Patent Publication Number: US-6906299-B2

Title: Cooperating paperboard blanks for forming a microwave heating food container

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional application of U.S. patent application Ser. No. 09/944,285 (issued as U.S. Pat. No. 6,586,715 on Jul. 1, 2003), filed Aug. 30, 2001, which was a divisional application of U.S. patent application Ser. No. 09/523,493 (issued as U.S. Pat. No. 6,320,172 on Nov. 20, 2001), filed Mar. 10, 2000, which was a non-provisional of U.S. Provisional Patent Application Ser. No. 60/124,243, filed Mar. 12, 1999, the contents of all of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to microwavable containers for food products, and methods of fabrication thereof. The present invention relates more specifically to a microwavable tub for storing, shipping, heating and serving food products such as, for example, popcorn, puffed cheese snacks and pork rinds. 
   2. Description of Related Art 
   The increasing popularity of microwave cooking has lead to the development of several types of containers for microwave heating of food products. For example, a number of bag-type containers for microwave popping of popcorn are available. These containers are typically formed of paper or other flexible materials, and often include heating elements of microwave interactive susceptor material that absorb microwave energy to generate heat, which pops the popcorn. Such containers are typically shipped and stored in a folded configuration and, upon heating and popping of the corn, unfold into an expanded configuration. 
   More rigid containers have also been developed, such as cup-shaped containers for microwave heating of popcorn. These containers typically are in the form of generally frustoconical paperboard tubs. A microwave susceptor is installed on or around the floor of the tub, and a quantity of unpopped corn and cooking oil or shortening is placed in the tub&#39;s interior. A plastic film or other barrier material is often applied over the corn in an effort to seal out external contaminants, seal in moisture, and preserve freshness. It has been found, for example, that moisture loss from popcorn inhibits popping and reduces popped volume. Efforts to form a hermetic seal around the food product in previously known containers have proven less than fully satisfactory. For example, one previously known container provides a concave cooking tray formed of coated paperboard stock for containing the food product and heating through microwave absorption. The tray includes a number of folds or corrugations, which present discontinuities in any seal attempted to be formed with a barrier material. These discontinuities form air channels that allow moisture loss from the food product. In an attempt to overcome this problem, a moisture impervious liner has been provided around the exterior of the tray, which liner is sealed to the barrier film around the lip of the tray. The multiple components required in previously known containers, however, typically results in increased costs of materials and assembly. Another disadvantage found to inhere in previously known containers incorporating a tray for containing the food product is the potential for the food product to be displaced inadvertently from the tray onto the shelf or ledge formed by the lip of the tray, removing the food product from thermal contact with the microwave susceptor material, often resulting in incomplete cooking or popping. 
   Previously known paperboard tub containers for microwave cooking are typically assembled by depositing the food product onto the floor of an upright container, or into a heating tray placed within the container, and then installing liners and/or film in various configurations in an attempt to form a seal around the food product. As discussed above, the provision of cooking trays and liners undesirably increases expense and may adversely affect cooking performance. If the food product is distributed across the container floor, attempts to form a seal around the food product by sealing a cover film to the container walls typically are unsuccessful, as moisture may escape through the walls and floor of the container, and/or through the seam between the walls and floor. 
   Thus it can be seen that a need exists for a container for microwave cooking of food products, which overcomes disadvantages of previously known containers. A need further exists for an economical and efficient method of fabricating a microwavable container for food. It is to the provision of a container and method of fabrication meeting these and other needs that the present invention is primarily directed. 
   SUMMARY OF THE INVENTION 
   The microwavable container and method of assembly of the present invention provide a number of improvements over previously known containers and methods of assembling such containers. For example, the container of the present invention prevents moisture loss and contamination by means of a simplified hermetic sealing arrangement described in greater detail below. This sealing arrangement results in a fresher food product, thereby enhancing consumer enjoyment. Shelf life of the product is also extended. The simplified sealing arrangement reduces material and assembly costs, resulting in a more commercially viable product. The container is easy and convenient for consumers to use, has an attractive shelf appearance, is nestable for ease of shipping and handling and reduced shelf space requirements, and provides large visible surface areas for the display of point-of-sale marketing features. 
   Briefly described, one aspect of the present invention provides a container for microwave heating of a food product. The container preferably includes a substantially upright sidewall assembly defining an interior volume, the sidewall assembly having a base, a mouth and at least one wall panel extending between the base and the mouth. The container preferably also includes a moisture-impervious floor extending from the at least one wall panel adjacent the base. The floor has an interior face defining the lower extent of the interior volume and an exterior face opposite the interior face. The container preferably also includes a sheet of barrier material forming a pocket for containing a food product, the sheet of barrier material being continuously sealed to the interior face of the floor around the pocket. 
