microns, whilst still maintaining an adequate gas permeability in the composite. An alternative approach is the use of two or more layers of ionomer with a reinforcing mesh of rigid flexible or extensible plastic net (e.g. that sold in the United Kingdom under the Registered Trade Mark "Netlon"). Such a multiply structure gives better puncture resistance and the mesh limits to some extent the progressive extension and thinning of the ionomer.
However, it is possible to use two plies of polymethyl pentene provided that an adequate seal can be effected between them as by high temperature heat sealing or a lacquer coating to improve the heat-seal.
In order to enable the invention to be more readily understood, an example thereof will now be described in greater detail.
In this example, packages of fresh meat, such as beef steak, roasting joints, mince or offal are to be formed on a conventional vacuum packaging--gas flush machine. A web of ionomer having a thickness in the range 25 micron to 250 micron to suit the forming depth required is fed into the machine. The web may be heated to 60°C-70°C before passing to either a vacuum-forming station or to a pressure-forming station where it is formed into a shrinkable retractile receptacle with a suitable thickness to give the permeability required. The meat produced to be packaged is placed in the receptacle. The receptacle is then brought into contact with a tensioned substantially inextensible composite web. The two webs are combined in a vacuum chamber so that the mount of the receptacle is covered by this composite which can be of ionomer/paper board (opaque structure) or ionomer/polymethyl pentene (transparent). The ionomer layer should be 20 to 25 microns thickness ideally to permit adequate oxygen transmission therethrough. In opaque structures the ionomer may be pigmented white to give a suitable display backing surface. The package is then evacuated in the chamber structure and a hard vacuum drawn. The vacuum is broken by the feeding in of oxygen or an oxygen-rich gas mixture (80% 0₂+20% CO₂ say) to alter the pressure in the package to 80% of the atmospheric pressure after peripheral sealing has been effected. The package may be subjected to a heating stage intergral with heat-sealing to effect secondary sealing and shrinkage on the machine. This gives particularly effective retention of drip and exuded juices by sealing the two film webs together. Alternatively the shrink/ secondary seal can be effected by hot water at 70°C to 80°C. The package is then ready for sale with the substantially more rigid composite covering film acting as a base on which the package may rest.
Instead of using a single web of ionomer as one of the materials of which the package may be formed, it is possible to use a composite formed of co-extruded plies of an extensible grade of polymethyl pentene and ionomer, and such a composite material can be used as either or both of said first and second materials or webs.
This composite material may consist of a web of polymethyl pentene having a thickness of 25 to 900 microns laminated to a web of ionomer having a thickness of 15 to 250 microns. The material may be hot- or cold-drawn to form a receptacle in which the fresh meat may be placed to be covered by a lid of the same material. The gas transmission rate of the ionomer component of the composite material is suitable for the requirements of the package of fresh meat and, for oxygen, would be at least 5,000 ml/m^z/24 hours/atmosphere differential.
The use of polymethyl pentene as a component of the material enables the material to be considerably thicker, and thus stronger and more puncture-resistant than when the material is an ionomer monofilm, because polymethyl pentene is sixteen times more permeable to oxygen than ionomer. Thus it is possible to make the receptacle of the package mechanically stronger and more resistant to puncture by bone.
In the use of such composite material in packaging equipment, the minimum thickness of the ionomer layer is determined by the necessity for obtaining adequate heat-sealing of the material at the periphery of the receptacle and this minimum thickness is about 15 microns. However, the formation of the receptacle causes the ionomer layer to be thinned pro-rata by the drawing of the receptacle so that a thin highly resilient membrane of ionomer of very high gas transmission rate can be produced in the body of the receptacle while still maintaining the required minimum thickness of 15 microns required for adequate heat-sealing at the periphery. For example, if the formation of the receptacle results in a threefold increase of area of the drawn region, the resulting 5 micron layer of ionomer has a gas transmission rate of 25,000 ml/m²/24 hours/atmosphere differentials.
The tough, resilient nature of the composite material and the fact that it can be of greater thickness than an ionomer monofilm facilitates the use of the material on packaging machines which employ gripper fingers or chains to hold and transport the receptacles both unfilled and when subsequently filled with fresh meat.
Although it is possible to make the receptacle and the lidding material of the ionomer/polymethyl pentene composite material, it is preferred to make the lidding material either of a board/ polymethyl pentene/ionomer triple composite or one of the other stiffer composite lidding materials referred to above, especially where the lidding material is to be pigmented to give an attractive background for display purposes.
The use of the ionomer/polymethyl pentene composite is particularly useful in the case where the fresh meat is packaged as a vacuum-packed anaerobic package, and the pressure is cut back with nitrogen instead of oxygen.
The invention will now be further illustrated by the following Examples:

Example 1

A composite material comprising a layer of an extensible grade of polymethyl pentene 50 microns thick laminated by co-extrusion in conventional manner to an ionomer layer 25 microns thick is formed by drawing into a receptacle with a wall thickness at its edge of 25 microns where heat-sealing to a lidding material is to occur. This results in an ionomer thickness of 8 microns at the base of the receptacle with a gas transmission rate of 15,000 ml/m²/24 hours/atmosphere. After filling with fresh meat, the receptacle is heat-sealed with a board/ionomer lidding material in the manner described above.

