Determination of drying kinetics and quality changes of Panax quinquefolium L. dried in hot-blast air

Abstract

The drying kinetics and quality changes of American ginseng dried in hot-blast air were studied at three temperatures (30, 40 and 50 °C). Low-field nuclear magnetic resonance (LF-NMR) and MRI techniques were used to study the water conditions and its distribution during the drying process. The experimental results show that the Henderson and Pabis model provided the best prediction of the drying process. The times at which the second phase began in the drying kinetic curve corresponded exactly with the turning point at which T2 free water decreased to its equilibrium with times of 168, 48 and 24 h at drying temperatures of 30, 40 and 50 °C, respectively. MRI image analysis showed that the internal moisture distribution of American ginseng was uneven, and that the water diffused inwards and outwards during the drying process. Based on the analysis of T2, color parameters and the HPLC data, the free water in American ginseng dissipated slowly at 30 °C, its color was brighter, and the content of ginsenosides was higher than at 40 and 50 °C. The LF-NMR technique can monitor the changes of the water state and predict the turning point of the sample drying curve during the drying process.