Thawing of Foods in a Microwave Oven: II. Effect of Load Geometry and Dielectric Properties

Non-uniformity of temperatures in thawing of food and biological materials inside a microwave oven is affected by size, shape, and dielectric properties of the load (food). The objective of this study was to relate the time to thaw and the non-uniformity of heating to the shape, size, and the dielectric properties of the load. The details of the heat transfer analysis, experimental measurement of thermal and dielectric properties, and temperature measurements are described in a companion paper. Results show that both the aspect ratio and the volume have a significant effect on the heating rates, time to thaw and the non-uniformity of temperatures during thawing. A "shield" develops from surface thawing and leads to reduced microwave penetration. This "shield" develops more readily at higher dielectric loss, thereby effectively increasing the thawing time even more for a lossy material at higher power levels. Thawing time increases linearly with volume. As the load aspect ratio decreases (it is made flatter), it thaws faster since the energy decays relatively less in a thinner material. These new results could provide a more rational and quantitative approach to development of frozen food products to be heated in a microwave oven.