Abstract:
The disclosed invention provides a microwavable package for a food product. The package comprises a base with an active microwave energy heating element to support the food product. The active microwave heating element comprises energy collecting resonant loops, tuned structures, and transmission lines to collect incident microwave energy and redirect it to other parts of the food product. The microwave package also includes a cover comprising a microwave energy interactive layer including one or more apertures within the cover. The apertures promote localized fields to promote browning of the food product in the local areas around the apertures. The cover may also comprise a susceptor layer that is heated in localized areas around the apertures due to the fields promoted by the apertures and impingement by incident microwaves energy through the apertures, thereby providing localized browning of the food product.

Description:
This application is a continuation of U.S. application Ser. No. 08/703,098, filed Aug. 16, 1996 now abandoned and is the National Phase (371) and PCT/CA97/00597 filed Aug. 26, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to packages for 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 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 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. Pat. 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 product into specific regions. This is purported to produce a localised charring of the food product. Overall, however, 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. Pat. 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 foodstuff. 
     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 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 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 microwavable package comprising: 
     a base to support a food product; 
     an active microwave energy heating element on said base to effect heating of a 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 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. 
     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 loops. The resonant loops have a perimeter sufficient to limit 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 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 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 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 limit 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: 
     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 DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which: 
     FIG. 1 is an exploded side elevational view of a microwavable package in accordance with the present invention; 
     FIG. 2 is a top plan view of a tray having an active microwave energy heating element thereon for the microwavable package of FIG. 1; 
     FIG. 3 is cross-sectional view of FIG. 2 taken along line  3 — 3 ; 
     FIG. 4 is a top plan view of a cover forming part of the microwavable package of FIG. 1; 
     FIG. 5 is a cross-sectional view of FIG. 4 taken along line  5 — 5 ; 
     FIG. 6 is a top plan view of an alternative embodiment of a cover for a microwavable package in accordance with the present invention; 
     FIG. 7 is an enlarged part cross-sectional view of FIG. 6 taken along line  7 — 7 ; 
     FIG. 8 is an enlarged top plan view of a portion of FIG. 6; 
     FIG. 9 is a top plan view of yet another alternative embodiment of a cover for a microwavable package in accordance with the present invention; 
     FIG. 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; 
     FIG. 11 is a top plan view of another alternative embodiment of a tray having an active microwave energy heating element thereon for a microwavable package in accordance with the present invention; 
     FIG. 12 is a top plan view of 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; 
     FIG. 13 a  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 
     FIG. 13 b  is a cross-sectional view of FIG. 13 a.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, 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  12 . 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 FIGS. 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  32   a  of the substrate is adhered to the base  12  and an opposed surface  32   b  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 circumferentially 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 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  56  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  54  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 islands are reactive with the incident microwave energy and so the 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 such that the enclosed circuit length is close to λ where λ 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  76   b.    
     The outer ring  76  has a circular outer peripheral edge  76   a  and an undulating inner peripheral edge  76   b.  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 FIGS. 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 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 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 and the food product  16  is placed in the microwave oven supported by the base  12  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 form of a dome to overlie the top of the food product  16  as well as its sides. 
     Referring now to FIGS. 7 and 8, 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  100  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  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. 
     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 FIG. 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 FIGS. 4,  6  and  9  illustrate different embodiments of the 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 FIG. 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 FIG.  2 . As can be 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 lines  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  492  at the center of the tray. Three 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 FIG. 11, another embodiment of a tray  511  is shown. Tray  511  is very similar to that shown in FIG.  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 FIG. 2, the tray  511  does not include an island at its center. 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 . 
     FIG. 12 shows yet another embodiment of a tray  611 . In this embodiment, the transmission lines  646  extend closer to the center 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 FIGS. 13 a  and  13   b,  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  713 . 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  752  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 FIG. 6 are formed in the outer ring about the circumference of the sidewall. 
     In each of the embodiments of FIGS. 10 to  13   b  and similar to the embodiment of FIG. 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 microwave energy heating elements may be 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&#39;s co-pending application filed on Sep. 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.