Microwave cooking relies upon dielectric heating of foods responsive to microwave radiation. Because of the nature of microwave cooking, the heating characteristics in a microwave oven for some food products are dramatically different from those experienced in a conventional oven. Moreover, the use of microwave ovens can result in undesirable temperature differentials for a variety of food products. For example, some food products when cooked in a microwave oven, will heat to a greater extent on the interior of the product rather than on the product surface because of dielectric microwave heating which favors heating of the product interior.
The above problems are well-known in the art of microwave cooking and numerous attempts have been made to solve them, none of them entirely satisfactory. In conventional microwave packaging and cooking containers, this problem of uneven heating is ameliorated by instructing the user to leave the material unattended for a few minutes after the normal microwave cooking time in order for thermal conduction within the food to redistribute the heat evenly. Alternatively, the material may be stirred, if it is of a type which can be stirred.
The problem of uneven heating in microwave ovens can be exacerbated by typical packaging systems which employ microwave susceptors that are designed to heat the food. Microwave susceptors typically contain one or more thin metallic layers designed to absorb substantial amounts of microwave energy and to convert this energy into thermal energy.
The resistivity of a thin, metallic structure is a function of the thickness of the structure. Thin metal layers of typical susceptors have resistivities greater than about 20 ohms per square. At these values, the metallized layer reflects a smaller percent of microwave energy than a layer having lower resistivity. Thus, layers with high surface resistivities such as the metal layers used in typical susceptors, will absorb a significant portion of the impinging electromagnetic energy when illuminated with microwaves. A higher percentage of impinging microwaves will therefore penetrate the susceptor layers than will penetrate layers formed of very low surface resistance values.
These susceptors often suffer physical deterioration when exposed to microwave radiation, the result of very rapid microwave penetration and subsequent heating which occurs during the early stage of heating cycle. This heating causes the material to undergo dimensional changes which can damage the structure and the resulting reflective and transmissive properties of the susceptor material. Susceptors undergoing physical change from this intense internal heating often show an increase in microwave transmission into a food product. This results in either burning of the food or uneven heating.
Moreover, structures such as those used in many potato chip bags and drip coffee pouches incorporate a continuous, metallic layer having low surface resistance (i.e. less than about 2 ohms per square) layer. These structures disintegrate almost as soon as they are irradiated with microwave radiation. These structures presently unsuitable for microwave use.
Current attempts to control the amount and spatial distribution of microwaves during microwave cooking include using, on at least one surface of a container, one or more electrically conductive diffuser plates and/or microwave-transparent diffuser apertures that are not themselves designed to heat the food product.
At least one such attempt has been made in the prior art to control distribution of heating, described in detail in U.S. Pat. No. 4,927,991 to Wendt et al., the disclosure of which is incorporated herein by reference. Although addressing a number of significant issues relating to microwave diffusers, the Wendt patent describes an aluminum foil grid diffuser that is a very costly and complex addition to a microwaveable food package. Moreover, it is well known that as the resistivity of a metallic coating such as aluminum decreases, a surface charge accumulates which can result in severe arcing on the metal surface. Arcing is an inherent possibility at the site of any lowered resistivity such as nicks or sharp edges in reflecting grid diffusers.
Thus, a significant need exists for a simple, inexpensive microwave diffuser having low resistivity that will not arc, will not itself heat up enough to directly heat food but will distribute a microwave field within the food packaging in a well-defined way.