Electric heating – Microwave heating – Cookware
Reexamination Certificate
2000-05-24
2002-11-26
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Cookware
C219S728000, C219S729000, C219S734000, C219S762000, C099SDIG014, C426S234000, C426S243000
Reexamination Certificate
active
06486455
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a container for reheating frozen food products in a microwave oven. The invention is particularly useful for reheating large size frozen meals that typically require excessively long heating times.
BACKGROUND AND PRIOR ART OF THE INVENTION
The long length of time required to reheat large size frozen meals in a microwave oven is a real concern in the food service and catering business. For individual portions or small size frozen meals, reheating in a domestic microwave oven, can be carried out in a relatively short period of time, generally in the range of 2 to 6 minutes, depending on such factors as, for example, the type of foods, the size of the food components, and the lay-out of the various food components in the tray. For large size frozen meals, however, microwave reheating has proven to be excessively long, for example, up to 30 minutes. These long reheating times for large frozen meals renders the use of microwave ovens less attractive.
Another problem with re-heating frozen products in a microwave oven is that temperature gradients occur in the food when it is reheated in most known containers. Before the food product is thawed, the frozen product is essentially transparent to microwaves so that the microwaves are only absorbed at a very low rate, or not absorbed at all. When a frozen product is reheated in a regular microwave transparent container the microwave energy is not properly absorbed by the frozen mass. Instead the major portion of the energy is concentrated at the interface region where the container contacts the frozen product. This uneven energy distribution is not equalized by convection heat transfer and results in excessive heating at the edges of the container with the core of the frozen mass remaining at a very low temperature. The microwave heating pattern of a large frozen dish is generally characterized by the presence of large cold spots in the center of the upper surface, by a very late thawing of the inner parts of the product, and by overheating at the edges and corners of the product.
EP 348 156 to Hewitt relates to an improvement in microwave heating wherein a microwave mode is generated from underneath the food product. The food product is disposed in a tray that is transparent to microwaves and the tray is placed on a stand so that a predetermined elevation is maintained between the bottom surface of the food product and the internal bottom surface of the stand. Heating from underneath occurs by placing separated electrically conductive plates at the bottom of the stand which are made of a microwave transparent material, or by making apertures in the electrically conductive bottom of the supporting stand. The purpose is to have a majority of the microwave energy enter through the undersurface of the container and maximize the bottom heating effect.
EP 185 488 to Sugisawa discloses a container, made of a material that is transparent to microwaves for use in a microwave oven. The container has a microwave reflecting strip that partly covers the region of the container where the upper surface of the material contacts the side of the container to prevent local over-heating of the food product. The container, however, brings no significant improvements in reheating of frozen foods and simply proposes a solution to the problem of local burning at the edges of the product when the product is reheated in a conventional transparent container.
EP 471 969 to Payne relates to the use of a microwave susceptor sleeve for pizza and the like onto which the food items are placed. The susceptor, with the food product on it, is placed on a supporting base. The supporting base is elevated with respect to the bottom of the microwave oven by the use of pre-cut legs. The elevation of the base supporting the susceptor is dictated by the need to separate the susceptor from the bottom of the microwave oven sole (i.e., the bottom surface of the oven cavity) to eliminate the risk of arcing when the oven does not have a glass shelf.
WO 93 23971 A to the Campbell Soup Company relates to a microwave metallic container wherein the bottom and the whole lateral walls are externally insulated using a polymeric or glass thin layer that completely isolates the container from the metallic parts of the microwave oven. The main features of the container are that it prevents arcing by insulating the aluminum inner tray. For better convenience and for a better heat distribution within foodstuffs that do not retain their initial shape, such as liquid foodstuffs, the bottom of the aluminum container may be slightly raised or domed so that the thickness of the product in the center of the container is reduced, since it is predominately the center of the product that has a cold spot upon microwave heating. Variations in the thickness of foodstuffs are, however, generally undesirable as it might create problems when removing the foodstuff from the container. In particular, the center of the foodstuff becomes more fragile than the periphery and this may lead to portions of the foodstuff breaking off when the foodstuff is removed from the tray. The slanted bottom of the tray also results in a more acute angle between the bottom and the sidewalls of the tray that further renders it more difficult to remove the foodstuff from the tray. Finally although the thickness of the foodstuff to be heated is reduced in the center part of the tray, the slanted bottom portion of the domed tray has a tendency to reflect the microwaves in an upward diverging direction and away from the center which causes a reduction in the microwave absorption in the center part of the foodstuff, and consequently cold spots in the center part of the foodstuff.
U.S. Pat. No. 5,310,980 to Beckett discloses the incorporation of metallic patches on a microwave transparent tray in order to orient the impinging microwave energy beams selectively towards parts of the product that do not heat-up appropriately.
EP 350 660 A2 to Jaeger relates to a susceptor sheet with a microwave transparent packaging.
U.S. Pat. No. 4,642,434 to Cox et al. relates to a microwave reflecting energy concentrating spacer that includes in its lower part a microwave reflector separated from the food base by a distance of about ¼of a wave length, i.e., about 3 cm, since the free space wave length at the microwave emitted frequency in the microwave oven (2.45 GHz) is about 12 cm.
EP 242 026 A2 to Swiontek discloses an assembly between a susceptor which is described as a “microwave interactive layer” and the whole package.
U.S. Pat. No. 4,656,325 to Keefer refers to “cold susceptors” by placing metallic patches disposed in a regular array on the cover of a pan containing the food product.
U.S. Pat. No. 4,888,459 to Keefer also refers to “cold susceptors” in addition to optimizing the thickness and the dielectric permittivity of the material constituting the non-reflecting part.
U.S. Pat. No. 5,270,502 to Brown et al. relates to a combination of a microwave interactive layer that is in fact a susceptor and a supporting stand made of a microwave transparent material.
WO 95 24 110 to Gics relates to an ovenable food package comprising a microwave susceptor placed beneath the food base in order to induce crispiness in the food base.
U.S. Pat. No. 4,496,815 to Jorgensen relates to a microwave browning utensil comprising a metallic base with a ferrite layer that is a highly microwave absorbing material.
U.S. Pat. No. 4,542,271 to Tanonis et al. relates to a microwave tray comprising an absorbing material placed beneath the bottom surface of the tray.
U.S. Pat. No. 4,927,991 to Wendt et al. relates to a microwave oven package comprising a combination of a grid and susceptors inside a microwave-transparent tray that behaves like a conventional frying pan as it is heated by microwave radiation that passes through the tray.
EP-A-0 451 530 to Peleg proposes to combine a susceptor sheet and a layer of heat absorbing material to control the heat flux to the bottom surface of the food product that is pl
Leung Philip H.
Nestec S.A.
Winston & Strawn
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