## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.3 Freeze-Drying  
Also known as lyophilization, freeze-drying (FD) is one of the most relevant dehydration methods for foods rich in bioactive compounds. This is due to the nature of the process, which involves the sublimation of water from a previously frozen product under a low pressure. It is reported that typical FD conditions involve temperatures below 0 \({}^{\circ}\)C and pressures lower than 2 mmHg [58; 70]. According to [PERSON] and [PERSON] [71], the process consists of three stages: the freezing of the product (usually under atmospheric pressure), primary drying, and secondary drying. During freezing, ice crystals form, producing benefits such as reduced physical, chemical, and microbiological changes; the immobilization of ingredients; the prevention of foaming during the next phase (caused by the reduced pressure in the drying chamber); and the structural stabilization of cells, minimizing ruptures and deformations. The freezing rate directly influences the properties of the final dried product, as it determines the morphology of the ice crystals, which in turn affects the sublimation rate in the next stage [71; 72]. In the first drying stage, the frozen product is placed in the freeze-dryer, and sublimation occurs under a reduced pressure at a temperature about 2-3 \({}^{\circ}\)C below the product's collapse temperature (the temperature at which physical alterations compromising quality may occur). In the second stage, most of the sublimation is complete, and the desorption of bound water molecules begins. This is the longest stage and is essential for reducing the moisture content to the desired level [71; 72]. Regarding the structure of the final product, [PERSON] et al. [4] reported that porosity is directly influenced by the ice crystal formation during the freezing stage. Rapid freezing results in finer powders, while slow freezing produces coarser particles. In this regard, FD has been noted as a favorable option for producing highly porous powders, which improves solubility but may reduce the storage stability. Table 5 summarizes the application of this technology in food matrices. When it comes to thermosensitive bioactive compounds, FD has proven to be an excellent preservation method, mainly due to its use of low temperatures that minimize thermal degradation. According to [PERSON] et al. [73], this drying technique is essential for preserving antioxidant compounds in Amazonian fruits, such as _Clidemia hirta_ L. and _Clidemia jupensis_ DC. The study demonstrated that FD played a significant role in retaining the antioxidant activity in these fruits, likely due to the low temperatures used during the process. Similarly, [PERSON] et al. [60] reported higher levels of polyphenols and anthocyanins in _Bletilla striata_ (_Thumb._) _Reichb.f._ flowers compared to other drying methods. However, they noted that anthocyanins tend to degrade more as a result of prolonged drying times rather than the temperature used, suggesting that while FD minimizes the heat exposure, extended drying durations can still contribute to the degradation of sensitive compounds. [PERSON] et al. [76] found similar results in raspberries (_Rubus ideaus_ L.), observing that although FD reduces the thermal degradation of some bioactive compounds, polyphenols may still be exposed to oxidative conditions during the long drying time, potentially reducing their concentrations. This highlights that even though the low temperatures of FD limit heat-induced damage, the process still poses risks for oxidative degradation during prolonged drying times. [PERSON] et al. [70] conducted a study on pumpkin (_Cucurbita moschata_) residues and reported that FD yielded samples with a higher antioxidant activity when measured by the DPPH method compared to oven drying. However, when the reducing power method was applied, FD showed a lesser effect.