## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.1 Microwave
Particularly during the falling-rate period, microwave drying proves useful because this phase is limited by diffusion, which typically leads to structural shrinkage and reduced surface moisture. Microwave heating, by generating internal vapor pressure, facilitates water migration and helps avoid material shrinkage [48]. Microwave drying is considered an interesting approach for food processing due to its efficient drying rate, lower energy consumption, and capacity to preserve heat-sensitive compounds. However, prolonged microwave exposure may cause overheating, as reported by [PERSON] et al. [49] and [PERSON] [9]. A solution is intermittent microwave drying, in which the magnetron is turned on and off periodically, resulting in a discontinuous process. This approach allows for a more uniform temperature and moisture distribution throughout the drying cycle, reducing the risk of overheating [50]. Intermittent drying is a promising alternative to continuous heating, as it reduces the processing time, optimizes energy use, and preserves the product quality. Unlike continuous drying, which may cause thermal damage and energy waste, heat modulation in intermittent drying allows for controlled moisture migration, minimizing negative effects [43]. Nevertheless, a key limitation of microwave drying is uneven heating caused by temperature differentials between cold and hot spots. Due to the dielectric nature of microwave interactions, heating concentrates in regions with higher water contents, forming hot spots. To mitigate this, it is essential to improve the radiation uniformity and control the hot spot temperature. Strategies include using modulated stirrers, hot-air-assisted agitation, the precise control of the surface temperature and microwave power, and rotating trays to promote homogeneous heating [46; 51]. Table 3 summarizes the application of microwave technology in various food matrices. Microwave drying (MW) stands out for significantly reducing processing times compared to conventional methods. [PERSON] et al. [52] reported that MW shortened the drying time by up to 100-fold compared to hot air drying, while the combination of MW with a heat pump also led to shorter times than conventional dryers [53]. This intensified process minimizes polyphenol oxidation [54] and enhances the moisture removal both on the surface and internally [47; 59]. This enhancement is attributed to the volumetric heating and high vapor pressure gradients generated inside the product, which increase the internal water migration and reduce the resistance to mass transfer, as also confirmed in the microwave-assisted pulsed fluidized bed drying of okara [47]. Regarding bioactive compounds, the effects of MW can vary. Intermittent microwave drying has been shown to help preserve phenolic compounds, increasing their retention by up to 80%. For turmeric, MW caused as low as a 60% degradation of phenolics, which may be associated with the inactivation of polyphenol oxidase [11]. In ginger, the microwave pretreatment reduced the total phenolic content compared to ultrasound and ohmic heating, but increased their availability due to the cell membrane rupture, which promotes a greater release and conversion into smaller, more bioavailable structures [56]. Similarly, the antioxidant activity was enhanced, possibly due to the release of previously retained compounds or the formation of new bioactive molecules. Nonetheless, an excessive MW power and exposure time can lead to the thermal degradation of sensitive compounds. [PERSON] et al. [47] reported significant losses of isoflavones. The microwave drying of _Catharanthus roseus_ leaves resulted in higher phenolic contents, due to the combination of the temperature and vapor pressure generated at 300 W, which facilitated cell wall ruptures and the release of previously vacuole-trapped compounds. Similar findings were reported by [PERSON] et al. [58]. Compared to sun and tray drying, microwave-treated samples had higher phenolic contents.