Microwavable package and process

The present invention provides packaging capable of room-temperature storage of foods followed by simplified microwave heating. The package also permits heating in conventional ovens. The package includes a multi-ply container portion having a central recess surrounded by a sealing rim to which is heat sealed a lid which is flexible and non-extensible and has good barrier proporties. The inner surface of the container is amorphous polyethylene terephthalate to assure good heat sealing. The out ply of the container is crystallizable to provide sufficient dimensional stability during heating. The package can be retorted and vents itself when microwave heated, thus simplifying storage and preparation of foods.

TECHNICAL FIELD 
The present invention relates to microwavable packaging which is suitable 
for protecting foods during handling and storage and which is also 
suitable for heating for serving with maximum product protection and ease 
for the consumer. 
Microwave cooking is especially advantageous because of its ability to 
rapidly heat small portions of food. Consumer convenience is important, 
and simplified instructions can help assure the gustatory satisfaction 
promised for a well-prepared product. Unfortunately, to date, a number of 
conflicting safety, packaging, esthetic and convenience priorities and 
limitations have deterred the development of a shelf-stable, retorted food 
product which could be simply taken from the grocery shelf and reheated 
briefly in a microwave oven. 
For most microwavable, prepared foods it is necessary to perform one or 
more success-determinant operations in addition to simply popping the food 
into the oven--the apparent promise of microwave cooking. Consumers are 
often disappointed to find, especially when they learn too late, that they 
must specially program the oven or perform a sequence of distinct heating 
operations, use a special cooking container or device, remove a packaging 
lid only to replace it with a cooking lid, vent, interrupt heating, and/or 
specially orient the package in the oven. Some consumers never determine 
the cause for failure and never purchase the product again, and some are 
deterred from purchasing products they perceive to be similar. 
There is a present need for a package and method which improves the 
delivery to the consumer of foods which can be simply, safely, swiftly, 
and effectively prepared for serving by microwave heating. And, ideally, 
this package should be retortable to provide a shelf-stable food. 
BACKGROUND ART 
Many attempts have been made to provide containers for processing and 
storing foods which permitted heating by microwave and some, also, by 
conventional ovens. For example, in U.S. Pat. No. 3,865,302, Kane 
discloses a container consisting of a polyethylene terephthalate tray 
having a polyethylene terephthalate lid bonded to it with an adhesive 
blend of an ethylene copolymer and a wax. The tray is formed of a single 
ply film employing titanium dioxide as an opacifier and talc to enhance 
the rate of crystallization. The lid, made of a material having properties 
similar to the tray, is bonded to a flange on the tray by a heat-activated 
adhesive. The adhesive does not flow or leak even after cooking at 
temperatures of 400.degree. F. Unfortunately, containers which remain 
sealed when oven heated to 400.degree. F. develop significant internal 
steam pressures. 
The problem with steam buildup can be especially severe during microwave 
heating because of the very nature of the heating phenomenon. Microwave 
heating virtually always causes steam generation from hot spots before the 
food is heated desirably throughout. The continued generation of steam at 
these hot spots and the continued heating of the steam itself cause 
pressures which must be released in a controlled manner. In U.S. Pat. No. 
4,210,674, Mitchell proposes applying a small strip of metal to the lid to 
melt a vent during heating. Similarly, in U.S. Pat. No. 3,997,677 Hirsch 
and Spiegel disclose a covered tray-like package which includes a weakened 
seal area to provide pressure relief. Also, several patents have disclosed 
ovenable polymer-coated paperboard packages which would self vent when 
heated. For example, in U.S. Pat. No. 3,863,832 Gordon and Siegele 
disclose that the seal can be made to fail at a preselected temperature 
where the coating is polyethylene or polypropylene for both tray and lid. 
Similarly, in U.S. Pat. No. 4,261,504, Cowan discloses a self-venting 
package having polymer coated paperboard upper and lower portions. One is 
given a coating of thermoplastic polyester and the other, a thermosetting 
polyester to provide a package which is said to vent by steam buildup just 
prior to completion of the cooking cycle. 
Another concern, but one which is not addressed by the above patents, is 
the development of a package which can be used for retorted foods which 
are able to be stored at room temperature. For example, in U.S. Pat. No. 
