## 3 Results and Discussion
### Experimental Results
1.3 Effect of Intermittency Ratio (A) on Moisture Diffusion Coefficient, Color Change and Saponin Content of Dried Materials  
To evaluate the effect of intermittency on Bo Chinh ginseng, experiments at a temperature of 45 \({}^{\circ}\)C and intermittency ratios of 0.0-0.2-0.4-0.6-0.8 were carried out. The intermittency ratio A is calculated as follows:  
\[\text{A}=\frac{\text{T}_{\text{OFF}}}{\text{T}_{\text{ON}}+\text{T}_{\text{ OFF}}} \tag{7}\]  
where  
TON: drying time with ultrasound;Figure 10: Effect of intermittent drying on (**a**) moisture diffusion coefficient and (**b**) color change. Figure 9: Drying curves at 45\({}^{\circ}\)C with different intermittency ratios. During intermittent drying, the heating of the material is sporadic, and the moisture inside is re-distributed, which will facilitate the diffusion of moisture, speeding up the diffusion rate inside and evaporation rate from the material surface [1,2]. At the same conditions of drying temperature and ultrasonic power (45 \({}^{\circ}\)C + 120 W), the results reveal that it brings about a powerful effect on drying time and moisture diffusion coefficient, and the drying time with continuous ultrasound generator drying (A = 0) is not the shortest one. Figure 11(a) is the result of heat pump drying at 45 \({}^{\circ}\)C, while Figure 11(b)-f is the results of ultrasound-assisted drying with different intermittency ratios (A). Ultrasonic wave propagation helps to create microchannels and break the moist boundary layer of the material. Based on the obtained SEM images with a magnification of 200 times, it is observed that the material surface with ultrasound-assisted drying is porous, while heat pump drying results in an almost non-porous material (Figure 11(a)). This porous property stimulates the process of moisture transfer, thereby shortening the drying time. Notably, it is observed that the density and size of pores depend on the intermittency ratio. With intermittency ratio A = 0.2 (Figure 11(e)), the greatest number of pores and the largest pore size can be observed compared to those of other intermittency ratios. Thus, this drying mode A = 0.2 results in the shortest drying time with the best value of moisture diffusion coefficient Deff. Drying temperature has a strong effect on the color and saponin content of the product. By adjusting the drying time, we can minimize the exposure of ginseng to heat, and ultrasound-assisted intermittent drying helps to improve color and retain as much saponin content with an appropriate intermittency ratio. Ultrasonic wave propagation increases the temperature of the material due to mechanical vibration [6]. The temperature rise caused by ultrasonic waves in the air ranges from 1 to 10 \({}^{\circ}\)C, depending on specific drying conditions. During intermittency (T\({}_{\text{OFF}}\)), the thermal effect caused by energy transfer of ultrasound in the material is OFF, thereby limiting the color change and retaining as much saponin content. Therefore, it is necessary to choose an appropriate intermittency ratio to optimize the parameters of moisture diffusion coefficient, saponin content and color change. A = 0.0-0.4 was chosen. Figure 11: Effect of intermittent drying on saponin content. Figure 4:0 SEM images of Bo Chinh ginseng after drying: (**a**) heat pump drying 45 \({}^{\circ}\)C; (**b**) drying with intermittent ultrasound generator 45 \({}^{\circ}\)C + 120 W + 0.8; (**c**) drying with intermittent ultrasound generator 45 \({}^{\circ}\)C + 120 W + 0.6; (**d**) drying with intermittent ultrasound generator 45 \({}^{\circ}\)C + 120 W + 0.4; (**e**) drying with intermittent ultrasound generator 45 \({}^{\circ}\)C + 120 W + 0.2; (**f**) drying with intermittent ultrasound generator 45 \({}^{\circ}\)C + 120 W + 0.0.