Software Technology for “Container for active microwave heating”

Food Science Packaging – Phillip L. Minerich, Bryan C. Hewitt, Cindy M. Lacroix, Melville D. Ball, Hormel Foods LLC

Abstract for “Container for active microwave heating”

An microwave container with synergistic active components provides uniform heating and is more sensitive to variations in food product, load, and heating conditions. Active elements, which are both microwave opaque and conductive, include an annular band in the base of a container, a band that extends from the base to the sides of the walls, a lip that extends from the bottom of side walls onto the base and at least one, preferably, three cooperative active elements within the lid of a container. These containers are suitable for cooking frozen, uncooked meats, and other foods that have not been achieved with prior art containers.

Background for “Container for active microwave heating”

“This invention is a container for active microwave heating food products. This invention is a better active container system that can heat or cook a variety food products. The containers of the invention can also be used to cook and thaw frozen foods, such as meat. These products can all be cooked evenly and heated in an efficient manner, without any overcooked, dried, or scorched areas.

Microwave heating has significant advantages when it comes to thawing or reheating food products. The most important benefit for consumers is the shorter time it takes to heat frozen foods. However, there are some drawbacks. The microwave heating process is uneven in conventional packaging. This means that certain areas of the food product are not adequately heated. Meanwhile, food at the edges of containers tend to be overheated or dried out.

This problem has been addressed with a variety of approaches and designs. To “shield’ overdone food, some designs use microwave reflective materials such as metallic foils. However, this reduces the energy required to cook these portions, and increases the cooking time and energy efficiency.

U.S. Pat. Nos. Nos. EPO application 92105572.9 was filed to Saunier. These containers can reduce the overheating of food around the edges, but they have not been widely used commercially. The potential benefits of containers have often been outweighed by their increased cost.

“A newer approach uses materials in or parts of the package to modify microwave radiation. This type of packaging is disclosed in U.S. Patent. Nos. Nos. “Active microwave packaging” is packaging that alters the electric (or magnet) field configuration and the heating pattern of the product within. Active packaging includes susceptors and heater boards that are used to brown or crisp products.

“Active packaging that alter the electrical field makes more efficient use of microwave energy and provides more even heating of food and other materials inside the container. They make microwave heating possible for many products that cannot be heated in any other packaging. However, these designs have not been able to handle particularly complicated products like large quantities (more than 300g) of frozen, uncooked meat products.

“It is the object of the invention to provide an improved active microwave container that modifies microwave radiation in the container to better distribute energy for heating, defrosting and cooking food.”

“Another object is to provide a container that produces satisfactory results with a wider range of food products than the currently available containers.” Another object is to provide a container that freezes uncooked meats and other food can be cooked in a microwave oven.

“An additional objective is to provide a container capable of providing good results in a variety of microwave oven styles and types, as well as that is sensitive to load and fill variations like those found in commercial products.” Another objective is to make the container relatively insensitive to changes in microwave oven position.

These and other advantages can be achieved by a container with an open-ended tray, a closed-base and side walls that extend from the closed base to open ends, and a lid covering the open edge. The tray’s closed base is made of microwave transparent material, such as paper-board or another suitable plastic material for microwave cooking and reheating (e.g. Polypropylene, Polyester or the like) and an annular circle of conductive material and microwave opaque material. Side walls are covered with a band of conductive or microwave opaque material such as foil. This extends from the sidewalls onto the closed bottom, creating a lip around it.

The lid consists of a microwave transparent material, preferably heat sealable polyester film, and at most one conductive, microwave opaque element. These elements are separated from the sidewalls by an annular area made of microwave transparent material. It is preferable to have one or more additional conductive microwave opaque elements between each element and the sides walls. These are separated from the side walls using microwave transparent material.

These active elements of microwave opaque material and the boundary conditions created by the container walls and food act to modify the microwave fields that are incident on the food within the container. Side walls are made of conductive, microwave-opaque material that prevents food from overheating. This conductive sidewall material creates well-defined boundary conditions for the incoming microwave energy.

The conductive elements in the lid, base, and side walls are designed to work together. The lid’s active elements modify microwave fields incident on the food’s upper surface. The heating behavior of lower portions of the food is also dominated by the elements in the tray. The overall uniformity of heating is enhanced by the synergistic effects between upper and lower active elements. The containers of the invention are suitable for products that cannot be heated satisfactorily using prior art containers.

These and other benefits and objectives of the invention will be apparent in the following description.

“DRAWINGS”

“FIG. “FIG.

“FIG. “FIG. 1. FIG. FIG. 2A shows a cross-sectional view of the lid. FIG. 2A is a cross sectional detail view of lid, and FIG. 2.”

“FIG. “FIG. 1, 2 and 3.

“FIG. “FIG. The lateral axis of the container is shown in FIGS. 1-3. These curves show the variation of the electric field intensity at both the top and base of the food (AA) and the container’s bottom (BB).

