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Comparison of microwave drying and microwave mixed-bed drying of red peppers

Seçmeler, Özge
The main objective of this work is to investigate whether the time required to dry red peppers in the diffusion controlled period can be reduced. For this purpose, the possibility of microwave drying in this period was studied. As comparison parameters the drying rates and color of the products were selected. The conventional drying was conducted under constant external drying conditions with air at 60 0C and 3.5 % RH and 1.0 m/s velocity in a batch dryer. For the microwave drying a domestic microwave oven with three power levels, 286, 397 and 560 W was used. Pre-experiments were done to obtain an insight on the possibility of moisture distribution within the structure of the red peppers and electrical field distribution inside the oven cavity. The non-uniformity due to the latter effect was tried to be overcome by designing and installing a six blade impeller into the bed having the symmetrical pair of blades fixed at three levels. Samples for the microwave drying experiments were prepared by drying the peppers from approximately 90% of moisture to 20% in 7 hours in the conventional dryer at 600C and then cutting them into pieces about 1x1 cm. Thus prepared samples were dried further in the microwave oven to 14% moisture content without and with the agitator. To compare the results, the effective diffusivity coefficients were calculated for all of the methods and parameters to enable the aimed comparison. These indicated that by using microwave drying, the drying time can be reduced by about 120 times. Further, by mixing the uniformity of drying and the drying rate could be improved. Considering color of the product, there was no significant difference with respect to the fresh samples after all operations.