Analysis of heat and moisture transfer in the microwave drying of potatoes

In order to gain a deeper knowledge of the heat and mass transfer processes in the case of starchy fruits and vegetables during microwave drying, we have conducted an experimental study based on a developed microwave apparatus for drying potato. The changes in the temperature, drying rate, moisture content, thermal parameters, moisture state and distribution of the potatoes samples during the microwave drying process were obtained. The results show that, according to the temperature changes of the samples dried at different microwave powers (600 W, 800 W, 1000 W), the microwave drying process can be divided into three stages: heating stage, constant temperature stage, and cooling stage. The moisture content reduction of the samples in the constant temperature stage accounted for more than 90% of the total reduction. Meanwhile, as the microwave drying process goes by, the thermal conductivity and thermal diffusivity of the samples both decrease first and then increase, while the volumetric heat capacity presents an opposite trend. Moreover, the range and amplitude of transverse relaxation time of the samples reduce over the drying time, which confirms the existence of massive, outward moisture-migration in the dried samples. The loss of the weakly bound water in the moisture-migration, which is closely related to the starch, has a significant effect on the heat and mass transfer in the dried samples.