Study on the comparison of bioactive components by drying methods of Rehmannia glutinosa

The roots of Rehmannia glutinosa (Scrophulariaceae) have been used in the traditional medicine for treatment of diabetes and fever in Korea. Steamed roots of Rehmannia glutinosa have been also used for the treatment of auditory diseases such as tinnitus and hearing loss. The main classes of bioactive components are iridoid glycosides, phenyl propanoid glycosides and flavonoids, while catalpol is the most abundant iridoid glycoside in fresh roots of the plants. To have a contrast study on bioactive components of Rehmannia glutinosa, the roots of plants have been prepared by air-drying, freeze-drying, blanching, vacuum-drying, and hot-air-drying methods. RP-HPLC has been applied for the analysis of three kinds of components, catalpol, aromadendrin and verbascoside from Rehmannia glutinosa. According to RP-HPLC chromatogram, the amount of catalpol was higher in freeze-drying, and it has been decreased by heat processing. As a result, at same concentrations of samples, the contents of major bioactive components showed different peak intensities. Those results suggest that the processing methods of extracting medicinal plants like Rehmannia glutinosa should be always considered in order not to loose the bioactive components.