Abstract:
In this research, convective and microwave drying characteristics, energy requirement and color changes of black cabbage leaves (Brassica oleracea L.) were reported. Black cabbage leaves were dehydrated in a computer connected parallel air flow type dryer and in a microwave oven dryer. The convective dryer mainly consisted of an electric heater, temperature adjuster, centrifugal fan, air speed adjuster, corrosion resistant chromium mesh, corrosion resistant chromium sheet, glass wood insulator, a digital balance, RS232 connection, a PC, data processing software, drying air inlet and outlet channels as well as thermostat, hot-wire anemometer, humidity and temperature sensors, wattmeter and wheels. The microwave oven dryer mainly has a magnetron tube, oven cavity, filter, step-up transformer, power plug, wave guide, mode stirrer and oven tray. Samples of freshly harvested black cabbage leaves were dehydrated under three air temperatures of 50 degrees C, 60 degrees C and 70 degrees C and at three microwave power levels of PL-1, PL-2 and PL-3. Selected drying air velocity was 0.30 m/s for all temperatures. This is coming from the fact that it was understood from the preliminary studies that the temperature less than 50 degrees C and the air speed more than 0.30 m/s increase the drying time and energy requirement, extremely for these products. Black cabbage leaves were dehydrated from the initial moisture content of 402 (percentage dry basis) to a final moisture content of 8 ... 9 %. During convective drying experiments, product were weighted automatically by the balance per 5 ... 10 minutes. Data were transferred to the computer and processed by a software. During microwave drying, the product were weighted and data recorded manually per 15 ... 60 minutes. The influence of drying method, drying air temperature and microwave power level have also been studied. Hunter L, a, b values system was also used to evaluate changes in total color difference (Delta E) on dried products. The results showed that convective drying air temperature and microwave oven power levels influenced the total drying time, total energy requirement, specific energy requirement and color difference for black cabbage leaves. The minimum specific energy requirement were determined as 3.66 kWh/kg and 7.35 kWh/kg for 70 degrees C and PL-3 respectively. 70 degrees C drying air temperature and PL-1 was found to yield better quality product in terms of color retention of Hunter L, a, b and Delta E. As a result, to reduce drying energy consumption and to keep better color retention, convective drying can be recommended for this application.
