Note: Descriptions are shown in the official language in which they were submitted.
CA 02564627 2006-10-30
MICROWAVA)itj~E PACK~,CrE
Fieln~Of The ~(nventian
The present invention relates to packages foc food products and in
particular to a microwavable package and an active microwave energy heating
element for the same.
Background Of The Invention
Microwave ovens have become a principle form of cooking food in
a rapid and effective manner and the number of food products available for
preparation in a microwave oven is constantly increasing. As the market for
microwavable food products has increased, so the sophistication required from
such food products has also increased. There is, therefore, a continuing
demand
to improve the quality of food prepared in a microwave oven and to ensure that
IS when it is presented to the consumer, the food product is attractive and
meets the
standards normally associated with such food.
Foods that are specially prepared for cooking within a microwave
oven are delivered to the consumer in containers that may be used directly
within
the microwave oven to facilitate preparation. These containers must therefore
not
only be capable of containing the food product during transport in an
effective
manner but must also be capable of contributing to the cooking of the food
product
within the microwave oven and the subsequent presentation of the food product.
As the demand for more sophisticated food products increases, so
the demand for effects, particularly appearance, normally associated with food
preparation also increases. For example, it is desirable for a food product
that
includes a pastry shell or lid to have a browned appearance, so that it
appears to
have been baked. While these effects can be produced in isolation, it becomes
more difficult to produce such an effect in combination with a container that
can
also uniformly heat the food product within a time that offers advantages over
conventional cooking techniques.
Typically, the areas in which browning or crisping are required are
those on the outer surfaces of the food product. Those areas typically receive
the
highest proportion of incident microwave radiation and therefore cook or heat
the
CA 02564627 2006-10-30
2._
quickest even though the power distribution is very non-uniform over these
surfaces. On the other hand, there are areas of the food product that are
relatively
shielded from incident microwave radiation or exist in a region of a minimum
RF
field and which therefore require longer cooking periods. If, however, a
longer
cooking period is provided, the outer surfaces of the food product tend to
char and
burn, leading to an unacceptable food product.
Various attempts have been made in the past to provide containers
that will, produce effects normally associated with cooked foods. For example,
U.S. Patent No. 5,322,984 to Habeger, Jr. Et al. and assigned to The James
River
Corporation suggests a container having heating devices on the bottom wall and
possibly the top wall of the container. The heating devices are designed to
provide a charring effect normally associated with barbecuing by directing
energy
normally not incident upon the food prcxluct into specific regions. This is
purported to produce a localised charring of the food product. Overall,
however,
!5 such containers have not been successful. The charring effect produced on
the
food product may be attributed to the high field intensities and associated
induced
currents that result from the concentration of energy at particular Locations.
In
practice it is found that those induced currents may also cause charring and
burning of the container itself.
U.S. Patent No. 4,927,991 to Wendt et al and assigned to The
Pillsbury Company discloses a microwavable package for foodstuffs and in
particular pizza. The package includes a tray on which a grid in combination
with
a susceptor are located. The grid and susceptor combination act together as a
microwave energy heating element. The package also includes an aluminum top
having apertures provided in it. The apertures allow microwave energy to
penetrate the top thereby to heat the focxistuff.
It has also been found that in order to produce the required results
for the preparation of the food product, the container must be capable of
controlling distribution of energy about the food product, to utilize the
energy in
the most efficient manner, and at the same time ensure that the food product
and
the container provide a pleasant and acceptable finished product. Also, the
containers must be able to hold the food product securely to avoid damage to
the
CA 02564627 2006-10-30
- 3 ~-
food product during transport. It has been found that in the case of pizza
containers, conventional designs have not been adequate resulting in
separation
between the pizza crust and the toppings during transport.
It is therefore an object of the present invention to provide a novel
S food product package and active element for the same which obviates or
mitigates
at least one of the above disadvantages.
Summary Of The Invention
According to one aspect of the present invention there is provided a
IO microwavable package comprising:
a base to support a food product:
an active microwave energy heating element on said base to effect
heating of a ford product upon impingement by microwave energy; and
a cover spaced from said active microwave energy heating element
15 to overlie said food product, said cover including a microwave energy
interactive
material layer extending substantially over said food product, and a plurality
of
apertures in said microwave energy interactive material spaced about a
peripheral
margin of said cover, said apertures being sized to promote localised fields
to
promote browning of said food product.
