Dehydration behaviour, mathematical modelling, energy efficiency and essential oil yield of peppermint leaves undergoing microwave and hot air treatments

This paper aims at presenting a comprehensive perusal on hot air and microwave drying of medicinal herbs through investigating the dehydration behaviour, mathematical modelling, energy characteristics and essential oil yield of peppermint leaves drying. For our study the leaves were dried using microwave at power levels of 200, 400, 600, and 800W, and hot air at temperatures of 50, 60 and 70°C. The dehydration process of the leaves in microwave drying method occurred in an accelerating rate period at the start followed by a falling rate period while; convective drying took place in the falling rate period entirely. Six mathematical models were fitted into the experimental data and the Midilli and Kucuk model best described the drying curves. The lowest effective moisture diffusivity (1.809×10−9m2s−1) belonged to hot air drying at 50°C, while the highest (110.552×10−9m2s−1) belonged to microwave drying at 800W. The activation energy for microwave and hot air drying was determined to be 1.89Wg−1 and 12.46KJmol−1, respectively. Specific energy consumption for microwave and convective drying of the peppermint leaves were ranged from 4.581 to 5.890MJkgw−1, and from 42.721 to 64.712MJkgw−1, respectively. The maximum and minimum oil yields were obtained from the leaves dried at air temperature of 70°C and microwave power at 800W, respectively.