Investigation of water variation and surface crust of Polygonati Rhizoma extract during vacuum drying

In order to better understand the drying process, the water variation and surface crust of Polygonati Rhizoma extract during drying were investigated. Water variation investigation showed that the total water content decreased gradually over drying time, mainly attributed to the removal of most of the immobile and all of the free water; As drying progressed, the mobility of immobile water and free water increased, while the bound water increased; water in different status underwent mutual transformation. Surface crust investigation showed that the extract surface gradually shrunk and became roughness during drying; crust with a dark color circle appeared on the extract surface, and some cracks connected between pores were found; as drying progressed, the porosity, total pore number and pore volume increased, the number of pores for all volume distributions below 1 mm3, 1–2 mm3 and over 2 mm3 initially increased and then decreased.