Research status of drying technology and equipment for Chinese medicinal materials

The drying process is one of the most essential steps in the initial processing of Chinese medicinal materials. Current natural drying (such as sun and shade drying) approaches have been widely used to improve the quality of medicinal materials, due to their simplicity and low cost. However, the natural environmental conditions can dominate the quality of dried medicinal materials, resulting in low drying efficiency. It is still lacking in the strengthened research of controllable drying. In this study, a critical review was given on the research status of drying technology and equipment for Chinese medicine materials. Firstly, a systematic investigation was performed on the moisture migration, diffusion and removal of Chinese medicinal materials during drying, as well as the changes in color, appearance, microstructure, and physical and chemical properties. The moisture removal rate and drying quality depended mainly on the water content, volume, and tissue structure of Chinese medicinal materials in the conditions of drying, such as temperature, relative humidity and wind speed. Secondly, the drying mechanisms, equipment structure and working principle were performed on various drying, including hot air, heat pump, infrared, microwave, vacuum and high voltage electric field drying. The performances of drying equipment were analyzed, such as uneven drying, high-energy consumption and low efficiency. The structure of the drying chamber was optimized to improve the airflow distribution using heating with multiple heat sources. Chinese medicine materials remained in a dynamic state during drying, in order to improve drying uniformity. At the same time, the drying performances were also evaluated on various drying technologies under different conditions. Some parameters were then considered, including drying temperature, slice thickness, and vacuum degree. The drying temperature generally improved the drying rate while shortening the drying time. Much higher temperature led to reduce the sensory quality and efficacy of medicinal materials. Pretreatment, staged variable temperature and humidity, and intermittent drying were also discussed to obtain comprehensive drying quality, efficiency and cost, and application scopes. Among them, the hot-air drying equipment was suitable for the large-scale drying of cheap Chinese medicinal materials, due to its simple, low cost and large scale, but with the low drying efficiency and quality, as well as the high energy consumption. Vacuum drying was suitable for small batch drying of Chinese medicinal materials, which were easy to oxidize and brown with high added value. The low temperature and oxygen environments were used to improve the drying quality of Chinese medicinal materials, but with the low drying efficiency, the high investment and the operation cost of equipment. Moreover, the drying mechanisms were analyzed for the microwave-vacuum, microwave-hot air, and far infrared-heat pump drying in the coupling or series combination modes. The research progress on the development of combined drying equipment was summarized to compare the combined and single drying, in order to explore the limitations of combined drying equipment and processes. Finally, the research directions were predicted from the evaluation standards, the drying-related models, and the drying equipment of Chinese medicinal materials. This finding can provide the theoretical basis and technical support for the research and innovation of drying technology and equipment for Chinese medicinal materials.