“FIG. FIG. 5 shows the temperature distribution in a container incorporating this invention.

“FIG. “FIG. 1-3.”

“The package illustrated at FIGS. The package illustrated in FIGS. 1-3 contains an oval tray, commonly referred to by 20 and covered with a lid generally referred as 10. FIG. FIG. 2 shows that the package contains a 50 g food product. This may include a portion of uncooked, frozen turkey meat and gravy. Between the top of the food products 50 and the lid 10, there is 60 mm. The head space (distance between food product 50 and lid 10) should be approximately 2-20 mm.

The tray has a rectangular base 24 with side walls 26 that taper upwards and outwards from the closed base 24, to an open oval top. A flange 28 extends outwardly beyond the top of 26 side walls. The preferred embodiment has an open oval top measuring 185mm in length and 125mm wide. The inside dimensions of the oval base are 165mm long and 105mm wide. Side walls measure 42mm high along the sidewalls.

“Tray 20” is made from a paperboard blank or another microwave transparent material such as foil-label plastic or foil with apertures. The tray’s paperboard shell is coated with active elements. Additional active elements are attached to the lid. These active elements can be conductive or microwave opaque. The elements are conductive and microwave opaque because they are made of materials with a combination thickness and conductivity at microwave frequencies. This means that most of the microwave energy that is incident on them will reflect back. Practically, the amount of incident microwave energy that these elements absorb or transmit will be negligible. The following criteria should be met for reflection (R), absorption(A) and transmission coefficients (T):

“R>0.9” (i.e. more than 90% should be reflected);

“A+T0.1” (i.e. less than 10% should be absorbed/transmitted).

In the illustrated embodiment, active microwave elements are made from aluminum foil that is at least 5 microns thick. As is well-known in the art, active microwave elements can also be made by deposition of a metallic pattern or using conductive inks.

“A foil band 32 is attached or embedded in the 26-inch side wall of the paperboard shell 22. The foil lip 34 is an integral part the foil band 32. It extends from tray 20’s side wall 26 to the base 24. The oil band 32 protects food from overheating and allows for the establishment of well-defined boundaries for microwave energy.

“Preferably the foil band extends along the sidewalls to a height of 26 to 31mm. This is about the height of the food inside the container. The foil should not be more than 5mm below the top edge of the food. This will cause the foil to overheat. Strong localized fields may form if the foil is more than a few millimeters higher than the top of food. These fields can heat or char the container. If the food is not at least twice the height of the foil, these fields will be absorbed by food.

Alternately, you can continue the foil band 32 to the top of your sidewalls and onto flange 28. It is best to extend the foil to the edge of flange. The foil should extend to the edge and not touch any metal walls. This will allow the heat generated by the foil to be dissipated into air. This can increase the risk of arcing in unusual circumstances. The preferred arrangement is to extend foil band 32 to the expected food level.

The foil lip 34 should be between 2 and 10 millimeters wide. Foil lip 34 should not be wider than 10mm. This will result in excessive shielding and poor heating of food at the lower corners. Manufacturing irregularities can result in foil lip that is less than 2mm wide. This could cause foil band 32 to stop short of 26 at the bottom. This can lead to undesirable field intensification at your side wall, as we have already mentioned.

“The enclosed base 24 of tray 20 also contains an annular ring 36 made of conductive, microwave-opaque aluminum foil. The shape of the annular ring 36 should match that of the base. The ratio of the minor and major axes (or widths) of the annular ring 36 should be equal or similar to that of base 24’s major axis. The ratio of the annular foil ring 36’s width to the base 24’s length is 0.65. For the oval container illustrated, it is 0.65. The ring’s dimensions (the average of inner and outer dimensions) should be approximately constant. They should move angularly around a ring with the same dimensions as the container base aperture (i.e. The aperture is defined by the inner edge 34 of the lip. This ratio should be between 0.4 and 0.0.7 for circular containers and between 0.5 to 0.6 for elliptical ones. This ratio is approximately 0.55 in the illustrated container. This ring’s maximum diameter is the distance between the outer edges and the outer edges of the ring along its major direction. It measures approximately 90mm. The base’s 165mm overall length is 0.55. The base’s minimum overall diameter is 60mm. This is 0.55 times 105mm. The ratio can vary from one point to another around the container due to manufacturing distortions. Variations of 0.15 or more will be acceptable in some applications. However, it is preferable to keep the ratio relatively constant.

The preferred annular ring 36 width (the distance between the outer edge and inner edge at any point around a ring) is between 2 mm to 10 mm. This allows for sufficient interaction with microwave energy, but not so small as to create a shielded area of any significance beyond the ring 36. Annular rings smaller than 2 mm can function satisfactorily provided that the electrical conductivity is high enough to cause desired field modification. However, this will require increased manufacturing difficulties and higher costs for consistent and reliable production. The alignment of material during container pressing is also critical and costly if the lip 34 is less than 2mm.