4,367,312, Bontinck, Jacobs and D'Hondt discuss as important, the ability 
to form a peelable seal which withstands retort conditions. To achieve a 
retortable, but peelable seal with packaging films including polyester, 
polyvinylchloride, polyamide, and polyacrylonitrile, they disclose a blend 
of an ethylene polymer with polystyrene and other optional components. 
Additional polymers are disclosed to enhance interpolymer compatibility, 
fillers can be added for opacity or hardness, and adjuvants and the like 
can be added for their functions. The heat sealable films are prepared by 
extrusion followed by corona discharge pretreatment. 
The retorting process for non-rigid containers typically entails heating 
them in water or steam to a predetermined temperature, holding them at 
that temperature for a time required to thoroughly heat the contents, and 
then cooling. The pressure inside the containers varies in response to 
external temperature changes. Unless the proper materials of construction 
are employed and unless this pressure is compensated for by causing the 
external pressure to approximate that on the interior, a variety of faults 
and failures will be produced, among which are: bursting of the packaging 
material; wrinkles in flexible film portions, especially at corners; 
partial release of the seal along seams; distortion of printed matter; and 
other defects. 
One commercial packaging system employs three distinct members which 
cooperate to deliver a room temperature storable, retorted, prepared food 
product which can be prepared for serving by microwave or conventional 
oven heating. This package has a shallow dish-like bottom portion, a 
multi-ply lid heat-sealed to a top flange on the dish, and a vented dome 
cover which snaps over the top of the dish. To achieve good strength and 
heat sealability to the lid, the tray is formed of two layers of 
unpigmented polyethylene terephthalate. The outer layer is partially 
crystallized to improve toughness and barrier properties while the inner 
layer is highly amorphous for better heat sealing. The multi-ply lid is a 
sandwich of polyethylene terephthalate films bonded to an inner aluminum 
foil core. One side of the lid is coated with an adhesive to provide a 
peelable seal with the tray flange. In use, the tray is filled with a 
prepared food, the film is heat-sealed to the tray, and the sealed tray is 
then heated in a retort as required while applying an external pressure to 
counterbalance the internal pressure. To prepare the product for use by 
microwave oven, the heat-sealed lid is removed and discarded and the 
vented dome cover is snapped onto the tray. In contrast, when the food is 
prepared for serving by conventional oven heating, the vented dome is 
discarded and the heat-sealed lid peeled back for a short distance. 
The use of both the heat-sealed lid and a separate vented cover is an 
inefficient use of packaging material, inconvenient, and the possible 
cause of accidents arising during preparation. Accordingly, while the art 
has long sought a package which would fulfill the needs for simplicity and 
effectiveness for foods which were to be capable of final preparation by 
microwave ovens, it has yet to achieve this. The need is especially acute 
and unfulfilled for foods which must be retorted to provide 
room-temperature, shelf stability. 
DISCLOSURE OF INVENTION 
It is an object of the invention to provide a package for foods which can 
be prepared for serving by microwave oven heating in a simple and 
effective operation. 
It is another object of the invention to simplify use of a microwavable 
food package by enabling one step preparation simply heating for the 
desired time--with the package having a self-venting feature. 
It is a further object of the invention to provide a self-venting microwave 
package which is capable of withstanding retorting. 
It is a more detailed object of the invention to provide a self-venting 
package for storing food at room temperature for extended periods and 
permit one-step preparation for eating by simply briefly heating in a 
microwave oven. 
It is a further object of the invention to provide processes for packaging 
and storing and heating foods utilizing a package of the type described. 
These and other objects are achieved by the present invention which 
provides a package and methods for obtaining and utilizing the package 
which is capable of highly simplified preparation of packaged food 
products for serving. The package of the invention comprises: a formed 
lower container portion having a recessed area surrounded by a peripheral 
sealing rim and a flexible lid heat-sealed to the sealing rim. The 
container is formed from a multi-, preferably two-, ply laminate of 
polyethylene terephthalate wherein the outer ply is crystallizable and the 
inner ply is substantially amorphous. The sealed container is capable of 
dimensional stability during the retorting necessary to sterilize the 
contents. The flexible lid is non-extensible under conditions of use, 
comprises a barrier layer substantially impervious to oxygen and water, 
and includes a heat-sealable layer which is capable of forming a seal with 
the substantially amorphous surface of the sealing rim which is both 
retort stable and self-venting during microwave heating of the package. 
