## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.1 Microwave
However, higher powers (450 W) led to phenolic degradation due to the temperature rise. Studies indicate that the phenolic retention is optimal at temperatures between 50 and 60 \({}^{\circ}\)C. The microwave drying at 450 W provided the highest flavonoid retention, while 600 W caused significant degradation though still superior to sun or tray drying. At 900 W, the degradation was lower, likely due to the rapid deterioration of bioactives at high powers. These findings are consistent with previous studies, which indicate that microwave drying can accelerate the disruption  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No. ** & **Drying Method** & **Material** & **Main Findings** & **Reference** \\ \hline \multirow{4}{*}{1} & \multirow{4}{*}{Microwave} & \multirow{4}{*}{Minion (_Allium cpa_) and ginger (_Zingther officinal_)} & Higher antioxidant activity content (9\% and 6\% for onion and ginger, respectively) compared to conventional drying (2\% and 4\%). A similar pattern was observed for total phenolics: microwave (18\% and 12\% for onion and ginger, respectively) vs. conventional (15\% and 11\%). Microwave preserved phenolic compounds better, resulting in \(\sim\)60\% degradation. It also caused less degradation of tannins and antioxidant activity (ascorbic acid equivalent). & \\  & & & & \\  &  
of the cellular matrix, thereby promoting a faster release of flavonoids previously bound to cellular structures [57]. However, the microwave intensity and exposure time must be carefully managed to avoid excessive compaction or the collapse of the structure, which may negatively affect the rehydration capacity and mechanical properties. [PERSON] et al. [54] observed that _Pllysalis_ fruits dried under microwaves at 480 W exhibited an increased hardness and resistance to cutting compared to convective drying. MW also affects other compounds, such as ascorbic acid, organic acids, and pigments. The microwave technique yielded the highest increase in vitamin C contents in passion fruit residue--up to a 700% increase compared to fresh material. This was attributed to the inactivation of degrading enzymes and the release of ascorbic acid due to the cell rupture. However, a high microwave power reduced the vitamin C retention, highlighting the need to optimize the time and power levels [58]. This behavior was also observed for isoflav in okara: while MW promoted the interconversion of conjugated forms into bioavailable aglycones, it also led to notable losses when high temperatures were reached inside the drying chamber [47]. Such dual effects underscore the importance of fine-tuning MW parameters according to the target compound's thermal sensitivity. The citric acid content in _Spirulina_ decreased by up to 30%, with the lowest degradation observed at lower microwave powers [43]. Conversely, the phycocyanin retention was higher at elevated powers, suggesting that the exposure time plays a crucial role. In the case of _Bletilla striata_ flowers, MW promoted a greater retention of polysaccharides (10.55 mg/g) [60]. These differences across compounds and matrices suggest that MW outcomes are highly specific to the chemical nature of each bioactive. As observed in different studies, the antioxidant capacity, vitamin retention, and enzyme inactivation respond variably to the MW intensity and duration. Therefore, predictive modeling and experimental validation are essential for process optimization. Moreover, combining MW with other treatments can enhance the preservation of bioactives. Ginger samples pretreated with MW showed a higher total flavonoid content than controls, due to the cellular matrix disruption [56]. Similarly, samples pretreated with citric acid and subjected to microwave-assisted drying showed a lower degradation of total phenolics, since citric acid inhibited polyphenol oxidase and facilitated compound release [9]. In this context, hybrid systems, such as microwave-assisted fluidized beds or vacuum microwave drying, have shown promising results. [PERSON] et al.