Example 2

A composite material comprising a layer of an extensible grade of polymethyl pentene 300 microns thick laminated to an ionomer layer 25 microns thick is formed by drawing into a receptacle with a wall thickness at its base of 65 microns and a gas transmission rate of 25,000 ml/ m²/24 hours/atmosphere. After filling with fresh meat the receptacle is heat-sealed with a board/ polymethyl pentene/ionomer composite in the manner described above to give a package with a high degree of puncture and penetration resistance.
In addition to board base (or other composite structures resembling paper board) an alternative presentation is possible in which a packaging adjunct (e.g. foamed polystyrene board or tray, foil-board structure or other presentation surface structure) can be incorporated within the package (placed on top of the meat in the receptacle) to give a rigid base to the package when this is inverted at the point of sale. A wide range of shapes and sizes of meat cut can be accommodated on the machine, within the size limits of the cavities in the forming/sealing dies since the surplus film is drawn out of the way on the vacuuming/sealing station and subsequently shrinks away. In the case of chops and bone-in meat products likely to cause puncture, the base- forming web can be made of a co-extruded composite of ionomer and a non-extensible grade of polymethyl pentene to give the puncture resistance needed in such case.
Unlike in the case of non-shrink laminates, the forming die shape and contour are not highly significant. With conventional non-shrink laminates these have to be accurately tailored to give snug-fitting packages free from wrinkles and excess film material. When the meat is correctly handled (in respect to temperature specifically) in the abattoir, chilling, preparation and cutting stages and then held at 31.5° Fahrenheit (-0.25°C) substantial shelf life approaching that of controlled atmosphere packaged products can be obtained.
The pack is also capable of a dual role and use as a frozen meat package. In this case the meat will generally be packed fresh and the resulting pack may be frozen for storage. In its use as a frozen meat package, significantly, no freezer burn will occur due to the skin packaging effect.
The present pack is also of value in cases where the animal from which the meat is obtained has been treated, before slaughter, with a tenderiser, such as a papin-type tenderiser. In cases where no such treatment has occurred, the meat is normally vacuum-packed and tenderising of the meat occurs over a three week period in the vacuum pack. However, by packing tenderiser- treated meat in the pack of the present invention, tender-eating meat can be made available earlier than the three week period required for non- treated meat.

Claims

1. A method of packaging fresh meat by forming a first web of material into a receptacle to receive fresh meat, placing the meat in the receptacle and sealing the receptacle with a second web of material, wherein the meat, placed in the receptacle, is subjected to a vacuum treatment, characterised in that the vacuum is thereafter partially released under controlled conditions and the second web of material is sealed to the receptacle to form a package containing the fresh meat, the vacuum treatment and sealing being controlled so as to ensure that the condition of critical oxygen tension does not develop, and in that at least one of the webs is made of a material which is permeable to oxygen under the influence of the partial pressure obtaining in the package and which allows carbon dioxide to leave the package faster than oxygen enters, and at least one of the webs is transparent.

2. A method as claimed in Claim 1, wherein one of said webs is both transparent and permeable to oxygen.

3. A method as claimed in Claim 1 or 2, wherein one of said webs is flexible and the other is more rigid, and optionally pigmented, to provide a base for the package.

4. A method as claimed in any one of Claims 1 to 3, wherein one of said webs comprises an oxygen-permeable ionomeric polymer selected from the sodium and zinc salts of ethylene-acrylic acid and ethylene-methacrylic acid copolymers.

5. A method as claimed in Claim 4, wherein the web of ionomeric polymer is combined with a substantially non-extensible film or ply.

6. A method as claimed in Claim 4 or 5, wherein the web of ionomeric polymer is combined with an oxygen-permeable layer of polymethyl pentene.

7. A method as claimed in any one of Claims 4 to 6, wherein the web of ionomeric polymer has a thickness of 25 to 250 microns alone or 15 to 250 microns when combined with a layer of polymethyl pentene of a thickness of 25 to 900 microns.

8. A method as claimed in Claim 3 or any one of Claims 4 to 6 when appended to Claim 3, wherein the more rigid web is oxygen-permeable and is a composite web comprising an ionomeric polymer combined with board, oriented polystyrene, polymethyl pentene, unplasticised polyvinyl chloride or oriented polypropylene.

9. A method as claimed in any one of Claims 1 to 9, wherein sealing at reduced pressure is effected in an oxygen-rich gas, preferably comprising 80% oxygen and 20% carbon dioxide.

10. A method as claimed in any one of Claims 1 to 9, wherein sealing at reduced pressure is effected in an oxygen-free atmosphere to provide an anaerobic package capable of being stored under vacuum or in an inert gas atmosphere.