Refer to FIGS. 3 and 6. 3, and 6. The tray is made from shell 22 of 282# milk carton stock newspaperboard. The side wall band 32, lip 34 and annular ring 36 are applied to shell 22 by adhesively laminating 8 micron foil to a film 38 of 48 gauge PET, or polyethyleneterephthalate, demoralizing the foil to form the desired patterns for sidewall band 32, lip 34 and annular base ring 36, and adhesively bonding the foil/PET laminate to the paperboard. FIG. 46 shows the Pleats. The side walls 26 and the flange 28 are then drilled with pliats 46 (shown in FIG. 2, and 3.

The container lid contains additional microwave active elements. The container lid contains a sheet made of microwave transparent material 12, an oval-shaped active element of aluminum foil 16 and two ring segments 17 also made of aluminum foil. The ring segments 17 are separable from the central oval 16 and the sides walls by microwave transparent material. The ring segments form an annular, interrupted lid ring that is interrupted by gaps of microwave transparent material at the ends.

The die-cutting of adhesive coated pressure sensitive foil creates the central oval 16 and annular segments 17. The oval 16 and the ring segments 17 are placed on a large piece of adhesive-coated pressure sensitive paper stock (or label 14), which acts as a carrier. It keeps the active elements in a proper relationship. The transparent film 12 is adhered to label 14. Film 12 is heated sealed to flange 28 after the food product 50 is placed in the container. This seals the open end of tray 20, with a minimum bond strength of 100 grams.

The central oval 16 should have a similar shape to the container’s top-inner dimensions. Additionally, the ratio of principal dimensions of oval and top-inner container dimensions should remain approximately constant. This means that the ratio between the oval’s length and the container’s top-inner length should roughly equal the ratio between the oval width and the container’s top-inner width. An oval lid with three active elements such as the illustrated container has a preferred ratio of between 0.2 to 0.3. The container’s oval element 16 measures approximately 0.27 inches in length and oval element 16 measures approximately 0.23 inches in width. It is desirable to maintain a consistent ratio between the diameters of the oval element 16 and the diameters of the open ends of the containers at all angles around the container, as with the annular rings 36 in the tray’s base 24.

The size of the central oval is not affected by the microwave energy enhancement or modification. The oval becomes smaller, but it is less sensitive to headspace variance. A lower concentration of microwave field intensity can result in intense heating of a narrower, central area of the food rather than a more diffuse, larger region that heats deeper into the food. The overall performance is affected by increasing the size of the central oval.

A lid with one central foil element can be quite effective in increasing the overall heating performance of the container. To distribute microwave energy evenly, more elements are needed, such as the annular rings segments 17.

“Annular rings segments 17 and the gaps between them are segments of an annular ring interrupted with dimensions that are determined in relation the central oval 16 in this way:

“1. The distance between the inner edge and the edge of central oval 16 should be roughly constant. This distance should be approximately 10-25 mm for the illustrated container. It is preferred to be around 10 mm.

“2. “2.” The width of the annular rings defined by ring segments 17 should be constant. This ring should measure approximately 10 to 20 millimeters wide for the illustrated container (preferably 15 millimeters).

“3. To avoid local field intensification, the corners of the annular rings segments should be rounded to a radius of 2 mm or more.

“4. “4.

“The size, shape, and spacing of the annular rings segments 17 were determined empirically. Small adjustments to position and size were made to “fine tune? the container performance and achieve the desired degree or uniformity, while avoiding any potential for arcing or dielectric breakage between the annular segments or one of the segments and the central oval 16.

These dimensions have been experimentally determined to be very critical. Excessive fields around the edges and active elements can result in deviations greater than 2 mm. If the gaps are significantly larger, effectiveness will also decrease.

The effects of active elements in this container on uniform distribution can be better understood when there is a discussion about the performance of the container with no active elements. The interaction between the rapidly changing electrical field and the molecules or ions in the food causes microwave heating. In the unfrozen state, water molecules and salts play a significant role. The incident field (by vibrating or rotation) is not as effective in freezing water and salts.

“Power absorption per unit volume is proportional to square of the electric fields.”

“P=2?? “P=2?”

“?o is electric permittivity of space”

“?r” is the relative permissivity (e.g. “?r is the relative permittivity (e.g.

“E” is the electric field (magnitude), at the point of interest (V/m).

“In a container of the same dimensions and shape as the illustrated one, but with no central oval 16 or annular rings segments 17 in it, the foil band 32 in its sidewalls, and the foil lip 34 in the base, the dominant field intensity pattern will be determined based on the size and shape the food load, and the field distributions in the oven. Most foods have dielectric properties that are significantly different from those in free space or air. For example, the microwave wavelength of unfrozen food will be approximately 12 mm. In air, the equivalent wavelength is 120 mm. These large dielectric changes at the food-air interfaces can cause reflections and other refraction effects that alter the overall field distributions within the food and the oven cavity. This means that the field distributions reaching the food surfaces must conform to the “boundary conditions” created by food in the container.

