[Mechanism of moistening process of Rehmanniae Radix based on kinetic process]

The moistening process of Rehmanniae Radix was characterized quantitatively by moisture phase, texture properties, and component content based on water absorption kinetics and expansion kinetics. Non-linear fitting of water absorption kinetics and expansion kinetics in the moistening process of Rehmanniae Radix was carried out. Low-field nuclear magnetic resonance and imaging(LF-NMR/MRI) technology was used to investigate the phase state and distribution changes of water during the moistening process. The Texture Analyzer was used for the determination of texture properties. The correlations between water absorption rate, expansion rate, water phase state, hardness, and compression cycle work of Rehmanniae Radix at different moistening time were analyzed. The results showed that the water absorption kinetics and expansion kinetics of Rehmanniae Radix were in accordance with the first-order kinetics. Moreover, the water absorption rate and expansion rate increased with the increase in temperature but decreased with the increase in the size of the medicinal materials.In the moistening process, the moisture was transferred from the outside to the inside, and the proportion of the moisture phase changed significantly.Within 16 hours, free water increased from 0.825% to 97.7%,while bound water decreased from 99.2% to 2.33%.Within 28 hours, the texture properties, such as hardness and compression cycle work, decreased gradually with the prolongation in moistening time.At 32 hours, water was evenly distributed throughout the whole medicinal material, and the texture properties also tended to be stable.Pearson correlation bivariate analysis showed that moistening time, water absorption rate, expansion rate, the relative content of free water and bound water, hardness, and compression cycle work were significantly correlated, suggesting that water absorption kinetics and expansion kinetics, LF-NMR/MRI,and Texture Analyzer could directly and quantitatively characterize the moistening process.This study is expected to provide a scientific basis for clarifying the scientific connotation of the moistening process of Rehmanniae Radix.