Thawing of foods in a microwave oven: I. Effect of power levels and power cycling

Microwave thawing is faster than other methods, but it can produce signific ant non-uniformity of heating. The objective of this study was to perform c omprehensive experimentation and heat transfer modeling to relate the time to thaw and the non-uniformity of thawing to power cycling, power level and the surface heat transfer coefficient. The governing energy equation was f ormulated with an exponential decay of the microwave flux from the surface. Surface microwave flux was obtained from the measured temperature rise usi ng inverse heat transfer analysis. Gradual phase change was formulated as a n apparent specific heat, and was obtained for the experimental material ty lose from differential scanning calorimetry (DSC) measurements. The tempera tures were measured immediately following heating with a fast response ther mocouple. Dielectric properties were measured above freezing. Results show that the microwave flux at the surface and its decay are affected by the ch anges in the power level. Power cycling has an almost identical effect as c ontinuous power at the reduced level of the average cycled power. As power level increases, the surface flux increases by the same fraction. At higher power levels, however, the outside thaws relatively faster. A "shield" dev elops due to a much reduced microwave penetration depth at the surface. Thi s thawing time at higher power levels is reduced considerably. Temperature increases initially are non-uniform since the surface is heated at a faster rate than the interior. In keeping with the assumption that once the tempe rature reaches 100 degrees C, all energy absorbed goes into evaporation, an d subsequent temperature is maintained at 100 degrees C. Thus, eventually, non-uniformity starts to decrease.