CA1142103A - Package for food products - Google Patents

Abstract

PACKAGE FOR FOOD PRODUCTS

ABSTRACT

This application is concerned with food pack-ages made from sheet or film into the forms of heat sealable pouches, bags, or covered trays in which the food product is placed, the package sealed, and the packaged food processed in a pressure cooker or retort to permit the packaged food to be stored without refri-geration. Certain food products can be rendered com-mercially sterile by hot filling the package at tem-peratures below boiling to permit storage for long periods without refrigeration. While still in the sealed package of the present invention, the consumer can heat or cook the food by immersion in boiling water or by microwave oven. The sheet or film is a multi-layer construction wherein each layer performs specific functions and the choice of materials for the layers is interdependent to provide a film structure having all desired properties at the least cost.

Description

11~2103 Background Heat sealed sheet or f ilm ood packages are in use for foods which ~r~ pteserved by refrigeration or for dried foods. The film o~ sheet structure used or these purposes must have a heat sealable inside sur-face, be relati~ely imperJiOUS to oxygen and moisture, and have an outside surface of sufficient durability to protect the integrity of the package. The materials must not adversely affect the food product.

1(~ The ability to hot fill or to retort foods in sheet or film packages permits the foods to be stored without refrigeration. Commercial sterilization by hot fillinq or retorting imposes several additional res-trictions on the choice of materials for the sheet or film structure. The heat seal must survive commercial steriliz-ation temperatures of over 180 or typical re-tort condltions of steam or water at 250~ or more ur-der pressure for one half hour or more. The structure must ~ot ~elaminate, shrink, or wrinkle as a result of the sterilization. The oxygen and water barrier pro-perties must not be permanently adversely ~ffected by the conditions of commercial sterilization. The str c-ture must be adequately strong to permit handling of the packag~ while still hot. The additional require-ments i~posed by hot filling or by retorting rule out many of the materi~ls and stcuctures commonly used for non-retorted film or sheet food packages.

Retortable sheet or film pa_kages ha~Je ~ePn tried in which the ~arrier layer is aluminum foil.

11~2103 Suitable materials for the inside and outside surfaces are bond-ed to the foil. An aluminum foil containing structure cannot be made in a single step as is the case with multi-layer all polymeric structures which can be made in a single coextrusion operation. An aluminum foil containing structure is opaque visually and is not suited to microwave oven cooking of the packaged food.
Summary of the Invention Food packages in the form of heat sealable pouches, bags, or covered trays are formed from coextruded, multi-layer, all polymeric film or sheet. The packages much withstand com-mercial sterilization conditions. The sheet or film structure should be capable of being transparent and the packages should be suited to microwave cooking of the food in the package by the consumer. The multi-layer film or sheet is of the general structure:
Outside layer/barrier layer/heat sealable inside layer.
Additional interior layers may be employed between the above noted layers.
Accordingly, the present invention provides a heat sealed package for commercial sterilization and unrefrigerated storage of food products, said package comprising multi-layer polymeric material, the layer which forms the outside surface of the package being relatively permeable to moisture at tem-peratures of unrefrigerated storage, the layer which forms the inside surface of the package being relatively impermeable to moisture at the temperatures of unrefrigerated storage, being relatively permeable to moisture at temperatures of commercial sterilization, and being a heat sealable polymer to produce a seal which will withstand commercial sterilization temperature, and an interior oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of acetal groups.

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- ~, 1.14Z103 Detailed Description In the drawings:
Figure 1 is a perspective view of a pouch according to the present invention, Figure 2 is a perspective view of a covered - 3a -tray according to the present invention, Figure 3 is an enlarged ccoss-sectional view of the film or sheet material used in packages accord-ing to the invention, Figure 4 is a graph showing the oxygen permea-bilitv of ethylene-vinyl alcohol copolymer film 3s a function of moisture content, Figure ~ is a graph comparing oxygen perme~bi-lity as a function of time of a film according to the invention with a similar film not according to the in-vention, and Figure 6 is a graph comparing the moisture content as a function of time of the film of a package according to the invention with a similar film not ac-cording to the invention.