The invention further provides the method for packaging a food which 
comprises: depositing a food product into the recessed area of a multi-ply 
polyethylene terephthalate container having a sealing rim surrounding the 
recessed area; sealing a non-extensible, flexible lid to the sealing rim; 
and retorting the package. 
The invention further provides a method utilizing a package of the 
invention for storing a food product at room temperature for extended 
times followed by heating for serving, the method comprising: depositing a 
food product into the recessed area of a multi-ply polyethylene 
terephthalate container having a sealing rim surrounding the recessed 
area; sealing a nonextensible lid to the sealing rim; retorting the 
resulting package; storing the package at room temperature for an extended 
time; and subjecting the food to microwave energy for a time sufficient to 
heat the food to a desired serving temperature and to cause the seal 
between said rim and said lid to partially release and form a vent. 
The invention in its various aspects has a number of advantages including 
true simplicity for the consumer. By virtue of the invention, foods can be 
stored without refrigeration or freezing, taken directly from the shelf 
and heated. Thus, there is no need to thaw, and heating time is minimized. 
There is no need to specially sequence heating in the microwave oven. And, 
because heating time is so short, there is no need to rotate after partial 
heating and restart the process. There is no need to remove protective 
sealing films from the tray and snap on special vented cooking covers. 
There is no need to specially vent because the package vents automatically 
during microwave heating. 
In addition to these benefits, the invention provides environmental 
advantages due to efficient overall energy requirements and the beneficial 
use of recyclable components.

BEST MODE FOR CARRYING OUT THE INVENTION 
The invention will be described in conjunction with the drawings to show 
its advantages with specific reference to preferred materials and 
parameters. In this regard, reference is made to FIG. 1 which depicts a 
sealed package 10 of the invention having a lower container portion 20 and 
a lid 30 sealed to it. Reference to FIGS. 2 and 3 enables viewing the 
principal components of the package in greater detail. 
FIG. 2 shows lower container portion 20 as having a central recessed area 
22 which holds a food. The recessed area 22 is defined by upstanding side 
wall 24 and slightly raised bottom panel 26. Desirably, the side wall 24 
flares outwardly from the vertical at an angle of at least 12.degree., 
preferably 13.degree. to 25.degree.. Both of these design features are 
believed to contribute to the dimensional stability of the preferred 
container, thereby facilitating retorting without damage to the package 
and self-venting during microwave heating. 
A rounded recess 27 between the bottom panel 26 and the side wall 24 
extends around the entire container 20. Also, the rim is extended at 
opposed ends to form handles 29 and 29'. A peripheral sealing rim 28 
surrounds the central recessed area 22. Reference to FIG. 3 more clearly 
shows the preferred embodiment where sealing rim 28 has a slight crown to 
enhance the reliability of the seal with flexible lid 30. 
The container 20 is formed of a multi-ply polymeric stock. As shown in FIG. 
3, the container stock comprises an inner ply 120 of a substantially 
amorphous polyethylene terephthalate and an outer ply 220 of a 
crystallizable polyethylene terephthalate. The inner ply 120 is 
sufficiently amorphous to provide effective heat sealing with the flexible 
lid 30, and the outer ply 220 is sufficiently crystalline to provide 
barrier properties and the desired dimensional stability during retorting 
and subsequent oven heating, whether by microwave or conventional oven. 
After forming and prior to retorting, the inner ply 120 will be less than 
10%, and preferably less than 5%, crystalline as formed to assure good 
heat sealing; and the outer ply 220 will typically be greater than 10% 
crystalline, typically about 12 to 25%. The average crystallinity of the 
two layers together at this point will preferably be within the range of 
from 9 to 19%. To achieve these averages it is preferred to maintain the 
inner layer as amorphous as possible while permitting the outer layer to 
achieve a relatively high level of crystallinity. 
The multi-ply stock for forming the container is preferably prepared by 
coextruding two plies of suitable resins as known in the art. The ply 
which is to form the inner ply 120 will be of sufficient thickness to 
enable presentation of a uniform sealing surface in the formed container. 