“Generally, many field patterns or modes will be compatible with the boundary conditions. For typical food containers, however, the most common field patterns (modes), have dense fields around the edges and minimal intensity in the middle. This is why many foods products have a difficult central region that can be heated efficiently, but the edges are more efficient.

These field distributions in rectangular containers can be described using combinations of sine or cosine functions (analogous to rectangular waveguides and cavities). The mathematical descriptions of elliptical and round containers are based on modified or Bessel functions.

“Active elements, such as foil and other conductive, microwave-obvious materials, are designed to alter this field distribution to produce a more desirable pattern of heating. The conductor will be infected by electric currents when microwave energy reaches it. These currents’ intensity and pattern will depend on how the microwave energy arrives at the conductor and what the foil is made of. ”

“Foil and other conductive elements can be used on a microwave transparent cover to modify the fields. They can also develop higher order modes close to the food surface. The modified field’s effect is affected by the size, shape, quantity, and distance between the food and the element. Element that mimic the shape of an oval container are ideal for modification. Similar shapes and elements can be used to modify rectangular, square and circular containers.

“In the case where an oval foil element is used (as in the central oval 16 of this container), the patterns and currents that are induced will be typical of the shape of the element. These rapidly changing, circulating currents will lead to the reradiation of microwave energie. The element acts as a “patch antenna”. The element is located close to the food surface so a significant portion of the energy will propagate to the surface.

“The central oval 16 increases the microwave intensity in central regions relative to outer regions. This greatly improves the heating uniformity. This improves heating uniformity significantly for oval containers. However, the dominant mode (or the field pattern) generated from the combined influence between the single label (or container) results in a heating distribution that is relatively uniform. There are some residual cooler areas (diffuse zones in the annular area between the zone covered with the central label, and the outer container walls, mainly towards one end of the container).

The structure was also equipped with two annular segments 17 to improve heating uniformity. As can be seen in curve A-A of FIG. 4 The ring segments alter the field distribution created by central oval 16 and provide a localized heating effect (field enhancement) in the area immediately below the annular rings segments 17.

“Plot B-B-B in FIG. 4. The active elements in the tray alter the energy distribution at bottom of container. This plot shows the schematic representation of field intensity along a line through the bottom. The active elements in the tray alter the energy distribution at bottom of container. It produces a central field maximum as well as two subsidiary maxima. These maxima correspond to the aperture that is defined by the foil lip 36 and the foil ring 34. The annular ring 36 is the location of the minimum field intensity, while the inner edge and foil lip 34 are the positions for the minimal field intensity. Because conductive foil components cause components of the electric fields parallel to conductors to be null, any non-zero electrical field causes charge flow within conductors until an equal and opposite field is created to cancel the original field. Foil edges are boundary conditions for microwaves arriving at base of container. Some key field components will therefore be zero at annular base ring 34, and inner edges of foil lip34.

FIG. “As you can see from FIG. 4, and the following example, the active elements of the lid 10 and tray 20, work together to efficiently distribute energy so that even heating, defrosting and cooking can happen. This distribution allows for relatively large food loads (25-40mm) as well as foods that aren’t homogeneous such as meat, gravy, entrees, and side dishes to be prepared for microwave heating, defrosting, and/or cooking. Many products can be microwave heated. This is possible because prior art packaging was not designed for smaller food loads or foods with more homogeneous ingredients, such as pasta and macaroni, cheese, and sliced meats.

“EXAMPLE 1”

“The tests were done in a Kenmore oven 750 watt with the container centered in it. The container was heated on high power for 12 minutes. The heating cycle was completed. Temperatures were taken as soon as possible after the expiration of the 12 minutes heating time. Twelve thermocouples, each calibrated at three levels, were used to record the temperature. FIG. 52 shows the figures. FIG. 5 shows bottom temperatures. They were measured approximately 2 to 3 millimeters higher than the container’s base. Oval 54 shows the bottom temperatures at about 15 millimeters above container base (at the estimated food-depth). “Oval 56’s top temperature measurements were taken from just below the food surface (about 2 millimeters).

The product, which contained approximately 8 ounces turkey breast meat and 8 ounces gravy, weighed in at 16 ounces. The product was weighed before and after cooking to determine its weight loss. The 11.8% weight loss is well within the range of 15% for uniformly heated products.

“This invention offers a number significant advantages over previous microwave heating containers. Heating is uniform and energy is used more efficiently. The container described above can be modified by those skilled in the art. The dimensions shown here are the preferred dimensions of the oval container when it is used to store uncooked or frozen meat products with gravy. Final adjustments might be required for other products. It may be possible to heat one portion more than the other if the products are not homogeneous or the fill depth is different in different parts of a container. This could happen with vegetables, meats, and other side dishes. Small adjustments to the dimensions and spacings of active elements can make a significant difference in heating behavior. A small decrease in the size of the central oval 16 or annular rings segments 17 on the lid can result in more energy being concentrated into the central area. A slight increase in the size of the central oval 16 or annular ring segments 17, will result in more diffuse and lower intensity heating in central parts of the container.

