## 3 Results and Discussion
### Experimental Results
Ultrasound assisting the drying process at power levels of 40-160 W affects positive to these parameters, drying time is shorter and product quality is enhanced significantly. In  
Figure 8: Effect of ultrasonic power on saponin content. Figure 7: Effect of ultrasonic power on (**a**) moisture diffusion coefficient and (**b**) color change. conclusion, ultrasound assists effectively in the drying process, shortens the drying time and improves product quality. This is consistent with the research results of previous authors [2; 8; 15]. At the same conditions of drying temperature and speed (45 \({}^{\circ}\)C, 0.5 m/s), the results show that drying time decreases with increasing ultrasonic power. The drying time of 8.8 h at 40 W ultrasonic power reduced to 7.8 h at 80 W ultrasonic power, and reduced to 7 h at 160 W ultrasonic power. As the drying time decreases, the moisture diffusion coefficient increases. In general, ultrasonic waves provide energy to break the moist surface boundary layer. Wave propagation also causes the molecules of materials to vibrate, expand and contract continuously. This produces a vibration leading to the creation of more microchannels in the material, as well as decreasing the intermolecular force from which the moisture diffusion process in the material is greatly facilitated. By increasing the ultrasonic power, ultrasound can affect the surface and internal structure of the material and make the cell walls extend, which contributes to moisture diffusion inside the cells [9]. When the power is high, the efficiency of ultrasonic assistance is impaired. From 40 to 80 W, the drying time is shortened from 8.8 h to 7.8 h, reduced by 11.36%; from 80 to 120 W, it is shortened to 7.2 h, reduced by 7.7%; and at 160 W, the drying time is 7 h, reduced by 2.8%. The remaining saponin content after drying tends to increase gradually at the power level of 40-120 W (from 89.2% to 95.2%) and decrease when the power increases from 120 to 160 W (from 95.2% to 90.5%). At the low power level of 40-120 W, the ultrasonic energy that causes molecular vibration is not sufficient to break the microstructure of the material, but just enough to create microchannels in the material, as well as decrease the intermolecular force. This helps to shorten the drying time, thereby reducing the time that the material is exposed to heat and preserving product quality. Nonetheless, when the power increases to 120-160 W, the energy supplied is sufficient to break the microstructure of the material, leading to a decreasing trend in the remaining saponin content of the material. This is also consistent with previously published work on the influence of ultrasonic power [9; 10; 11]. Ultrasonic-assisted convective drying results in less color change of the product compared with convective drying alone. This indicates that the long hot-air drying time produces significant color change [11]. With the assistance of ultrasound at the power level of 40-120 W, drying time is reduced. Bo Chinh ginseng is less exposed to heat, which helps to preserve the product color. However, at the power level of 120-160 W, although the drying time is shortened, the color change of the product is substantial, indicating that the mechanical vibration of ultrasonic waves also affects the color of the product, especially at high power levels. This can be explained as at high power levels, the amount of ultrasonic energy is extremely large; when propagating to the surface of the material, it can break the moisture boundary layer, leading to a phenomenon known as \"atomization\". This results in the \"uncovered\" surface of the material, and it is exposed to hot air, thereby causing color change [20]. Therefore, it is necessary to limit an appropriate power range to achieve the best value of electricity cost, saponin content and color change; therefore, the power range of 80-160 W was chosen.