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EP 0 116 394 B1 |
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EUROPEAN PATENT SPECIFICATION |
(45) |
Mention of the grant of the patent: |
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25.03.1987 Bulletin 1987/13 |
(22) |
Date of filing: 05.01.1984 |
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(54) |
Packaging of fresh meat
Verpacken von Frischfleisch
Emballage pour viande fraîche
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Designated Contracting States: |
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BE DE FR GB NL |
(30) |
Priority: |
14.01.1983 GB 8301067 07.07.1983 GB 8318425
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(43) |
Date of publication of application: |
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22.08.1984 Bulletin 1984/34 |
(71) |
Applicant: BUNZL FLEXPACK LIMITED |
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Bury
Lancashire BL9 7PA (GB) |
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(72) |
Inventors: |
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- Ferrar, Andrew Nicholas
Bolton
Lancashire (GB)
- Jones, Arthur Neville
Beaconsfield
Buckinghamshire (GB)
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(74) |
Representative: Thiemann, Peter Albert William et al |
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LLOYD WISE, TREGEAR & CO.
Norman House
105-109 Strand London WC2R 0AE London WC2R 0AE (GB) |
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[0001] 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.
[0002] Such a method is known from DE-A-2364565.
[0003] 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.
[0004] Factors 1 and 2 can be controlled by the choice of packaging system and subsequent
handling procedures.
[0005] 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% C0
2), 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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).
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] In order to enable the invention to be more readily understood, an example thereof
will now be described in greater detail.
[0019] 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% 0
2+20% CO
2 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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/m
2/24 hours/atmosphere differentials.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] The invention will now be further illustrated by the following Examples:
Example 1
[0028] 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/m
2/24 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
[0029] 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/ m
2/24 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
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.
1. Verfahren zur Verpackung frischen Fleisches, mit folgenden Schritten: Formen einer
ersten Folie zu einem Behälter, der frisches Fleisch aufnimmt, Einlegen des Fleisches
in den Behälter und Versiegelung dieses Behälters mit einer zweiten Folie, wobei das
in dem Behälter liegende Fleisch einer Vakuumbehandlung unterworfen, wird, dadurch
gekennzeichnet, daß das Vakuum danach teilweise unter kontrollierten Bedingungen nachgelassen
wird und daß die zweite Folie an dem Behälter angesiegelt wird, um eine frisches Fleisch
enthaltende Verpackung zu bilden, daß die Vakuumbehandlung und die Versiegelung derart
gesteuert werden, daß gesichert ist, daß der Kritische Sauerstoff-Druck sich nicht
entwickelt, daß wenigstens eine der Folien aus einem Material hergestellt ist, das
für Sauerstoff unter dem Einfluß des in der Verpackung vorhandenen Partialdruckes
permeabel ist und das dem Kohlendioxid ermöglicht, die Verpackung schneller zu verlassen
als daß der Sauerstoff eindringt, und daß eine der Folie transparent ist.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß eine der genannten Folien
sowohl transparent als auch für Sauerstoff permeabel ist.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß eine der genannten
Folien flexibel ist und die andere relativ starr und gegebenenfalls pigmentiert ist,
um die Verpackungsbasis zu liefern.
4. Verfahren nach einem der Ansprüch 1-3, dadurch gekennzeichnet, daß die eine der
genannten Folien aus einem sauerstoffpermeablen, ionomerischen Polymer besteht, das
ausgewält ist, aus Natrium- und Zinksalzen der Ethylenacrylsäure- und Ethylenmethacrylsäure-Copolymeren.
5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß die Folie aus ionomeren
Polymeren kombiniert ist mit einem im wesentlichen nicht-dehnbaren Film oder einer
nicht-dehnbaren Folienlage.
6. Verfahren nach Anspruch 4 oder 5, dadurch gekennzeichnet, daß die Folie aus ionomerischen
Polymeren verbunden ist mit einer sauerstoffpermeablen Schicht eines Polymethylpentens.
7. Verfahren nach einem der Ansprüche 4-6, dadurch gekennzeichnet, daß die Folie der
ionomerischen Polymere eine Dicke von 25-250 Mikrometer (gm) allein oder 15-250 Mikrometer
(Ilm) dann hat, wenn sie mit einer Lage aus Polymethylpenten mit einer Dicke von 25-900
Mikrometer (um) verbunden ist.