20 In one embodiment, the apertures are in the form of elongate slots
arranged in concentric rings. Microwave energy interactive material islands
may
be located within the slots to enhance further the cooking performance. In
this
embodiment, the active microwave energy heating element includes a plurality
of
energy collecting structures, each energy collecting structure having resonant
25 loops. The resonant loops have a perimeter sufficient to Iimit currents
induced
therein to below a predetermined level upon impingement by incident microwave
energy. The energy collecting structures distribute energy towards a central
region of the food product to heat the food product generally uniformly and to
inhibit charring of the base. In one form, the active microwave energy heating
30 element further includes tuned structures at spaced locations each of which
is
located between a pair of the resonant loops.
According to another aspect of the present invention there is
CA 02564627 2006-10-30
-4-
provided a microwavable package comprising:
a base to support a food product;
an active microwave energy heating element interposed between said
food product and said base to effect heating of said food product upon
impingement by microwave energy; and
a cover spaced from said active microwave energy heating element
to overlie said food product, said cover including a substrate and microwave
energy interactive material on said substrate to cover at least a portion of
said food
product, said substrate extending beyond the peripheral edge of said microwave
IO energy interactive material to isolate electrically said base and said
cover.
According to still yet another aspect of the present invention there is
provided a packaged food product comprising:
a base to support said food product;
a flexible cover to overlie and conform to said food product; and
a flexible wrap to constrain said base and cover and inhibit relative
movement therebetween.
According to still yet another aspect of the present invention there is
provided an active microwave energy heating element for a microwavable package
to heat generally uniformly a food product within said package, said active
microwave energy heating element comprising:
a plurality of energy collecting structures, each of said energy
collecting structures including resonant loops having a perimeter sufficient
to Iimit
currents induced therein to below a predetermined level upon impingement by
incident microwave energy; and
a plurality of tuned structures at spaced locations and positioned
between adjacent resonant loops, said energy collecting and tuned structures
distributing energy across said active microwave energy heating element to
heat
generally uniformly said food product and inhibiting charring of said
microwavable
package.
In still yet another aspect of the present invention there is provided
a microwavable package comprising:
CA 02564627 2006-10-30
_ 3
a tray having a base and an active microwave energy heating
element on said base to effect heating of a food product on said tray upon
impingement by microwave energy; and
a plurality of spaced apertures in said tray to permit moisture
released from a food product to pass through said tray.
The present invention provides advantages in that the microwavable
package design is such to heat generally uniformly the food product while
browning the outer periphery of the food product. This design is particularly
suited to cooking pizzas.
Brief Descriution Of The Drawings
Embodiments of the present invention will now be described more
fully with reference to the accompanying drawings in which:
Figure 1 is an exploded side elevational view of a microwavable
package in accordance with the present invention;
Figure 2 is a top plan view of a tray having an active microwave
energy heating element thereon for the microwavable package of Figure 1;
Figure 3 is cross-sectional view of Figure 2 taken along line 3-3;
Figure 4 is a top plan view of a cover forming part of the
microwavable package of Figure 1;
Figure 5 is a cross-sectional view of Figure 4 taken along line 5-5;
Figure b is a top plan view of an alternative embodiment of a cover
for a microwavable package in accordance with the present invention;
Figure 7 is an enlarged pan cross-sectional view of Figure 6 taken
along line 7-7;
Figure 8 is an enlarged top plan view of a portion of Figure 6;
Figure 9 is a top plan view of yet another alternative embodiment of
a cover for a microwavable package in accordance with the present invention;
Figure 10 is a top plan view of an alternative embodiment of a tray
having an active microwave energy heating element thereon for a microwavable
package in accordance with the present invention;
Figure 11 is a top plan view of another alternative embodiment of a
CA 02564627 2006-10-30
-6=
tray having an active microwave energy heating element thereon for a
microwavable package in accordance with the present invention;
Figure 12 is a top plan view of yet another alternative embodiment
of a tray having an active microwave energy heating element thereon for a
S microwavable package in accordance with the present invention;
Figure 13a is a top plan view of still yet another alternative
embodiment of a tray having an active microwave energy heating element thereon
for a microwavable package in accordance with the present invention; and
Figure 13b is a cross-sectional view of Figure 13a.
