Note: Descriptions are shown in the official language in which they were submitted.
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MICROWAVE CORN POPPING PACKAGE
FIELD OF THE INVENTION
The invention relates to microwave packaging and
especially to a package suited for popping'popcorn in a .
microwave oven.
BACKGROUND OF THE INVENTION
U.S, patents 4,861,958 and 4,678,882 describe a
popcorn popping container in the farm of a paperboard cup .
with a microwave lossy element added to the bottom of the
cup either as a separate disc or as a laminate and with a
lid having apertures for releasing moisture produced
during popping. While the lossy element is intended to
assist in popping the corn, neither the volume of popped
corn produced nor the percentage of unpopped kernels have
been as good as desired. Users are particularly
disturbed when a substantial number of unpopped kernels
or "old maids" remain after the popping process. It is
therefore one object of the invention to make an
improvement in the volume of popped corn and unpopped
kernels produced in a package having the general form
described in the aforesaid patents.
Patent 4,586,649 describes a popping package in
which the bottom of the package slopes centrally toward a
central panel of small size. The carton resembles two
truncated pyramids which are joined base-to-base and
supported on a separate stand in the form of a
collapsible tube. The sloping lower portion of the
carton causes the corn to bunch together in a clump. In .
addition, the supporting stand must be manipulated by
hand. This requires extra attention by the user and in
the present state of development of the art is considered
to be unsatisfactory because of the extra,time, care and
attention needed in setting up the package for popping.
Moreover, many users have trouble following directions.
Popping will be less satisfactory because the package
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also contains no susceptor. Finally, the distance
between the package and the floor of the oven varies from
one point to another because the bottom part of the
package is sloped.
It is also known to provide a foil pan with an
expandable cover for popping popcorn on a stove top.
These packages are not suitable for use in a microwave
oven because the metal foil pan acts as a shield.
Examples are patents 3,519,439; 3,782,976 and 3,969,535.
Another popcorn package is described in patent
4,584,202. This package includes a paperboard carton
with a top panel having a removable portion that is
detached from the rest of the package just prior to
popping. The unpopped popcorn is placed in a separate
pouch which as it expands is pushed outside the carton
through the top opening. The package contains no
susceptor, and accordingly, popping of the corn will
proceed at a relatively slow rate and may be incomplete.
The use of a separate inner pouch makers the container
more expensive. By now, the art has developed to a point
at which the removal of a panel prior to popping is a
deficiency that may render the produce unacceptable to
some users. In the course of developing the present
invention it was found that the large bubble of
superheated steam which collected under the flexible
cover could burn the fingers when the package was opened.
An attempt was made to vent the steam through vent holes.
Vent holes, however, allow moisture to enter the corn and
reduce popping performance. Moisture could also enter to
a degree even without vent hales. Changes in corn
moisture due to the gain or loss of moisture from the
atmosphere was found to be a troublesome obstacle to good
popping performance, both with respect to the number of
unpopped kernels remaining as well as the volume of
popped corn produced.
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It is therefore a major object of the invention to
provide an improved microwave popping package which
requires no manipulation prior to use, from which popped
corn can be eaten directly, which is capable of being -
stacked one inside another, and which provides a
substantial improvement both in the volume of popped corn
produced and also in the number of kernels that remain
unpopped. A further object is to provide a popping
package of the type described having a microwave
,interacti've susceptor material distributed in heat
conductive relationship with the popcorn and to find a
way to strategically locate the entire susceptor in
relationship to the supporting surface of the oven. ,
Another object is to provide a spacing or gap between the '
susceptor and the oven floor that is related to the
electrical characteristics of the oven for maximum energy
transfer to the susceptor. It is also an object to find
a way to provide a reliable hermetic seal for the corn to
promote good popping of the corn a5 well as to vent steam
without allowing moisture to enter the food through the
vent holes.
These and other more detailed and specific objects
of the invention will be better understood by reference
to the following detailed description and figures which
illustrate by way of example but a few of the various
forms of the invention within the scope of the appended
claims.
THE FIGURES
Figure 1 is a perspective view of the invention
partly broken away;
Figure 2 is a partial vertical sectional view of a
portion of the side and bottom wall of the package;
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Figure 3 is a central vertical sectional view of the
package of Figure 1;
Figure 4 is a partial vertical sectional view of the
center portion of the package on a greatly enlarged
scale;
Figure 5 is a vertical sectional view of another
form of the invention; and
Figure 6 is a partial vertical sectional view of
another form of the invention.
