Packaging sheet, packaging and use of such a packaging sheet

Abstract

A packaging sheet (1) which is intended for packaging a cheese product. The packaging sheet (1) comprises an internal membrane (5) and a printable external surface (10). The internal membrane comprises at least one internal plastic layer (7) intended to be oriented towards the cheese product and at least a first water-absorbing layer (12) made of a first water-absorbing material. The external surface (10) is intended to be oriented away from the cheese product. The or each internal plastic layer (7) has a plurality of microperforations (8, 9) arranged in a random fashion on the or each internal plastic layer (7) in such a manner that the aft permeability of the internal membrane (5) is comprised between 5 mL/min Bendtsen and 500 ml/min Bendtsen, the first water-absorbing material having a water absorption rate comprised between 1 and 30 g/m.sup.2 as measured using the COBB test C.sub.3600.

Claims

1. Packaging sheet intended for packaging a cheese product, the packaging sheet comprising: an internal membrane comprising at least one internal plastic layer intended to be oriented towards the cheese product and at least a first water-absorbing layer made of a first water-absorbing material; a printable external surface, the external surface being intended to be oriented away from the cheese product; characterized in that the or each internal plastic layer has a plurality of microperforations arranged in a random fashion on the or each internal plastic layer in such a manner that the air permeability of the internal membrane is between 5 mL/min Bendtsen and 500 mL/min Bendtsen, the plurality of microperforations including through-holes that extend through the entire thickness of the at least one internal plastic layer and blind-holes whose length is smaller than the thickness of the at least one internal plastic layer, the first water-absorbing material having a water absorption rate is between 1 and 30 g/m.sup.2 as measured using the COBB test C.sub.3600; wherein the density of microperforations on an internal surface of the internal plastic layer is between 500 and 5000 microperforations per dm.sup.2; and wherein the density of micropeforations is an average value of at least 50% of the surface area of the packaging sheet.

2. Packaging sheet according to claim 1, wherein the first water-absorbing material is a cellulose-based material.

3. Packaging sheet according to claim 1, wherein the first water-absorbing material has a surface weight of between 20 and 70 g/m.sup.2.

4. Packaging sheet according to claim 1, wherein the plastic material forming the or each internal plastic layer comprises one of a petroleum-based plastic, a plastic derived from renewable sources or a blend of a petroleum-based plastic and a plastic derived from renewable sources.

5. Packaging sheet according to claim 1, wherein the greatest dimension of the microperforations of the or each internal plastic layer in the plane of the internal plastic layer is between 10 and 500 μm.

6. Packaging sheet according to claim 1, comprising an external layer at least partially bonded to the internal membrane, the external surface being formed by a surface of the external layer oriented away from the internal membrane.

7. Packaging sheet according to claim 6, wherein the external layer is made of a plastic material or an aluminum foil.

8. Packaging sheet according to claim 6, wherein the external layer is not perforated.

9. Packaging sheet according to claim 6 wherein the external layer is perforated.

10. Packaging sheet according to claim 6, wherein the external layer and the internal membrane are adhesively bonded to one another along a set of bonding points and/or lines, the adhesive being in particular a hot melt adhesive, a pressure-sensitive adhesive or a water-based adhesive.

11. Packaging sheet according to claim 6, wherein the external layer and the internal membrane are bonded to one another along their entire surface.

12. Packaging sheet according to claim 1, wherein the external surface is formed on the internal membrane.

13. Packaging sheet according to claim 1, the first water-absorbing layer being made of paper having a grammage between 30 and 40 g, the internal plastic layer being made of polyethylene having a surface weight between 10 and 12 g/m.sup.2, wherein the air permeability of the internal membrane is between 10 mL/min Bendtsen and 50 mL/min Bendtsen, the first water-absorbing material having a water absorption rate of between 10 and 22 g/m.sup.2 as measured using the COBB test C.sub.3600.

14. Packaging comprising the packaging sheet according to claim 1 and a cheese product, arranged in the packaging sheet in such a way that the internal plastic layer is oriented towards the cheese product, the cheese product being one of a soft cheese, a fresh cheese or a blue cheese.

15. Packaging sheet according to claim 1 wherein the packaging sheet slows maturation of a cheese product in such a way that the NPT/NT ratio is lower than 13 after 45 days of maturation, NPT being the quantity of nitrogen soluble in phosphotungstic acid, NT being the total quantity of nitrogen, the ratio NPT/NT being measured using the Kjeldhal method, according to the amended standard NF IS08968-1.