Detailed Description Of The Preferred Embodiments
Referring now to Figure l, a microwavable package for a food
product is shown and is generally indicated to by reference numeral 10. The
package 10 in this particular example is best suited to contain uncooked
pizzas
having raw dough crusts.
As can be seen, in this particular example the package 10 includes a
tray 11 having a base 12 formed of suitable material such as for example,
paperboard. The base is in the form of a circular disc sized to the dimension
of
the food product to be held in the package 10. The base can of course take
other
geometric shapes if desired. An active microwave energy heating element 14 is
bonded or adhered to one surface of the base I2. The food product, in this
case a
pizza 16, contacts the microwave energy heating element and is supported by
the
base 12. A flexible cover 18 overlies the top of the food product 16 and
conforms
with its surface. The cover 18 can be folded at its periphery to overlie at
least
part of the sides of the food product. A plastic wrap 20 encompasses the base
12,
cover 18 and food product 16 to maintain the base 12 and cover 18 in secure
contact with the food product 16 and inhibit relative movement therebetween.
Referring now to Figures 2 and 3, the active microwave energy
heating element 14 is better illustrated. As is shown, the microwave energy
heating element 14 is in the form of a laminate 30 and includes a substrate 32
formed of suitable material such as for example paper, paperboard or polymeric
film. One surface 32a of the substrate is adhered to the base 12 and an
opposed
CA 02564627 2006-10-30
_ ').
surface 32b has a pattern 34 of microwave interactive material deposited
thereon.
The microwave energy interactive material 34 may be electroconductive or
semiconductive material such as metal foil, vacuum deposited metal or metallic
ink. The electroconductive material is preferably aluminum although other
metals
such as copper may be employed. In addition, the electroconductive material
may
be replaced with a suitable electroconductive, semiconductive or non-
conductive
artificial dielectric or ferroelectric. Artificial dielectrics comprise
conductive
subdivided material in a polymeric or other suitable matrix or binder and may
include flakes of electroconductive metal such as aluminum.
A susceptor 36 including at least one layer of suscepting material
covers the microwave energy interactive material 34 and the substrate 32 and
produces.a heating effect upon excitation by incident microwave energy as is
well
known. The susceptor 36 may be in the form of a printed ink or alternatively,
a
coating sputtered or evaporated over the active element 14. The susceptor 36
may
not be utilized or additional layers of suscepting material may be provided
depending on the heating effect required.
The pattern of microwave energy interactive material 34 and
susceptor 36 constitute a microwave energy controlling structure which permits
a
controlled degree of penetration of incident microwave energy through the base
12
and channels microwave energy towards a central region of the food product.
Specifically, the design of the active microwave energy heating element 14
moderates penetration of microwave energy in the peripheral region of the food
product 16 and directs microwave energy towards its central region. This
allows
the food product to cook more uniformly.
Looking at the pattern of microwave energy interactive material 34
more closely, it can be seen that the pattern includes a plurality of
circumferentialiy spaced transmission elements 40 arranged in a ring about a
circular island 42 positioned at the center of the microwave energy heating
element
14. Each transmission element 40 includes a pair of resonant loops 44
interconnected by a pair of transmission lines 46. In this particular example,
the
loops 44 are generally circular. The loops 44 have a perimeter sufficient to
limit
currents induced therein to below a predetermined level and which is as close
to
CA 02564627 2006-10-30
_ 8 .-
an integer multiple of the effective wavelength of the incident microwave
energy.
The loops 44 are tuned to collect microwave energy from the
peripheral region of the microwave energy heating element 14 and distribute
the
energy to a central region of the food product to heat the food product
generally
uniformly and to inhibit charring of the base 12. The transmission lines 46
are
selected to provide a progressive power loss from each of the tuned loops 44
and
are of such length that the power decays towards zero at the mid-point of the
transmission lines. This is achieved by matching the energy fed by the loops
44 to
the absorption characteristics of the transmission lines 46.