SUMMARY OF THE TNVENTION
The invention provides a package for popping popcorn
in a microwave oven. One major component is an open-
topped wide mouth container in the form of a tub or bowl
which is formed from self-supporting, preferably stiff
microwave transparent sheet material such as paper or
paperboard. The tub includes an upstanding side wall
terminating at the top in a free edge which defines an
upper wide mouth. The side walls of the tub slope
outwardly to permit stacking of a plurality of such
packages within one another during shipment and storage.
The side walls have a lower edge which rests on the floor
of the microwave oven during cooking.
The tub also includes a flat bottom wall. Above the
bottom wall is a first compartment containing unpopped
popcorn and optionally a quantity of vegetable oil or
shortening. The bottom wall is connected to the side
wall at a point spaced upwardly from its bottom edge so
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as to define a second compartment within the package
providing an air space between the bottom wall of the
package and the oven floor that has a substantially
uniform height throughout. A layer of microwave
5 interactive susceptor material is connected to the bottom .
wall and is distributed across the bottom wall so that
the entire susceptor has a constant gap from the bottom
wall of the oven to assist in popping the popcorn. The
uniform height of the air gap between the susceptor and
the oven floor facilitates the efficient transfer of ,
microwave energy via the susceptor to the corn for
enhancing popping of the corn.
The lower edge of the side wall of the tub is
uninterrupted to make possible a sealing engagement with
the floor of the microwave oven so that the air in the
compartment is held below the package. As a result, the
air in the air gap becomes heated above the temperature
of the air in the oven which surrounds the package. This
assists in the transfer of microwave energy to the
susceptor. The unpopped popcorn contained in the upper
compartment is in heat conductive relationship with the
susceptor. A flexible cover is sealed to the top of the
tub aid expands upwardly as the kernels pop.
The susceptor can comprise any of a variety of
microwave interactive materials such as a thin layer of
metal, e.g. vapor deposited metal, metal oxide, carbon
and the like known to the art. The susceptor can be
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applied directly to the bottom wall of the tub or can be
supported upon a sheet of paper or plastic which is
subsequently bonded to the bottom wall of the tub. One
potentially suitable susceptor is a thin layer of plastic
or paper having on it a dried layer of a liquid microwave
interactive coating containing a microwave interactive
component, e.g, carbon particles bonded together with a
film former or matrix and microwave transparent
electrically nonconductive attenuator particles. The
susceptor layer can also be composed of a self-supporting
coating adhered to the bottom wall and containing
microwave interactive particles supported within a cured
binder or matrix which holds the particles together and
also bonds the susceptor coating to the surface of the
bottom wall. Examples of such coatings are described in
patents 4,26?,420 and 4,190,757 and USA patent
application serial numbers 456,159 filed 12/22/89 or
261,380 filed 10/24/88.
In another form of the invention, the food is held
in a sealed compartment, e.g, a hermetically sealed
compartment, and vent means are provided in a second
compartment separated from the sealed compartment by a
releasable seal. In one preferred package the food is
held in a rimmed dish or tray at the bottom of the tub or
forming the bottom wall thereof. The cover is a flexible
plastic film releasably sealed to the rim of the dish.
During heating the seal is released, allowing the
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flexible cover to expand upwardly. A circular lid can be
sealed to the dish beneath the cover.
DETAILED DESCRIPTION OF THE INVENTION '
As shown in the figures, the package 10 comprises a
bowl or tub-shaped container having an upstanding side '
wall 12 and a horizontal bottom wall 16 both of which can
be farmed from a stiff or semi-flexible material such as
paper or paperboard. The side wall 12 is in this case
circular, but other shapes are possible. As shown in
Figures 1 and 2, the top edge of the tub is turned
outwardly at 14 to define a reinforcing lip forming a
circular wide mouth at the top of the side wall 12. The
15' bottom wall 16 which is also circular is spaced upwardly
from the oven floor 18 by a distance 20. The bottom wall
16 is thus bonded to the side wall 12 at a point above
its lower edge 21. The bottom wall 16 can be made of
stiff or flexible material such as paperboard or paper.
As shown, its outer edge is folded downwardly and bonded
to the side wall 12. Above the bottom wall 16 is a first
compartment within the package which contains unpopped
popcorn optionally mixed with a quantity of cooking oil
or other shortening, indicated generally at 24.
