Note: Descriptions are shown in the official language in which they were submitted.
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.
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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
3Q
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114Z103
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
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103
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
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2103
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.
1142103
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-
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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
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successEul.
Films similar to those abo~e were used to hot
fill packaging of syrup and tomato paste. These pack-
ages were successful.
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