# Experimental Study of the Effect of Ultrasound on the Freezing Process of Bo Chinh Ginseng
## 1 Introduction  
Bo Chinh ginseng (Abelmoschus sagitifolius ([PERSON]) [PERSON]) is one of the precious ginseng species grown in Vietnam. Bo Chinh ginseng has important medicinal ingredients, including triterpene saponins, which are considered to be a group of compounds that determine the typical pharmacological characteristics of the medicinal plants in the ginseng family. In addition, Bo Chinh ginseng also contains other important components, such as mucilage, starch, phytosterols, coumarin, fatty acids, organic acids, reducing sugar and ionic compounds, lipids, proteins and protids. Hence, the process of preservation and processing is very important in order to keep the medicinal ingredients in ginseng intact. Currently, ginseng is dried after it is harvested and before it is stored. The medicinal ingredients in ginseng are easily lost when they are dried at high temperatures. Therefore, freeze-drying is a promising method for preserving high levels of the valuable properties of ginseng. In particular, the first stage of freeze-drying involves freezing the material in order to completely crystallize the water in the material, which is also an important stage in determining the quality of the material after drying [1]. This study aimed to investigate theeffects of ultrasound parameters on the freezing process of Bo Chinh ginseng. In addition, in order to reduce the drying times in the freeze-drying process and enhance the quality of the dried material, having more control over the freezing stage is important. This is because freezing phenomena, such as nucleation and ice crystal growth, are related to ice crystal morphology parameters, and these parameters directly affect drying rates. In recent years, ultrasound irradiation has been widely studied and applied in the field of food preservation and processing, including research into ultrasound irradiation methods in order to assist in the freezing process of materials. Ultrasound irradiation effectively assists in the phase transition to nucleate and promote ice crystal growth, and to reduce the size of ice crystals. This is very important in ensuring the quality of the material after freezing; smaller ice crystals will help to maintain the protein structure, and the biochemical properties of the material will be preserved [2]. [PERSON] et al. [3] indicated that ultrasound irradiation accelerates heat and water transfer because ultrasound irradiation creates microchannels by disrupting the bubbles generated during nucleation. In addition, some studies show that ultrasound irradiation increases the nucleation temperature of materials compared to samples of materials frozen without ultrasound support, thereby accelerating the crystallization process, reducing the crystal size and shortening the freezing time. Some other studies have indicated that the nucleation step can be divided into primary nucleation and secondary nucleation stages when assisted by ultrasound [4; 5; 6; 7; 8; 9; 10; 11]. In particular, at the primary nucleation stage, ultrasound irradiation breaks the bubbles to create cavitation bubbles, which increases the nucleation temperature. In the secondary nucleation stage, ultrasound irradiation helps to promote heat and mass transfer. At the stage of crystal growth, the crystal size is reduced and uniform crystals are formed because ultrasound irradiation creates microchannels that increase the rate of heat transfer, which shortens the crystallization time. However, not all intensities and durations of ultrasound irradiation produce positive effects in the freezing process. Ultrasound irradiation helps to increase the nucleation temperature during freezing compared to freezing without ultrasound support. However, if the ultrasound intensity is too large and the transmitting time is long, it will have an adverse effect on the freezing process. This is due to the ice crystals crystallizing again after the thermal effect melts them, which increases the freezing time [12].