Dehydration characteristics and evolution of physicochemical properties of Platycodon grandiflorum (Jacq. A.DC.) roots (PGR) during pulse-spouted microwave vacuum drying (PSMVD)

Platycodon grandiflorum (Jacq. A.DC.) root (PGR) is a famous edible and medicinal herb which contains lots of bioactive components that were beneficial for people health. Drying process is a necessary post-harvest technology for fresh PGR to lower the moisture content and extend shelf-life of PGR. But it also causes irregular cellular deformation and dynamically changing viscoelastic properties of PGR, which are critical for the drying kinetics and the quality attributes of materials. Herein, it is essential to understand the micro-level cellular changes, viscoelastic properties, and their relationship with the moisture loss during drying. PGR slices at different drying stages were investigated for the evolution of physicochemical properties (moisture changes, micro-level cellular attributes, and viscoelastic properties) and their correlations during pulse-spouted microwave vacuum drying (PSMVD). Low field nuclear magnetic resonance (LF −NMR) results showed that the removal of moisture from the PGR mainly depends on the loss of free water instead of bound water. Furthermore, magnetic resonance imaging (MRI) demonstrated the uniform removal of water from PGR slices during PSMVD process. As the drying proceeding from stage I to V, irregular cellular enlargement and intracellular cavities appeared in the tissues of PGR slices, and the fractal dimension (Df) values of PGR slices increased from 1.62 to 2.08. Through dynamic mechanical analysis (DMA), both the equilibrium modulus (Ee) and viscosity (μ0) exhibited a descending trend first then ascending trend, indicating that the hardness and stiffness of PGR slices were decreased first then increased during PSMVD process. This study further established the correlation of microcellular structure and viscoelastic properties with moisture loss during PSMVD process. Thus, this work identified the evolution of physicochemical properties of PGR during PSMVD process and future would help to build a predicted machanic model for the material deformation during drying.