   In preferred embodiments, the container of the present invention includes a generally rectangular tub assembly having four wall panels. Alternatively, the tub assembly can be a generally cylindrical or frustoconical tub having one wall panel closed upon itself. The tub assembly of the present invention can be fabricated from paperboard blanks folded and assembled in manners described in greater detail below. The floor of the container comprises a moisture barrier, preferably provided on the interior face of the floor to prevent migration of moisture into or through the floor material. The floor preferably, but not necessarily, also comprises a microwave susceptor material to enhance heating of the food product. The microwave susceptor material is preferably disposed to the interior face of the floor, but can be disposed to its exterior face or be formed integrally with the floor. The microwave susceptor and the moisture barrier can be one and the same through appropriate material selection, or can be separate materials or layers functioning in combination. The at least one wall panel preferably slopes outwardly from the base to the mouth, whereby multiple containers can be nestably stacked. 
   In another aspect, the present invention provides a container for microwave heating of a food product, the container preferably including a generally polygonal tub having three or more generally flat wall panels, a moisture-impervious floor and an open mouth. In a further preferred embodiment, the tub is generally rectangular, having four wall panels. The container preferably also includes a sheet of barrier material, such as a moisture-impervious balloon film, forming a pocket for containing a food product, the sheet of barrier material being continuously sealed to the floor around the pocket. Support legs preferably extend below the floor from corners defined by the intersections of adjacent wall panels, the support legs being separated by notches formed by removal of portions of the wall panels. A microwave susceptor is preferably disposed to the floor, and may be disposed to the interior or exterior face of the floor, or be integrally formed with the floor. The wall panels preferably slope outwardly from the support legs to the mouth of the tub, whereby multiple containers can be nestably stacked. 
   In yet another aspect, the present invention provides one or more cooperating paperboard blanks for forming a container for microwave heating of a food product. In a preferred embodiment, a sidewall blank is provided having three or more wall panels, and more preferably four wall panels, adjacent wall panels joined along score lines for folding to form a sidewall assembly. A floor blank is also provided, having edges adapted to be attached to a respective wall panel of the sidewall blank. The floor blank preferably is formed from a moisture-impervious material and optionally includes a microwave susceptor. 
   In another aspect, the present invention provides a container for microwave heating of a food product, the container including a tub having at least one wall panel and a floor providing a moisture barrier. The container further includes a sheet of barrier material deformed to comprise a pocket for containing a food product, the sheet of barrier material sealed to said floor about the periphery of the pocket. The floor of the container preferably also includes a microwave susceptor to enhance heating performance. 
   In another aspect, the present invention provides a container for microwave heating of a food product, the container including a sidewall assembly having a base, a mouth, and at least one wall panel extending between the base and the mouth. The container further includes a floor extending horizontally from the at least one wall panel adjacent the base, the floor having an interior face and an exterior face and the at least one wall panel having an interior face and an exterior face as well. The container also includes a sheet barrier material, which cooperates with the interior face of the at least one wall panel and the interior face of the floor to form a volume for containing the food product. The sheet of barrier material is sealed to the interior face of the at least one wall panel. 
   In still another aspect, the present invention provides a method of assembling a container for microwave heating of a food product. The method preferably includes forming a pocket in a sheet of barrier material, depositing a quantity of a food product within the pocket, placing a tub assembly having a floor and at least one wall panel over the food product, and sealing the barrier material to the floor of the tub assembly around the pocket to encapsulate the quantity of food product between the barrier material and the moisture-impervious floor of the tub assembly. In a further preferred embodiment, the barrier material is a moisture-impervious film, and the sealing step of the method is preferably carried out by heat sealing the sheet of moisture-impervious film to the floor of the tub assembly. Preferably, the heat seal is formed by applying heat from the outside of the container, through the paperboard or other material of construction of the tub assembly. The forming step can be carried out by deforming the barrier material as with a mandrel and die, vacuum forming, heat forming, folding, crimping, and/or through the provision of a preformed pocket. Heat can be applied to the sheet of barrier material during the forming step, as through the use of a heated vacuum platen, to plastically deform the material. The method may further include attaching the barrier material to a wall panel of the tub assembly at one or more locations. 
   In another aspect, the present invention provides a method of assembling a container for microwave heating of a food product, the method entailing fabricating a tub assembly by folding at least one blank to form a floor and at least one wall panel, forming a pocket in a sheet of barrier material, depositing a quantity of a food product within the pocket, inserting at least a portion of the barrier material within the tub assembly whereby the pocket of food product is adjacent the floor of the tub assembly, and forming a continuous seal between the barrier material and the floor of the tub assembly around the pocket. 