Each of the several layers of the sheet or film structure serves particular purposes and must sa-tisfy particular requirements. Certain requirements of the materials are shared in common by all layers. All layers must be of sufficientl~y flexible materials to result in z flexible structure which will not fracture in use. For economical manufacture, the materials of all layers should be coextrusible, that is, the rheo-logy of each material in the molten state should allow all the layers to be extruded simultaneously to join in an integr~ted film structure. To produce an nPxpen-sive film, each material must be re'2tivPly ine~pensive as measured by the relative zmount of tnat material present in the finishod structure. S nc~ ~he struc~ure ll~Z103 should be capable oE being made reasonably transparent to microwaves in an oven, and ~or some apDlications transpar~nt visually, each layer must be. A decision to use a particular material for one layer affects the choice of ~aterials ~or other layers. The following description of requirements and preferred materials be-gins with the interior barrier layer.
The barrier layer must provide a sufficient barrier ~o oxygen to provide adequate shelf-life for the food in the package. In the thickness requieed for adequate barrier properties, the material must not be expensive. Ethylene-vinyl alcohol copolymer (EVOH~
pro~ides superior oxygen impermeability swhen com ared C~ ~ e ,n c. r~ ~
with other polymeric materials such as ~L~ and acry-lonitrile which have been employed in packages for oxy-gen barrier qualities. A very thin layer of EVOH will provide an adequate barrier to oxygen. Extrusible grades of EVOH are available under the ~ EVAL from Kuraray Co., Ltd. of Japan. The oxygen barrier quality of EVOH is adversely af~ec~ed by the oresence of water in the EVOH layer. A small quantity of water will raise the moisture content of a thin layer of EVOH to an extent where the oxygen barrier quality of the layer is severely affected. The oxygen barrier quality of EVOH is restored when the moisture is removed.
Polyolefins such as polyethylene, polyprooy-lene, and blends and copolymers of the two are .orsi-dered to be excellent moisture barriers. Polioleins are heat sealable. High density ?olyethylene, oolypro-pylene, and blends and co?olymers of poly?ropylene and polyethylelle melt at ~:emperatures sufriciently high toprovide heat seals whlch survive hot ~illing at temper-atures ln the neighborhood of 190~ or retorting at up to 260F, yet seal at temperatures betwe2n 350F an{l ~00F attainable with existing heat seal equipment ~t normal sealing pressures and ,imes. Consequently, high mel~ing temperature polyolefins are suitable for use as the inside heat seal layer. However, the permeability of polyolcfins to water is greatly increased a. the elevated temperatures encountered in commercial steri-lization. Thus, a polyolefin layer may permit intru-sion of water into the EVOH oxygen barrier layer during hot filling or retorting. Since polyolefins again be-come relatively impermeable to water upon cooling, moisture in the EVOH layer cannot escape through the polyolefin layer. Thus, we have concluded that the layer or layers which are on the outside of the package from the EVOH layer must be relatively permeable to moisture under normal storage conditions to permit the escape of moisture in the EVOH layer.
A suitable polymeric material for the package outside layer is nylon. Nylon is sufficiently permea-ble to moisture to permit the esca?e of moisture Erom the EVOH layer to restore the oxygen barrier quality, yet is sufficiently waterproof to perrnit acci.lental wetting of the package without harm. Nylon possesses all of the other desired ?roperties 'or the ou~side o' the package. Nylon is tough, flexible, not ~-e3tly affected by heat or cold, aorasion resistant, trans?a-rent, and can be printed upon for lable pur.aoses 1On is not e~pellsive ~uld is one oE the ~ew po'ymers s~3itable ~or Eilm struc'ures to which EVOH will ,~de-quately adher~ ylon can be coextruded with EVOH and with polyolefins. Thus, nylon is we11 suited ~or use as the outsid3e s~lr~ace 1;3yer ov~rlying the EVOII barrier layer.
Since polyoleEins do not adhere well to EVOH, steps mus~ be t~ken to providQ adequate adhesion to preJent delamination of the film structure. ~n inter-layer o a polymeric material adherent to EVC~i and ~opolyole~ins may be used between the polyolein inside heat seal layer and the EVOH oxygen barrier 13yer.
hhere the inside layer heat sealable high melting polyolein is high density polyethylene, suit-able adherent, extrusible polymeric m~teri~la for ~he TP~ c~ c interlayer are those sold under the .~e Plexar by Chemplex Company of Rolling Meadows, Ili. These ma-terials are blends of a polyolefin sach as ethylene ~-inyl acetate with a graft copolymer of high density polyethylene and an unsaturated fused ring carboxylic acid anhydride.
~ here the inside layer high temperatlre poly-olein is polypropylene or a blend o polypropylene a1o polyethylene or a copolymer of propylene and ethylene, suitable adherent, extrusable polymeric mater~als for~

