Vibro-fluidized bed heat pump drying of mint leaves with respect to phenolic content, antioxidant activity, and color indices

Abstract

Because of high porosity and stickiness of mint leaves, they could not be fluidized well during fluidization. In this study, a vibro-fluidized bed dryer assisted heat pump system was designed and fabricated to overcome this problem. The drying experiments were carried out at temperatures of 40, 50 and 60 ?C. The quality of the dehydrated samples was assessed based on color indices, antioxidant activity, and total phenolic content. Drying process primarily occurred in falling rate period. The effective coefficient of moisture transfer of the samples was increased with air temperature and varied from 4.26656?10-11 to 2.95872?10-10 m2 s-1 for heat pump drying (HPD) method, and 3.71918?10-11 to 1.29196?10-10 m2 s-1 for none-heat pump drying (NHPD) method. The color indices for temperatures of 40 and 50 ?C were very close to each other, whereas by increasing temperature to 60 ?C, a remarkable loss of green color was observed. The highest phenolic content was found in methanolic extract for HPD at 60 ?C, and NHPD at 50 ?C contained the lowest amount of phenolic compounds. NHPD treatments showed lower antioxidant activity compared to HPD treatments at the same temperature due to the longer drying times.