NO873952L - PROCEDURE FOR PACKAGING A FOOD. - Google Patents

Description

This invention relates to a method for packaging foodstuffs, where the foodstuffs can be subjected to a sterilization or pasteurisation treatment or also a cooking treatment.

Simple packages of foodstuffs that can be sterilized by heat treatment with steam in pressure vessels - so-called retort packages - have so far been in the form of rigid packages with inherent mechanical stability, such as e.g. cans or glass containers with rigid lids. These packages are either filled with hot food or treated with steam when they are closed. The steam treatment involves the use of excess pressure in the range of 1.05 kg/cm<2>, and the seals undergo thermal shock when the treatment pressure is relieved and/or when they are exposed to cooling. The heat shock is absorbed by bending the ends of the boxes or the lids on the jars or, in cases where the goods are vacuum-packed, counteracted by the vacuum-packing process.

Complicated retort seals are those that require a further overpressure during the steam treatment to prevent the seals from bursting, as such overpressure is supplied by compressed air. Here, too, problems with thermal shock are often encountered when the pressure is relieved and/or the seals cool.

In addition to the above, simple closed packaging of baked goods, such as e.g. cake and bread, which can be pasteurized, arrangement of leaky channels to allow relief of pressure and steam when the packages are subjected to pasteurization under heat. After cooling, these channels must be sealed to prevent recontamination of the contents of the package, as there is clearly a significant risk of contamination due to air being sucked back into the package. It is also difficult to achieve full steam pressure in the seal, close to the boiling point, without sealing or seal failure.

There is a need for a method for producing packages that are free of heat-sensitive, vegetative versions of microorganisms, and in particular forms, intended to contain moist baked goods, e.g. those with high water activity, e.g. bread and muffins.

An object of the present invention is to provide a package where the above-mentioned problems are reduced or solved, and where the package is a retort package or simple closed package where the contents can be subjected to a heat treatment.

According to the present invention, a method for packaging a food product is provided, comprising the following steps: placing the food product in a container formed from a molded fibrous material and lined with a thermoplastic layer or formed from heat-resistant plastic material, heat sealing a bacteria-impermeable and water- and gas-permeable lid layer to the edges of the container under such conditions that the material of the thermoplastic layer extends through the lid layer to form a hermetic seal, heat treating the package to sterilize or pasteurize its contents, cooling and drying the package, heat sealing a moisture barrier layer -impermeable or which allows controlled moisture permeability and, possibly, exposure of the finished package to radiation sterilization. The radiation sterilization may be particularly necessary for sterilizing the upper space between the lid layer and the barrier layer, particularly for products that are very moist and have a strong active water content. In many baked goods, however, the final water activity (% equilibrium relative humidity) is sufficiently low to prevent mold formation in the space between the lid layer and the barrier layer.

The heat treatment can take the form of a treatment with steam in an autoclave or similar container with the aim of subjecting the package's contents to a steam sterilization treatment. Alternatively, the package may be heated, preferably by dielectric heating, to generate steam in the package for pasteurization or sterilization of its contents.

The containers used in the present method are preferably formed of porous fibrous material, such as e.g. molded fiber pulp, paper, cardboard or fiberboard produced in a conventional way by molding ("moulding") of fibers deposited by papermaking techniques. Alternatively, the containers can be made of glued ("bonded") wood chips or glued fiber material, provided that the adhesive material is able to withstand the heat treatment conditions under which the sterilization or pasteurization takes place. The containers can also be made of foamed plastic or fibrous constructions of plastic that can contain fillers and that can withstand the sterilization or pasteurization conditions or they can be made of molded plastic that can withstand the sterilization or pasteurization conditions, such as e.g. nylon, polyethylene terephthalate or polymethylpentene, the latter being particularly applicable if transparent gaskets are desired.

