# The Impact of Freeze Drying on Bioactivity and Physical Properties of Food Products
## 1 Introduction  
For centuries, various techniques, including fermentation and drying, have been applied by people to preserve foods that require protection from the degradation of their bioactive and physical properties [1, 2]. With increasing demands in the consumption and preparation of functional foods and their derivatives, drying has become essential to ensure the preservation of the bioactive components of foods, especially throughout their shelf life [3, 4, 5]. Drying has been beneficial for centuries in protecting food and enabling long-term storage, especially in the science and food industry [6, 7]. In other words, drying ensures that the bioactive content and flavor of the foods are conserved over a long time [8, 9]. In recent years, one of the newly applied drying techniques is the freeze drying (FD) technique, which has been used to maintain the quality of products by preserving the physical and bioactive characteristics of the products [10, 11].  
For a long while, foods have been used as spread product ingredients in several areas, including medicine and industrial manufacturing [12, 13, 14]. Foods were dried using different techniques, primarily to enhance their long-term storage potential while minimizing the decomposition of their bioactive components [15, 16]. Hot air drying, sun drying, and microwave drying are frequently used drying techniques in several industries and laboratories to produce high-quality dried materials [17]. Recently, FD has become popular in studies aiming to obtain maximum product quality from the samples [18]. Even though FD successfully acts on the prevention of the bioactive property, physical property, and flavor of the material, it also involves high costs and significant energy consumption due tothe low temperature and pressure required [19]. Low temperature and pressure can be a source of the high product quality of the freeze-dried samples.  
Freezing, primary drying, and secondary drying are the main processes required to achieve the maximum product quality in FD [20; 21]. In the application of the FD technique, these processes are pursued, respectively. The duration of each drying process is crucial for managing energy consumption and cost. FD has some parameters, including temperature and time, that influence the impact of FD on the bioactive content and flavor properties of the products [22]. For example, in one study, eggsplants were freeze-dried using a laboratory freeze-dryer at \(-10\), \(0\), and \(10\)\({}^{\circ}\)C as parameters, for \(50\) h. As a result of the study, the drying time was decreased while the temperature was increased, and that conclusion explained that temperature is a parameter of freeze drying. Also, the degradation of total phenols, total flavonoids, total anthocyanins, and ascorbic acid of the eggplant increased while temperature increased. Additionally, the freeze-drying of plants and foods, including fruits, vegetables, and other food products, as well as waste and by-products, is mentioned [23; 24; 25; 26; 27; 28; 29]. Also, FD is used for the conversion of waste products, which is an enormous problem for the global environment and cost [30]. Waste products, including peels, seeds, and pomace, with FD, transformed the effective materials, including microencapsulated bioactive content and powder, with high bioactive ingredients, to enhance the quality of the other products that are manufactured in the industry [31].  
This review article examines the preservation effects of FD on bioactive components of food products. It has been investigated how the parameters affect the bioactivity and flavor properties of the dried food and provide the optimal conditions to obtain high-quality product. Additionally, it has been examined how the waste products are valorized by FD.