## 4 Effect of Drying on the Nutritional, Biochemical, and Physiochemical Properties of Sea Cucumbers
### Freeze Drying  
Freeze drying is a modern drying processing method in the food industry and is often known as lyophilization. lyophilization is utilized not only for preserving food particles but also for maintaining the original colour [48; 90]. Generally, drying starts with frozen products that are maintained below low temperatures and give a crystalline form, depending on the type of product. In freeze drying, solid particles are transformed into vapour particles through sublimation, and then the water and the suspension medium become crystallized under low temperatures [91]. During sublimation, the pressure of the surrounding vapour is lower than the ice vapour [29]. The energy is provided in the form of heat, which is lower than the product's low temperature. After that, the product's surrounding pressure is increased to form a crystallized structure [29]. Freeze drying is a more beneficial and effective preserving method than the other drying methods because it maintains the yield and retains the morphological, biochemical,and immunological characteristics, as well as reduces microbial activity and weight, and provides longer shelf life, along with the recovery ability of volatile substances [48; 79]. However, the maintenance cost, energy consumption rate, drying time (i.e., 18 h), and operation cost are higher in freeze drying than in other drying methods, but it is free from any cell rupture or deformation. Even heat-sensitive molecules become stable in this method, and the quality of the dried products looks similar to fresh products since they maintain their actual colour [48; 79]. For example, freeze-dried sea cucumber viscera contained 6.15% less moisture when compared to hot air drying [1], and protein content (6.58%) was also higher in freeze-dried viscera (Table 5) [3]. Moreover, the saponin and polysaccharide content increased by 0.07 and 6.25%, respectively, in the body wall of sea cucumbers using freeze drying after infrared radiation (Table 4) [60]. Based on the above literature, freeze drying with a temperature range of \(-40\) to \(-80\)\({}^{\circ}\)C for 48-72 h could be used for better appearance, texture, colour, and odour, with reduced fungal infection, in sea cucumbers (Figure 4). Therefore, processing strategies for sea cucumbers, including pre-treatments and drying methods, are crucial parts of obtaining quality products. Different pre-treatments combined with different processing techniques and drying methods can greatly affect the proximate composition as well as the content of bioactive compounds such as phenolics, saponin, polysaccharides, and collagen (Tables 4 and 5). These compounds possess a significant potential to be a candidate in the food, pharmaceutical, and nutraceutical industries due to their wide range of bioactivities [92]. Figure 4: Processing of sea cucumbers using a freeze dryer. \begin{table} \begin{tabular}{c c c c c c c} \hline \hline
**Species** & **Drying Methods** & **Moisture (\%)** & **Ash (\%)** & **Protein (\%)** & **Lipid (\%)** & **References** \\ \hline _Holothuria tubulosa_ & HAD & 6.86 & 38.16 & 56.12 & 1.36 & \\ (BW) & FD & 6.00 & 30.39 & 62.70 & 1.93 & [3] \\  & MD & 6.04 & 32.78 & 59.79 & 1.65 & \\ \hline _Cucumaria fondosa_ & HAD & 7.84 & 12.49 & 45.79 & 25.69 & [1] \\ Viscera & FD & 1.79 & 12.53 & 46.96 & 23.19 & [1] \\ \hline _Apostichopus japonicus selenka_ & HAD & 5.15 & 32.99 & 51 & 5.2 & \\ (BW) & SD & 5.3 & 37.18 & 46.50 & 5.10 & [37] \\  & VFD & 3.99 & 31.89 & 48 & 5.22 & \\ \hline _Holothuria atra_ & SD & 7.5 & 8.8 & 78.6 & 2.7 & \\ (BW) & Oven-Dried at 40 \({}^{\circ}\)C & 7.2 & 8.8 & 78.8 & 2.6 & [30] \\  & Oven-Dried at 40 \({}^{\circ}\)C & 8.4 & 8.5 & 80.6 & 2.4 & \\  & Oven-Dried at 40 \({}^{\circ}\)C & 4.5 & 7.3 & 82.9 & 2.4 & \\ \hline \hline \end{tabular}