These containers, which are made of porous fibrous material, are lined with a thermoplastic material which can melt under heat and pressure to bond to the lid layer, but which can also withstand the heat treatment conditions. Suitable liner material includes nylon, polyesters, polypropylene and certain other polyolefins and other polymer materials. A preferred carrier material is an ionomeric polymer, which is a polymer in which ionized carboxyl groups form ion cross-links in the molecular structure, which bonds are reversibly broken at melting temperatures. Such a polymer is hereinafter appropriately termed "ionomer" and a preferred ionomer is the one sold under the registered trademark

"SURLYN".

The lid material is permeable to gas and moisture, but has the ability to exclude microorganisms. The preferred material is so-called "medical paper" which is sold under the trade mark "STERISHEET" where the fibers are laid in such a way that they prevent the passage of bacteria and other microorganisms. Instead of medical paper, it is possible to use a microporous polypropylene film, such as the one sold under the registered trademark "VALMIC".

After the food to be packaged is placed in the container, the package is heat-sealed using a shaped support and heating tool under such conditions that the thermoplastic lining melts and flows through the lid material at the edges to form a hermetic seal that moisture cannot penetrate through wicking action. Suitable heat sealing temperatures are 170°C for ionomer and polypropylene linings and 220-230°C for nylon and polyester linings.

The seals are then exposed to heat treatment. In one embodiment of the present method, the packs are placed in an autoclave or pressure vessel and subjected to steam sterilization for a suitable time/temperature combination. In accordance with the food regulations, this treatment can be from Fl conditions, namely a vapor pressure of 1.05 kg/cm<2>in 15 minutes to F4 conditions, namely a vapor pressure of 1.05 kg/cm<2>in 45 minutes, the temperature in each case being 121.5°C in accordance with the requirements of the contents of the package or the microbiological status of the contents. During sterilization of the package, distortion of the package may occur and the lid material may form a dome on the package.

In another embodiment of the present method, the packs are heated using dielectric energy in a microwave oven or in a microwave or radio frequency tunnel for the development of steam in the pack to thereby raise the atmospheric temperature in the pack to a desired value between 65° and 100 °C. The quality, thickness, substance and physical properties of the lid material and in particular its permeability to gas and vapour, will be such that the correct and undesirable amount of moisture will be maintained in the product at the end of the heating cycle. Also, the degree of pressure developed in the pack by the steam generated during the heating step which can cause the cap material to warp to form a dome, and the temperature can be chosen appropriately. The strength of the seal between the lid material and the box or the box's polymer lining, and the elastic nature of the box and the moist lid paper make it possible to allow a certain excess pressure due to the internal development of steam in the package without breakage or deformation. An "overpressure" develops e.g. in a 700 watt microwave oven with a small damp baking sheet for 30-40 seconds. A short time at a temperature above 70°C is sufficient to kill mold and associated spores. Some regulation of the humidity condition may be necessary to replace the loss of water as steam through the cover material.

After the sterilization or pasteurization heat treatment, the package is cooled and any steam in the package can flow out through the lid material, while the container's elasticity and flexibility as well as its bearing enable the pressure to be relieved naturally and without any unfavorable distortion, as any distortion that may have occurred during the heat treatment is corrected naturally by itself. The packs can then be allowed to dry in air or quickly dried, preferably under sterile conditions, as lid materials prevent the penetration of microorganisms, and when the packs are dry they are heat-sealed with a suitable moisture-proof barrier film which can be a composite polyester/polyvinylidene chloride/polypropylene film. Here, too, it is important to provide a hermetic seal at the edge of the package to prevent the infiltration of moisture between the barrier film and the lid material.

To prevent the formation of molds, some of which have hyphae capable of growing through fibrous cellulosic materials and consequently capable of penetrating the lid material, the packages may finally be subjected to sterilization by radiation, preferably using ultraviolet light of appropriate wavelength to kill microorganisms and mold spores that may occur in the upper space between the lid material and the barrier film. This is particularly important for foods that have a high water content and foods where mold formation can be a problem.

The resulting pasteurized packs can be stored at room temperature for a long time and are microbiologically stable. Permissible storage time depends on the gas permeability of the bearing to the container and the barrier film.

