Moisture transformation and transport during the drying process for Radix Paeoniae Alba slices

Moisture content and drying rate during the drying process for a 4 mm-thick RPA slice under 80 °C. The drying rate (R) exhibits a nonlinear relationship with the moisture content (M) of RPA slices, and the slope of R (dR/dM) changes non-monotonically with M. When the diffusion law of FW as well as the mutual transformation between FW and BW are considered, the drying process of the slices can be divided into three stages: FW drying, mix-drying, and BW-drying periods. During the FW-drying period, the drying rate is determined from the rate of FW diffusion and transmission. During the mix-drying period, the drying rate is affected by both diffusion and mutual transformation. By contrast, the drying rate is only influenced by the conversion rate of BW to FW during the BW-drying period.133