~ a G~4 ~
the interlayor are those sold under the d~ ~dmer by Mitsui Petrochomical Company of Japan. Thoso mate.ials are acid anhydride grated polyole~ins.

The adherent polyneric materia1s sold under the names Plexar and ~dner alaO adhere ~ell to nvl~n.

` ~142103 Since EVOH adheres well to nylon, a ~urther interlayer of nylon on the inside face of the E~10H layer m2y be used. The adherent polymer is located between that ny-lon interlayer and the polyolefin -inside surface layer.
- Figure 1 shows a heat sealed food pouch ac-cording to the invention. The pouch is formed by over-lying t-~o rectangles of film with the heat sealable high temperature polyolefin layers confronting each other and the nylon surface layer of the film on the outside. A conventional heat seal bar is used to 3uto-genously weld the films together along seal line 12 e;~-tending along three of the four side flanges 1~. After the food product is placed in the pouch, the fourth side flange is heat se31ed.
The pouch of Figure 1, after filling 3nd sealing is ready for retorting. Filled Pouches are loaded in a pressure cooker or retort and cooked under pressure in water or steam at temperatures up to about 260F. for times extending up to two hours. The times and temperature are related such that higher tempera-tures generally require shorter times. I~any food pro-ducts require about one-half hour a. about 250F.
Other commercial sterilization techniques such as mi-crowave or hot air heating can be employed.
The pouch of Figure 1 can be hot filled witn certain food products which do not require high tem-perature processing to achieve commercial sterility for adequate storage life. ~xamples of such foods are su-gar syrups, ketchup, fruit juices, jellys ana preserves, and high acidity foods. ~ot filling is 11~2103 usually done at temperatures in excess of 180F. The pouch may be immersed in a hot water bath during or after filling and sealing for a period of time sufficient to assuee the destruction of molds 3nd yeasts.
Figure 2 shows a covered tray according to the invention. The tray 20 is thermoformed from relatively thick sheet stock which may be formed by ex.rusion coating the polyolefin side of the multi-layer film structure described above with an additional thickness of polyolefin on the inside surface of the tray. The nylon surface is on the outside of the tray. After deposit of the food product in the tray 20, a cover 26 of the multi-layer film is positioned over the tray with the heat sealable polyolefin surface confronting the polyolefin surface which is uppermost on the flanges 24 which form the periphery of the tray. Con-ventional heat sealing heater bars are used to heat seal the cover sheet to the tray flanges 24 along seal lines 22. The trays can be hot filled or retorted as is described above.
Figure 3 is an enlargod cross-sectional vie~
of a film structure according to the invention. The outside of the package is the nylon layer of the film.
The barrier layer is the oxygen barrier material which preferably is a layer of ethyleno-vin11 alcohol copo-lymer (EVOH). The inside layer is tne he~ se313ble layer, Qreferably a polvolefin ~hich heat seals at a temperature higher than that encountered in hot filling or retorting.

