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

The moisture-binding type and change in moisture content of Radix Paeoniae Alba (RPA) slices were determined during their drying process. Water in RPA slices exists in two types: bound water (BW) and free water (FW). During the drying process, FW diffuses from inside the sample to outside it; simultaneously, it is mutually transformed with BW. 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. Mutual transformation between FW and BW is simplified as a reversible conversion process, and a drying model is established for RPA slices based on Fick's second law. (C) 2016 Elsevier Ltd. All rights reserved.