8. Verfahren nach Anspruch 3 oder einem der Ansprüche 4-6 (bei Abhängigkeit von Anspruch
3) dadurch gekennzeichnet, daß die relativ starre Folie sauerstoff-permeabel und eine
zusammengesetzte Folie ist, die ein ionomeres Polymer umfaßt, welches verbunden ist
mit Pappe, orientierte Polystyrol, Polymethylpenten, hartem Polyvinylchlorid oder
orientiertem Polypropylen.
9. Verfahren nach einem der Ansprüche 1-8, dadurch gekennzeichnet, daß das Versiegeln
bei reduziertem Druck in einem sauerstoffreichen Gas durchgeführt wird, das vorzugsweise
80% Sauerstoff und 20% Kohlendioxid enthält.
10. Verfahren nach einem der Ansprüche 1-9, dadurch gekennzeichnet, daß das Versiegeln
bei reduziertem Druck in einer sauerstofffreien Atmosphäre bewirkt wird, um eine anaerobe
Verpakkung zu erzeugen, die unter Vakuum oder in einer inerten Gasatmosphäre lagerbar
ist.
1. Un procédé pour l'emballage de viande fraîche par formage d'une première nappe
de matériau en un récipient, dans lequel on place la viande, et soudage du récipient
avec une seconde nappe de matériau, dans lequel la viande placée dans le récipient
est soumise à un traitement sous vide, caractérisé en ce que le vide est ensuite partiellement
casé en conditions contrôlées et le seconde nappe de matériau est soudée au récipient
pour former un emballage contenant la viande fraîche, le traitement sous vide et le
soudage étant contrôlés de manière à assurer que la tension critique d'oxygène n'apparaisse
pas, et en ce que l'une au moins des nappes est faite d'un matérieau qui est perméable
à l'oxygène sous l'influence de la pression partielle obtenue dans l'emballage et
qui permet au dioxyde de carbone de quitter l'emballage plus rapidement que l'oxygène
n'y entre, et l'une au moins des nappes est transparente.
2. Un procédé selon la revendication 1, dans lequel l'une desdites nappes est à la
fois transparente et perméable à l'oxygène.
3. Un procédé selon la revendication 1 ou 2, dans lequel l'une desdites nappes est
flexible et l'autre est plus rigide, et facultativement pigmentée, pour réaliser une
base de support pour l'emballage.
4. Un procédé selon l'une quelconque des revendications 1 à 3, dans lequel l'une desdites
nappes comprend un polymère ionomère perméable à l'oxygène choisi parmi les sels de
sodium et de zinc des copolymères éthylène-acide acrylique et éthylène-acide méthacrylique.
5. Un procédé selon la revendication 4, dans lequel la nappe de polymère ionomére
est combinée avec un film ou une couche non extensible.
6. Un procédé selon la revendication 4 ou 5, dans lequel la nappe de polymère ionomère
est combinée avec une couche de polyméthylpentène, perméable à l'oxygène.
7. Un procédé selon l'une quelconque des revendications 4 à 6, dans lequel la nappe
de polymére ionomère a une épaisseur de 25 à 250 gm, si elle est seule, ou de 15 à
250 um, si elle est combinée avec une couche de polyméthylpentène d'une épaisseur
de 25 à 900 µm.
8. Un procédé selon la revendication 3 ou l'une quelconque des revendications 4 à
6, rattachée à la revendication 3, dans lequel la nappe plus rigide est perméable
à l'oxygène et constituée d'une nappe composite consistant en un polymère ionomère
combiné avec du carton, du polystyrène étiré, du polyméthylpentène, du chlorure de
polyvinyle non plastifié ou du polypropylène étiré.
9. Un procédé selon l'une quelconque des revendications 1 à 9, dans lequel le soudage
sous pression réduite est effectué dans un gaz riche en oxygène, contenant de préférence
80% d'oxygène et 20% de dioxyde de carbone.
10. Un procédé selon l'une quelconque des revendications 1 à 9, dans lequel le soudage
sous pression réduite est effectué dans une atmosphère exempte d'oxygène pour donner
un emballage anaérobie pouvant être stocké sous vide ou dans une atmosphère de gaz
inerte.