Intelligent control of microwave vacuum drying based on online aroma monitoring

Astragalus is a common medicinal food that has many benefits for the human body. Some medicinal components in the astragalus would decompose or volatilize at high temperature. Therefore, vacuum drying at a low temperature is a common post-harvest processing for the astragalus. The components and the internal structure of the astragalus change along with the planting range and the harvest year. Therefore, only trained experts with years’ experience can adjust the vacuum drying parameters properly based on their aroma perception, which is backward in the industrial age. In this study, astragalus aroma in microwave vacuum drying was online measured with an electronic nose. Volatiles were verified with Gas Chromatography-Mass Spectrometry (GC-MS) analysis. Drying temperature was PID controlled to maintain constant values. Microwave power was automatically regulated through a Triac circuit. Vacuum degree was adjusted instantly with a step motor. The physical and chemical quality changes of astragalus in drying process were investigated at various temperature and vacuum conditions. To optimize the drying effects, an Artificial Neural Network-based predictive control was designed to adjust the vacuum degree intelligently. The astragalus from Dingxi City, Gansu Province, China was conducted for experiments. By applying different control strategies, the astragalus quality was greatly improved, including more contents of total polysaccharides，astragaloside, total flavonoid, total saponin as well as the enhancement of beany flavor.