Performance evaluation of an experimental radio frequency heating system designed for studying the solid-liquid extraction

Radio frequency (RF) heating has been extensively used in food processing. However, most RF equipment used for food processing studies is for general-purpose on a pilot scale, rather than being designed on specific processes. This study aimed to develop an experimental 50 Ω RF system equipped with vertical parallel plate electrodes to facilitate the liquid heating in the solid-liquid extraction (SLE) studies, and its heating performances and electromagnetic (EM) leakage were also evaluated. Results showed that compared with the cylindrical container, rectangular one had better heating rate and uniformity, and was more suitable for the vertical parallel plate system. For solid-liquid mixture, RF heating rate increased with increasing particle size within a certain range, but the heating uniformity of the sample surface decreased. Both RF heating rate and uniformity increased with increasing liquid-solid ratio. Furthermore, air bubble stirring at a flow rate of 0.8 L/min resulted in a larger heating rate with better heating uniformity. Shielding using 4 mm stainless steel plate in the self-designed metallic enclosure ensured that the EM leakage was much smaller than the limiting value required by International Commission on Non-Ionizing Radiation Protection. The results from this study would provide some basic information and guidelines for application of vertical parallel plate RF system in the study of SLE of bioactive compounds from agri-food by-products.