“Adjustments to the dimensions of containers with different dimensions and shapes may be required.” These modifications and others may be made within this invention’s scope, as defined in the following claims.

Summary for “Container for active microwave heating”

“This invention is a container for active microwave heating food products. This invention is a better active container system that can heat or cook a variety food products. The containers of the invention can also be used to cook and thaw frozen foods, such as meat. These products can all be cooked evenly and heated in an efficient manner, without any overcooked, dried, or scorched areas.

Microwave heating has significant advantages when it comes to thawing or reheating food products. The most important benefit for consumers is the shorter time it takes to heat frozen foods. However, there are some drawbacks. The microwave heating process is uneven in conventional packaging. This means that certain areas of the food product are not adequately heated. Meanwhile, food at the edges of containers tend to be overheated or dried out.

This problem has been addressed with a variety of approaches and designs. To “shield’ overdone food, some designs use microwave reflective materials such as metallic foils. However, this reduces the energy required to cook these portions, and increases the cooking time and energy efficiency.

U.S. Pat. Nos. Nos. EPO application 92105572.9 was filed to Saunier. These containers can reduce the overheating of food around the edges, but they have not been widely used commercially. The potential benefits of containers have often been outweighed by their increased cost.

“A newer approach uses materials in or parts of the package to modify microwave radiation. This type of packaging is disclosed in U.S. Patent. Nos. Nos. “Active microwave packaging” is packaging that alters the electric (or magnet) field configuration and the heating pattern of the product within. Active packaging includes susceptors and heater boards that are used to brown or crisp products.

“Active packaging that alter the electrical field makes more efficient use of microwave energy and provides more even heating of food and other materials inside the container. They make microwave heating possible for many products that cannot be heated in any other packaging. However, these designs have not been able to handle particularly complicated products like large quantities (more than 300g) of frozen, uncooked meat products.

“It is the object of the invention to provide an improved active microwave container that modifies microwave radiation in the container to better distribute energy for heating, defrosting and cooking food.”

“Another object is to provide a container that produces satisfactory results with a wider range of food products than the currently available containers.” Another object is to provide a container that freezes uncooked meats and other food can be cooked in a microwave oven.

“An additional objective is to provide a container capable of providing good results in a variety of microwave oven styles and types, as well as that is sensitive to load and fill variations like those found in commercial products.” Another objective is to make the container relatively insensitive to changes in microwave oven position.

These and other advantages can be achieved by a container with an open-ended tray, a closed-base and side walls that extend from the closed base to open ends, and a lid covering the open edge. The tray’s closed base is made of microwave transparent material, such as paper-board or another suitable plastic material for microwave cooking and reheating (e.g. Polypropylene, Polyester or the like) and an annular circle of conductive material and microwave opaque material. Side walls are covered with a band of conductive or microwave opaque material such as foil. This extends from the sidewalls onto the closed bottom, creating a lip around it.

The lid consists of a microwave transparent material, preferably heat sealable polyester film, and at most one conductive, microwave opaque element. These elements are separated from the sidewalls by an annular area made of microwave transparent material. It is preferable to have one or more additional conductive microwave opaque elements between each element and the sides walls. These are separated from the side walls using microwave transparent material.

These active elements of microwave opaque material and the boundary conditions created by the container walls and food act to modify the microwave fields that are incident on the food within the container. Side walls are made of conductive, microwave-opaque material that prevents food from overheating. This conductive sidewall material creates well-defined boundary conditions for the incoming microwave energy.

The conductive elements in the lid, base, and side walls are designed to work together. The lid’s active elements modify microwave fields incident on the food’s upper surface. The heating behavior of lower portions of the food is also dominated by the elements in the tray. The overall uniformity of heating is enhanced by the synergistic effects between upper and lower active elements. The containers of the invention are suitable for products that cannot be heated satisfactorily using prior art containers.

These and other benefits and objectives of the invention will be apparent in the following description.

“DRAWINGS”

“FIG. “FIG.

“FIG. “FIG. 1. FIG. FIG. 2A shows a cross-sectional view of the lid. FIG. 2A is a cross sectional detail view of lid, and FIG. 2.”

“FIG. “FIG. 1, 2 and 3.

“FIG. “FIG. The lateral axis of the container is shown in FIGS. 1-3. These curves show the variation of the electric field intensity at both the top and base of the food (AA) and the container’s bottom (BB).

“FIG. FIG. 5 shows the temperature distribution in a container incorporating this invention.

“FIG. “FIG. 1-3.”

“The package illustrated at FIGS. The package illustrated in FIGS. 1-3 contains an oval tray, commonly referred to by 20 and covered with a lid generally referred as 10. FIG. FIG. 2 shows that the package contains a 50 g food product. This may include a portion of uncooked, frozen turkey meat and gravy. Between the top of the food products 50 and the lid 10, there is 60 mm. The head space (distance between food product 50 and lid 10) should be approximately 2-20 mm.

