Abstract:
A food package and method of controlling heating of a foodstuff within the food package using microwave energy wherein the food package has a microwave shielding layer containing a plurality of apertures therein sized to permit penetration of evanescent microwave energy into the interior of the package with the microwave shielding layer being moved outward as the package expands due to generation of water vapor such that an interior volume of the package is subsequently protected against substantial microwave irradiation of the foodstuff upon and beyond completion of the microwave heating cycle.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of microwave heating of foodstuffs, in particular to packaging designed for influencing the heating of the foodstuff as it is irradiated with microwave energy. 
     BACKGROUND OF THE INVENTION 
     With respect to microwave foods, it is often desirable that the microwave heating be controlled in order to prevent overheating of the food. One example is microwave heating and popping of popcorn. If popped kernels are subjected to prolonged microwave heating, scorching occurs. Currently, microwave popcorn is packaged in flexible paper bags. Embedded in the popcorn bag is a susceptor used to absorb microwave energy and aid popcorn heating and popping. Typically in packaging microwave popcorn, a slurry including popcorn kernels are located on top of the susceptor, the bag is folded over itself to a compact size. When the bag is placed in the microwave oven, instructions typically call for at least partial unfolding of the bag and placing the bag on a microwave transparent shelf or floor of the oven with the susceptor below the popcorn. When the popcorn bag is heated in the microwave oven, steam or water vapor from the popping popcorn causes the bag to further unfold and inflate. With the current bag designs, popped kernels are unprotected from microwave irradiation after popping. When heated above about 210° C., popped kernels begin to scorch. The present invention overcomes this shortcoming of prior art popcorn bags (and other microwave-related food packages) by providing a bag or package that initially exposes the unpopped popcorn (or other food load) to microwave irradiation to pop the kernels or otherwise heat the food load and thereafter protects the bulk of the popped kernels (or other heated food load) from microwave energy, thus reducing or eliminating the scorching (and other undesirable results of overheating) that would otherwise occur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a popcorn bag useful in the practice of the present invention shown in a first state prior to popping under the influence of microwave irradiation. 
     FIG. 2 is the popcorn bag of FIG. 1 shown in a second state with a substantial amount of popcorn popped. 
     FIG. 3 is a simplified perspective view of a conducting sheet with apertures useful in the practice of the present invention. 
     FIG. 4 is a side view of the perforated conducting sheet of FIG. 3, along with a simplified graph of evanescent microwave propagation power decay after microwave energy transits the sheet. 
     FIG. 5 is a schematic or simplified pictorial view of a generic version of the bag of FIG. 1 corresponding to the first state to illustrate certain features of the present invention. 
     FIG. 6 is a schematic or simplified pictorial view of a generic version of the bag of FIG. 2 corresponding to the second state to illustrate certain aspects of the present invention 
     FIG. 7 is a perspective view of an alternative embodiment of a package useful in the practice of the present invention and shown in an expanded condition. 
     FIG. 8 is a top plan view of the package of FIG. 7 illustrating a microwave shielding layer with apertures therein in an unfilled, flat condition, with portions broken away. 
     FIG. 9 shows a cross sectional view of the package of FIG. 7 according to section line  9 — 9  of FIG. 7, with the popped popcorn removed and the microwave shielding layer with apertures therein shown for illustration. 
     FIG. 10 shows a side view of the package of FIG. 7 in an opened condition. 
     FIG. 11 is a view similar to FIG. 1, except that the popcorn bag is generally enclosed by a microwave shielding layer with apertures only in a limited region thereof and with the unpopped popcorn load omitted for clarity. 
     FIG. 12 is a view according to FIG. 1, except showing the popcorn bag in the second state and with the popped popcorn load omitted for clarity. 
     FIG. 13 is a bottom plan view of the bag of FIG. 12 in the second state. 
     FIG. 14 is a plan view of an alternative lattice arrangement for an aperture pattern useful in the practice of the present invention. 
     FIG. 15 is a perspective view of the popcorn bag of the embodiment of FIG. 1 shown in an a completely folded state. 
     FIG. 16 is a perspective view of the popcorn bag of FIG. 15 shown in a partially unfolded state. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the Figures, and most particularly, to FIGS. 1,  2 ,  11 ,  12 ,  15  and  16 , a food package  20  useful in the practice of the present invention may be seen. Food package  20  is in the form of a modified conventional microwave popcorn bag having wings  21 ,  23  in which the unpopped popcorn  22  is vended or sold for consumers to place in a microwave oven and pop the popcorn. It is to be understood that the unpopped popcorn load  22  typically will include fat, oil, salt, colorings, flavorings or the like in addition to the popcorn kernels, forming a mass or slurry  24 , typically positioned on a microwave susceptor  26 . Susceptor  26  may be a conventional susceptor as is well known to use for microwave heating, especially for popping popcorn. 
     Referring now also to FIGS. 5 and 6, as well as the Figures already referred to, in this embodiment, the package  20  is preferably a flexible, inflatable bag. Bag or package  20  can be made from any desired material but is preferably formed of paper, one or more polymers, or a combination thereof, including but not limited to base coated paper or similar polymer structures or the like. It is to be understood that FIGS. 5 and 6 show an “idealized” package to illustrate certain aspects of the invention. 
     The package  20  preferably includes one or more septic layers  28  such as paper or plastic to provide a clean or sanitary environment and a suitable external appearance for the foodstuff during vending and handling. In addition, as part of the septic layer, (or as a separate layer) package  20  also has a water vapor barrier layer (e.g., interior layer  29 ) for reasons which will become apparent. It is to be understood that the water vapor barrier layer is desirably similar or identical to that used in conventional popcorn packaging intended for use heating in microwave ovens. It is to be further understood that this layer is sealed sufficiently to cause or allow the bag to inflate as is conventional in the microwave popping of popcorn, for reasons to be explained infra. 
     Unlike conventional packages for microwave popcorn, the package  20  of the present invention further includes a microwave shielding layer  30  which may be formed of metal. Referring now also to FIGS. 3 and 4, the microwave shielding layer  30  has a plurality of apertures  32  therein, with each aperture sized to permit substantially only evanescent or non-propagating microwave energy to enter the package. In the preferred embodiment, layer  30  is desirably thick enough to prevent the transmission of microwave energy therethrough {and is desirably thick enough to avoid layer  30  functioning (generally) as a susceptor}. It is believed that conventional susceptors are in the range of tens to hundreds of Angstroms in thickness. For conventional metals such as copper and aluminum (not acting as susceptors, but instead providing microwave shielding) the penetration depth is about a few microns. 
     The shape and size and pattern or lattice of the apertures are preferably chosen to limit transmission of microwave energy to substantially only an evanescent mode when the microwaves transit the layer  30 . This is achieved primarily by maintaining the maximum dimension  36  of each aperture  32  to be sufficiently small to prevent transmission of propagating modes of microwave energy through layer  30 . In comparison, and as a figure of merit, for a long waveguide with square cross section, the microwave energy is limited to an evanescent mode when: 
     