In order to make the invention more understandable, reference must now be made to the accompanying drawings which schematically and as an example show an embodiment of this, and where,

Figure 1 shows part of a plant for packaging food products, Figure 2 shows a further part of the plant including a sterilization step, Figure 3 shows an alternative part of the plant including a pasteurization tower, Figure 4 is a partially spread section through a pack produced at the plant shown in Figures 1 and 2 or Figures 1 and 3, Figure 5 shows a continuous, overlying sealing device, and

Figure 6 is a perspective view of a wrapped package.

Figure 1 shows a plant for the production of packages of a food product to be sterilized or pasteurized. Preformed containers in the form of flanged boxes 1 of porous fiber material, e.g. molded fiber boxes are taken from a box store 2 by means of a destacking device indicated by an arrow 3 and placed in rows across a conveyor 4 so that they form a link of closely spaced boxes on the conveyor. The boxes

1 is fed to a hot forming station 5. A roll 7 of ionomer film 6 in which a barrier of polyvinylidene chloride (not shown in Figure 1) is layered and which has a thickness of 100 pm is mounted above the conveyor 4 and the film is joined to the boxes at the hot forming station 5 so that the boxes are covered with the film and join the boxes together. The film is continuously connected to the boxes by being heated by means of a heating device 8 and by being pulled down by means of a vacuum which acts at a vacuum outlet 9.

After they have left the hot forming station, the boxes are taken to a filling station 10 where they are filled with the food 11 to be packed. A lid material 12 in the form of a web of medical paper which is sold under the trade mark "STERISHEET" A5/43 and has a basis weight of 59 g/m<2> is unwound from a roll 15 located above the conveyor path of the boxes and brought to cover them. The two webs are then joined together in a heat sealing machine 16 using a profiled support tool 17 which is shaped to receive the boxes and an overlying sealing plate 18. The heat sealing is carried out under pressure at a temperature of 170°C for a sufficient time to ensure that the ionomer lining of the box melts sufficiently to flow through the paper to form a hermetic seal with the box flanges.

The filled boxes leave the heat sealing machine and are separated by a knife 19 into individual packs 20 which are taken to sterilization in the part of the plant shown in Figure 2.

As shown in figure 2, the gaskets 20, which are separated by means of the knife 19, are fed on a conveyor 21 to an autoclave 22 where they are stacked on a base 23 and subjected to a steam sterilization treatment at a temperature of 121.5°C, under a pressure at 1.05 kg/cm<2>for 15 minutes. At the end of this period, the autoclave is cooled by injecting cold water through a pipe 24. After cooling, the packages are stacked in a drying tower 25 on a base 26 and dried by blowing air over them using a fan 27.

When the gaskets are dry, they are taken by means of a conveyor 28 to another heat sealing machine 29. A barrier film 30 in the form of a web of a composite material 43 pm thick is unwound from a roll 31 located above the gaskets' feed path and brought to cover them. The barrier film comprises an inner layer of ionomers, a middle layer of polyvinylidene chloride and an outer layer of polypropylene. The barrier film is then heat-sealed by its ionomer layer to the edges of the gaskets in the heat-sealing machine using a profiled support tool 32 and an overlying sealing plate 33. The heat-sealing is carried out under pressure at a temperature of 170°C for a sufficient time to ensure that a hermetic seal is formed at the edge of the gasket edge so that no absorption of moisture in the area between the medical paper and the barrier film can occur.

The joined gaskets 34 are separated by means of a further knife 35 and guided by means of a conveyor 36 under an ultraviolet ray device 37. The sterilized gaskets are then delivered for storage or for sale. If desired or if more appropriate, the packages can be irradiated before they are separated.

In an alternative embodiment, the filled boxes that leave the heat sealing machine and are separated by means of a knife 19 into individual packages 20 are taken from the part of the plant shown in Figure 1 to pasteurization in the part of the plant shown in Figure 3.

