Electronic ovens heat items within a chamber by exposing them to strong electromagnetic fields. In the case of typical microwave ovens, the electromagnetic fields are a result of microwave radiation from a magnetron, and most often take the form of waves with a frequency of either 2.45 GHz or 915 MHz. The wavelength of these forms of radiation are 12 cm and 32.8 cm respectively. While heating, the electromagnetic waves in the chamber of a magnetron-powered microwave oven may drift or hop in frequency for short periods of time, generally within a range of +/−5%. For purposes of this disclosure, the mean temporal wavelength of an electromagnetic wave is referred to as the “dominant wavelength” of the associated electromagnetic wave, and dimensions of an electronic oven that are given with respect to a frequency or wavelength of an electromagnetic wave refer to the frequency or wavelength of the dominant wavelength of that electromagnetic wave.
The waves within the microwave oven that are not absorbed by the heated item reflect within the chamber and cause standing waves. Standing waves are caused by the constructive and destructive interference of waves that are coherent but traveling in different directions. The combined effect of the reflected waves is the creation of local regions of high and low microwave field intensity, or antinodes and nodes. The waves may interfere destructively at the nodes to create spots where little or no energy is available for heating. The waves interfere constructively at the antinodes to create spots where peak energy is available. The wavelength of the radiation is appreciable compared to the length scales over which heat diffuses within an item during the time that it is being heated. As a result, electronic ovens tend to heat food unevenly compared to traditional methods.
Electronic ovens are also prone to heat food unevenly because of the mechanism by which they introduce heat to a specific volume of the item being heated. The electromagnetic waves in a microwave oven cause polarized molecules, such as water, to rotate back and forth, thereby delivering energy to the item in the form of kinetic energy. As such, water is heated quite effectively in a microwave, but items that do not include polarized molecules will not be as efficiently heated. This compounds the problem of uneven heating because different portions of a single item may be heated to high temperatures while other portions are not. For example, the interior of a jelly doughnut with its high sucrose and water content will get extremely hot while the exterior dough does not.
Traditional methods for dealing with uneven cooking in electronic ovens include moving the item that is being heated on a rotating tray and homogenizing the distribution of electromagnetic energy with a rotating stirrer. These approaches prevent an antinode of the electromagnetic waves from being applied to a specific spot on the item which would thereby prevent uneven heating. However, both approaches are essentially random in their treatment of the relative location of an antinode and the item itself. They also do not address the issue of specific items being heated unevenly in the microwave. In these approaches, the heat applied to the chamber is not adjusted based on the location, or specific internal characteristics, of the item being heated.