       
           a &lt;λ/2  (1) 
       
     
     where “a” is the linear dimension of the waveguide cross section, and λ is the free space wavelength. 
     In general in the prior art, “evanescent mode” has been used to refer to operation below cutoff, i.e., λ&gt;λ c , where λ c  is the cutoff wavelength, and the guide wavelength λ g  is given by Equation (2): 
     
       
         λ g =λ 0 /(1−v 2 ) ½   (2) 
       
     
     where ν is the normalized wavelength, given by Equation (3) as the ratio of the wavelength of interest, λ, to the cutoff wavelength: 
     
       
         ν=λ/λ c   (3) 
       
     
     The free space wavelength is about 12.24 cm for 2450 MHz. As used herein, the term “evanescent” is believed to be consistent with, but an extension of, the use of that term in the prior art. Typically, in a microwave oven, the cavity is “overmoded,” unlike conventional waveguide operation. Since the food package of the present invention is exposed to the overmoded field in order to carry out the present invention, the term “evanescent” here is used by analogy or extension to prior art use and refers to decaying, as opposed to propagating microwave energy passing through the grid or aperture pattern of the microwave shielding layer  30 . 
     Returning again to conventional prior art systems, below cutoff, the microwave energy decays generally exponentially with a depth of penetration  49  given by Equation (4): 
     
       
           D   p =( a /2π)[1−(2 a /λ) 2 ] −½   (4) 
       
