The extraction of functional components from natural plant is one of important processing in the development and further practical application of plant product. Microwave assisted extraction (MAE) has been widely used in the extraction of many samples for its unique heating mechanism, short extraction time and high yield of extract. Astragalus slice is a special and typical plant porous media. We describe an approach by scanning electronic microscope (SEM) to characterize the trachea and aperture of Astragalus slices irradiated 20 min by microwave at 600 W and 900 W, with the aim to analyze the effect of the microwave power on the extraction yield by SEM and discuss further the relationship between the microstructure characteristics of sample and mechanism on mass transfer in micro-scale. It is found that extract with the 20 min irradiation of microwave at 600 W is higher than that at 900 W because the apertures on the trachea wall remain open at 600 W, but shrink at 900 W. Moreover, we confirm the important role of the aperture in the extraction of plant materials. Therefore, this study has significant meanings to develop the optimized extraction technology of plant porous media and maintain or improve the quality of extract.

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