Typically, the inner ply 120 will comprise from 20 to 40% of the thickness 
of the multi-ply stock. The outer ply 220 will typically comprise from 60 
to 80% of the thickness of the multi-ply stock. The stock is preferably 
formed into a sheet having a thickness of about 0.04 inch. The exact 
thickness will of course depend on the specific product application, as 
well as the size of the final container and the extent to which the stock 
will be stretched during subsequent forming. 
Both plies 120 and 220 are desirably of polyethylene terephthalate and can 
be of essentially the same starting resin. The inner ply material is 
desirably virgin resin. In addition to providing good heat sealing 
qualities, it is an advantage of the invention that the inner ply, which 
is also the food contacting surface, is virgin polymer with no additives. 
The outer ply material, on the other hand, can include reworked scrap 
stock and preferably includes a nucleating agent as well as a stiffening 
additive which is in the form of a pigment, color at or other additive. 
Typically, a dry inorganic particulate material such as calcium carbonate, 
titanium dioxide, or magnesium silicate alone or as a carrier for a color 
additive, typically organic, is employed as the stiffening additive. In 
addition to providing a desirable color, stiffening additives such as 
titanium dioxide increase the density and crystallinity of the outer ply 
material. This can be important to the provision of dimensional stability 
in the container during retorting and subsequent heating. While the added 
dimensional stability during retorting puts added stress on the seal at 
this point and may be counterindicated for that reason, the increased 
dimensional stability achieved by virtue of the additive assures 
self-venting during subsequent microwave heating for preparation of the 
packaged food product. 
The amount of rework which can be tolerated as a component of the resin for 
the outer, crystallizable ply can be readily determined on a trial and 
error basis. Typically, as much is employed as is possible while not 
adversely affecting the functionality of the container. 
The stock can be formed into the containers by molding as known in the art. 
For example, the stock is heated to about 115.degree. to 140.degree. C. 
and upwardly forced into a vacuum mold assisted by air from a plug which 
stretches the stock to achieve uniform wall thickness in the finished 
container. The inner mold surface is heated to about 140.degree. to 
170.degree. C. The sealing rim 28 will be thicker than the rest of the 
container wall and therefore may tend to have a slightly lower 
crystallinity at the sealing surface. 
FIG. 3 shows some detail in the area of the seal between the sealing rim 28 
and the flexible lid 30. The flexible lid detail is shown in exaggerated 
scale for the sake of illustrating a preferred embodiment. The lid 30 is 
flexible and substantially non-extensible. By non-extensible, we mean that 
the lid does not significantly increase in dimensions under the stresses 
to which it is subjected during retorting and microwave heating. For 
example, preferred materials will require forces in excess of 10, e.g., 12 
to 15, pounds per inch to stretch the material 1%. Importantly, the lid 
should not stretch during retorting to cause cosmetic or seal defects, and 
it should not stretch during microwave heating so as to relieve the stress 
on the seal which is necessary to effectively vent the package. 
The lid 30 must also provide a barrier to oxygen and moisture. The food is 
preferably sealed in the package after flushing the container with 
nitrogen and evacuating to reduce oxygen which can cause deterioration, 
especially of a room-temperature, shelf-stable product. The barrier is 
desirably suitable to prevent the permeation of oxygen into the package. 
Conversely, moisture must be kept in the package to prevent the food from 
drying out. A preferred barrier will comprise an aluminum foil layer 32. 
Also preferred are polyethylene terephthalate layers 34 and 34' which are 
adhered to the foil layer 32 as a sandwich by adhesive layers 36 and 36'. 
The composite, multi-ply lid 30 is shown adhered to sealing rim 28 by a 
heat seal formed between the amorphous polyethylene terephthalate 
container inner ply 120 and a sealant layer 38 on the lid 30. The sealant 
is desirably one known in the art to: effectively seal with amorphous 
polyethylene terephthalate, be resistant to creep during retort, and 
provide a peel strength which enables relative ease in removal under the 
conditions of use. Typical of the peel strengths desired after retorting 
are from about 3 to about 10 pounds per inch at 21.degree. C. One type of 
sealant suitable for use in the invention, comprises a blend of one or 
more copolyester resins with inorganic fillers. The sealant can be applied 
to the lid stock as a suspension of solids in a carrier such as toluene or 
methyl ethyl ketone. Typically, suspensions comprising from 25 to 40%, 
e.g., 32%, solids are applied and then heated to dry to a thickness on the 
order of about 0.7 to 2, e.g., 1.4, pounds per 1000 square feet. 