The cover 12' consists of a circular sheet of
flexible microwave transparent material (Figure 3) having
a portion 26 at the edge which is bonded by means of a
suitable adhesive to the rim 14. The cover 12' has a
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peripheral portion 28 which conforms to the inner surface
of the side wall 12 and a central portion 30 which rests
against the upper surface of the charge of unpopped
popcorn and shortening 24. During popping, the cover 12'
expands upwardly as the kernels pop until it reaches a
position approximately at the elevation of the upper edge
of the rim 14 or slightly above it. The size of the tub
and the charge of popcorn 24 are proportioned so that the
popped corn will approximately fill or slightly exceed
the volume of the tub. To serve the popcorn, all that is
necessary is to remove the cover 12' by separating it
from the upper rim 14 of the side wall 12. The tub then
functions as a serving dish or bowl from which the
popcorn can be eaten directly.
Since the side wall 12 of the tub is sloped out-
wardly, several of such packages can be stacked within
one another during shipment and storage.
A suitable microwave interactive susceptor 22 is
bonded to the bottom wall 16 of the tub and distributed
across the center portion but not the edge portion that
is folded downwardly and bonded to the lower part of side
wall 12. It will be seen that both the bottom wall 16
and the susceptor 22 are positioned horizontally during
use and are parallel to the floor of the oven 18. It
will also be seen that the spacing between the bottom
wall 16 and the bottom edge 21 of the side wall 12
defines a second compartment within the package below the
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bottom wall 16 to provide an air space between the bottom
wall 16 of the package and the oven floor 18. This air
space has a substantially uniform height throughout. As
a result, the microwave interactive susceptor 22
connected to the bottom wall is positioned such that
substantially the entire susceptor 22 has a constant gap
G from the floor 18 of the microwave oven.
The lower edge 21 of the side wall 12 is uninter-
rupted to provide a sealing engagement with the floor of
the microwave oven. As a result, air in the lower
compartment is held within the confines of the side wall
12 and below the bottom wall 16. Air in the lower
compartment will become heated during cooking above the
temperature of the air in the oven. Because the gap G is
of constant height their will be provided beneath the
susceptor 22 a heated layer of air that serves as a
dielectric layer having a constant height (gap G)
throughout the entire susceptor 22. It is known that the
dielectric constant of a substance will affect the
microwave energy distribution through it. When the air
in the gap G becomes heated, its dielectric constant will
change primarily as a result of the expansion due to
heating and the resulting reduction in the moisture
content of the heated air. It is believed that the
uniformity of the heated air layer in the gap G acting
with the reduction in the dielectric constant of the air
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WO 91 /15096 PCT/US91 /0133 % .
as it becomes heated helps to assure maximum microwave
energy transfer to the susceptor 22.
Because microwave energy is reflected from surfaces,
it has been found that energy peaks or nodes can occur at
5 predetermined distances from a surface, as for example a
predetermined distance from an oven wall that is
partially or completely reflective of microwave energy.
In one form of the invention bottom wall 16 and
susceptor 22 are placed at a strategic distance above the
10 bottom edge 21 of the side wall 12 which is related to
the electrical characteristics of the microwave energy.
For example, in one form of the invention, the gap G is
about one-eighth wavelength of the microwave energy
supplied to the oven so that when the oven has a
wavelength of 12 cm (which is typical of almost all
ovens), the gap G is about 1.0 cm to 1.5 cm and
preferably about 1.25 cm. In this way substantially the
entire susceptor 22 has a constant gap G between itself
and the floor of the oven 18, namely, about one-eighth of
the wavelength of the microwave energy supplied to the
oven. With respect to energy reflected upwardly onto the
susceptor 22, there can thus be an energy reinforcement
region or node which coincides with the location of the
susceptor 22 which helps further to transfer the
microwave energy efficiently to the susceptor 22 and then
to the popcorn.
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WO 91 / 15096 PCT/US91 /0133 i
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The effectiveness of the invention is shown in the
following comparative examples. In each example a tub
was formed from food grade paperboard. A cover was
formed from a sheet of polyester film bonded with
adhesive to the rim of the tub. The tubs were each
loaded with 56 grams of unpopped popcorn and 23.5 grams
of vegetable oil. In each case popping was accomplished
in a 700 watt Litton microwave oven for 2 minutes and 35
seconds.
Example 1 - The Invention
This example demonstrates the results achieved with
the invention as shown in the figures. The bottom wall
16 was placed about 7/16 inches (1.25 cm) above the lower
edge 21 of the side wall 12 to provide a gap G between
the susceptor 22 and the floor of the oven 18 of 1.25 cm.
The oven had a wavelength of 12 cm. The results obtained
in all examples are shown in Table 1 below.
Comparative Example 2
A package was prepared the same as in Example 1
except that the portion of the side wall 12 extending
below the bottom wall 16 was removed so that the bottom
wall and susceptor rested against the floor of the oven.