     
     As is illustrated generally in FIG. 4, the microwave power transiting sheet or layer  30  having apertures  32  therein is evanescent when the maximum dimension of the apertures  32  is below a length permitting propagating power to pass through such apertures. For square or rectangular apertures, the maximum dimension is a diagonal  36 . For apertures of other geometries, the maximum dimension is characteristically the longest “free” dimension of the aperture, e.g., for an ellipse, the chord through the two vertices (along the major axis) is the maximum dimension. Curve  38  is an illustration of the power decay of evanescent microwave energy plotted with energy on the ordinate axis  40  and distance from the layer  30  along the abscissa  42 . It is to be generally understood, that the smaller the maximum dimension of the apertures, the more rapid the power decay, provided that other design parameters are held constant. The evanescent mode of microwave energy transiting the apertured layer  30  will form a spatially limited zone of microwave energy beyond the outer surface of layer  30 . The depth of the zone beyond the layer  30  can be adjusted by varying the dimensions (especially the maximum dimension) of the apertures in the layer, or by adjusting the shape or pattern of the apertures. In the practice of the present invention, apertures  32  in layer  30  create a spatially controlled “penetration zone”  44  (see FIGS. 5 and 6) for microwave heating within package  20 . 
     In FIG. 5 it will be noted that when the package or bag  20  is collapsed in its initial configuration, the penetration zone may extend substantially across the entire interior of the package, thus permitting microwave irradiation both from above and below, in effect providing an “overlap” of the penetration zone extending down from the top layer with the penetration zone extending up from the bottom layer. In the alternative, the upper and lower penetration zones may abut each other, or it may be desirable (for other reasons) to have the penetration zones not overlap, e.g., in the event the food load is desirably or necessarily thicker than the sum of the depths or thicknesses the desired penetration zones. 
     In FIG. 6, with the bag expanded or inflated, the penetration zone  44  extends only a predetermined, limited distance within layer  30 , with the boundary of the penetration zone  44  indicated by dashed line  46 . In the idealized images shown in FIGS. 5 and 6, it is to be understood that apertures  32  extend across substantially all of the surface of layer  30  of package  20 . 
     While the pattern of apertures  32  may extend across the entire package (as is illustrated in an alternative embodiment in FIGS.  8  and  9 ), alternatively, the microwave shielding layer  30  may extend across substantially all of the surface  62  of the food package  20 , with one or more patterns of apertures  32  extending across only one or more predetermined, limited regions, for example, a region made up of sub-regions  34 ,  48  of the food package  20 , as is shown in FIGS. 11 and 12. As a still further embodiment, various regions may have different sized or shaped or spaced apertures to selectively control the microwave energy passing through layer  30  and into the interior of package  20 . To that end, it has been found that altering the spacing between apertures can be used for such microwave energy control. Furthermore, it has been found that using a regular lattice i.e., one having constant spacing between apertures and between the rows and columns of apertures, is the most restrictive to the passage of microwave energy through the grid of layer  30 . As used herein, it is to be understood that “lattice” refers to the geometrical arrangement of apertures, particularly the spacing between adjacent rows or columns (or both) of the apertures  32  in layer  30 . 
     It is to be understood that it is within the scope of the present invention to use offset lattices in the practice of the present invention. Such offset lattices can be periodic or non-periodic, and different regions of the microwave shielding layer can have different lattice arrangements in addition, or as an alternative, to changing the shape and size of individual apertures. In FIG. 14, a triangular lattice  64  is formed by the pattern of individual apertures  32 , and is illustrative of an alternative to the regular lattice or pattern of apertures shown with respect to the earlier Figures. It is also within the scope of the present invention to use other aperture shapes in such alternative lattice arrangements, as well. 
     Turning now to the embodiment shown in FIGS. 11,  12  and  13  (which correspond to the embodiment of FIGS.  1  and  2 ), the evanescent mode microwave energy penetrates layer  30  in an upper surface only in a region  48  corresponding to the food load  22 . At the same time, microwave energy is continuously applied through region  34  of a lower surface to heat the food load  22 . In this embodiment, package  20  thus includes a predetermined region containing the plurality of apertures that includes both isolated sub-regions  48  and  34  on more than one surface of the food package or bag  20 . Initially, in this embodiment, the predetermined region is preferably generally congruent to the food load  24  as it exists prior to being heated. As the food load  24  is heated, the bag  20  inflates due to the steam or water vapor generated by the popcorn popping, moving region  48  away from the food load  22 , thus limiting penetration of microwave energy through apertures  32  to a penetration zone adjacent the interior of region  48 . In this embodiment, the popped popcorn will be shielded by layer  30  from further exposure to microwave energy while the food load  22  will be continuously exposed through sub-region  34  to the microwave energy to complete popping. Furthermore, gravity will move the popped kernels away from the sub-region  48 , even though continued popping will jostle the popped kernels. Referring now again to FIG. 6, the depth  49  of the penetration zone  44  can be controlled and varied from place to place along the bag or package  20  (or  50 ) by using different sizes or shapes or numbers or spacing of apertures  32  in different sub-regions of layer  30  around the bag  20 . For example, and not by way of limitation, penetration zone  44  can have a depth of penetration or thickness of ¼ inch adjacent sub-regions  48  and  34 , and a lesser depth of penetration  51  of ⅛ inch in the remainder of the interior of the food package  20 . Referring now again to FIG. 4, the example numerical values for the depths of penetration  49 ,  51  are relative figures of merit, for example, and not by way of limitation, the half-power points corresponding to distance  55  away from ordinate axis  40  (representing the outer surface of layer  30 ) where level  53  is one half the peak power  57  of curve  38 . 