16. Packaging sheet according to claim 11, wherein the external layer and the internal membrane are adhesively bonded to one another along their entire surface.

17. A package comprising: a cheese product; and a packaging sheet folded around the cheese product, the packaging sheet including: an internal membrane comprising at least one internal plastic layer oriented towards the cheese product and at least a first water-absorbing layer made of a first water-absorbing material; a printable external surface, the external surface being oriented away from the cheese product; characterized in that the or each internal plastic layer has a plurality of microperforations arranged in a random fashion on the or each internal plastic layer in such a manner that the air permeability of the internal membrane is between 5 mL/min Bendtsen and 500 mL/min Bendtsen, the plurality of microperforations including through-holes that extend through the entire thickness of the at least one internal plastic layer and blind-holes whose length is smaller than the thickness of the at least one internal plastic layer, the first water-absorbing material having a water absorption rate is between 1 and 30 g/m.sup.2 as measured using the COBB test C.sub.3600; wherein the density of microperforations on an internal surface of the internal plastic layer is between 500 and 5000 microperforations per dm.sup.2; and wherein the density is an average value on at least 50% of the surface area of the packaging sheet.

18. Package according to claim 17 comprising an external layer at least partially bonded to the internal membrane, the external surface being formed by a surface of the external layer oriented away from the internal membrane, the air permeability of the external layer being greater than the air permeability of the internal membrane.

19. Packaging sheet according to claim 6 wherein the air permeability of the external layer is greater than the air permeability of the internal membrane.

20. Packaging sheet according to claim 7 wherein the air permeability of the external layer is smaller than the air permeability of the internal membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features of the invention will be apparent from the following description with reference to the accompanying drawings wherein:

(2) FIG. 1 is a diagrammatic cross-section of a packaging sheet according to a first embodiment of the invention;

(3) FIG. 2 is a diagrammatic plane view from below of the internal plastic layer of the packaging sheet shown in FIG. 1;

(4) FIG. 3 is a diagrammatic cross-section of a packaging sheet according to an alternative of the first embodiment;

(5) FIG. 4 is a diagrammatic cross-sections of a packaging sheet according to a second embodiment of the invention;

(6) FIG. 5 is a diagrammatic view of a packaging according to the invention; and

(7) FIGS. 6 and 7 show respectively the weight of the packaging and the weight of the cheese product, as a function of the maturation time, for several packagings.

DETAILED DESCRIPTION

(8) The packaging sheet according to the invention is intended for packaging cheese products, typically a soft cheese, a fresh cheese or a blue cheese. The packaging sheet is particularly suited to packaging a soft cheese with a white rind, such as Camembert or Brie.

(9) It is particularly adapted as well for packaging a moisture-sensitive cheese products, the packaging sheet controlling the moisture and/or the exudates originating from the cheese product after it has been packaged, and allowing for an extended conservation time.

(10) A packaging sheet 1 according to a first embodiment of the invention is shown on FIG. 1. It comprises an internal membrane 5 intended to be oriented towards the product to be packaged. The internal membrane 5 is therefore intended to be on the inside of the finished package.

(11) It also comprises an external surface 10, intended to be oriented away from the product to be packaged. The external surface 10 may be the outermost surface of the packaging sheet 1. However, the packaging sheet 1 may further comprise a layer of varnish and/or ink superimposed onto the external surface 10. The varnish is intended for protecting the external surface 10 against aggressions from the environment. It may also be used to provide a glossy aspect to the external surface 10. The external surface 10 is visible from the outside.

(12) The external surface 10 is a printable surface. A printable surface is a surface that can be printed upon without the need of additional treatments, for example for increasing the adherence of the print on the surface. For example, a paraffin coated surface is not a printable surface.

(13) The internal membrane 5 comprises an internal plastic layer 7. The internal plastic layer 7 is the layer of the internal membrane 5 that is intended to be oriented towards the product wrapped in the packaging sheet.

(14) The internal plastic layer 7 has a plurality of randomly distributed microperforations. “Randomly distributed” in particular means that the distance between two adjacent microperforations, taken in the plane of the internal plastic layer 7, is not constant. At least over a surface of the internal layer 7 having an area substantially equal to that of the active surface of the tool used for making the microperforations, the microperforations do not define a pattern that is repeated periodically. Further, the shapes of the microperforations are variable. The dimensions of the microperforations in the plane of the internal plastic layer 7 are also variable from one microperforation to the other. This feature is illustrated on the diagrammatic representation of FIG. 2.