Comparative Example 3
A tub was prepared as in Example 1 except that
portions of the side wall 12 below the bottom wall 16
were cut away and removed to leave four spaced apart
WO ~~~~~'~ ~ ~ PCT/LS91/01337
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12 '
remnants of the side wall which served as legs to hold
the susceptor 22 the same distance from the oven floor as
in Example 1 but allowed air to circulate freely beneath
the susceptor.
Comparative Example 4
A sample was prepared as in Example 1 except that no
susceptor was provided. The results are shown in the
following table.
R E S U L T
S
TABLE 1
Volume of Unpopped Unpopped
Popped Corn Kernels Kernels
(ml) (a) (~)
Example 1 - Invention 2100 1.04 1.9
(G - 1.25 cm)
Comparative Example 1900 7.43 13.3
2:
Bottom wall rests
on oven floor
(G = 0 cm)
Comparative Example 1950 4.05 7.2
3:
Air allowed to cir-
culate below susceptor
(G = 1.25 cm)
Comparative Example 1950 4.80 8.6
4:
No susceptor
(G - 1.25 cm)
As can be seen in the table, the volume of popped
corn that resulted from the invention was 2100 ml, or 150
ml greater than examples 3 and 4 and 200 ml greater than
Example 2. This improvement provides the consumer with a
substantially greater volume of popped corn. Even more
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importantly, the invention results in only 1.9$ unpopped
kernels. By comparison, Examples 2, 3 and 4 result in a
much greater percentage of unpopped kernels, namely
13.3$, 7.2$ and 8.6$, respectively. In other words, the
number of kernels that remained unpopped in Example 3 is
about 370$ greater than the invention, and Example 4 is
about 450$ greater than the invention.
The air in the lower compartment beneath the
susceptor in Example 1 reached a temperature of 198.4'C
after being heated for 2 minutes, 35 seconds. When no
susceptor was used, the air in the compartment reached a
temperature of 128.7°C. In the meantime, the air 'in the
oven outside the package was raised from a temperature of
about 20°C before cooking to about 49.5°C. This shows
that the air in the compartment beneath the susceptor is
about 195°C hotter than the air in the oven and about 70°C
hotter than achieved with a similar package having no
susceptor. This provides a change ~n the dielectric
constant of the air through a gap of uniform height G
between the susceptor and the oven floor which assists in
the efficient transfer of microwave energy to the
susceptor and to the popcorn.
It will be noted that the package does not have to
be manipulated in any way prior to popping. It also
provides a package from which the popcorn can be easily
consumed directly, i.e. without the necessity of
transferring it to another container. In addition, the
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popping performance as measured by the volume of popped
kernels and the reduction in the number of unpopped
kernels is substantially improved.
Refer now to Figure 5 which illustrates a modified
form of the invention wherein the same numerals refer to
corresponding parts already described. As shown in the
figure, the package indicated generally at 40 includes a
circular sidewall 12 which tapers outwardly proceeding
toward the top edge 41 which defines an upper open wide
mouth similar to that already described. It will be
noted that the upper edge of the flexible cover 12' is
sealed at 43' to the upper edge of the side wall 12 to
provide a first seal for the cover 12'. The package 40
also includes a bottom wall 16 having a downwardly turned
peripheral collar 20 that is sealed to the lower edge
portion 21 of the side wall 12 to define an air chamber
of constant height for providing an air gap G between the
floor of the oven 18 and the bottom wall 16 of the
container. In this case, food, e.g, a popcorn-
containing dish 42, is provided within the package 40 .
with its lower surface resting on the bottom wall 16.
The dish 42 can be composed of any suitable microwave
transparent self-supporting material such as paper,
paperboard or plastic. In the form shown, the dish 42
comprises a pair of inner and outer paper or paperboard
layers 44, 46. The outer paperboard layer 46 in this
case comprises a 282 lb. per ream paperboard layer, while
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WO 91 / 15096 PCT/ US91 /01337
the inner paper layer comprises 21 lb. greaseproof paper.
Between the layers 44 and 46 is an intermediate susceptor
layer 48 composed in this case of plastic film, such as a
0.48 mil metallized polyester film which is
5 semiconductive and adapted to absorb microwave energy to
heat the food (popcorn) indicated at 50. The susceptor
48 can comprise any of the susceptor materials described
above or generally known to those skilled in the art.
One suitable susceptor comprises a semiconductive
10 aluminum layer applied by vacuum electrodeposition to one
surface of a 2 mil polyester film. The layers 44-48 of
the dish are bonded together with a suitable adhesive
such as a polyvinylacetate emulsion adhesive, e.g.