   In another aspect, the present invention provides a method of hermetically sealing a food product within a microwave cooking container, the method entailing depositing the food product between a sheet of barrier material and a moisture barrier portion of the container, and forming a continuous seal between the barrier material and the moisture barrier portion of the container. 
   These and other features and advantages of preferred forms of the present invention are described herein with reference to the drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross-sectional elevation of a container according to one embodiment of the present invention. 
       FIG. 1   a  shows a cross-sectional elevation of a container according to another embodiment of the present invention. 
       FIG. 2  shows a perspective view of a container according to yet another embodiment of the present invention. 
       FIG. 2   a  shows a partially cut-away, exploded view of the container of the present invention shown in FIG.  2 . 
       FIG. 2   b  partially shows a partially cut-away, exploded view of the container according to one embodiment of the present invention shown in FIG.  1 . 
       FIGS. 3   a  and  3   b  show top plan views of blanks used to fabricate the container of  FIG. 2 , according to one form of the invention. 
       FIG. 4  shows a perspective view of a container according to yet another embodiment of the present invention. 
       FIGS. 5   a  and  5   b  show top plan views of blanks used to fabricate the container of  FIG. 4 , according to another form of the invention. 
       FIG. 6  depicts schematically a method of assembly of a container according to one form of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawing figures, wherein like reference numerals represent like parts throughout unless specifically indicated otherwise, preferred forms of the present invention will now be described. With reference first to  FIG. 1 , the present invention is related to a container  10  for microwave heating, as with a standard microwave oven, of a food product  12 . The food product  12  may be, for example, popcorn, pork rinds, puffed cheese snacks, or other food product. The container of the present invention is particularly well-suited for, but is not limited to, the heating of food products that expand or puff when cooked. In alternative embodiments, the container of the present invention can be used to heat beverages, such as coffee or tea. Cooking oil, shortening, spices, preservatives, flavorings, stabilizers, colorants, or other substances may be included with the food product  12 . Moreover, one or more surfaces of the container  10  can be printed, labeled or otherwise provided with text, graphics or other features for marketing, informational or source indicating purposes. 
   The container  10  preferably includes a tub assembly  13  having a substantially upright sidewall assembly  14 , formed of paperboard, paper, cardboard, plastic, or other foldable, moldable or deformable material. Acceptable results may be obtained, for example, using 15, 18 or 24 point SBS (solid bleached sulfate) paperboard. The material(s) of construction used to form the sidewall assembly  14  are selected to result in a container  10  that is substantially rigid (i.e., capable of supporting the weight of the container  10  and its contents in normal use by a consumer without undue deflection), and to provide economy and ease of fabrication. One or both of the interior and exterior faces of the sidewall assembly can comprise a coating, laminate, coextrusion or other treatment, such as for example polyethylene or other polymer(s), flourocarbon treatment or wax, to provide a barrier against staining or absorption of oils, water or other liquids from the food product  12 . As a representative example, a flourocarbon treatment sold under the tradename FC807 by the 3M Company can be applied to the sidewall assembly. 
   The sidewall assembly  14  includes at least one wall panel  16 . A single, curved wall panel  16  can be formed into a generally cylindrical or frustoconical container  10 , or multiple flat wall panels can be formed into a multi-walled, polygonal container  10  as will be described more particularly with reference to  FIGS. 2-5 . For example, in the embodiment shown in FIG.  2  and described in more detail below, the sidewall assembly  14  includes wall panels  16   a - 16   d . The sidewall assembly  14  preferably further includes a base portion  18  at the lower edge of the wall panel(s)  16 , which is adapted to rest on a support surface such as the floor of a microwave oven (not shown), and maintain the container  10  in a stable, upright position. The upper extent of the sidewall assembly  14  preferably comprises an open mouth  20  providing access to the interior volume  22  bounded by the sidewall assembly  14 . The upper edge(s) of the wall panel(s)  16  may be rolled, folded, or otherwise formed to provide a lip  24 , to enhance the structural integrity of the container  10 , and/or to assist in handling the container  10 . 
