Astragalus slice is one species of stem and root medicinal herb with the widely curative effects, also a special and typical plant porous material, and the drying operation is one of important processing technologies in its storage and further practical application. By using scanning electronic microscope (SEM), we mainly characterize the micrographics of parenchyma cell, trachea and aperture of Astragalus slices dried by hot air at 60°C, with the aim to discuss the relationships among the microstructure of sample, macro-drying property and drying mechanism in micro-scale. Results show that in hot air dried sample, the distribution status of cytoplasm inside parenchyma cells appears obvious different with that of untreated sample, behaving agglomeration and leaving wide space with cell wall. Furthermore, we find that in comparison with the untreated sample, the shape of aperture in hot air dried sample basically maintains un-changed, but most of apertures are fully filled with some perceived substance. It is demonstrated that the flow phenomenon of cytoplasm occurs inside matrix accompanied by the moisture diffusion in the mode of liquid water, particularly biological macro-molecular, which directly influence the permeability of moisture transfer path inside matrix and the quality of dried products. Hence, this study has significant meanings to develop the optimized drying technology of plant porous material focused on micro-mechanism and the quality of dried products.

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