     Referring now most particularly to FIGS. 15 and 16, the embodiment of FIGS. 1 and 2 is shown in fully folded and partially folded configurations. FIG. 15 shows bag or package  20  with first and second wings  21 ,  23  in a fully folded configuration. FIG. 16 shows bag  20  in a partially folded configuration with wing  21  folded and wing  23  unfolded. It is to be understood that bag  20  is preferably fully folded when packed for shipment and sale. In the practice of the present invention, bag  20  may be placed in a microwave oven fully or partially folded, or fully unfolded (as illustrated in FIGS. 1 and 11) prior to exposure to microwave irradiation. However, it is preferred that the bag  20  be fully unfolded as shown in FIG. 1 prior to microwave irradiation. As with conventional bags, if a susceptor  26  is used, bag  20  is preferably oriented with the surface containing the susceptor located on the bottom. 
     The present invention, in the embodiments shown, provides a bag for reducing scorching while still enabling popping of popcorn, or popping, puffing, or otherwise heating other foodstuffs, by allowing significant penetration of microwave energy into the bag, delivering sufficient energy to pop the popcorn while the bag is in a collapsed or folded condition. After popping has inflated the bag, the majority of the food package interior (i.e., the region beyond, or interior of, the penetration zone) is protected from further entry of significant microwave energy. This is accomplished by selecting one or more sizes of apertures  32  to permit passage of substantially only evanescent (i.e., non-propagating) microwave energy modes into the interior of the bag. In the practice of the present invention wherein the susceptor  26  is interior of layer  30 , there is preferably a region  34  in layer  30  on the bottom surface of the package  20  at least substantially congruent to the susceptor  26  to permit microwave energy to reach and heat the susceptor  26  as the energy enters from the bottom of the package. If susceptor  26  is located exterior of layer  30 , it may still be preferable to have a grid or perforated region  34  on the bottom of the package to enable microwave energy to pass through susceptor  26  and heat the food load located inside the package. In either event, the lattice or grid of region  34  is desirably arranged to prevent entry of microwave energy other than evanescent mode energy into the interior of package  20 . This may be accomplished by providing a pattern of apertures  32  adjacent to the susceptor  26 . It is to be understood that the susceptor  26  may be located interior or exterior of the microwave shielding layer  30 , (or even may be omitted) as desired. 
     Referring now to FIGS. 7 through 10, an alternative embodiment of the present invention may be seen. In FIG. 7, the package  50  of this embodiment is shown in an expanded condition. The package or bag  50  is generally circular in plan as may be seen most clearly in FIG.  8 . As with the previously described embodiment, bag  50  is preferably formed of a flexible, but non-extendable material such as paper or similar cellulose material  52 , with a microwave shielding or reflective layer  54  laminated thereto. The various panels or walls making up bag  50  are preferably sealed to trap the water vapor created within the bag  50  during microwave heating thereof, while at the same time allowing selective rupture when desired to permit access to the interior of the bag when the food is to be consumed. It is preferred to provide an annular adhesive strip  56  to secure the walls of bag  50  together, using heat and or pressure. 
     It is to be understood that it is preferable to form bag  50  as a generally planar assembly when collapsed. FIGS. 8 and 9 illustrate that the microwave shielding layer  54  is perforated with apertures  32  across substantially all of the surface thereof, with the possible exception of the adhesively secured seams  58  and  59 . As in the first embodiment, it is to be understood that the microwave shielding layer may be invisible to a consumer user, being laminated between other layers forming a sanitary or septic food package. In FIG. 9 a susceptor  60  is shown, preferably secured to bag  50 . As with the first embodiment, susceptor  60  can be exposed to the full effect of microwave irradiation by being located exterior of the microwave shielding layer  54 , or it may be attached interior of the apertured microwave shielding layer  54 . Bag  50  is preferably loaded with a charge of unpopped popcorn, and fat or oil, with flavorings and colorants optionally included. Bag  50  is preferably folded into a generally rectangular configuration for shipping and vending, and, in its folded configuration, may be of a size and shape similar to the first embodiment or other conventional microwave oven ready popcorn packages. 
     Bag  50  also preferably has a removable cover  92  overlapping an opening  94  in the upper surface thereof Cover  92  preferably has an adhesive seam  59  which is openable by a consumer once the popcorn is popped, as is illustrated in FIG. 10. A non-adhered flap  96  preferably is formed integrally with cover  92  to assist in opening the bag  50 . It is to be understood that cover  92  may have an aesthetically pleasing outer layer  52  formed, for example of a heat stable polymer or paper and an inner microwave shielding layer  54 , with apertures therein, as is illustrated in FIGS. 8 and 9. 
     It is to be understood that the contents of the food package of the present invention may be popcorn kernels or any suitable grain such as rice, maize, barley, sorghum, or the like for being popped or puffed when heated or reheated in a microwave oven. 
     When subjected to microwave heating, the susceptor will convert microwave energy to heat, and the food load will be subjected to direct heating until sufficient water vapor is released to expand the bag sufficiently to move the upper apertured microwave layer away from the food load by a distance greater than the depth of penetration of the evanescent microwave energy. As popping or puffing continues, the food package will inflate or expand further, enlarging the volume protected from substantial microwave irradiation interior of the penetration zone. It is to be understood that the penetration zone extends substantially across the entire interior surface of package  50 . Nevertheless, the protected volume will eliminate scorching of the popped popcorn therein, and the jostling of the popped popcorn will constantly move peripheral popped kernels into and out of the penetration zone, further reducing the chance of scorching. 
     The grid pattern for square apertures in the practice of the present invention is preferably in the range of ½ to 1½ inches in linear dimension (the length of each side of an aperture). In order to create evanescent microwave energy interior of the microwave shielding layer, the thickness and width of the grid pattern forming the apertures must be greater than the penetration depth δ of the conducting material. For a material of conductivity σ, the penetration depth is given by Equation (6): 
     
