1. Field of the Invention
The present invention relates generally to cooking containers which can be used in microwave ovens and, more particularly, to covers for containers which focus the microwave energy at the center portions of the food.
2. Description of the Prior Art
The heating and cooking of food in microwave ovens has become commonplace in todays society. Although the speed and convenience of microwave cooking is a great advantage, the quality, thoroughness and consistency of the temperature of the resulting food is less than ideal. Typically, because the center of the food represents a large heat sink when compared to the outer portion of the food, the temperature at the center, even if heated by the same amount of microwave energy as the rest of the food, is lower than that of the outer portions of the food. This normally results in a food product with a center that is too cold and outer portions which are too hot. If the time is increased to heat the center to the desired temperature. The outer portions are overexposed and burn.
This inability of microwave ovens to produce food of a uniform temperature was recognized early-on in the development of microwave containers and there have been several alternative approaches attempting to correct this problem, each meeting with its own level of success.
In recognition of the speed and convenience of microwave ovens as food warmers, there have been proposals to package food products in boxes or containers formed in part of a microwave reflective material such as aluminum foil having holes in selected areas. This was based on the idea that the microwave radiation would enter the package through the holes and be reflected about within the package by the aluminum foil, thereby facilitating the heating of the product. The microwave energy actually acting on the food was moderated or attenuated in the hope of improving its distribution within the food thereby uniformly heating the food. These devices are generally expensive to produce and do not focus the microwave energy acting on the food and therefore do not improve the center heating characteristics of the resulting food. U.S. Pat. Nos. 4,190,757 and 4,230,924 are exemplary of disposable covers of this type.
Another proposal is to use a special cover which is spaced a distance form the surface of the food in a metal foil container. The cover is designed such that it will not transmit reflected energy, making it possible to heat food in a metal foil container which otherwise could damage the microwave oven. Thus, the cover will allow passage of the microwave energy into the container holding the food, while substantially preventing escape of microwave radiation reflected from the food surface and the container bottom, to retain and concentrate energy within the container. This invention, while providing for efficient heating of the food and allowing use of a metallic container in the microwave oven, does not solve the problem of center heating. The resulting food has an undesirable temperature gradient, being colder in the center than at the edges, it merely reaches that undesirable temperature gradient fastener than it would without the cover, U.S. pat. No. 4,656,325 is exemplary of such a cover.
There have also been proposed microwave oven cooking container covers incorporating a microwave reflective layer to set up microwave energy modes of a higher order than is normally present in the container. The reflective layer has an irregular shape including a number of protuberances distributed around its perimeter for diffusing the heating effect of the higher order mode microwave energy. The devices of this type are designed to allow for an even heating of the food placed within the container. However, even heating of the food does not result in an even or uniform temperature due to the large heat sink represented by the center of the food as compared to the outer portions. Additionally, these devices suffer from the expense and difficulty of manufacturing reflective layers of the proper dimensions as well as the inability to heat the center of the food to a proportionally higher level than the remaining food. U.S. Pat. No. 4,814,568 is exemplary of covers of this type.
As consumer demand increases, there still exists a need for a cover for a microwave container which will concentrate and retain microwave energy within the container to heat the food proportionally more at the center than elsewhere such that the resulting food has a even temperature throughout.