Quality characteristics of Rehmannia glutinosa dried at different drying temperature

Rehmannia glutinosa (RG) is a widely cultivated medicinal crop in Korea. Its rhizomes are commonly distributed in dried form as a food or traditional medicine. However, very little research into the drying methods for R. glutinosa has been conducted. This study was performed to determine the effect of the drying temperature on the quality characteristics of R. glutinosa. The famous domestic cultivar ‘Dagang’ was used for the study. The drying experiments were carried out at 40, 50, 60, 70, and 80°C in a hot air dryer. All samples were dried until their moisture content was less than 12%; the moisture content of the raw root was 78.97±1.27%. Under the hot-air drying conditions, the drying process required 288 h at 40°C, 144 h at 50°C, 45 h at 60°C, 22 h at 70°C, 12 h at 80°C. The physical properties of RG were different for each drying temperature. When the temperature was increased, the lightness color value decreased. Hardness which is an important texture factor, was high for a drying temperature of 50–60°C and relatively low for 40°C, 70°C, and 80°C. The catalpol content increased with increasing temperature up to 70°C, and then decreased at higher temperature. Considering the overall characteristics of the dried RG, hot-air drying at 60°C is optimal to achieve desirable physical properties and high active content. These results should provide useful information for the manufacture of high-quality dried RG.