(15) The greatest dimension of the microperforations of the internal plastic layer 7 in the plane of the internal plastic layer 7 is comprised between 10 and 500 μm, preferably between 10 and 250 μm, and more particularly between 20 and 200 μm. This means that the width of any microperforation taken along any direction comprised in the plane of the internal plastic layer 7 is smaller or equal to a maximum value comprised in the above mentioned ranges.

(16) The length of the microperforations, taken in a transverse direction, perpendicular to the plane of the internal plastic layer 7, is also variable. Some microperforations 8 are through-holes that extend through the entire thickness of the internal plastic layer 7. Other microperforations 9 are blind-holes whose length is smaller than the thickness of the internal plastic layer 7. Each microperforation 9 of the latter type thus forms a recess or a cavity in the internal surface 11 of the internal plastic layer 7. As a whole, the microperforations confer microporosity to the internal plastic layer 7.

(17) The presence of cavities or recesses on the internal surface 11 is particularly advantageous. Indeed, the internal surface 11 is the surface intended to be in contact with the cheese product wrapped in the packaging sheet 1. The cheese flora, and in particular the penicillium, can develop in these cavities, which has a very positive effect on the maturation of the cheese.

(18) The microperforations may be obtained using a cylinder fitted with randomly distributed projections of different shapes and lengths, these projections being suitable for indenting or perforating the internal plastic layer 7. “Randomly distributed” in particular means that the distance between two adjacent projections is not constant. More particularly, the projections fitted on the cylinder do not define a pattern that is repeated periodically over the cylinder. The microperforations on the internal layer 7 are distributed randomly on the internal layer 7, i.e. in such a way that, at least over an area that is equal to that of the lateral surface of the cylinder, they do not define a pattern that is repeated periodically. A suitable tool is for example a cylinder fitted with diamond points.

(19) The density of microperforations on the internal surface 11 of the internal plastic layer is high. The density of microperforations on the internal surface of the internal plastic layer is comprised between 500 and 5000 microperforations per dm.sup.2, preferably comprised between 1000 and 4000 microperforations per dm.sup.2, and even more preferably between 2000 and 3500 microperforations per dm.sup.2.

(20) Said densities are average values, on at least 50% of the surface area of the packaging sheet.

(21) The random distribution of the microperforations contributes to obtaining this high density of microperforations.

(22) The microperforations are distributed on the internal plastic layer 7 in such a manner that the air permeability of the internal membrane 5 is comprised between 5 and 500 mL/min Bendtsen. Advantageously, the air permeability of the internal membrane 5 is comprised between 10 and 200 mL/min Bendtsen, and more preferably between 10 and 50 mL/min Bendtsen, or even between 10 and 30 mL/min Bendtsen.

(23) The air permeability of the internal membrane 5 can for example be modified by varying the pressure exerted by the tool, e.g. the cylinder, on the layer 7 to be microperforated.

(24) The internal plastic layer 7 has areas having different densities of microperforations.

(25) In another embodiment, all the areas of the internal plastic layer 7 have the same density of microperforations.

(26) The internal plastic layer 7 comprises one of a petroleum-based plastic material, a plastic material derived from renewable sources or a mixture of a petroleum-based plastic material and a plastic material derived from renewable sources.

(27) The petroleum-based plastic material is for example chosen among: polyethylene (PE), in particular low density polyethylene, linear low density polyethylene or metallocene, polyamide (PA), polypropylene (PP), polyester, ethylene methyl acrylate (EMA), ethylene butyl acrylate (EBA), ethylene acrylic acid (EAA) or a ionomer and their blends.

(28) The plastic materials derived from renewable sources are for example: polylactic acid (PLA), polyhydroxybutyrate (PHB) or their mixtures, polyhydroxyalkanoates (PHA) or blends of PLA with Mater-Bi® sold by the company Novamont

(29) The internal plastic layer 7 for example has a weight per square meter comprised between 3 and 20 g, more particularly between 6 and 12 g, even more particularly between 10 and 12 g.

(30) The zones in which the thickness of the internal plastic layer 7 is reduced due to the presence of microperforations are zones of preferred water vapour passage. The microperforations 8 which extend through the entire thickness of the internal plastic layer 7 allow the passage of air, water vapour and liquid water.

(31) The distribution and shape of the microperforations described above is particularly advantageous. Indeed, the high density of micrometric perforations allows for a homogenous distribution of the exchange sites over the internal surface and therefore for a homogenous flow of air and water through the internal membrane.

(32) On the contrary, when the perforations are made using needles that are regularly distributed on a cylinder, the resulting arrangement of perforations is regular and not random. The density of perforations is also much lower than with the microperforations according to the invention. Therefore, an internal layer which is perforated with needles presents localized surface areas of high flow of water or air through the internal membrane where the perforations are located and large areas of zero flow through the internal membrane between the perforations. The presence of such local areas in the internal membrane results in a degradation of the cheese flora. Thus, the absence of such localized surface areas which is achieved in the packaging sheet according to the invention is very advantageous for the maturation of the cheese.

(33) This distribution of microperforations according to the invention also allows for a complete and rapid removal of the liquid and gaseous water originating from the cheese. Thus, the risk of formation of condensation on the inside of the packaging is significantly reduced.

(34) As shown on FIG. 1, the internal membrane 5 further comprises a first water-absorbing layer 12 made of a first water-absorbing material and superimposed on the internal plastic layer 7. The first water-absorbing material is for example a cellulose-based material, such as cellulose film or paper. Advantageously, the first water-absorbing layer 12 is a layer of paper, typically Kraft paper. Preferably, the layer of paper is machined-glazed or calendered.

(35) The first water-absorbing layer 12 is bonded to the internal plastic layer 7. For example, the internal plastic layer 7 is extruded onto the first water-absorbing layer 12. According to an alternative, the or each internal plastic layer 7 is formed separately from the first water-absorbing layer 12 and is then bonded to the first water-absorbing layer 12 before forming the microperforations 8, 9 in the or each internal plastic layer 7. According to another alternative, the plastic material forming the internal plastic layer is dissolved in a water-based solvent or organic-based solvent, and the first water-absorbing layer 12 is coated with the solvent.

(36) The internal plastic layer 7 provides support to the first water-absorbing layer 12, which is in particular a layer of paper and thus increases its wet strength. It is therefore possible to use for the first water-absorbing layer 12 paper of different quality or paper which has not been previously treated with wet-strength resin.

(37) The first paper layer 12 for example has a grammage of 20 to 70 g, and in particular a grammage of 20 to 40 g, more particularly of 32 to 40 g, and for example 32 g.

(38) The water absorption rate of the first water-absorbing material is comprised between 1 and 30 g/m.sup.2 as measured using the COBB test C.sub.3600. A preferred range for the water absorption rate of the first water-absorbing material is comprised between 3 and 25 g/m.sup.2 as measured using the COBB test C.sub.3600, and a more preferred range is between 10 and 22 g/m.sup.2 as measured using the COBB test C.sub.3600.

(39) In the first embodiment, the packaging sheet 1 further comprises an external layer 15, which is at least partially bonded to the internal membrane 5. In this embodiment, the external surface 10 is formed by a surface of the external layer 15 intended to be oriented away from the packaged product. Alternatively, the external, printable surface 10 is formed by a surface of the external layer 15 intended to be oriented toward the packaged product, and the external layer 15 is transparent.

(40) The external layer 15 is adhesively bonded to the internal membrane 5. The external layer 15 is for example adhered only partially to the internal membrane 5. The adhesive is for example sprayed or coated onto one or both of the surfaces of the internal membrane 5 and of the external layer 15 intended to be bonded to one another. The adhesive may be transparent or coloured. The external layer 15 is for example adhered to the internal membrane 5 along a set of bonding points and/or lines 16. Adequate adhesives are for example a hotmelt adhesive, a pressure-sensitive adhesive or a water-based adhesive, such as a dextrin or a casein based adhesive.

(41) The adhesive may also form a graphic symbol, such as a brand mark, a letter, a word or a drawing. In this case, the adhesive is chosen in such a manner that, when dry, it contrasts with the underlying and/or overlying layers. The graphic symbol formed by the adhesive is visible from the exterior and/or the interior of the package.

(42) The external layer 15 has a plurality of microperforations distributed in a random fashion. The microperforations are arranged in such a way that the air permeability of the external layer 15 is comprised between 0.1 and 40000 mL/min Bendtsen.

(43) The microperforations on the external layer 15 are arranged in such a way that the air permeability of the external layer 15 is greater than the air permeability of the internal membrane 5. This feature promotes the extraction of the water and prevents the formation of condensation at the interface between the product and the internal membrane, especially for cheese products which have a high weight loss during maturation.

(44) The microperforations are of the same type as those formed on the internal layer plastic layer 7.

(45) In an alternative, the microperforations form a regular pattern on the external layer 15. All the microperforations have a similar shape. All the microperforations are through-holes, i.e. they extend through the entire thickness of the external layer 15 and are open at both ends. These regular microperforations may for example be formed by means of a cylinder fitted with a plurality of identical needles arranged on the cylinder according to a regular pattern. In this alternative, the microperforations are therefore not randomly distributed.

(46) Advantageously, the external layer 15 is made of a plastic material, chosen among the materials listed above in relation with the internal plastic layer 7.

(47) An alternative of the first embodiment shown in FIG. 3 will be described below. Only the aspects by which the packaging sheet of FIG. 3 differs from the packaging sheet of FIG. 1 will be detailed below.

(48) As shown on FIG. 3, the external layer 15 is bonded to the internal membrane 5 along its entire surface facing the internal membrane 5 (“full surface bonding”). The internal membrane 5 and the external layer 15 are for example adhered to one another along their entire facing surfaces using a layer of adhesive 17. Adequate adhesives are waxes, for example petroleum-based waxes or waxes derived from renewable sources, pressure sensitive adhesives, water-based adhesives or adhesives comprising polyester and/or polyurethane or polyurethane derivates. The adhesive may be coated or sprayed onto one or both of the entire facing surfaces of the internal membrane 5 and of the external layer 15. In another embodiment, the full surface bonding between the external layer 15 and the internal membrane 5 is obtained by extrusion laminating a layer of resin, for example of polyethylene, onto at least one of the facing surfaces of the internal membrane 5 and of the external layer 15.

(49) In the case of a full surface bonding, the water-absorbing layer should have a higher water absorption capacity than in the case of a partial bonding in order to prevent water retention between the packaging sheet 1 and the packaged product.

(50) The external layer 15 is made of an aluminium foil.

(51) Alternatively, the external layer 15 is made of a plastic material, chosen among the materials listed above in relation with the internal plastic layer 7.

(52) In this case, a first possibility is that the external layer 15 does not have any perforations or holes. As a consequence the air permeability of the external layer 15 is very small and can be considered equal to zero. The air permeability of the external layer 15 is thus smaller than that of the internal membrane 5.

(53) A second possibility is that the external layer 15 has a plurality of microperforations distributed in a random fashion, of the same type as those of the internal layer 7. The air permeability of the external layer 15 is comprised between 0.1 and 80000 mL/min Bendtsen.

(54) A packaging sheet 1 according to a second embodiment is illustrated on FIG. 4. Only the aspects by which the packaging sheet of FIG. 4 differs from the packaging sheet of FIG. 1 will be detailed below.

(55) The packaging sheet of FIG. 4 differs from those of FIGS. 1 and 3 only in that it does not comprise an external layer 15. In the fourth embodiment, the packaging sheet 1 consists of the internal membrane 5. The external surface 10 is formed by the outer surface of the internal membrane 5, i.e. by the outer surface of first water-absorbing layer 12. A layer of varnish and/or ink (not shown) may be applied onto the external surface 10.

(56) The invention also relates to a packaging 25, illustrated diagrammatically on FIG. 5, comprising a packaging sheet 1 as described above and a cheese product 28. The cheese product 28 is wrapped in the packaging sheet 1 in such a way that the internal surface 11 of the internal membrane 5 faces, or is even in contact with, the cheese product 28. The cheese product 28 is wrapped in the packaging sheet 1 in such a way that all the exchanges of air and water between the cheese product 28 and the environment 30 surrounding the packaging 25 take place through the packaging sheet 1, and are therefore regulated by the packaging sheet 1.

(57) The behaviour of several examples of packaging sheets according to the invention has been tested and compared with existing packaging sheets.

(58) Three series of tests were carried out in parallel, in the same conditions. The results presented below correspond to the average of the three series.

(59) Each packaging sheet was wrapped around a piece of freshly prepared Camembert, and stored in a wooden box of the type usually used for conditioning Camembert. The box was stored in a refrigerated room at 4° C. for 35 days, and at 8° C. for 10 additional days (between day 35 and day 45).

(60) Four packaging sheets according to the invention were tested (C to F sheets), along with a standard packaging sheet (T sheet) and a so-called Expeco sheet (A sheet).

(61) The standard sheet has an external layer of OPP 20 μm thick, adhesively bonded by lines of glue to a water-absorbing layer of Kraft paper, and an internal layer of paraffin.

(62) The Expeco sheet has an external layer of OPP 20 μm thick, adhesively bonded by lines of glue to a water-absorbing layer of Kraft paper, an internal layer of polyamide, with an intermediate layer of polyethylene between the paper and polyamide layers.

(63) All the packaging sheets according to the invention have an external layer of oriented polypropylene (OPP) 20μ thick, adhesively bonded by lines of glue to a water-absorbing layer of Kraft paper, and a polyethylene (PE) internal layer.

(64) The OPP is microperforated, with an air permeability between 1000 and 3000 mL/min Bendtsen.

(65) Sheet C has a paper layer with a grammage of 30 g, and a PE layer with a grammage of 10 g. The PE internal layer is treated to form microperforations, with an indicia of 40. The indicia is representative of the density of microperforations on the internal plastic layer, in other words of the number of microperforations per dm.sup.2 of the plastic internal layer.

(66) Sheet D has a paper layer with a grammage of 30 g, and a PE layer with a grammage of 10 g. The PE internal layer is treated to form microperforations, with an indicia of 20.

(67) Sheet E has a paper layer with a grammage of 40 g, and a PE layer with a grammage of 12 g. The PE internal layer is treated to form microperforations, with an indicia of 50.

(68) Sheet F has a paper layer with a grammage of 40 g, and a PE layer with a grammage of 12 g. The PE internal layer is treated to form microperforations, with an indicia of 20.

(69) For each packaging sheet, the air permeability of the internal membrane was measured using the Bendtsen method. The water absorption rate of the water-absorbing material was measured as well, using the COBB test.

(70) The results are indicated in the table below.

(71) TABLE-US-00001 Bendtsen Cobb C.sub.3600 ml/min .Math. 50 cm.sup.2 g/m.sup.2 T 16.3 — A 1.43 — C 21.50 23.92 D 20.93 13.02 E 13.03 26.82 F 11.40 10.28

(72) The weight of the packaging (including the packaging sheet and the wooden box) was monitored periodically, every 7 days. The weight curve is shown on FIG. 6.

(73) The weight of the cheese alone (not including the packaging sheet and the wooden box) was monitored periodically. The weight curve is shown on FIG. 7.

(74) The cheese of each packaging was analyzed after 45 days in the packaging. The results are indicated in the table below.

(75) TABLE-US-00002 T A C D E F EST g/kg 448 436 453 452 443 455 MG g/kg 220 214 222 219 216 221 Ca g/kg 4.78 4.46 4.87 4.88 5.00 5.03 NaCl g/kg 20 19.58 20.12 19.95 20.25 19.66 pH H2O 7.40 7.64 7.36 7.42 7.43 7.40 HFD % 70.76 71.75 70.25 70.13 71.05 69.95 G/S % 49.05 49.10 48.90 48.45 48.80 48.60 Ca/ESD % 2.10 2.01 2.10 2.10 2.20 2.15 NaCl/H2O % 3.62 3.47 3.68 3.64 3.63 3.60 NS/NT % 90.0 87.8 89.6 88.5 86.6 89.6 NPT/NT % 13.7 15.0 12.3 12.5 11.9 11.3

(76) EST is the total dry extract. MG is the fat content. HFD is the moisture content of the defatted cheese. G/S is the fat to lean ratio. ESD is the dry extract excluding fats. NS is the quantity of nitrogen soluble in water. NT is the total quantity of nitrogen. NPT is the quantity of nitrogen soluble in phosphotungstic acid. The ratio NPT/NT is measured using the Kjeldhal method, according to the amended standard NF ISO8968-1.

(77) A key parameter for the conservation time is the ratio NPT/NT, which is representative of the secondary proteolysis reaction. Said parameter is significantly lower for the packaging sheets according to the invention. The ratio NPT/NT is below 13%, and even below 12.5% for the four packaging sheets C to F. It is above 13.7 for the two other packaging sheets.

(78) Furthermore, the appearance of the packaging is improved with the packaging sheets of the invention.