Duracet-12 adhesive by Franklin International, Inc. of
15 Columbus, Ohio.
It will be noted that the dish 42 includes a bottom
wall 53, an outwardly tapering sidewall 52, and a
horizontal circular rim 54. In one preferred form of the .
invention, only the bottom wall 53 and the rim 54 are
provided with microwave absorbing susceptor material, the
sidewall 52 being free of susceptor material, e.g. by
having the susceptor substance, e.g. metal, removed from
susceptor layer 48.
Between the rim 54 and the center portion 30 of the
flexible cover 12' is a second seal, in this case an
annular heat-releasable seal 56. The adhesive 56 can be
any suitable thermoplastic adhesive which is adapted to
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16
soften at elevated temperature. One suitable adhesive is
a temperature activated, i.e. hot-melt, adhesive
designated Scotch Pack~ #122 by The 3M Company of
St. Paul, Minnesota, which is activated at about 225°F to
375°F. It will be noted that the second cover seal 56
extends all the way around the circular rim 54 at the
mouth or upper edge of the dish 42. The releasable seal
56 thus provides a hermetic seal for the food-containing
space to reduce moisture transfer to or from the popcorn
50. It has been found that the popcorn 50 will, as a
result, keep its good popping qualities after being
stored for a substantial period of time.
Located in the flexible cover 12' just above the
dish 42 is an empty, in this case collapsed, compartment
51 (located between the wall 12 and the cover 12') having
a plurality of steam vent openings 60, e.g. 1/l6th inch
diameter holes. It will be seen that since the steam
vent openings 60 are in the compartment 51 above the seal
56, they do not provide a path for moisture to enter~or
leave the sealed dish 42.
To use the package of Figure 5, it is placed in a w
microwave oven and exposed to microwave energy. This
heats the popcorn 50, causing it to pop. Additional heat
is provided by the susceptor 48 in the dish 42 to enhance
the popping of the corn. The bottom of the dish 42 will
typically become heated to between 300°F and 450°F by the
susceptor layer 48. In addition, the susceptor material
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48 within the rim 54 will heat the releasible adhesive in
seal 56, allowing the cover 12' to rise as moisture vapor
expands within the package during the heating process.
It should also be noted that once the seal 56 is
broken, the evolved steam can easily escape through the
vent holes 60. Prior to this time, however, the hermetic
seal 56 between the cover 28 and the periphery of the
food containing compartment of the dish 42 will prevent
moisture transfEr to or from the popcorn 50 so as to
assure reliable popping. In addition, the controlled
venting of the container made possible by the openings 60
dissipates the bubble of steam which otherwise forms in
the container and, if present, could burn the fingers as
the package was opened. It can also be seen that the
sidewall 12 of the tub itself does not have to be a vapor
barrier since the popcorn 50 is enclosed between the
center portion 30 of cover 12' and the dish 42. The tub
can consequently be made of a less expensive material.
Refer now to Figure 6. The package of Figure 6 is
generally similar to Figure 5 except that the bottom wall
16 has been eliminated by providing a downward cir-
cular collar 64 at the outer edge of the rim 54. The
dish in this case is designated 43. Additionally, a
circular lid 66 of any suitable barrier material such as
plastic film, e.g. polyvinyladene coated polyester film,
is sealed to the rim 54 by means of the releasable seal
56 comprising any suitable thermoplastic adhesive. The
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18
lid 66 can be composed of any other suitable plastic
material known fox its low moisture vapor transmissivity
to reduce moisture vapor transmission to or from the
popcorn 50. The cover 12', however, can be composed of a
less expensive plastic material which need not have vapor
barrier qualities. While it is not essential, it is
preferred that the flexible cover 12' be bonded, e.g. by
means of adhesive 67, to the edge or to other portions of
the lid 66.
The package of Figure 6 operates in the same manner
as that described in Figure 5. When the microwave energy
heats the package and pressure develops within the
chamber containing the popcorn 50, the seal 56 will
eventually rupture, allowing the cover 12' to rise
upwardly due to the expansion of hot air and vapor within
the package. As the cover 12' rises to accom-
modate the expansion of the popping corn, steam escapes
through the vent openings 60. Prior to popping, however,
the popcorn 50 within the dish 43 is hermetically sealed
to insure good popping characteristics.
The embodiment of Figure 5 requires somewhat more
material than that of Figure 6 but can be assembled more
readily from preformed tubs and will usually be somewhat
stronger due to the presence of the bottom wall 16. The
embodiment of Figure 6, however, requires less material
and is therefore lower in cost.
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19
Many variations of the present invention within the
scope of the appended claims will be apparent to those
skilled in the art once the principles described herein
are understood.