   One or more handles, projections or other surface features may be provided to assist in handling the container  10 . For example, at least one optional handle  17 , as best shown in  FIG. 2 , is especially helpful after the heating of food products and the container  10  may be hot. Optional handle  17  is a flap extending away from the wall panel  16   d  and can be folded along an upwardly extending axis that is substantially non-horizontal, such as edge  17   a , so as to position the handle  17  against an adjacent wall panel, such as wall panel  16   a  in the embodiment shown in FIG.  2 . This foldable characteristic of handle  17  allows handle  17  to move between an inoperative position where handle  17  is co-planar with the wall panel  16   a  and an operative position where handle  17  extends away from the wall panel  16   a  and therefore not to significantly affect the stacking of one container  10  into another one. Preferably, handle  17  is an integral part of the wall panel  16   d  in one embodiment as shown in  FIG. 3   a . Alternatively, handle  17  can be a separate element and attached or fixed to the wall panel  16   d  at a location of the user&#39;s choice. For instance, handle  17  can be a separate piece of paperboard, paper, cardboard, plastic, or other foldable, moldable or deformable material having a sticky end that can be stuck to the wall panel  16  prior to use by the consumer. In this manner, the stackability of the container  10  is preserved. Note that although the handle  17  is associated with the wall panel  16   d  in the embodiments shown in  FIGS. 2 and 3   a , obviously, handle  17  can be associated with any of the wall panels  16   a - 16   d . Additionally, the substantially upright or vertical axis about which handle  17  pivots can be located on any of the wall panels  16   a - 16   d  at any desired location thereon. Also, more than one handle can be utilized to further facilitate handling of the container  10 . 
   Additionally, at least one projection  19  can be formed to facilitate stacking two or more containers  10  together. For the embodiment shown in  FIG. 2 , projection  19  is formed by cutting the wall panel  16   a  along the solid line l and then pushing flap portion  21  outwardly away from the wall panel  16   a  so that the flap portion  21  pivots upwardly along axis a to assume its operative position. In the inoperative position of projection  19 , the projection  19  is co-planar with the wall panel  16   a  and the bottom of the projection  19  merges with the wall panel  16   a  seamlessly and, in the embodiment shown in  FIG. 2 , the projection  19  is part of the wall panel  16   a . The formed projection  19  can be considered as a “stacking ear” projecting from the wall panel  16   a  and movable along the substantially horizontal axis a. The projection  19  is located at a distance h from the upper edge of the wall panel  16   a . The distance h is variable to accommodate variable uses of the container  10 . The container  10  can have more than one stacking ear, as for instance in the embodiment shown in  FIG. 2 , and more clearly shown in  FIG. 2   a , wherein the container  10  also has a projection  23  or stacking ear formed on the wall panel  16   c , opposite the projection  19  with projection  23  being similarly dimensioned to projection  19 . Although it is not necessary, it is preferable that if container  10  has two or more projections, they are formed on opposing wall panels. It is also preferable that the projections be similarly shaped, each extending the distance h from the upper edge of the wall panel  16   a  and pivoting about a respective axis a. By providing the wall panel(s)  16  with a slight outward slope from the base  18  to the mouth  20 , two or more containers  10  as described herein may be nestably stacked, one within another. If the container  10  is of a type having at least one projection  19 , the combination of the outward slope of the wall panel(s)  16  and the projection(s)  19  in the operative position facilitates the stacking of multiple containers in a nested array. The nested array of containers  10  can be packaged as a unit, as by applying a shrink-wrapped sleeve or other overwrap. 
   Referring to  FIG. 2   a , the tub assembly  13  preferably further includes a floor portion  30  extending generally horizontally from the wall panel(s)  16 . The floor portion  30  has an interior face  31   a  defining the lower boundary of the interior volume  22  of the container  10 , and an exterior face  31   b  opposite the interior face  31   a . The floor  30  comprises a moisture-impervious material to prevent moisture loss from food product  12  encapsulated thereby, as will be described below. The floor  30  is preferably formed from a moisture barrier material or is provided with a moisture barrier coating or layer along substantially its entire interior face. Acceptable results have been obtained, for example, using 20 or 24 point SBS paperboard with a 2 mil polyester laminated on its interior face. Alternatively, acceptable results can be obtained by using 12.5-13 point SBS paperboard laminated on its interior face with 8 lb./ream nylon. Other polymer coatings, laminates, coextrusions or layerings, such as for example: polypropylene; polyvinyl dichloride (PVDC)-coated nylon; PVDC-coated polyester; and/or polyester and polypropylene composites, may be used to provide substrate materials such as paperboard, cardboard, paper or plastics with acceptable barrier properties. 
   The floor  30  can be integrally formed with the wall panel(s)  16 , or can be a separate component attached to the wall panel(s) by adhesive, folding, crimping, or other standard attachment means. A microwave susceptor  32 , such as a 48-gauge or 2 mil metallized polyester film, vacuum deposited metal, carbon or metallic based coatings, laminates, inks or print, other microwave interactive material(s), or any combination of them, is preferably disposed to the floor  30 . The susceptor  32  is preferably laminated or otherwise affixed to the interior face  31   a  of the floor  30 . Alternatively, the susceptor  32  can be laminated or otherwise affixed to the exterior face  31   b  or be integral with the floor  30 . The susceptor  32  is preferably sized and placed to be underlying at least the portion of the floor  30  upon which food product  12  is initially placed.  FIGS. 2   a  and  2   b  show two embodiments of the susceptor  32  of the present invention. 