Two arrays 50 and 52 of tuned structures 54 and Sb respectively are
also circumferentially spaced in a ring about the circular island 42. The
tuned
structures 54 of array 50 are positioned between adjacent transmission
elements 40
while the tuned structures 56 of the array 52 are positioned between the two
loops
44 of each transmission element 40. The tuned structures 54 and 56 each
include
nested loops and islands as will now be described.
Each tuned structure S4 and 56 includes a deltoid ring 60 having
rounded corners. Within the deltoid ring 60 is an annular ring 62 joined to
opposed corners of the deltoid ring by a pair of bridges 64. A circular island
66
is positioned within the annular ring 62. A sagittal island 68 is also
positioned
within the deltoid ring 60. The arrowhead 70 of the sagittal island 68 points
toward the center of the microwave energy heating element 14. The shaft 72 of
the sagittal island 68 extends radially from the arrowhead 70 crossing the
annular
ring 62 and terminating at the circular island 66.
The deltoid rings 60 of the tuned structures 54 are more elongate
than the deltoid rings of the other tuned structures 56 and therefore are more
pointed towards the center of the microwave energy heating element 14. The
arrowheads 70 of the sagittal islands 68 within the deltoid rings 60 of the
tuned
structures 54 are also more pointed than the arrowheads of the tuned
structures 56.
As a general principle, the Loops and istands are reactive with the incident
microwave energy and so their nature and extent of their coverage of the
microwave energy heating element determines the amount and distribution of
microwave energy. The radial spacing between the deltoid and annular rings is
CA 02564627 2006-10-30
-9-
such that the enclosed circuit length is close to ~ where J~ is equal to the
effective
wavelength of the incident microwave energy. The islands principally inhibit
transmission of microwave energy but provide a local excitation at their outer
edges.
The outer-most corners of the deltoid rings 60 are joined to an outer
ring 76 which covers the peripheral margin of the microwave energy heating
element 14 by bridges 74. The bridges 64 and 74 permit the tuned structures 54
and 56 to be excited by the antenna formed by the inner circumference of
peripheral edge 76b.
The outer ring 76 has a circular outer peripheral edge 76a and an
undulating inner peripheral edge 76b. Two concentric rings of
circumferentially
spaced apertures 78 are formed in the outer ring. The apertures 78 are in the
form of elongate slots having cambered major edges. In the specific embodiment
shown, the elongate slots 78 of the two rows are staggered.
Referring now to Figures 4 and 5, the cover 18 is better illustrated.
The circular cover 18 is also in the form of a laminate 80 and includes a
substrate
82 formed of suitable material such as for example, paper, paperboard or a
polymeric film. Microwave energy interactive material 84 of one of the types
previously described is on one surface of the substrate 82. A susceptor 86
including at least one layer of suscepting material overlies the microwave
energy
interactive material 84 and the substrate 82 although the susceptor 86 is
optional.
The substrate 82 extends beyond the peripheral edge of the microwave energy
interactive material 84 to ensure that the cover 18 and the microwave energy
heating element 14 remain electrically isolated if the edge of the cover 18
contacts
the microwave energy heating element. Spaced apertures 88 are formed in the
microwave energy interactive material 84 about its peripheral margin. The
apertures 88 are in the form of elongate slots having cambered major edges. In
the particular example shown, the slots are arranged in three concentric rings
with
the slots in the various rings being staggered. The elongate slots 88 are
sized to
promote localized fields to enhance the susceptor 86 and promote browning of
the
food product 16 when penetrated by microwave energy. In addition, the
circumference of the shielding may be designed to enhance or limit the
electrical
CA 02564627 2006-10-30
10=
activity at its edge.
During packaging, the food product 16 is placed on the microwave
energy heating element 14 and is supported by the base 12. The flexible cover
18
is then placed over top the food product 16 with the susceptor 86 in contact
with
the food product. Since the cover 18 is flexible it generally conforms to the
shape
of the food product. Following this, the base 12, cover 18 and food product 16
are shrink wrapped with the plastic film 20 to hold securely the food product
16
between the base 12 and the cover 18 and inhibit relative movement between
them. Because the wrap 20 holds the cover, base and food product securely, in
IO the case of pizzas, separation between the crust and the pizza toppings is
unlikely
to occur.
