# Experimental Study of the Effect of Ultrasound on the Freezing Process of Bo Chinh Ginseng
## 3 Results and Discussions
### Experimental Effect of Ultrasound Irradiation on the Microstructure of Bo Chinth Ginsing  
The microstructure plays an important role in assessing the quality of materials during food processing.  
Previous studies have shown that ultrasound irradiation aids in freezing, and besides promoting early nucleation and reducing the size of ice crystals, it also creates microchannels that enhance transmission heat and mass transfer [3]. [PERSON] et al. [17] presented their findings showing that an appropriate ultrasonic power applied to the assisted freezing process can decrease the ice crystal size. [PERSON] et al. [9] showed that the crystal size of frozen samples treated by ultrasound was smaller and the distribution of ice crystals was uniform.  
The SEM images of the materials are shown in Figure 11 with two different magnification ratios (50 and 200 times). Through the image, it can be seen that the surface of the material sample frozen without the support of ultrasound irradiation (0 W-control) has very few holes. Meanwhile, in the ultrasound-assisted freezing mode, a number of holes appeared on the surface of the dense material. This proves that super waves greatly influence the creation of the microchannels, which are shown through holes on the surface of the material. The fact that holes appear on the surface of the material proves that this experimental study produced results consistent with previous studies. In addition, the creation of microchannels in the material after freezing is of great significance for the sublimation drying phase, as the microchannels promote the heat transfer process, helping to shorten the drying time and improve the quality of the material.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatment** & **L*** & **a*** & **b*** & **WI** \\ \hline Fresh sample & 79.65 \(\pm\) 0.83 & \(-\)2.27 \(\pm\) 0.09 & 9.34 \(\pm\) 0.07 & 77.49 \\ Control & 86.28 \(\pm\) 0.18 & 0.34 \(\pm\) 0.04 & 9.62 \(\pm\) 0.12 & 83.24 \\ US-100-c & 85.65 \(\pm\) 0.16 & \(-\)0.44 \(\pm\) 0.05 & 8.5 \(\pm\) 0.26 & 83.32 \\ US-100-0.6i & 84.93 \(\pm\) 0.35 & \(-\)1.17 \(\pm\) 0.13 & 6.66 \(\pm\) 0.24 & 83.48 \\ \hline \hline \end{tabular} Note: fresh sample—sample of fresh material; control—frozen material samples without ultrasound assistance; US-100-c—ultrasound-assisted frozen material samples with continuous transmittance; US-100-0.6i—ultrasound-assisted frozen material samples with intermittent transmittance at a rate of 0.6.  
\end{table}
Table 2: Color index of material samples.  
Figure 11: SEM images of ginseng microstructures under different freezing conditions: (**a**) 0W-control sample at \(\times\)50 and \(\times\)200 magnifications; (**b**) 100W-IR1—continuous ultrasound-assisted freezing at \(\times\)50 and \(\times\)200 magnifications; (**c**) 100W-IR 0.6—intermittent ultrasound-assisted freezing at \(\times\)50 and \(\times\)200 magnifications; (**d**) 100W-IR0.4—intermittent ultrasound-assisted freezing at \(\times\)50 and \(\times\)200 magnifications.  
The surface of the continuously frozen ultrasound-assisted (100 W-IR1) sample was much more porous than that of the non-ultrasound (0 W-control) sample. This may be attributed to the cavitation bubbles produced by ultrasound-induced nucleation, and to microstreaming that may have fragmented pre-existing ice crystals into smaller nuclei, promoting secondary nucleation and resulting in more ice nuclei and a shorter freezing time [26]. However, compared with the 100 W-IR1 sample, the intermittently frozen ultrasound-assisted (100 W-IR0.4 and 100 W-IR0.6) samples had more pores on their surfaces at the 500 \(\upmu\)m magnification. In the corresponding image at a 100 \(\upmu\)m magnification, it is easy to see that the pores appear evenly and more frequently in the material sample with an intermittency ratio of 0.4 compared to that with a ratio of 0.6.