Thawing of Foods in a Microwave Oven: I. Effect of Power Levels and Power Cycling

Abstract

Microwave thawing is faster than other methods, but it can produce significant non-uniformity of heating. The objective of this study was to perform comprehensive 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 formulated with an exponential decay of the microwave flux from the surface. Surface microwave flux was obtained from the measured temperature rise using inverse heat transfer analysis. Gradual phase change was formulated as an apparent specific heat, and was obtained for the experimental material tylose from differential scanning calorimetry (DSC) measurements. The temperatures were measured immediately following heating with a fast response thermocouple. Dielectric properties were measured above freezing. Results show that the microwave flux at the surface and its decay are affected by the changes in the power level. Power cycling has an almost identical effect as continuous 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" develops due to a much reduced microwave penetration depth at the surface. This 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 temperature reaches 100 degrees C, all energy absorbed goes into evaporation, and subsequent temperature is maintained at 100 degrees C. Thus, eventually, non-uniformity starts to decrease.

Cooking Pattern and Quality Properties of Ground Pork Patties as Affected by Microwave Power Levels

“…Patties cooked at 900 W had the shortest cooking time, while patties cooked at 360 W had the longest. This result was consistent with the trend observed by Chamchong and Datta (1999). The rate of increase in temperature was not proportional to the increase of power.…”

“…A sharp edge or corner will act as an antenna and attract more energy than the surrounding areas (Ohlsson and Bengtsson, 2001;Ohlsson and Risman, 1978). These effects are well documented in many literatures (Chamchong and Datta, 1999;Ni et al, 1999;Ohlsson et al, 1974;Zeng and Faghri, 1994). Zeng and Faghri (1994) studied thawing patterns for cylindrically-shaped tylose samples with different aspect ratios.…”

“…The boiling temperature of water is 100 o C; thus, when this temperature is reached, all additional absorbed microwave energy is expected to contribute to the evaporation of water. Therefore, the temperature is maintained near 100 o C (Chamchong and Datta, 1999;Ni et al, 1999). Differences in mean temperature between the edge position and the center position and their standard deviation resulted in non-uniformity in microwave cooking (Fig.…”

Coupled electromagnetic and heat transfer model for microwave heating in domestic ovens

“…Typically, frozen portion of the food absorb considerably less MW energy than the thawed food portion, because dielectric properties of ice are much less than that of liquid water ( [Chamchong and Datta, 1999a]; [Chamchong and Datta, 1999b]). In microwave oven, electric field distribution is highly sensitive to changes in the dielectric and thermal properties of foods.…”

“…Researchers have made assumptions to simplify the problem and reduce the computational time. For example, instead of modeling Maxwell's equation, several researchers simplified the problem by using Lambert's law, which calculates dissipated power by assuming that the energy decays in the food exponentially from the material surface to deep inside of the material ( [Campanone and Zaritzky, 2005]; [Chamchong and Datta, 1999a]; [Chamchong and Datta, 1999b]; [Chen et al, 1993], [Khraisheh et al, 1997]; [Zhou et al, 1995]). Lambert's law does not represent the true electromagnetic field distribution inside the MW oven cavity (Datta & Anantheswaran, 2001).…”