When the food product 16 is to be cooked, the wrap 20 is removed
arid the food product 16 is placed in the microwave oven supported by the base
IZ
and with the cover 18 overlying the top of the food product. The outer edge of
the cover 18 is preferably folded down over at Least a portion of the sidewall
of
the food product to provide some edge heating. The design of the microwave
energy heating element 14 and cover 18 are such to heat uniformly the food
product 16 while ensuring that the crust of the food product is cooked and
browned.
Although the cover 18 is shown as being circular and planar, the
cover can take other geometric shapes and may be in the farm of a dome to
overlie the top of the food product 16 as well as its sides.
Referring now to Figures 6 and 7, another embodiment of a cover
for a microwavable package is shown. In this embodiment, two concentric rings
of apertures 188 are formed in the peripheral margin of the microwave energy
interactive material 184. The apertures in this case are rectangular in
appearance
and have rounded corners. Islands 100 are located within each aperture 188.
Each island 10(? itself has a flattened decussate aperture 102 formed in it.
Although, the cover 18 has been described as being flexible to allow
it to be folded over at least a portion of the sides of the food product 16,
those of
skill in the art will appreciate that the peripheral margin of the base 12 may
also
be made to be flexible so that the active microwave energy heating element 14
CA 02564627 2006-10-30
_ 11~_
may be folded over at least a portion of the side of the food product together
with
or instead of the cover 18. In these instances, the cover 18 and base 12
should be
dimensioned to inhibit electrical coupling of the microwave energy interactive
material on the cover and base.
S In addition, although the microwave energy heating element and
cover have been described as a laminate with the microwave energy interactive
material deposited on one surface of the substrate and covered by a susceptor,
it
should be realized that the pattern of microwave energy interactive material
can be
deposited on one surface of the substrate and the susceptor can be deposited
on an
opposite surface of the substrate. In this case, the surface of the substrate
on
which the microwave energy interactive material is deposited, is bonded or
adhered to the base 12.
Referring now to Figure 9, yet another embodiment of a cover 218
for a microwavable package is shown. In this embodiment, three concentric
rings
of apertures 288 are formed about the peripheral margin of the microwave
energy
interactive material 284. The apertures 288 are in the form of elongate slots
and
are arranged so that the apertures of the various rings are staggered. Within
the
inner most ring of apertures 288, is an array of additional apertures 300. The
apertures 300 are in the form of elongate slots and are arranged in two
alternating
patterns 302, 304 about the center of the cover 218. Each pattern 302 of
apertures
300 includes three radially directed apertures arranged to form a triangle
with a
tangentially oriented aperture between the inner aperture and the two outer
apertures. The apertures that are arranged to form a triangle taper in width
towards the center of the cover 218. Each pattern 304 of apertures 300
includes
an outer tangentially oriented aperture and an inner radially directed
aperture 300.
The radially directed aperture has cambered major edges. An annular aperture
308 is formed at the center of the cover and surrounds a circular island 310.
Depending upon the depth of the crust, the toppings appearance and
design on the crust and the size of the pizza, a cover of the types
illustrated may
or may not be used. Although the cover will assist heating of the food
product,
due to cost in many applications, a transparent cover or no cover will be
used.
Although Figures 4, 6 and 9 illustrate different embodiments of the
CA 02564627 2006-10-30
- 12 ~-
cover, those of skill in the art will appreciate that other configurations of
microwave energy interactive material on the cover can be used. For example,
the
cover may include islands of microwave energy interactive material in the
shape of
circles or polygons. Alternatively, the microwave energy interactive material
may include annular or polygonal loops surrounding correspondingly shaped
islands.
Referring now to Figure 10, another embodiment of a tray 411 is
shown. In this embodiment, the configuration of the tuned structures 450 and
452
and the outer peripheral ring 476 is different from that of Figure 2. As can
be
1U seen, each tuned structure 450 and 452 includes a generally circular loop
480
joined to the outer ring 476 by a bridge 474. The loop 480 is connected to a
triangular island 482 by way of a pair of transmission Iines 484. Nested loops
486
are positioned between the transmission lines 484 adjacent the triangular
islands
482 and include an annular ring 488 surrounding a circular island 490. The
triangular islands 482 of the tuned structures 450 are longer than those of
tuned
structures 452 and point towards a circular island 49~ at the center of the
tray.