Alternatively, the copolyester resin can be coextruded with the polymer 
for layer 34 to form a two-ply film. 
The polyethylene terephthalate layers 34 and 34' can be the same and can be 
of the same type as employed for the container stock. Desirably, at least 
one layer, in this case outer layer 34', is biaxially oriented. The 
adhesive sealing plies 34 and 34' to the foil ply 32 can comprise 
polyurethane. The thickness of the adhesive layers 36 and 36' will be as 
needed, typically from 0.7 to 1.8, preferably about 1.3, pounds per 1000 
square feet. Where polyurethanes are employed, curing is conducted as 
needed, e.g., for one week at 55.degree. C. 
It is an advantage of the invention that foods can be packaged to protect 
them during room temperature storage for extended time periods and then 
simply prepared for serving by placing in a microwave oven for the desired 
period of time. The package will self-vent as the serving temperature is 
reached. A food is deposited into the recessed area 22 of the container 
20. A lid 30 is then sealed to the sealing rim 28 by the simultaneous 
application of heat and pressure in the area of the seal. 
The sealed package is then subjected to retorting under conditions of heat 
as necessary to assure room-temperature shelf-stability. Preferably, this 
is accomplished by heating in water or steam while balancing the pressure 
outside of the package to approximate that within. Typically, for a single 
serving of a meal entree, the temperature will be brought up to a suitable 
level within the range of from about 115.degree. to 140.degree. C., 
preferably 120.degree. to 130.degree. C., maintained at that temperature, 
and then cooled. The heating is conducted within a pressure chamber which 
allows adjusting the pressure in the chamber to balance the pressure 
within the container. While balance is not typically exactly achieved, the 
pressures should be close enough to prevent seal creep, partial seal 
release or stress deformations to either the container 20 or lid 30. 
Retort times at the target temperature of from 20 to 60 minutes will be 
typical following a similar time for bringing it up to temperature. A 
similar cooling period is also typical. After retorting, the crystallinity 
of the container typically increases to 33 to 52%, as an average of both 
layers. 
The packaged food can be a full-moisture food, e.g., having a normal 
moisture content of greater than 40% by weight, typically for dishes 
containing meat, vegetables, or pasta and sauce, from 65 to about 85% by 
weight. The moisture content and water activity are not important because 
the package of the invention will protect even higher moisture foods at 
water activities of 0.85 and above for extended periods of 
room-temperature storage. Storage for greater than one month is typically 
required for commercial distribution and minimum stabilities of from three 
to twelve months are desired. Preferably, shelf-stability for eighteen 
months is desired. In practical terms, the product should remain stable 
even beyond these time periods. 
To prepare the packaged food for consumption, it is necessary only to heat 
it in a microwave oven to a desired serving temperature. The package will 
self-vent. During microwave heating steam pressure is created within the 
package. This pressure is permitted to build within the package, which has 
a dimensionally-stable container portion sealed to a non-extensible lid, 
until it becomes sufficient to rupture the seal, usually at a single 
location such as 40 in FIG. 2. This typically occurs at a corner of the 
rim. The remainder of the seal is easy to peel such that the lid is 
readily separated from the container after the package is removed from the 
microwave oven. Thus, while the peel strength is from 3 to 10 pounds per 
inch at room temperature, it will be less than 2 pounds per inch at 
serving temperature of 140.degree. to 200.degree. F., preferably less than 
1.5 pounds at 200.degree. F. 
The invention provides a number of advantages. In addition to superior 
convenience to the consumer, the invention provides high overall energy 
efficiency and recyclability of the materials employed. The invention 
eliminates the need for special venting accessories, i.e., a plastic dome, 
useful only for heating. It eliminates the high energy consumption 
required for freezing foods, maintaining them frozen, and then thawing. 