The tray has a rectangular base 24 with side walls 26 that taper upwards and outwards from the closed base 24, to an open oval top. A flange 28 extends outwardly beyond the top of 26 side walls. The preferred embodiment has an open oval top measuring 185mm in length and 125mm wide. The inside dimensions of the oval base are 165mm long and 105mm wide. Side walls measure 42mm high along the sidewalls.

“Tray 20” is made from a paperboard blank or another microwave transparent material such as foil-label plastic or foil with apertures. The tray’s paperboard shell is coated with active elements. Additional active elements are attached to the lid. These active elements can be conductive or microwave opaque. The elements are conductive and microwave opaque because they are made of materials with a combination thickness and conductivity at microwave frequencies. This means that most of the microwave energy that is incident on them will reflect back. Practically, the amount of incident microwave energy that these elements absorb or transmit will be negligible. The following criteria should be met for reflection (R), absorption(A) and transmission coefficients (T):

“R>0.9” (i.e. more than 90% should be reflected);

“A+T0.1” (i.e. less than 10% should be absorbed/transmitted).

In the illustrated embodiment, active microwave elements are made from aluminum foil that is at least 5 microns thick. As is well-known in the art, active microwave elements can also be made by deposition of a metallic pattern or using conductive inks.

“A foil band 32 is attached or embedded in the 26-inch side wall of the paperboard shell 22. The foil lip 34 is an integral part the foil band 32. It extends from tray 20’s side wall 26 to the base 24. The oil band 32 protects food from overheating and allows for the establishment of well-defined boundaries for microwave energy.

“Preferably the foil band extends along the sidewalls to a height of 26 to 31mm. This is about the height of the food inside the container. The foil should not be more than 5mm below the top edge of the food. This will cause the foil to overheat. Strong localized fields may form if the foil is more than a few millimeters higher than the top of food. These fields can heat or char the container. If the food is not at least twice the height of the foil, these fields will be absorbed by food.

Alternately, you can continue the foil band 32 to the top of your sidewalls and onto flange 28. It is best to extend the foil to the edge of flange. The foil should extend to the edge and not touch any metal walls. This will allow the heat generated by the foil to be dissipated into air. This can increase the risk of arcing in unusual circumstances. The preferred arrangement is to extend foil band 32 to the expected food level.

The foil lip 34 should be between 2 and 10 millimeters wide. Foil lip 34 should not be wider than 10mm. This will result in excessive shielding and poor heating of food at the lower corners. Manufacturing irregularities can result in foil lip that is less than 2mm wide. This could cause foil band 32 to stop short of 26 at the bottom. This can lead to undesirable field intensification at your side wall, as we have already mentioned.

“The enclosed base 24 of tray 20 also contains an annular ring 36 made of conductive, microwave-opaque aluminum foil. The shape of the annular ring 36 should match that of the base. The ratio of the minor and major axes (or widths) of the annular ring 36 should be equal or similar to that of base 24’s major axis. The ratio of the annular foil ring 36’s width to the base 24’s length is 0.65. For the oval container illustrated, it is 0.65. The ring’s dimensions (the average of inner and outer dimensions) should be approximately constant. They should move angularly around a ring with the same dimensions as the container base aperture (i.e. The aperture is defined by the inner edge 34 of the lip. This ratio should be between 0.4 and 0.0.7 for circular containers and between 0.5 to 0.6 for elliptical ones. This ratio is approximately 0.55 in the illustrated container. This ring’s maximum diameter is the distance between the outer edges and the outer edges of the ring along its major direction. It measures approximately 90mm. The base’s 165mm overall length is 0.55. The base’s minimum overall diameter is 60mm. This is 0.55 times 105mm. The ratio can vary from one point to another around the container due to manufacturing distortions. Variations of 0.15 or more will be acceptable in some applications. However, it is preferable to keep the ratio relatively constant.

The preferred annular ring 36 width (the distance between the outer edge and inner edge at any point around a ring) is between 2 mm to 10 mm. This allows for sufficient interaction with microwave energy, but not so small as to create a shielded area of any significance beyond the ring 36. Annular rings smaller than 2 mm can function satisfactorily provided that the electrical conductivity is high enough to cause desired field modification. However, this will require increased manufacturing difficulties and higher costs for consistent and reliable production. The alignment of material during container pressing is also critical and costly if the lip 34 is less than 2mm.

Refer to FIGS. 3 and 6. 3, and 6. The tray is made from shell 22 of 282# milk carton stock newspaperboard. The side wall band 32, lip 34 and annular ring 36 are applied to shell 22 by adhesively laminating 8 micron foil to a film 38 of 48 gauge PET, or polyethyleneterephthalate, demoralizing the foil to form the desired patterns for sidewall band 32, lip 34 and annular base ring 36, and adhesively bonding the foil/PET laminate to the paperboard. FIG. 46 shows the Pleats. The side walls 26 and the flange 28 are then drilled with pliats 46 (shown in FIG. 2, and 3.