   The susceptor  32  includes a film of polyester  33  and a layer of metal  35 . A metallic material such as aluminum is deposited onto the polyester film  33  to form a very thin metal layer  35  over the polyester film  33 . The deposition process is controlled so that the metal layer  35  substantially occupies, but not necessary fully, the portions of floor  30  underneath the food product  12 , as shown in  FIG. 2   a . Alternatively, the deposition process can be controlled so that the metal layer  35  fully occupies the portions of floor  30  underneath the food product  12 . Then, portions of the metal layer are removed in areas where the metal layer is not needed. In other words, the distance s between the edge of the metal layer  35  and the line  37  representing the location of the bottom of a wall panel has a nonzero value in the preferred embodiment. Obviously, the distance s can have a zero value as well. The susceptor  32  is disposed to the floor  30  such that the polyester film  33  is in contact with the floor  30  and indeed, covers substantially the entire floor  30 , with the metal layer  35  to be in contact with the food product. Preferably, the polyester film  33  covers the whole interior surface  31   a  and extends upwardly along the inner surfaces of the panel  16   a - 16   d  as shown in  FIG. 2   a . The food product is placed over the metal layer  35  and during cooking remains at all times in thermal contact with the susceptor  32 . In another embodiment, the susceptor  32  has an additional film of polyester or similar material (not shown) covering the metal layer  35  so that the metal layer  35  is sandwiched in the lamination. In this embodiment, the food product  12  is not in direct contact with the metal layer  35 . In a further embodiment as shown in  FIGS. 1 and 2   b , for examples, the susceptor  32  is disposed to the floor  30  such that the metal layer  35  is in contact with the floor  30 , with the polyester film  33  to be in contact with the food product  12 . Adhesive materials can be applied to the floor  30  prior to the application of the susceptor  32  to bind the metal layer  35  with the floor  30 . Although acceptable microwave heating of the food product  12  can be obtained without the inclusion of the microwave susceptor  32 , the inclusion of a microwave susceptor has been found to provide faster and more consistent heating. 
   The floor  30  can be raised a distance above the base  18 , forming a lower chamber  34  between the exterior face  31   b  and the support surface such as the floor of a microwave oven. The lower chamber  34  is preferably open to heat and air transfer to and from the container&#39;s surroundings, through the provision of one or more notches or openings, as is described in greater detail below. It is believed that placement of the exterior face  31   b  of the floor  30  of the container 10 approximately around 1.905 cm or smaller, but in no case greater than 3 cm, above the floor of a microwave oven or a similar supporting surface optimizes cooking performance, as this distance places the microwave susceptor  32  applied to the interior face  31   a  of the floor  30  approximately one-quarter wavelength of the microwave energy above the oven floor. 
   Referring to  FIGS. 1 and 6 , the container  10  of the present invention preferably further comprises a sheet of barrier material  40  forming a pocket  42  for containing a quantity of the food product  12 . The sheet of barrier material  40  can go up along the wall  16  anywhere between the bottom of the wall  16  and the top of the wall  16 . Indeed, in one embodiment as shown in  FIG. 1 , the sheet of barrier material  40  goes up to near the top of the wall  16 . In another embodiment (not shown), the sheet of barrier material  40  just covers the floor  30  and does not go up the wall  16  at all. The pocket  42  is preferably generally centered on the floor  30 , and does not normally extend to the exterior edges of the floor  30  where it joins with the bottom of wall panel  16 . In the embodiments where a microwave susceptor  32  with a sized metal layer  35  is provided, the pocket  42  substantially covers the sized metal layer  35  as shown in FIG.  1 . In this manner, the floor  30  comprises a generally central portion  30   a  underlying the pocket  42  containing the food product, and an annular or peripheral outer portion  30   b  not having food product supported thereon. The interior face  30   c  of this annular or peripheral outer portion  30   b  provides a sealing surface for contacting and forming a seal with the barrier material  40 . 
