## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.4 Ultrasound  
Ultrasound (US) is an emerging technology that uses mechanisms such as cavitation and the sponge effect to modify the structure of food products. This leads to an improved heat and mass transfer during processing, enabling drying at lower temperatures and/or shorter times [35; 81]. This technique has been applied both as a pretreatment prior to drying and as an in-process aid [82]. US refers to sound waves with frequencies equal to or greater than 16 kHz, which is above the range of human hearing. These waves can propagate through solids, liquids, or gases depending on the wavelength and medium composition. US can induce physical, chemical, and mechanical changes in fruits and vegetables depending on the power and frequency applied. A high-power US at frequencies between 20 and 100 kHz can promote physicochemical modifications, microbial inactivation, enhance heat and mass transfer, accelerate chemical and biochemical reactions, and activate or inactivate enzymes. In contrast, low-power US at frequencies above 100 kHz is used for nondestructive and/or noninvasive analyses. It is important to note that, in addition to affecting food structures, the composition and structure of the foods themselves influence how US propagates and how ultrasonic energy is converted to modify the matrix [83; 8]. Commercial US equipment includes ultrasonic baths and probes. Baths are more affordable and easier to maintain, operating with submerged piezoelectric transducers (40-130 kHz) that transmit sound waves to the material and enhance mass transfer, although at a low intensity (0.1-1 W/cm\({}^{2}\)). Probes, while more expensive, enable higher intensities (>5 W/cm\({}^{2}\)) and are used for material fragmentation and homogenization [8]. The primary effect of US in liquids is acoustic cavitation, characterized by the formation and collapse of bubbles due to pressure variations in longitudinal waves. During rarefaction, the bubbles grow past a critical threshold, and their collapse produces microjets and microstreaming that impact nearby cells and molecules, forming microchannels that increase water diffusivity and improve the drying performance [8; 84]. In solids, ultrasonic waves cause rapid compressions and expansions, known as the \"sponge effect\", that overcome the surface tension, releasing internal fluids and allowing osmotic solution penetration. This phenomenon facilitates mass transfer and can also structurally modify the material [85; 86]. While the sponge effect is a direct mechanism, the microchannel formation from acoustic cavitation represents an indirect structural change. Therefore, ultrasound not only enhances mass transfer but also alters the physical properties of treated materials, making it a relevant technology for various industrial applications [82]. Fruit and vegetable tissues may undergo physical and chemical changes due to the interaction between sonication and drying, which can alter food textures either positively or negatively. Additionally, ultrasound may minimize color degradation. Depending on the application conditions and the type of food, ultrasound may improve not only the physical quality of fruits and vegetables but also their nutritional quality. Ultrasound has been shown to aid in the preservation of the polyphenols, flavonoids, vitamin C, and antioxidant activity in dried fruits and vegetables [8] (Table 6). \begin{table}
\begin{tabular}{c c c c c c} \hline
**No. ** & **Pretreatment** & **Drying Method** & **Material** & **Main Findings** & **Reference** \\ \hline
1 & - & Convective drying & High-power ultrasound reduced kiwi & \\  & & with air & drying time by 55–65\%. Without & \\  & & recirculation, & ultrasound, samples dried at 15 \({}^{\circ}\)C showed & \\  & & airflow control, & & greater losses of bioactive compounds (39–54\%) and antioxidant activity (57–69\%) & [87] \\  & & & control, and & compared to those dried at 5 \({}^{\circ}\)C (14–43\% and 23–50\%). Applying ultrasound at 15 \({}^{\circ}\)C minimized these losses to 15–47\% and 47–58\%, respectively.