Figure 3 shows the seals 20 as they are fed on the conveyor 21 to a microwave or radio frequency tunnel 40 where they are heated in a dielectric field between an electrode 41 which is connected to a microwave or radio frequency source 42 and an electrode 43 which is connected to earth. As a result of the heating in the tunnel 40, steam develops in the packs 20 and the steam pressure can cause the medical paper lid to form a dome over each pack. Steam escapes through the dome, but as indicated above, the lid material and the heating temperature are chosen in such a way that the development of steam does not break the packing or the seal, and that the contents of the packing reach a temperature of from 65 to 100°C to ensure that the contents of the packing become properly pasteurized.

After the packs have left the tunnel 40 they are taken to a cooling station 44, which is schematically indicated, where cooling air 45, preferably sterile air, is blown over them to cool them and allow the dome of capping material to return to its normal planar state.

When the packs are cold, they are taken by means of the conveyor 21 to the second heat sealing machine 29 where they are covered with the barrier film 30 in the manner described in connection with figure 2.

Figure 4 shows the finished package in more detail with the molded fiber box 1 grooved with groove 6 including an inner layer 46 of ionomers, an outer layer 47 of ionomers and a gas-impermeable layer 48 of polyvinylidene chloride sandwiched between the two ionomer layers. Figure 4 also shows the package covered with the lid of medical paper 12 covered by barrier film 30 which comprises an inner layer 49 of ionomers which are heat-sealed to the medical paper at the edges, an intermediate, gas-impermeable layer 50 of polyvinylidene chloride and an outer protective layer 51 of polyester.

In an extension of the present method, the packs can contain a pie mixture, and after the heat treatment, but before applying the barrier film, the packs are subjected to a baking process in an oven where the normal crust is formed and development of the desired color and structure takes place. Furthermore, where applicable, this also has the effect of drying out the fiber box if it still contains some moisture. This extension of the method makes it possible to produce sterile pies and the barrier film is chosen so as to allow controlled loss of moisture vapor per unit of time to maintain the desired structure of the crust. Suitable materials for the barrier film include polyvinyl alcohol and polymers thereof, plasticized polyvinyl chloride and various coated regenerated cellulose films. The packages are finally exposed to ultraviolet radiation.

In this extension of the method or in any case where it is necessary to heat the product in a conventional oven up to a temperature of e.g. 240°C, the box or the box's lining material must be a polymer or a polymer system which can withstand the oven heating, but which also has the ability to bind to the fibrous structure of the lid material. Likewise, the barrier layer 50 can be of ethylene-vinyl alcohol copolymer to withstand higher temperatures, but the inner layer 49 should be temperature resistant.

When it comes to pork pies etc. products where jelly is included, the jelly in sterile and liquefied form can be injected into the pie and through the lid material after the pie is baked and before the barrier film is applied. The ability to produce a packaged pie in a package that allows controlled moisture loss (breathability) and where mold formation is prevented is a significant sales advantage.

It should be understood that many other variations in the method and the packages may come into play. Thus, e.g. the food product to be packed is placed at the filling station 10 in pre-loaded containers which have been produced elsewhere and fed to the filling station by means of a destacking mechanism.

The packaging itself and its components are of course chosen to suit the different treatment methods, e.g. sterilisation, pasteurisation, baking, etc., to which the packaging is subjected, and the different materials will preferably be chosen in compatibility with each other. Thus, if the lid material is made of microporous polypropylene instead of medical paper, the 6 outer layers of the lining should also be polypropylene.

It should be understood that many modifications and variations of the described packing process can be carried out. With such a modification, e.g. the food to be packed is placed warm in the boxes directly after it has left an oven where it has been cooked. The boxes are then immediately covered with a lid of medical paper. In this case, the heat retained in the food may be such that only a limited amount of heat is required from the dielectric heating to achieve the desired pasteurization temperatures above 70°C, in the required time.

In a further extension of the present method, the heat-treated, cooled and dried package, after the contents have possibly been baked, is flow-wrapped to form a so-called cushion package by covering the package in a tube of the barrier material, sealing the ends of the tube and then seal the barrier layer to the edges of the container. The finished package then takes the form of a bag that envelops the container, which is heat-sealed to the bag at its edges.