   For a variety of food product applications, the barrier material  40  preferably comprises a moisture-impervious (i.e., resistant to passage of water or water vapor) material such as, for example: a 50-gauge coated heat-sealable polyester film; a barrier-coated nylon film; or other heat-resistant and moisture impervious sheet polymers. A seal  44  is provided between the sheet of barrier material  40  and the moisture-impervious floor  30  around the pocket  42 , to hermetically seal the food product  12  within the pocket  42 . The seal  44  can be continuous. In this manner, moisture loss from the food product is minimized or eliminated. The seal  44  is preferably formed by heat sealing. Alternatively, adhesives or other sealants can be used to form seal  44 . Moreover, the seal  44  surrounding the pocket  42  of food product is preferably heat-releasable, such that the sheet of barrier material  40  will separate from the floor  30  upon heating to rise with expansion of the food product. One or more openings (not shown) can be provided through the sheet of barrier material  40 , outside of the pocket  42  beyond the seal  44 , to form release vents for allowing steam and expanding air to escape during heating. 
   Depending on the particular food product to be contained, the barrier material  40  may be impervious to air or other substances in addition to or instead of being moisture-impervious. For example, for containment of pork rinds, which are more susceptible to spoilage from exposure to oxygen than from moisture, the barrier material  40  may comprise an oxygen-impervious material. In alternative embodiments, the sheet of barrier material  40  may comprise a fluid permeable material that forms a barrier to external contamination, and/or that prevents release of materials contained in the pocket  42 . For example, a container for preparing and heating coffee or tea may comprise a barrier material  40  of paper filter material forming a pocket containing ground coffee beans or tea leaves. 
   Containment of the food product  12  within the pocket  42  in the manner of the present invention provides a number of advantages over containers wherein the food product is distributed over the entire floor or disposed within a tray. For example, the floor  30  forms a flat sealing surface, and does not present discontinuities-forming air channels to allow moisture loss from the food product, as may occur with the use of a cooking tray. Because the pocket  42  does not extend to the edges of the floor  30 , moisture cannot escape from the food product  12  through the joint between the floor  30  and the wall panel(s)  16 , as may occur with containers wherein the food product is distributed over the entire floor. Also, if a food product such as popcorn is packaged with cooking oil or shortening, the food product can be substantially encapsulated within the oil or shortening within the pocket  42 , thereby providing an additional barrier against moisture loss from the food product, and increasing the product&#39;s shelf life. The present invention also advantageously optimizes material usage and minimizes the number of components necessary to construct the container, thereby providing a more efficient and economical container. 
   It will be appreciated that, however, as shown in  FIG. 1   a , the food product  12  can be contained in a volume  142  defined by the sheet of barrier material  40 , the interior face  31   a  of the floor  30  and the wall panel(s)  16 . In this embodiment, a seal  144  is provided between the sheet of barrier material  40  and the interior face of the wall panel(s)  16 . The seal  144  preferably is a continuous seal formed by heat sealing to hermetically seal the food product  12  within the volume  142 . 
   If the container  10  is of a type having a raised floor  30 , the, generally central disposition of the pocket  42  of food product  12  on the floor  30  also enhances nestability when a number of containers  10  are stacked, as the pocket  42  of a lower container will nest within the lower chamber  34  of an upper container. Moreover, if the container  10  is of a type having at least one projection or stacking ear  19  as shown in  FIGS. 2 and 2   a , the distance h is chosen such that when a number of containers  10  are stacked, an upper container is supported at a selected position by an adjacent lower container through the engagement of the flap portion  21  of the projection  19  with the upper edge(s) of the panel wall(s)  16  with the bottom of the upper container barely in contact with the pocket  42  of the lower container. This avoids the situation wherein the food product  12  in the pocket  42  of the lower container is severely depressed by the upper container(s), thereby allowing more containers  10  to be stacked together without a concern that the pocket(s)  42  of food product of the lower container(s) will be damaged. 
   Referring now back to  FIG. 1 , a first attachment  50  is preferably provided between the sheet of barrier material  40  and the interior surface of the wall panel(s)  16  approximately midway up the height of the wall panel(s)  16 . Location of the first attachment  50  approximately midway up the height of the wall panel(s) prevents the sheet of barrier material  40  from rising a substantial distance above the mouth  20  of the container  10  upon inflation with steam or expanding air during heating. A second attachment  52  is preferably also provided between the sheet of barrier material  40  and the wall panel(s)  16  adjacent the mouth  20  of the container  10 . The second attachment  52  prevents contamination of the interior, food-contacting surfaces of the container  10  during shipping and storage. The first attachment  50  is preferably continuous about the container  10 , and will partially release upon heating to permit steam and expanding air to escape. The second attachment  52  can be discontinuous, in order to allow steam and expanding air to escape, and to facilitate removal of the barrier material  40  by the consumer. The first and second attachments  50  preferably do not fully release upon heating, but are readily released manually by a consumer after cooking of the food product  12 . Attachment of the sheet of barrier material  40  to the wall panel(s)  16  also prevents the sheet of barrier material  40  from interfering with the stacking of multiple containers in a nested array. Note that in the embodiment where the sheet of barrier material  40  only covers the floor  30 , optional one or more attachments can be provided between the sheet of barrier material  40  and the interior face  30   c  of the peripheral outer portion  30   b  of the floor  30 . Attachments can be formed in various kinds of means normally used in the art including glue, or heat sealing, etc. 
