Agri-food by-products are rich in valuable compounds, which require to be recovered by solid-liquid extraction (SLE). SLEs are performed using polar or non-polar liquids and their mixtures as extracting solvents in contact with insoluble solid matrix, and steam/hot water is commonly used to intensify the rupture/hydrolysis of cell wall and improve the mass transfer (Rodríguez García & Raghavan, 2022). Although traditional intensification is a well- established industrial process, high temperature and long treatment times are needed to achieve a desirable yield, resulting in some obvious disadvantages, such as thermal degradation, high energy consumption and environmental pollution (Cui et al., 2021).

To overcome the deficiencies of conventional methods, novel intensifications based on other forms and sources of energy have become a hotspot. Of the novel methods successfully used in laboratory studies, the introduction of physical fields, such as mechanical, electromagnetic (EM), electric, and acoustic energy for intensifying extraction processes has attracted considerable attentions, due to their high efficiency, eco-friendly characteristics, and also producing high quality products (Ling, Ramaswamy, Lyng, Gao, & Wang, 2023). Available studies suggest that physical field-based techniques can potentially replace the conventional intensification methods utilized in the SLE process.

Recently, radio frequency (RF) heating also attracts some attentions in the process intensification and is used to assist the extraction of polyphenol (Kochadai, Khasherao, & Sinija, 2022), pectin (Zheng et al., 2021), and edible oil (Fan, Xu, Guan, Li, & Wang, 2023) from different plant sources. Similar to microwave (MW) heating, RF treatment is a kind of EM field-based heating technology with volumetric, rapid and selective heating effects, and is more suitable for treating large volume samples both due to its deeper penetration depth and simpler EM field patterns (Ling, Cheng, & Wang, 2020). It is generally considered that EM energy can produce two coupled mechanisms that have positive effects on the extraction process: (i) selective heating of the plant matrix, which raises the intracellular pressure, leading to the rupture of cells wall; (ii) rapid increase in solvent temperature and enhancing the mass transfer (Mao, Robinson, & Binner, 2023).

Free running oscillator (FRO) and 50 Ω applicators are the two most widely used RF systems in the food processing studies (Gao et al., 2023). In the FRO system, the standard oscillation circuit formed by the triode generates RF energy. The output circuit of the RF generator is used as the primary circuit of the transformer. The RF applicator forms the secondary circuit of the transformer. RF power is controlled by adjusting the matching components of the applicator circuit (Li, Wang, Wang, & Ling, 2022). While in 50 Ω system, a weak RF signal is generated from a crystal oscillator, then amplified and transmitted through a coaxial cable to the applicator. To achieve the maximum power transmission, an impedance matching network is utilized to maintain a fixed impedance of 50 Ω (Tian, Guan, Li, Ramaswamy, & Wang, 2023).

Although the mechanism for the generation of EM energy is different, the applicator in FRO and 50 Ω RF system has similar structures, and both used to place the samples and transfer EM energy into them by a pair of electrodes. Among the RF equipment reported in the literatures, the one equipped with horizontal through-field type parallel plate electrodes is the most reported. The energy coupled to the sample can be adjusted by adjusting the distance (i.e., electrode gap) between the top and bottom electrodes (Gao et al., 2023). However, the horizontal parallel plate configuration could be not suitable for liquid samples since this orientation would be inconvenient to water vapor dissipation and installation of mixer. Furthermore, the electrode gaps and heating area cannot be utilized effectively since liquid samples are usually stored in elongated containers.

Although different RF systems are used in food processing studies for various purposes, most of them are for general-purpose in pilot scale, rather than the experimental devices designed for specific processes. Furthermore, most of them are equipped with horizontal parallel plates, which is not ideal configuration for liquid samples. Based on issues identified above, the general objective of this study was to develop a lab-scale dedicated RF heating system for intensify SLE processes. The specific objectives of this study were: (1) to develop a small-scale 50 Ω RF heating system equipped with vertical parallel plate electrodes, (2) to explore the effects of different experimental conditions including container shape, electrode gap, output power, solvent pH/electric conductivity (EC), particle size, liquid-solid ratio (LSR), and mixing on the RF heating rate and uniformity of liquid samples, and (3) to determine the EM leakage level around the openings in the self-designed RF heating system.