Between the barrier layer and the heat seal layer is an interlayer whose function is to securely bond the EVOH layer to the polyolefin n~at seal layer.
As illustrated, the interlayer is actually ~wo layers;
a nylon layer adjacent the ~VOH an an adhesive polymer such as Plexar or Admer modified polyolefins. The ny-lon layer of the interlayer bonds well to EVOH and the adhesive polymer bonds well to the polyolefin heat sealable layer and to the nylon. The above described structure can be simultaneously coextruded in a single step to provide an economical film. Typically, suit-able films range between 3 and 12 mils in thickness.
Figure 4 shows the effect of moisture in the EVOH barrier layer on its oxygen permeability. At high water contents the oxygen barrier quality of EVOH is seriously degraded. Removal of moisture from the ZVOH
layer restores the oxygen barrier quality. Since hot filling and retorting occur at temperatures of 190F c.
higher, the good moisture barrier properties of poly-olefins are impaired until lower temperatures are res-tored. During the time the package is exposed to nhot filling or pre-cooking temperatures, moisture can mi-grate through the polyolefin layer into the EVOH bar-rier layer. Return Oc the package to room temperature causes the moisture in the EVOH to result in a rela-tively high relative humidity which impairs the oxygen barrier quality. The restoration of moisture imper-meability of the polyolefin at room temperature pre-vents escape of the moisture.
Figure ; compares a film acco!ding to the in-ll~Z103 vention with a control film which was a si~milar ilm to which a further layer of modified HDPE (Plexar Il) was added outside the nylon layer. The film according to the invention was 7.5~ nvlon/20~ EVOH/7.5~ nylon/10 Plexar III/55~ HDPE. The control film ~as 32.~
HDPE/7.5~ nylon/20~ EVOH/7.5~ nylon/32.5~ HDPE. The films were made into pouches which were filled and subjected to retorting. Figure 5 shows that oxygen barrier quality improved because the nylon outside layer of the ilm of the invention permitted moisture entrapped in the EVOH layer to escape. The oxygen barrier quality of the film improved from a permeabi-lity of 650 cc~m2Jday to about 20 cc~m2/day within 6 hours tfter retorting, whereas the control film which had a further layer of HDPE over the nylon only dropped from about 400 cc~m2/day to about 300 cc/m2/day in 24 hours. After a full day the control film still was lS
times more permeable to oxygen than the film of the in-vention. The HDPE outer layer of the control film pre-vented the escape of moisture from the EVO~ barrierlayer. The continued presence of moisture degraded the oxygen barrier quality of the control film.
Figure 6 compares the relative weight as a function of ~ime of a package in accordance with the invention ~ith a control package having a layer of HDPE
over the nylon layer outside the EVOH barrier layer.
The films used were the same as tnose used in Figure ~.
The relative weight plot~ed is a measure of the ?e.cen-tage change in weight of ~ater in the fil.m. The rela-tive weight is:

~ 12103 weight at tlrl~ t - original weight orlglnal welgh~. X 100 The packages were in the form of ~ouches filled with tomato paste. The packages were weighed to establisn the original weight. The packages were then retoete~
at 250F foe one-half hour. The retorted pouches were then weighed and weighed again on the 1st, 2nd, 3rd, 6th, 7th, 10th, 13th and 15th day following retorting.
It is apparent that the packages according to the in-vention initially gained more weight during retorting than did the control packages. After one day the packages according to the invention had less moisture in the film than did tne control packages. For the next two weeks the packages according to the invention continued to dry out more rapidly than the control packages and at all times following the first few hours contained less moisture than the control packages The control packages were substantially unchanged in mois-ture content, ~lways remaining at an unacceptably high moisture content as is reflected by the high oxygen permeability shown for the control packages in Figure 5.
Transparent films according to the in~-ention comprising a coextruded structure of 7.5~, nylon/20?O
EVOH,/ 7.5~ nylonil0~O Plexar III/55~ HDPE ha~ing thick-nesses between 4.5 and 6 mils were formed into pouches which were filled with potatoes, tomatoes, corn, car-rots, beef stew, and green beans. Each of the ?ac~ages was subjected to retorting at temperatlres off at least

2 0F for at least one half hour. The packages were ` 1142103 successEul.
Films similar to those abo~e were used to hot fill packaging of syrup and tomato paste. These pack-ages were successful.