   As seen best with reference to  FIGS. 2-5 , the tub assembly  13  of the present invention may take the form of a generally polygonal (viewed from the top), hollow tub  60 , having three or more wall panels  16   a - 16   d . In a preferred embodiment, the tub assembly  13  is generally rectangular, having four wall panels  16   a - 16   d , and a floor  30 , the floor  30  indicated by broken lines in  FIGS. 2 and 4 . The term “generally rectangular” and any similar terms used herein are intended to describe a three-dimensional prismoidal or inverted frusto-pyramidal shape with corners of approximate right angles between adjacent walls. The rectangular tub  60  can take any of a number of particular embodiments, several of which will be described herein by way of example, but not by way of limitation. 
   In a first example embodiment, described with reference to  FIGS. 2 ,  2   a ,  3   a  and  3   b , the wall panels  16   a - 16   d  are formed from a unitary paperboard sidewall blank  70 , folded along score lines  72 . One end of the blank  70  can be provided with a gluing tab  74  for attachment to the opposite end upon folding to form a generally rectangular four-sided sidewall assembly  14 . Notches  76  can be formed along the base edge of the wall panels  16   a - 16   d  by removal of a cutout portion  78  of the wall panels  16   a - 16   d . In this manner, legs  80  are formed at corners of the container defined by the intersection of adjacent wall panels  16   a - 16   d . The notches  76  allow air circulation to and from the lower chamber  34  during heating, thereby preventing an excess buildup of heat. The size of the cutout portion  78  is variable. It should not be too large to affect the solidarity of the legs  80 . Nor should it be too small to affect air circulation to and from the lower chamber  34 . The floor  30  can be formed by folding a floor blank  82 . The floor blank  82  preferably comprises a generally rectangular floor panel  84 , and four edge panels  86   a - 86   d . The edge panels  86   a - 86   d  are folded to form approximate right angles with the floor panel  84 , and are preferably attached to respective wall panels  16   a - 16   d , as by adhesives or other standard attachment means, to provide a raised floor  30  supported a distance above a supporting surface such as the floor of a microwave oven. Although the corner webs  88  can be removed prior to folding the floor blank  82 , it is preferable that they be retained and folded along the score lines indicated in  FIG. 3   b , so that the edge panels  86   a - 86   d  and corner webs  88  form a continuous, leak-proof wall extending substantially upright from the floor panel  84  when the floor  30  is installed and attached into the wall assembly  14 . Although it is preferable that the floor  30  be installed with the edge panels folded upwardly, forming a tray-like containment structure, the floor  30  can alternatively be installed and attached into the wall assembly  14  with the edge panels oriented downwardly. As described in greater detail above, the floor  30  comprises a moisture barrier, and preferably further comprises a microwave susceptor, which, in a preferred embodiment, includes a sized metal layer to facilitate microwave heating. 
   In a second example embodiment, described with reference to  FIGS. 4 ,  5   a  and  5   b , a generally rectangular tub  60  is formed from a first paperboard blank  100  and a second paperboard blank  102 . The first blank  100  comprises second and fourth wall panels  16   b ,  16   d , and a floor panel  84  therebetween. The second blank  102  comprises first and third wall panels  16   a ,  16   c , and a substantially continuous base panel  104  extending therebetween. The wall panels  16   a - 16   d  are folded upwardly from the floor and base panels  84 ,  104  to form a pair of container subassemblies. These subassemblies are arranged in a crosswise configuration, with the floor panel  84  overlying the base panel  104 , and the wall panels  16   a - 16   d  forming a four sided sidewall assembly  14 . Base extensions  106  of blank  102  are adhesively affixed to the lower portions of wall panels  16   b ,  16   d , with the floor panel  84  raised a distance d above the base panel  104 , thereby forming a lower chamber  34  bounded on its bottom by base panel  104 , on its top by floor panel  84 , on two sides by base extensions  106 , and on two sides by wall panels  16   b ,  16   d . Openings (not shown) can be provided at the corners formed by the intersections of the wall panels  16   b ,  16   d  and the base extensions  106 , to permit air circulation to and from the lower chamber  34 . Gluing tabs  74  are preferably provided on wall panels  16   b ,  16   d  and on floor panel  84 , and are affixed to wall panels  16   a ,  16   c  to complete assembly of the container  10 . V-shaped notches  75  can be removed between the gluing tabs  74 ; or alternatively, the portion of the blank  100  between gluing tabs  74  can be left intact and folded, as shown in broken lines in  FIG. 5   a , to form a more liquid tight container. As described in greater detail above, the floor panel  84  comprises a moisture barrier, and preferably further comprises a microwave susceptor, which, in a preferred embodiment, includes a sized metal layer to facilitate microwave heating. 
