## 3 Results and Discussion  
### Experimental Results  
Effect of Drying Temperature (T) on Moisture Effective Diffusion Coefficient, Color Change and Saponin Content of Dried Materials  
To evaluate the effect of drying temperature on Bo Chinh ginseng, experiments with the assistance of 120 W ultrasonic power at various temperatures of 35-40-45-50-55 \({}^{\circ}\)C were carried out. Results are shown in Figures 3-5, respectively. Figure 3: Drying curves at different temperatures with 120 W ultrasonic power. Figure 4: Effect of drying temperature on (**a**) moisture diffusion coefficient and (**b**) color change. Figure 5: Effect of drying temperature on saponin content. Under the same conditions and 120 W ultrasonic power, the results show the higher the temperature, the shorter the drying time. Drying time is reduced from 12.5 h at 35 \({}^{\circ}\)C to 8.9 h at 40 \({}^{\circ}\)C and greatly reduced to 5.8 h at 55 \({}^{\circ}\)C. As the temperature increases, moisture pressure distribution in the material will increase, so the greater the drying kinetics, the shorter the drying time, and the higher the moisture effective diffusion coefficient Deff, which is consistent with previously published studies [15]. However, with the same rise in temperature of 5 \({}^{\circ}\)C, the reduced time is less when the temperature is higher. Particularly, from 35 to 40 \({}^{\circ}\)C, the drying time is shortened from 12.5 h to 8.9 h, reducing by 28.8%, while from 50 to 55 \({}^{\circ}\)C, the drying time is shortened from 6.1 h to 5.8 h, reducing by 4.9%. As the drying time decreases, the moisture diffusion coefficient Deff increases. However, considering the remaining saponin content in the product, from 35 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, the saponin content after drying tends to increase gradually, but if the temperature continues to rise from 45 \({}^{\circ}\)C to 55 \({}^{\circ}\)C, the saponin content decreases. This can be explained as the drying time affects the saponin content significantly. At the temperature of 35-45 \({}^{\circ}\)C, the drying temperature is not high, and the drying time is greatly reduced. This will support the further increase in saponin content. At higher temperatures (45-55 \({}^{\circ}\)C), a strong effect on saponin degradation can be observed, while drying time is not much reduced, and the drying temperature is high. So, the drying material contacts the hot drying air for a long time period, which leads to a downward trend for the remaining saponin content. Figure 4 shows that the increase in drying temperature increases the value of dE. This means that the color change of dried products increases. When the temperature rises from 35 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, the dE value changes slightly because the low temperature does not affect the color of the material significantly. However, when the drying temperature obtains the values of 40, 45 and 50 \({}^{\circ}\)C, the color change of the dried products increases appreciably because of the thermal effect. Therefore, based on the above results, it is necessary to limit an appropriate temperature range to achieve the best value of saponin content and color change; therefore, the temperature range of 40-50 \({}^{\circ}\)C was chosen. 1.2 Effect of Ultrasonic Power (P) on Moisture Diffusion Coefficient, Color Change and Saponin Content of Dried Materials  
To evaluate the effect of ultrasonic power on Bo Chinh ginseng, experiments at a temperature of 45 \({}^{\circ}\)C and ultrasonic power of 0-40-80-120-160 W were carried out. Results are shown in Figures 6-8, respectively. Figure 6: Drying curves at 45 \({}^{\circ}\)C with different ultrasonic power levels. The heat pump drying process is not assisted by ultrasound (45 \({}^{\circ}\)C + 0 W), results show that: it takes 9.9 h to dehydrate material, the moisture diffusion coefficient is low (3.69 \(\times\) 10\({}^{-10}\) m\({}^{2}\)/s), the color change is high (13.42) and the saponin content is low (51.7%).