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat sealed package for commercial sterilization and unrefrigerated storage of food products, said package compris-ing multi-layer polymeric material, the layer which forms the out-side surface of the package being relatively permeable to mois-ture at temperatures of unrefrigerated storage, the layer which forms the inside surface of the package being relatively imper-meable to moisture at the temperatures of unrefrigerated storage, being relatively permeable to moisture at temperatures of commer-cial sterilization, and being a heat sealable polymer to produce a seal which will withstand commercial sterilization temperature, and an interior oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of acetal groups.

2. A package for commercial sterilization and unrefrig-erated storage of food products, said package comprising multi-layer polymeric material, said multi-layer material comprising, in order: (1) a layer on the outside surface of the package, said layer being relatively permeable to moisture at temperatures of un-refrigerated storage, (2) an oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of groups, (3) a layer which is relatively permeable to moisture at temperatures of unrefrigerated storage, (4) a layer of polymeric material adherent to both said third layer and a layer on the inside surface of the package, and (5) a layer on the inside surface of the package which is relatively impermeable to moisture at temperatures of unrefrigerated storage and is relative-ly permeable to moisture at temperatures of commercial steriliza-tion.

3. The package of claim 1 wherein the layer which forms the outside surface of the package comprises nylon. I

4. The package of claim 1 wherein the layer which forms the inside surface of the package comprises a polyolefin polymer or copolymer.

5. The package of claim 3 wherein the layer which forms the inside surface of the package comprises a polyolefin polymer or copolymer.

6. The package of claim 5 wherein the multi-layer polymeric material comprises, in order, an outside surface layer of nylon, an oxygen barrier of ethylene-vinyl alcohol, a layer of nylon, an adherent layer of an acid anhydride grafted polyolefin, and an inside surface layer which comprises polypropylene polymer or copolymer.

7. The package of claim 2 wherein the layer which forms the outside surface layer of the package comprises nylon, the layer of adherent polymeric material is an acid anhydride grafted polyolefin, the layer which forms the inside surface of the package comprises polypropylene polymer or copolymer, and the layer which is relatively permeable to moisture and which is adjacent the layer of adherent polymeric material is nylon.

8. A package for commercial sterilization and unrefrig-erated storage of food products, said package comprising multi-layer polymeric material, said multi-layer material comprising, in order: an outside surface layer of nylon, an oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of acetal groups, a layer of nylon, an adherent layer of polyolefin blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride, and an inside surface layer of high density polyethylene.

9. A package for commercial sterilization and unrefrig-erated storage of food products, said package comprising multi-layer polymeric material, said multi-layer material comprising, in order: an outside surface layer of nylon, an oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of acetal groups, a layer of nylon, an adherent layer of an acid anhydride grafted polyolefin, and an inside sur-face layer which comprises polypropylene polymer or copolymer.

10. The package of claim 5 wherein the multi-layer polymeric material includes a layer of polymer adherent to the polyolefin layer and to the oxygen barrier-vinyl alcohol layer.

11 The package of claim 5 wherein the polyolefin layer comprises a polyolefin selected from the group comprising high density polyethylene, polypropylene, copolymers of propylene and ethylene, and Blends of polypropylene and polyethylene.

12. The package of claim 10 wherein the adherent polymer layer comprises a modified polyolefin.

13. The package of claim 12 wherein the polyolefin layer is high density polyethylene and the adherent layer is a polyole-fin blended with a graft copolymer of polyethylene and an unsatur-ated fused ring carboxylic acid anhydride.

14. The package of claim 12 wherein the polyolefin is polypropylene or a polypropylene blend or propylene copolymer and the adherent layer is an acid anhydride grafted polyolefin.

The package of claim 5 wherein the multi-layer polymeric material comprises, in order, an outside surface layer of nylon, an oxygen barrier of ethylene-vinyl alcohol, a layer of nylon, an adherent layer of polyolefin blended with a graft co-polymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride, and an inside surface layer of high density poly-ethylene.

16. The package of claim 15 wherein the high density polyethylene surface of the multi-layer sheet material is extru-sion coated with additional high density Polyethylene and the resulting sheet is formed into a tray which is covered by a film of the multi-layer sheet material,