The container lid contains additional microwave active elements. The container lid contains a sheet made of microwave transparent material 12, an oval-shaped active element of aluminum foil 16 and two ring segments 17 also made of aluminum foil. The ring segments 17 are separable from the central oval 16 and the sides walls by microwave transparent material. The ring segments form an annular, interrupted lid ring that is interrupted by gaps of microwave transparent material at the ends.

The die-cutting of adhesive coated pressure sensitive foil creates the central oval 16 and annular segments 17. The oval 16 and the ring segments 17 are placed on a large piece of adhesive-coated pressure sensitive paper stock (or label 14), which acts as a carrier. It keeps the active elements in a proper relationship. The transparent film 12 is adhered to label 14. Film 12 is heated sealed to flange 28 after the food product 50 is placed in the container. This seals the open end of tray 20, with a minimum bond strength of 100 grams.

The central oval 16 should have a similar shape to the container’s top-inner dimensions. Additionally, the ratio of principal dimensions of oval and top-inner container dimensions should remain approximately constant. This means that the ratio between the oval’s length and the container’s top-inner length should roughly equal the ratio between the oval width and the container’s top-inner width. An oval lid with three active elements such as the illustrated container has a preferred ratio of between 0.2 to 0.3. The container’s oval element 16 measures approximately 0.27 inches in length and oval element 16 measures approximately 0.23 inches in width. It is desirable to maintain a consistent ratio between the diameters of the oval element 16 and the diameters of the open ends of the containers at all angles around the container, as with the annular rings 36 in the tray’s base 24.

The size of the central oval is not affected by the microwave energy enhancement or modification. The oval becomes smaller, but it is less sensitive to headspace variance. A lower concentration of microwave field intensity can result in intense heating of a narrower, central area of the food rather than a more diffuse, larger region that heats deeper into the food. The overall performance is affected by increasing the size of the central oval.

A lid with one central foil element can be quite effective in increasing the overall heating performance of the container. To distribute microwave energy evenly, more elements are needed, such as the annular rings segments 17.

“Annular rings segments 17 and the gaps between them are segments of an annular ring interrupted with dimensions that are determined in relation the central oval 16 in this way:

“1. The distance between the inner edge and the edge of central oval 16 should be roughly constant. This distance should be approximately 10-25 mm for the illustrated container. It is preferred to be around 10 mm.

“2. “2.” The width of the annular rings defined by ring segments 17 should be constant. This ring should measure approximately 10 to 20 millimeters wide for the illustrated container (preferably 15 millimeters).

“3. To avoid local field intensification, the corners of the annular rings segments should be rounded to a radius of 2 mm or more.

“4. “4.

“The size, shape, and spacing of the annular rings segments 17 were determined empirically. Small adjustments to position and size were made to “fine tune? the container performance and achieve the desired degree or uniformity, while avoiding any potential for arcing or dielectric breakage between the annular segments or one of the segments and the central oval 16.

These dimensions have been experimentally determined to be very critical. Excessive fields around the edges and active elements can result in deviations greater than 2 mm. If the gaps are significantly larger, effectiveness will also decrease.

The effects of active elements in this container on uniform distribution can be better understood when there is a discussion about the performance of the container with no active elements. The interaction between the rapidly changing electrical field and the molecules or ions in the food causes microwave heating. In the unfrozen state, water molecules and salts play a significant role. The incident field (by vibrating or rotation) is not as effective in freezing water and salts.

“Power absorption per unit volume is proportional to square of the electric fields.”

“P=2?? “P=2?”

“?o is electric permittivity of space”

“?r” is the relative permissivity (e.g. “?r is the relative permittivity (e.g.

“E” is the electric field (magnitude), at the point of interest (V/m).

“In a container of the same dimensions and shape as the illustrated one, but with no central oval 16 or annular rings segments 17 in it, the foil band 32 in its sidewalls, and the foil lip 34 in the base, the dominant field intensity pattern will be determined based on the size and shape the food load, and the field distributions in the oven. Most foods have dielectric properties that are significantly different from those in free space or air. For example, the microwave wavelength of unfrozen food will be approximately 12 mm. In air, the equivalent wavelength is 120 mm. These large dielectric changes at the food-air interfaces can cause reflections and other refraction effects that alter the overall field distributions within the food and the oven cavity. This means that the field distributions reaching the food surfaces must conform to the “boundary conditions” created by food in the container.

“Generally, many field patterns or modes will be compatible with the boundary conditions. For typical food containers, however, the most common field patterns (modes), have dense fields around the edges and minimal intensity in the middle. This is why many foods products have a difficult central region that can be heated efficiently, but the edges are more efficient.

