# The Impact of Freeze Drying on Bioactivity and Physical Properties of Food Products
## 2 Freeze Dry Technique
### Process of Freeze Drying Technique  
The FD technique has processes such as freezing, primary drying, and secondary drying (Figure 1). The initial stage of FD is the freezing process, which is affected by manifold parameters to produce frozen materials at a special temperature for the used sample [43]. The parameters include a specific temperature, which varies depending on the content of protein, oil, and other components of the material that will be dried. Ice crystalsformed during this process depend on the cooling rate, which affects the size and amount of ice crystals [38]. Additionally, the freezing and primary drying processes are influenced by the size and amount of the ice crystals, especially their application times [20]. The large and few ice crystals that are obtained via a slow cooling process allow for rapid sublimation and reduce the drying time of FD [44]. Interestingly, the degradation of the cellular construction and cells of the material, which results from the large ice crystal formation, leads to an increase in the amount of some bioactive ingredients, which are present less in fresh food compared with freeze-dried material. In contrast, the application of rapid cooling causes the formation of smaller and larger quantities of ice crystals. This, in turn, leads to an increase in the time required for primary drying and the amount of consumed energy by FD; however, it naturally reduces the degradation of the cellular structures of the material [45]. The extent of degradation is also heightened by rapid cooling, as this process prolongs the primary drying phase. However, the cellular structure remains intact. Primary drying is the second main part of the FD technique. After freezing, ice crystals are sublimated directly to the vapor phase without passing through the liquid phase under low pressure and temperature [20; 46]. As the processed food enters the liquid phase, a valuable portion of its components degrade. The temperature and pressure are determined at the triple point for sublimation [47]. In other words, sublimation occurs below the triple point because only the gas and solid phases exist. According to these, 4.579 mmHg and 0.0099 \({}^{\circ}\)C conditions should be suitable for the sublimation of water because these values are below the triple point. The sublimated vapor is converted to the solid phase, which is a part of the FD device, so the moisture content of the food decreases, but a small amount of the moisture is still in the food. The last part of the FD technique is secondary drying. In this process, the temperature is increased and the pressure is decreased for efficient drying [21]. Higher temperatures are not harmful to the food at this stage because there are no ice crystals present, so no liquid phase occurs [38; 48]. The desorption term can be used for the secondary drying process. As a result of FD, 95-99.5% of the water in the food can be removed. Additionally, shorter drying times correlate with higher product quality. FD can be enhanced via utilizing assisted technologies [49]. The importance of these technologies was demonstrated in an experiment by [PERSON] et al., where FD was compared with  
Figure 1: Process of freeze drying technique. microwave-assisted FD [50]. Two different drying techniques were applied to the apple slices. In this study, vacuum microwave drying and FD were used as a hybrid system to obtain better results. Microwave-integrated FD was used to improve the efficiency of FD, reducing the drying time and promoting the nutritional value of the apple slices. Besides, greater puffing and better anti-moisture occlusion talents of the vacuum microwave and freeze hybrid dried apple slices were acquired, especially compared with apple slices that were freeze-dried alone. Another technique is ultrasonic treatment, which can be used as an assistant to the FD [51]. Ultrasonically assisted atmospheric FD can be applied to achieve higher-quality products than non-assisted FD.