## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems  
#### 4.1.1 Microwave  
Microwaves are electromagnetic radiations with long wavelengths (1 mm to 1 m) and high frequencies (300 MHz to 300 GHz), which interact with water molecules in materials, generating volumetric heating through dipolar and ionic mechanisms [43]. Microwave heating involves the conversion of electromagnetic energy into heat through dipolar reorientation and ionic conduction. Fruits and vegetables, for instance, contain approximately 80% water, whose molecules are dipolar and capable of aligning with the electric field of the waves. Since the electric field oscillates at a very high frequency, the dipolar molecules continuously attempt to realign themselves, leading to intense molecular movement [43; 44]. Thus, the rotation of dipolar molecules, especially water, creates friction and heat within the food matrix. In the case of ionic conduction, ions migrate under the influence of the electric field, and molecular friction also generates heat. Due to the electromagnetic field and the friction-induced internal heating, microwave radiation results in rapid temperature increases within the sample, with molecular expansion and porosity increases facilitating mass transfer [43; 44; 45]. In conventional thermal processing, energy is transferred to the food surface by convection and then conducted inward. However, due to the low thermal conductivity of food, this heating process is slow. In contrast, microwave processing delivers energy directly to the food through interactions with the electromagnetic field, generating heat throughout the volume without relying on surface diffusion. This mechanism results in faster, more uniform heating and is more energy-efficient while better preserving the sensory, nutritional, and functional properties of food [43; 46; 47]. In microwave drying, this principle results in more efficient moisture removal compared to conventional hot air drying, which is slower due to the limited heat transfer. Microwaves penetrate the product and generate internal heating that creates pressure gradients, accelerating the moisture migration to the surface. Additionally, due to its \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Drying Technology** & **Sample** & **Reference** \\ \hline
11 & Rotating tray dryer & Tomato seeds (_Solanum Lycopersicum_ L.) & [25] \\
12 & Microwave & Strawberries (_Fragaria x ananassa_) & [26] \\
13 & High-voltage electrical discharge & Blueberries (_Vaccinium sect. Cyanococcus_) & [27] \\
14 & Moderate electric field & Apples (_Malus domestica_) & [28] \\
15 & Radiofrequency & Canola seeds (_Brassica Napus_. L.) & [29] \\
16 & Pulsed electric field & Spinach (_Spinacia oleraea_) & [30] \\
17 & Vacuum impregnation and ohmic heating & Apple (_Malus domestica_) & [31] \\
18 & Micromization & Spinach (_Spinacia oleraea_ L) & [32] \\
19 & Ultrasound & Wheat starch (_Triticum aestivum_) & [33] \\
20 & Ultrasound & Potatoes (_Solanum tuberosum_) & [34] \\
21 & Ultrasound & Wheat (_Triticum aestivum_) and barley & [35] \\
22 & Infrared & Bocaiíva (_Arocomia Aculeata_) & [36] \\
23 & Ultrasound & Barley and barley malt (_Hordeum vulgare_) & [37] \\
24 & Spray drying & Pomegranate seeds (_Punica granatum_ L.) & [38] \\
25 & Spray drying & Cocoa shells (_Thebroma cacao_ L.) & [39] \\
26 & Spray drying & Sage (_Salvia officinalis_ L.) & [40] \\
27 & Spray drying & _L. nobilis_ L. leaves (_Laurus nobilis_ L.) & [41] \\
28 & Freeze-drying & Kernels pulp (_Prunus armenia_) and peach & [42] \\  & & pulp (_Prunus persica_ L.) & \\ \hline \end{tabular}
\end{table} 
Table 2: Cont. high dielectric loss factor, water efficiently absorbs microwave energy, enabling a selective drying that minimizes the degradation of certain bioactive compounds [47]. Microwave drying thus emerges as a promising alternative technology for thermal processing due to its volumetric heating nature [46].