# The Impact of Freeze Drying on Bioactivity and Physical Properties of Food Products
## 3 Parameters of Freeze Drying
### Pretreatmentments  
Pretreatments include any technique used to enhance the preservation of the physical properties of the samples via increasing the bioactive properties of the samples and reducing the costs or achieving other desired outcomes [54]. In a study, blanching, ultrasound (US), and pulsed electric field (PEF) pretreatments were applied to improve the FD of red bell peppers [55]. As a consequence, these pretreatments similarly reduced the bioactive compounds' degradation of the sample and reduced the FD time. Results of the study showed that the blanching technique reduced the drying time and the number of bioactive properties in the red bell pepper, while better preservation of the sample was observed when PEF and its combination with the US were applied. Hence, the protection of the product in all characteristics of the sample, including flavor, taste, color, and bioactive properties, was obtained using the appropriate choice of pretreatment techniques. In the FD application to the berries, fruits were used without cutting, peeling, or any physical changes to preserve their original shape and bioactivity; however, wrinkled products were obtained due to the outer skin barrier of the berries as an impediment to the flow of the water vapor [56]. Water vapor is generally produced during the sublimation period of FD. Also in this study, carbon dioxide (CO\({}_{2}\)) laser perforation was applied as a pretreatment to the skin of berries, which must be treated to obtain high-quality final products. The perforation improved the flow of the water vapor, which is produced by sublimation within the berries, and enhanced the drying processes. At the end of the study, the percentage of non-wrinkled products, which must be high to indicate that successful production occurred, increased from 47% to 86%. In other words, the product quality of the freeze-dried berries was improved with the perforation pretreatment. In brief, the low drying time and high product quality were obtained by pretreatment, which contributed to improving the FD of berries. Ultrasound pretreatment was applied to explain its impact on the drying rate and rehydration rate of apple slices, and a positive correlation between FD and the pretreatment was examined [57]. The ultrasound pretreatment enhanced the product quality and improved the capacity of the apple slices to absorb water after FD. In other research about ultrasound by [PERSON] et al., ultrasound pretreatment was applied for minutes to improve the product grade via increasing the bioactive properties, including the total phenolic compounds and antioxidant activity of the sample [58]. In this study, purple cabbage powder was prepared using FD with ultrasound pretreatment, and results were obtained from the implementation of FD compared with convective, spouted bed, and spray drying according to the appearance of the dried powder. Consequently, convective drying, FD, and spouted bed drying generated erratic and rigid surfaces, while spray drying induced globular particulates. Additionally, ultrasound-assisted FD was practiced to quintec slices [39]. Freeze-dried samples that were exposed to ultrasound pretreatment showed a higher retention strength of bioactive properties containing total phenols, antioxidant capacity, and ascorbic acid in the slices, compared with freeze-dried samples alone. Also, ultrasound pretreatment affected the physical properties, containing shrinkage, rehydration, color, water activity, and hardness of the freeze-dried samples, even more than dried samples without ultrasound pretreatment. Moreover, ultrasound was applied to improve the implications of FD on windfall apples [59]. The ultrasound application caused a high retention of the total phenolics, ascorbic acid, and antioxidant activity of the apples. Two different pretreatments can be used as a hybrid to obtain maximum-quality products. For instance, a hybrid system of ultrasound pretreatment and blanching, which were applied before the FD processes, was used in a study [60].