Step-down relative humidity convective air drying strategy to enhance drying kinetics, efficiency, and quality of American ginseng root (Panax quinquefolium)

As a measure to enhance drying kinetics, efficiency, and quality of the whole American ginseng root, step-down relative humidity (RH) convective hot air drying strategy was experimentally investigated. The effects of drying temperature (45, 50, 55, and 60 degrees C), RH (20, 30, and 40%), step-down RH, and continuous dehumidification, on hot air drying characteristics and quality attributes were examined. Results showed that the effective moisture diffusion coefficient varied from 1.155 x 10(-10) to 2.885 x 10(-10) m(2)/s while the activation energy was found to be 51.14 kJ/mol. Quality evaluation revealed that drying temperature, RH, and step-down RH had significant effect on color change. Additionally, the energy consumption was related to the drying time. The total ginsenosides content decreased with increasing drying temperature and decreasing RH. Microstructure comparison showed that a gel layer was formed on the sample surface when dried at temperature of 60 degrees C, which impeded internal moisture diffusion. Based on evaluation of the drying time and quality, air temperature of 55 degrees C and RH of 40% holding 5 h and then dropping it to 20% were proposed as the best drying conditions for whole root.