       
         δ= c /(2πσω) ½   (6) 
       
     
     where c is the speed of light, and ω is the microwave (radian) frequency. The width of the grid between apertures should not be so great as to prevent formation of significant evanescent waves interior of the microwave shielding layer to heat the food. 
     For this reason, the width of the grid is desirably greater than the penetration depth (a few microns, depending on material) and less than ⅜ inches. It is to be emphasized that the shape of the apertures can be regular or irregular, and can include, but is not limited to square, triangular, round, elliptic and even irregular or amorphous (if limited in its maximum dimension to achieve the evanescent microwave mode). The grid or aperture pattern can be regular across the surface of the package or it can be interrupted or irregular, as desired to achieve the proper heating effect for the particular food load carried by the package. The microwave shielding layer can be formed of any material capable of reflecting microwave energy, including, but not limited to, most metals and alloys, such as aluminum, nickel, copper, silver, iron, stainless steel, and the like. 
     The amount of microwave energy entering the food package can be controlled by varying the parameters of the apertures  32 , the grid bridge  72 , the thickness  82  of sheet  70  making up microwave shielding layer  30  in food package  20 , and the pattern (or lattice type) of the apertures  32 . It is to be understood to be within the scope of the present invention to vary (or hold constant) one, some or all of these parameters across the surfaces of the food package to obtain desired results by controlling the evanescent microwave energy entering the food package. This is true notwithstanding whether or not additional, non-evanescent microwave energy also enters the food package, provided that the food load is primarily influenced by the evanescent microwave energy at least in the regions where the present invention is being practiced. 
     It is to be further understood that the present invention is suitable for selective heating of foods other than popcorn and other puffed foodstuffs. For example, and not by way of limitation, a filled pastry that gives off water vapor when heated, may be heated and a topping such as frosting may be melted using a food package according to the teachings of the present invention. In such an application, the filling may be prevented from being overheated (to avoid scalding a consumer) while the outer surface of the foodstuff can be heated and even browned, if desired, using the penetration zone of the present invention to selectively heat the foodstuff, and prevent overheating by inflation of the package during microwave irradiation. 
     As another example, and not by way of limitation, the present invention may be used to selectively and controllably heat or cook a pizza using microwave irradiation, where the food package for the pizza may have relatively small apertures in a lower surface to admit evanescent energy only (or primarily) to the pizza crust below the toppings while the upper grid or region above the pizza food load may have apertures suitable for sufficient, but not excessive, heating or cooking of the toppings, followed by a movement of the upper grid away from the pizza (as a result of the water vapor generated) to prevent overheating of the toppings. This approach may be utilized with or without a susceptor to achieve desired browning of the crust, and to simultaneously achieve desired cooking of the toppings, without overcooking. 
     The invention is not to be taken as limited to all of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention.