While many packages for heating food in a microwave oven have been successful, heat loss often interferes with achieving optimum results. For example, in popping corn, heat losses can reduce the volume of the popped corn and increase the number of "old maids", especially in low powered ovens. This can result in consumer dissatisfaction. Insulation for microwave packaging has in the past been bulky and requires assembly steps and materials which make the package more expensive. In addition, the insulation has been located only in the bottom portion of the package, i.e. the portion resting on the floor of the microwave oven, thereby reducing heat loss only in that area. One example is described in U.S. Pat. No. 4,219,573 which provides enough insulation to prevent the loss of at least about 18 cal. per gram of popcorn. It has now been discovered that excellent results can be provided by providing the loss of only about 9 calories per gram of popcorn. In addition, the insulation materials--cork, wood, corrugated pad or ceramic paper--proposed in U.S. Pat. No. 4,219,573 are not required in the present invention. This reduces the cost of the package substantially. Insulation previously used reduces the cooling rate of the food when the package is chilled or frozen. It has now been discovered that the microwave energy itself can be used to induce the formation in situ of an insulation structure without adding material to the package. In developing the present invention, it has also been discovered that the most efficient formation of the insulation structure can be achieved by augmenting the heat supplied by the microwave energy alone to the unformed potential insulation before the insulation structure actually comes into being.
In view of the deficiencies of the prior art, it is a major object of the invention to reduce the loss of heat from microwave packaging without increasing the cost of the package or adding materials, to reduce heat loss in one or more selected areas or throughout the entire package, thereby permitting foods contained in the package to be cooked or heated more efficiently and to enable the package to be chilled or frozen as efficiently as a bag with no insulation. Our prior U.S. Pat. No. 5,081,330 provides insulation chambers only in limited areas and in contact with a susceptor. A major objective of the present invention is to achieve even more effective insulation for the food during microwave heating than can be accomplished following the teachings of U.S. Pat. No. 5,081,330.
These and other more detailed and specific objects of the invention will be better understood by reference to the following detailed description and figures which illustrate by way of example but a few of the various forms of the invention within the scope of the appended claims.