## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.4 Ultrasound
& \\  & & & Ultrasonic pretreatments significantly & \\  & & & & ultrasound, samples dried at 15 \({}^{\circ}\)C showed & \\  & & & & greater losses of bioactive compounds (39–54\%) and antioxidant activity (57–69\%) & [87] \\
2 & Dual-frequency & & controlled & 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. & \\  & & & Ultrasonic pretreatments significantly & \\  & & & & ultrasound, samples dried at 15 \({}^{\circ}\)C showed & \\  &  
Several studies have extensively investigated the influence of US as a pretreatment in reducing drying times and improving process efficiency (Table 6). In kiwi samples, ultrasound-activated drying reduced the conventional drying time by up to 65% [87], a behavior also observed in beetroot [91]. The enhancement in drying efficiency is mainly attributed to the cavitation effects induced by US, which disrupt cellular structures and promote a faster water removal, minimizing the exposure to heat and preventing the degradation of heat-sensitive bioactive compounds. The combination of US and ethanol as a pretreatment also proved effective in reducing the drying time and energy consumption, as demonstrated for cantaloupe melons [92] and bananas [94]. In these studies, ultrasound facilitated mass transfer by increasing the cell membrane permeability, which enhanced the release of soluble compounds, thereby improving the extractability of bioactive components. Moreover, vacuum-assisted conductive drying applied to ginger extract [56], and the vacuum drying of honeyberries [90], proved effective in both reducing the drying time and preserving bioactive compounds. These benefits are particularly noticeable in antioxidant compounds, which are typically vulnerable to heat stress during conventional drying. On the other hand, ultrasound-assisted convective drying applied to beetroot [91] showed that, although it accelerates mass transfer, the method may compromise the stability of certain compounds, such as betalains. The oxidative degradation of betalains was more pronounced when the US intensity was high, indicating the importance of carefully controlling process parameters to prevent the loss of sensitive compounds. US has shown beneficial effects in preserving bioactive compounds during the drying process. In kiwi, its application minimized the losses of bioactive compounds and antioxidant activity at higher temperatures [87], showing that US can enhance the thermal efficiency and compound retention. In cranberries, blanching combined with US better preserved color and enhanced the antioxidant activity [95], suggesting that US could act synergistically with other pretreatments to improve the final product's quality. Additionally, the US pretreatment resulted in higher levels of phenolic compounds and flavonoids in ginger extract samples [56], _Tradescantia zebrina_ leaves [89], and oregano [93], which is attributed to the increased solubility and extraction efficiency of these compounds.