   Other embodiments may alternatively be devised. For example, multi-walled tub assemblies having three wall panels  16 , or five or more wall panels  16 , are possible. Additionally, each wall panel  16  may be formed from one or more separate paperboard blanks, and attached to one another to form the sidewall assembly  14  by adhesive, folding and crimping, or other attachment means. Also, although the blanks used to form the tub assembly have generally been referred to a paperboard blanks, other materials of fabrication are possible, such as for example, cardboard and card stock, paper, plastic sheeting, and other foldable, moldable or formable materials. 
   Method of Assembly 
   The present invention is further related to a method of assembling a container substantially as described above. The method of assembly will be described according to a preferred embodiment, and with particular reference to FIG.  6 . 
   A sheet of barrier material  40  is provided. A pocket  42  is formed in the sheet of barrier material  40  by folding, crimping, or plastically and/or elastically deforming the sheet of barrier material  40 . The pocket is preferably formed by a vacuum platen  120 . Alternatively, the pocket  42  can be formed by mechanical folding or deformation. The vacuum platen  120  includes a recess  122  corresponding to the desired shape and size of the pocket  42  to be formed. A vacuum source  124  is in communication with the recess  122  to suction form the pocket  42  in the sheet of barrier material  40 . A male plug or mandrel (not shown) can be provided, cooperating with the recess  122  to form the pocket  42 . The vacuum platen  120  can further comprise heating means  126  to apply heat to the sheet of barrier material  40  to assist in forming the pocket  42 . More preferably, heat can be applied from an external source to assist in forming the pocket  42 . 
   A quantity of food product  12  is deposited in the pocket  42  formed in the sheet of barrier material  40 . The food product can be, for example, popcorn, pork rinds, puffed cheese snacks, or other food product. Cooking oil, shortening, spices, preservatives, flavorings, stabilizers, colorants, or other substances may be included with the food product. Metering means  128  are preferably provided for metering a predetermined amount of the food product, as by weight, quantity or volume. 
   An invented tub assembly  13  is placed over the food product  12 . The tub assembly  13  can include, for example, a generally rectangular or cylindrical tub assembly having a floor  30  and at least one wall panel  16 . The floor  30  of the tub assembly  13  has a moisture barrier, and preferably also comprises a microwave susceptor, which, in a preferred embodiment, includes a sized metal layer to facilitate microwave heating. The tub assembly  13  can be fabricated by folding at least one blank, as described above by way of particular examples, to form a floor and at least one wall panel. The tub assembly is placed over the food product, which is disposed in the pocket formed in the sheet of barrier material  40 , preferably in an upside-down orientation with the mouth  20  of the tub assembly generally downward. Barrier material surrounding the food product is brought into contact with the floor of the tub assembly, with the remainder of the barrier material draping downward along the interior of the walls of the tub assembly. 
   A seal  44  is formed between the sheet of barrier material  40  and the floor  30  of the tub assembly  13  to encapsulate the quantity of food product  12  between the sheet of barrier material  40  and the floor  30  of the tub assembly. The seal  44  is preferably continuous and formed by heat sealing the sheet of barrier material  40  to the floor of the tub assembly. For example, a heating element can be brought into contact with the exterior face of the floor panel  30  to form the heat seal. 
   According to the method of the present invention, the food product is hermetically sealed within the container by depositing the food product  12  between the sheet of barrier material  40  and a moisture barrier portion of the container, preferably the floor  30  of the container, and forming a continuous seal between the barrier material and the moisture barrier portion of the container, as described above. 
   The sheet of barrier material  40  can optionally be attached to one or more wall panel(s) of the tub assembly. For example a first attachment  50  can be made between the sheet of barrier material  40  and the wall panel(s) approximately mid-height along the wall panel(s), and/or a second attachment  52  can be made between the sheet of barrier material  40  and the wall panel(s) adjacent the mouth of the tub assembly. 
   While the invention has been described in its preferred forms, it will be readily apparent to those of ordinary skill in the art that many additions, modifications and deletions can be made thereto without departing from the spirit and scope of the invention.