Development of microwave assisted fluidised bed dryer for nutmeg mace

Abstract

Dried mace possess great importance in international trade and are used in the preparation of extractives and volatile oils. Among various hot air drying methods, the more efficient method is fluidized bed drying for drying of foods, fruits and vegetables. Microwave drying, an alternative method for the drying of food products has gained popularity and is widely adopted. In microwave heating, heat generation takes place from inside to outside of the material due to the absorption of microwave energy by the regions with higher moisture levels. Application of microwaves solely, can result in uneven heating of certain products, depending on their dielectric and thermo physical properties. Thus, combining microwave radiation with hot air fluidization also provides an effective means of overcoming the non-uniform heating problems in conventional microwave heating. The bulk density, L*, a* and b* values of the fresh mace sample were 1191 kg/m3, 21.28, 22.23, and 9.09 respectively. At 5.1 m/s the fresh mace sample attained the fluidization condition. The developed microwave assisted fluidized bed drying system consists of a fluidized bed dryer and a microwave oven unit. The microwave oven unit consists of control panel where cooking time, power indicators and clock time are displayed and controlled. This domestic oven was modified by making a hole of 14 cm at the bottom and inserting the fluidized bed dryer through the hole. The main components of a fluidized bed dryer are drying chamber, plenum chamber, heating chamber, blower with power source and an air flow control valve. Logarithmic model was found as the best fitting model for the drying data with highest R2 value of 0.9996 and lowest χ2, SSE and RMSE values of 1.2588e-05, 0.0003 and 0.01 respectively at 50°C drying temperature and 800 W microwave power. In order to evaluate the developed microwave assisted fluidized bed dryer for nutmeg mace, the process parameters such as microwave powers of 480, 640 and 800W and drying air temperatures of 40, 45 and 50°C were chosen as independent variables. The process parameters would influence drying rate, drying temperature, energy consumption and physico-chemical 120 parameters such as colour, moisture content, bulk density and essential oil yield. These parameters were chosen as dependent variables. The optimized conditions of drying temperature and microwave power for microwave assisted fluidized bed dryer were found to be 47.76 °C and 681.73W.