# Experimental Study of the Effect of Ultrasound on the Freezing Process of Bo Chinh Ginseng
## 1 Introduction
Different materials have different effects on the intensity and duration of ultrasound transmission. Therefore, a separate study is required for each type of material. In most of the studies on ultrasound-assisted freezing, the material is immersed in a refrigerant. [PERSON] et al. [13] experimented with freezing Agar gel dipped in an ethylene glycol solution. They found that ultrasound irradiation supports the freezing process at a frequency of 25 kHz with an intensity of 0.07, 0.14, 0.25, 0.35 and 0.42 W/cm\({}^{2}\) when the temperature starts the transmission process at \(-2\), \(-3\), \(-4\) and \(-5\)\({}^{\circ}\)C, respectively. These research results showed that the nucleation temperature of the agar gel formed in response to the super-cooling temperature was different and corresponded to the intensity and time of transmission. [PERSON] et al. [12] studied the freezing of radish cylinders dipped in a CaCl\({}_{2}\) solution with the help of ultrasound irradiation at 25 kHz with wave intensities of 0.09, 0.17, 0.26 and 0.37 W/cm\({}^{2}\). Transmittance started at temperatures of \(-0.5\), \(-1\), \(-1.5\) and \(-2\)\({}^{\circ}\)C, and there was a transmitting delay of 0, 3, 7, 10 and 15 s. A temperature of \(-0.5\)\({}^{\circ}\)C and a 7 s delay corresponding to a transmitting intensity of 0.26 W/cm\({}^{2}\) was the optimal condition for the nucleation of water in the material. [PERSON] et al. [14] studied the freezing of grass carp by applying a 40 kHz ultrasound with wave intensities of 0.3, 0.38, 0.48 and 0.6 W/cm\({}^{2}\) and an intermittent transmitting rate of 8 s on/2 s off. Their experiments showed that ultrasound irradiation at an intensity of 0.38 W/cm\({}^{2}\) helped to reduce the freezing time, as well as shorten the duration of the cooling phase and the transition phase. [PERSON] et al. [15; 16] studied the effect of ultrasound irradiation on the freezing process of broccoli at 20 kHz and 30 kHz with a power of 0, 125, 150, 175 and 195 W and an intermittency ratio of 60 s on/60 s off. Their research results showed that ultrasound irradiation significantly shortened the cooling time and supercooling time, especially at 150W (30 kHz) and 175 W (30 kHz). [PERSON] et al. [17] studied the freezing of common carp with the help of ultrasound irradiation at 30 kHz with a power of 175 W and a transmitting rate 30 s on 30 s off; ultrasound irradiation helped to shorten the freezing time and reduce water crystallization in the material. [PERSON] et al. [9] performed the ultrasound-assisted freezing of pork (porcine longissimus muscle) at a frequency of 30 kHz, with capacities of 120, 180, 240 and 300 W and the proportional intermittent ratio of 30 s on/30 s off. Experimental results showed that ultrasound irradiation helped to reduce freezing time, and at the 180 W power level, provided the most effective support with the shortest freezing time. [PERSON] et al. [6] conducted the direct freezing of mushrooms without dipping them in the refrigerant, and thus, the material was not contaminated. The form of ultrasound irradiation that supports the freezing process has a frequency of 20 kHz and a transmitting intensity of 0, 0.13, 0.27 and 0.39 W/cm\({}^{2}\). The results showed that at an intensity of 0.39 W/cm\({}^{2}\), the fungus kept its original hardness. However, the transmitting efficiency was reduced due to transmission in a cold-air environment. There are many studies on the ultrasound-assisted freezing process for many different materials, but no article has researched the application of ultrasound irradiation to support, in general, the freezing process of ginseng materials and, in particular, that of Bo Chinh ginseng. Therefore, this research will study the appropriate ultrasound parameters that support the freezing process of Bo Chinh ginseng, without it coming into direct contact with the transducer, in order to analyze the effects of the ultrasound power, irradiation temperature and intermittency ratio on the freezing time, nucleation temperature, color and microstructure of the materials.