Four concentric rings of apertures 496 are provided through the tray 411. The
apertures 496 allow moisture released from the food product during cooking to
pass through the tray 411. In use, a moisture absorbing towel or the like will
typically be placed beneath the tray to absorb moisture passing through the
apertures 496, The substrate 430 extends beyond the peripheral edge of the
active heating element 414.
Referring now to Figure 11, anther embodiment of a tray 51 I is
shown. Tray 511 is very similar to that shown in Figure 2. As can be seen, the
active microwave energy heating element 514 includes a plurality of
circumferentially spaced transmission elements 540 arranged in a ring about
the
center of the tray. An array of tuned structures 550 and 552 are also
circumferentially spaced in a ring about the center of the tray. Tuned
structures
550 are positioned between adjacent transmission elements 540 while tuned
structures 552 are positioned between the loops 544 of each transmission
element
540. In this case, the tuned structures 550 and 552 are the same. Unlike the
embodiment of Figure 2, the tray 5 l 1 does not include an island at its
center.
CA 02564627 2006-10-30
- 13-
However, the transmission lines 546 are longer and extend closer to the center
of
the tray. The loops 544 are generally diamond-shaped with rounded corners and
the tuned structures 550 and 552 are more elongate and have sharper corners.
Also, the substrate 530 extends beyond the peripheral edge of the active
heating
element 514.
Figure 12 shows yet another embodiment of a tray 611. In this
embodiment, the transmission lines 646 extend closer to the censer of the tray
obviating the need for an island at the center. Also, a bridge 680 joins the
transmission lines 646 of each transmission element 640 at their mid-point.
The
tuned structures 650 and 652 are the same and are in the form of loops
resembling
arrowheads. The tuned structures 650 and 652 are joined to the outer ring 676
by
bridges 674.
Referring now to Figures 13a and 13b, yet another embodiment of a
tray 711 is shown. In this embodiment, tray 711 includes a base 712, and
upstanding sidewall 713 about the periphery of the base 712 and a peripheral
rim
715 about the sidewall. The active heating element 714 extends over the base
and
the sidewall 7I3. The transmission elements 740 and tuned structures 750 and
752
are on the base 712 while the outer ring 776 runs about the periphery of the
base
and over the sidewall 713. As can be seen, similar to the previous embodiment,
bridges 780 join the transmission lines 746 at their mid-points. The tuned
structures 750 and 75 are the same and are in the form of diamond-shaped loops
782 joined to the outer ring 776 by narrow bridges 774. A triangular
projection
784 extends into each loop 782. A ring of apertures 778 is formed in the outer
ring 776 about the periphery of the base. A ring of apertures 788 similar to
those
provided in the cover of Figure 6 are formed in the outer ring about the
circumference of the sidewall.
In each of the embodiments of Figures 10 to 13b and similar to the
embodiment of Figure 2, the active microwave heating element on the tray
collects
microwave energy from the periphery of the tray and dissipates it
progressively
towards the center of the tray to provide a uniform heating effect.
While the above described embodiments show a tray and cover
separate from the tray, the active micrcwave energy heating elements may be
CA 02564627 2006-10-30
_ 14 -_
provided on opposed surfaces of a bag or pouch designed to accommodate the
food
product.
Although particular embodiments of the microwave energy heating
element 14 have been described and shown it should be apparent to those of
skill
in the art that other patterns of microwave energy interactive material may be
provided on the microwave energy heating element to achieve the desired
uniform
heating of the food product. Examples of alternative patterns of microwave
energy interactive material designed to heat uniformly a food product upon
exposure to incident microwave energy can be found in applicant's co-pending
t0 application filed on September 18, 1995 and issued serial number
08/529,450.
Also, although the tray 411 has been shown to include apertures 496
therein to allow moisture to pass through the tray, those of skill in the art
will
appreciate that the other embodiments of the trays may also include apertures.
In
addition, apertures may be provided through the covers if desired to allow
moisture to pass.
Those of skill in the art will also appreciate that variations and
modifications may be made to the present invention without departing from the
spirit and scope thereof as defined by the appended claims.