These field distributions in rectangular containers can be described using combinations of sine or cosine functions (analogous to rectangular waveguides and cavities). The mathematical descriptions of elliptical and round containers are based on modified or Bessel functions.

“Active elements, such as foil and other conductive, microwave-obvious materials, are designed to alter this field distribution to produce a more desirable pattern of heating. The conductor will be infected by electric currents when microwave energy reaches it. These currents’ intensity and pattern will depend on how the microwave energy arrives at the conductor and what the foil is made of. ”

“Foil and other conductive elements can be used on a microwave transparent cover to modify the fields. They can also develop higher order modes close to the food surface. The modified field’s effect is affected by the size, shape, quantity, and distance between the food and the element. Element that mimic the shape of an oval container are ideal for modification. Similar shapes and elements can be used to modify rectangular, square and circular containers.

“In the case where an oval foil element is used (as in the central oval 16 of this container), the patterns and currents that are induced will be typical of the shape of the element. These rapidly changing, circulating currents will lead to the reradiation of microwave energie. The element acts as a “patch antenna”. The element is located close to the food surface so a significant portion of the energy will propagate to the surface.

“The central oval 16 increases the microwave intensity in central regions relative to outer regions. This greatly improves the heating uniformity. This improves heating uniformity significantly for oval containers. However, the dominant mode (or the field pattern) generated from the combined influence between the single label (or container) results in a heating distribution that is relatively uniform. There are some residual cooler areas (diffuse zones in the annular area between the zone covered with the central label, and the outer container walls, mainly towards one end of the container).

The structure was also equipped with two annular segments 17 to improve heating uniformity. As can be seen in curve A-A of FIG. 4 The ring segments alter the field distribution created by central oval 16 and provide a localized heating effect (field enhancement) in the area immediately below the annular rings segments 17.

“Plot B-B-B in FIG. 4. The active elements in the tray alter the energy distribution at bottom of container. This plot shows the schematic representation of field intensity along a line through the bottom. The active elements in the tray alter the energy distribution at bottom of container. It produces a central field maximum as well as two subsidiary maxima. These maxima correspond to the aperture that is defined by the foil lip 36 and the foil ring 34. The annular ring 36 is the location of the minimum field intensity, while the inner edge and foil lip 34 are the positions for the minimal field intensity. Because conductive foil components cause components of the electric fields parallel to conductors to be null, any non-zero electrical field causes charge flow within conductors until an equal and opposite field is created to cancel the original field. Foil edges are boundary conditions for microwaves arriving at base of container. Some key field components will therefore be zero at annular base ring 34, and inner edges of foil lip34.

FIG. “As you can see from FIG. 4, and the following example, the active elements of the lid 10 and tray 20, work together to efficiently distribute energy so that even heating, defrosting and cooking can happen. This distribution allows for relatively large food loads (25-40mm) as well as foods that aren’t homogeneous such as meat, gravy, entrees, and side dishes to be prepared for microwave heating, defrosting, and/or cooking. Many products can be microwave heated. This is possible because prior art packaging was not designed for smaller food loads or foods with more homogeneous ingredients, such as pasta and macaroni, cheese, and sliced meats.

“EXAMPLE 1”

“The tests were done in a Kenmore oven 750 watt with the container centered in it. The container was heated on high power for 12 minutes. The heating cycle was completed. Temperatures were taken as soon as possible after the expiration of the 12 minutes heating time. Twelve thermocouples, each calibrated at three levels, were used to record the temperature. FIG. 52 shows the figures. FIG. 5 shows bottom temperatures. They were measured approximately 2 to 3 millimeters higher than the container’s base. Oval 54 shows the bottom temperatures at about 15 millimeters above container base (at the estimated food-depth). “Oval 56’s top temperature measurements were taken from just below the food surface (about 2 millimeters).

The product, which contained approximately 8 ounces turkey breast meat and 8 ounces gravy, weighed in at 16 ounces. The product was weighed before and after cooking to determine its weight loss. The 11.8% weight loss is well within the range of 15% for uniformly heated products.

“This invention offers a number significant advantages over previous microwave heating containers. Heating is uniform and energy is used more efficiently. The container described above can be modified by those skilled in the art. The dimensions shown here are the preferred dimensions of the oval container when it is used to store uncooked or frozen meat products with gravy. Final adjustments might be required for other products. It may be possible to heat one portion more than the other if the products are not homogeneous or the fill depth is different in different parts of a container. This could happen with vegetables, meats, and other side dishes. Small adjustments to the dimensions and spacings of active elements can make a significant difference in heating behavior. A small decrease in the size of the central oval 16 or annular rings segments 17 on the lid can result in more energy being concentrated into the central area. A slight increase in the size of the central oval 16 or annular ring segments 17, will result in more diffuse and lower intensity heating in central parts of the container.

“Adjustments to the dimensions of containers with different dimensions and shapes may be required.” These modifications and others may be made within this invention’s scope, as defined in the following claims.

Click here to view the patent on Google Patents.