This extension of the method will now be illustrated in connection with Figure 5 where the packages 20 that leave the drying tower 25 shown in Figure 2 or the radiant heating device 40 shown in Figure 3, possibly after the contents have been baked, are taken to a wrapping machine (not shown ) where each pack is inserted into a bag of the barrier film material, either by being wrapped with the material, which is then heat sealed along three sides to form a closed bag 53, or by being placed in a tube of the material which is likewise sealed at its ends to form a closed bag 53.

The bags 53, containing the gaskets 20, are then fed on a conveyor 54 to a continuous, overhead pressure sealer comprising an endless belt 55 which runs over end rollers 56 which are mounted on supports 57 and which are moved up and down by means not shown . Inside the endless belt 55, a hot box 58 and a cold box 59 are arranged. The wrapped packages are moved on the conveyor 54, so that each in turn lies under the hot box 58 and then under the cold box 59. When the package is under the hot box, the sealer is immersed in contact with the gasket, so that the barrier film in contact with the edges of the gasket is heat-sealed to it under a pressure and a temperature which is sufficient to ensure that a hermetic seal is formed at the edge of the gasket, so that no absorption of moisture in the area between the medical paper and the barrier film can occur. The sealer is then raised and the pack is advanced until it is under the cold box to cool the seal, after which the pack is taken to an ultraviolet ray device.

Figure 6 shows the enveloped package with the barrier film in the form of a tube 60 which is heat-sealed at its ends 61 and heat-sealed at 62 to the edges of the package 20.

Claims (11)

1. Method for packaging a food product, comprising placing the food product in a container, sealing a lid layer to the container, heat treating the resulting package, and subsequent cooling of the package, characterized in that the container (1) is formed from a cast fiber material and is covered with a heat-resistant thermoplastic layer (6) or is formed from heat-resistant thermoplastic material, that the lid layer (12) is of bacteria-impermeable but water- and gas-permeable material and is heat-sealed to the edges of the container under such conditions that the material in the thermoplastic layer (6) or in the box extends through the lid layer to form a hermetic seal, and that after the heat treatment and cooling, a barrier layer (30) which is moisture impermeable or which allows controlled moisture penetration is heat sealed to the lid material at the edges of the container. 2. Method according to claim 1, characterized in that the package is exposed to sterilizing radiation to sterilize the space between the lid layer and the barrier layer. 3. Method according to claim 1 or 2, characterized in that the lid is medical paper where the fibers are laid in such a way that the penetration of bacteria and other microorganisms is prevented. 4. Method according to claim 1 or 2, characterized in that the lid layer is a microporous polypropylene film and that the container (1) or its groove (6) is made of polypropylene. 5. Method according to one of claims 1 to 4, characterized in that the heat treatment comprises steam sterilization in an autoclave (22) or similar vessel, preferably at a steam pressure of 1.05 kg/cm <2> for a period of time from 15 to 45 minutes. 6. Method according to one of claims 1 to 4, characterized in that the heat treatment comprises heating the package using dielectric energy in a microwave oven or a microwave or radio frequency tunnel to raise the temperature of the atmosphere in the package to from 65 to 100° C. 7. Method according to one of claims 1 to 6, characterized in that the container is made of nylon, polyethylene terephthalate or polymethylpentene or is made of a porous fibrous material coated with nylon, a polyester, polypropylene or an ionomer polymer. 8. Method according to one of claims 1 to 7, characterized in that the barrier layer comprises polyvinyl alcohol, a polyvinyl alcohol polymer, an ethylene-vinyl acetate copolymer, softened polyvinyl chloride or a regenerated cellulose film. 9. Method according to one of the claims 1 to 8, characterized in that the packing, after heat treatment and cooling, is placed in a closed bag of the barrier layer material which is then heat sealed to the lid material. 10. Method according to one of claims 1 to 9, characterized in that the package, after the heat treatment, is subjected to a baking process in an oven to prepare the contents of the package. 11. Packaging characterized in that it contains a food product which has been produced by the method according to one of the preceding claims.