## 1 Introduction  
Bo Chinh ginseng, as well as other agricultural plants, are dried for long-term preservation and storage. Drying is the process of removing moisture from solid materials. Moreover, the drying process must ensure a certain level of quality of products with low operating costs. Energy efficiency during the drying process and the quality of products are the two main factors in drying technology. Drying with the intermittency ratio of the ultrasound generator is a highly promising method, which helps to improve drying efficiency and product quality without increasing drying costs [1, 2]. Another great advantage of intermittent drying is the uniform moisture content of the samples. [PERSON] et al. [3, 4] reported that the application of intermittent ultrasound is useful since it can prevent the temperature rise of products caused by the transfer of ultrasonic energy. Thus, in general, color, density, rehydration characteristics, ascorbic acid content and product quality are not changed after ultrasonic treatment. For valuable and precious ginseng, the conventional convection drying method is no longer employed due to the fact that the taste, color and nutritional values are damaged during the long period of high-temperature drying. In recent years, applying new technologies in combination with traditional drying techniques has become popular [5], among which ultrasound-assisted drying is recognized as a rising star. Compared with other alternative technologies applied in the drying process, such as infrared radiation, microwaves and radio waves, ultrasonic waves possess a weak thermal effect [6]. This property facilitates the drying of heat-sensitive materials where precise control of the process is required to avoid excessive heat of the drying material. According to [PERSON] et al. [7], the effectiveness of ultrasound depends not only on technical parameters, including wave frequency, ultrasonic intensity and pressure, but also closely relates to the experimental conditions, such as the drying temperature and initial moisture content of the drying material. To study the effect of ultrasound on product quality, [PERSON] et al. designed a hot-air convective dryer combined with ultrasound to dehydrate garlic slices. The drying process was significantly accelerated, retention of organosulfur compounds was better, and browning was effectively prevented [8]. Similar conclusions were also drawn by [PERSON] [9]. [PERSON] et al. conducted research on green pepper and concluded that convective drying combined with ultrasound and microwaves resulted in less color change of the product compared with convective drying alone, showing that long hot-air drying time may cause considerable color change [10]. Similar conclusions were also drawn by [PERSON] about the effect of drying temperature on color change [11]. According to [PERSON] et al. [5], drying temperature has a significant effect on the saponin content and color of white ginseng. Although high temperature helps to reduce drying time, it also leads to more saponin content loss and color change than low drying temperature. Similar results have also been reported by [PERSON] et al. [12; 13] for taegeuk ginseng and red ginseng. These results suggest that low drying temperature is beneficial for maintaining high saponin content in ginseng. In terms of time and cost for the drying process, research on freeze-drying combined with ultrasound on apples, carrots and eggplants has been carried out by [PERSON] et al. [14]. The study concluded that the assistance of ultrasound helps to reduce drying time and energy consumption and boost mass production without affecting product quality. Some previous studies focused on the continuity or intermittency of ultrasound-assisted drying, in which the intermittency of ultrasound-assisted drying is that the ultrasound will be operated intermittently in the drying process. [PERSON] et al.