## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.2 Radiofrequency
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Drying Method** & **Material** & **Main Findings** & **Reference** \\ \hline \multirow{4}{*}{1} & \multirow{4}{*}{Radiofrequency} & \multirow{4}{*}{Lotus pollen (_Nelumbo_ _nucifera_)} & RF drying reduced lipid oxidation by & \multirow{4}{*}{[63]} \\  & & & 31.58\% and better preserved & \\  & & & polyunsaturated fatty acids, such as & \\  & & & alpha-linolenic acid and docosahexaenoicenoic & \\  & & & acid (DHA), compared to hot air drying. & \\  & & & Pretreatment increased neochlorogenic & \\  & & & (241.48 \(\pm\) 4.12 mg/100 g) and & \\  & & & & isochrogenic acid & \\
2 & & coating, and vacuum RF drying & (_Prunus avium_) & (23.79 \(\pm\) 1.03 mg/100 g) levels by 1.25\(\times\) & \\  & & & and & 1.48\(\times\) and improved catechenin and & \\  & & & & kaempfer retention. & \\  & & & & \\  & & & & & \\  & & & & \\  & & & & & \\ \end{tabular}
\end{table} 
Table 4: Applications of radiofrequency in food matrices. process. The effects of RF on specific nutrients still require further investigation, particularly in relation to the long-term storage and transportation of RF-dried products. Treatments that combined ultrasound (US) with carboxymethylcellulose and cellulase significantly increased the total phenolics, flavonoids, and antioxidant activity (DPPH, ABTS, and FRAP). These effects were attributed to the mechanical and cavitation effects of US, which partially ruptured plant cells and facilitated the release and extraction of these compounds during drying. Additionally, drying under low pressures reduced the boiling point of water, allowing a more efficient moisture removal at lower temperatures and preventing heat damage to sensitive compounds [62]. While low-pressure drying is a promising approach, its application in large-scale food drying systems may present challenges in terms of the equipment cost and energy consumption. Further studies should evaluate the efficiency of these methods in different food matrices and the potential for energy savings in industrial settings. However, the interplay between US and RF in drying processes remains underexplored. The retention of compounds such as catechin, chlorogenic acid, neochlorogenic acid, and cyanidin-3-O-rutinoside was also improved by combining drying with protective coatings such as carboxymethylcellulose. These coatings formed a physical barrier that limited the oxygen exposure and oxidative degradation. Moreover, the combination of US and coatings helped preserve the cellular structure, inhibiting undesirable enzymatic reactions. The inhibition of polyphenol oxidase (PPO) and peroxidase (POD), enzymes responsible for polyphenol degradation, resulted in a higher antioxidant content and a better nutritional quality in the final product [62]. Despite these promising results, the use of protective coatings may increase production costs, and more sustainable alternatives should be explored, considering both the environmental impact and cost-effectiveness. [PERSON] et al. [69] explained that the effect of US on RF drying can be attributed to pretreatments increasing the \(\varepsilon^{\prime\prime}\) (dielectric loss factor) of the samples. This indicated that apples absorbed energy more effectively during drying, accelerating the process and improving the retention of bioactive compounds, particularly ascorbic acid. RF drying has emerged as an effective alternative for preserving bioactive compounds, reducing thermal degradation, and enhancing antioxidant release [64]. However, while the effectiveness of RF drying in preserving bioactive compounds is well-documented, further studies are needed to optimize the RF parameters (such as power and frequency) to enhance both the nutrient retention and process efficiency without incurring excessive energy costs. The studies analyzed demonstrate that radiofrequency is an effective alternative for drying processes, as it reduces operating times and, consequently, energy consumption.