The invention enables preparation from room temperature storage to serving 
temperature by simply placing the packaged food in a microwave oven. The 
invention provides long periods of shelf stability during which the food 
contacts only a virgin resin container surface. The lid does not have to 
be punctured, partially or totally removed, replaced or otherwise treated. 
The package is heated intact in a microwave oven. There is no need for 
special handling of the package prior to heating, special programming of 
the microwave oven, or special handling subsequent to heating. 
As another advantage, handles 29 and 29' may be shielded from microwave 
energy and remain cooler when a foil is employed in the lid and the lid 
extends to cover the handles. 
The following Example is provided to further illustrate and explain a 
preferred form of the invention and is not to be taken as limiting in any 
regard. Unless otherwise indicated, all parts and percentages are by 
weight. 
EXAMPLE 
A package as shown in the drawings is prepared from a two-ply container 
stock and a six-ply lid stock. The oval container portion has a recess 
with a length at the top of approximately 6.2 inches, a width of about 
4.25 inches, and a depth of about 1 inch. The bottom is raised by about 
3/32 inch. The angle of the side wall with the vertical is about 
15.degree.. A sealing rim (approximately 0.2 inch) surrounds the recess. 
Handles extend slightly at opposed ends, centered on the long axis. 
The two-ply container stock is prepared by coextrusion of crystallizable 
and amorphous polyethylene terephthalate layers after drying the resin and 
additives to less than 50 ppm residual moisture. The resin for the 
amorphous ply is virgin polyethylene terephthalate (I.V. 0.95, number 
average molecular weight of 32,500, weight average molecular weight of 
65,000, and crystalline peak melting point 245.degree. C.). The resin for 
the crystallizable ply is the same resin but with about 5% of a 
combination of polypropylene nucleating agent, container stock rework and 
a colorant comprising a color-coated fine particulate titanium dioxide. 
The resins are separately fed as melts to a coextrusion die from which 
they are extruded at about 560.degree. F. and chilled to form a composite 
sheet with a thickness of about 0.04 inch, with the crystallizable portion 
being about 0.028 inch and the amorphous portion being about 0.012 inch. 
The container stock is positioned with the pigmented, crystallizable ply 
nearest the interior of a vacuum forming mold, heated to about 125.degree. 
C., and drawn upwardly into the mold by vacuum with the aid of air flow 
from a plug assist. The formed sheet is then moved to a cooling mold and 
rapidly cooled to maintain heat-sealability for the inner ply and inhibit 
brittleness in the outer ply. 
A lid stock is prepared having a 1 mil thick aluminum foil layer sandwiched 
between two layers of 0.00048 inch thick oriented polyethylene 
terephthalate film, the layers adhered with a polyurethane adhesive 
applied at a thickness of 1 pound per 1,000 square feet. One side of this 
laminate is corona treated, and a heat sealing coating is applied as a 
suspension to achieve a dry thickness of 1.4 pounds per 1,000 square feet. 
The lid stock is cured for about seven days at 55.degree. C. The 
suspension comprises 32% solids (X17-3 available from Morton Chemical, a 
blend of copolyester resins with inorganic filler) in toluene. 
A serving of lasagna with a tomato sauce is deposited in the container 
recess, the container void space is flushed with nitrogen and then 
evacuated. The lid stock is heat sealed to the container rim by brief heat 
and pressure application to the lid stock against the rim. The sealed 
container is then retorted by raising the temperature over one half hour 
to 120.degree. C., holding at that temperature for about forty five 
minutes and then cooling to about 35.degree. C. over about one half hour. 
During heating and cooling, the sealed package is maintained under water, 
heated with steam and maintained at an external pressure effective to 
balance the pressure developed within the package. The peel strength of 
the lid seal is about 5 pounds per inch. 
After cooling and shelf storage, the package is placed in a 500 watt 
microwave oven and heated for 2 minutes, during which time a portion of 
the seal is ruptured at one corner. The lid is then easily removed by 
peeling to provide a meal portion ready for serving. 
The above description is for the purpose of enabling the person skilled in 
the art to make and use the invention and is not intended to describe each 
and every modification and variation of it which will be obvious to the 
skilled worker upon reading. All such modifications and variations are 
intended, however, to be included within the scope of the invention which 
is defined by the following claims.