Design and experiment of an auxiliary device for continuous-flow radio frequency heating of fluid/semi-fluid samples

Non-uniform heating has been one of the major challenges using radio frequency (RF) treatment in continuous-flow pasteurization, enzyme inactivation, extraction, or applications for fluid/semi-fluid foods. The objective of this study was to design an auxiliary device for the continuous-flow treatment of fluid/semi-fluid samples in the Strayfield SO6B RF heating system produced in UK. This device was composed mainly of peristaltic pumps, an electromotor, a sample trough, and a control system. The sample trough was equipped with a set of cut and folded flight screws to realize the mixing and stirring of samples. Among them, the sample trough was connected with the feed tank by a peristaltic pump to realize continuous flow treatment. The control system was used to realize the opening and closing of the motor and peristaltic pump, as well as the temperature measurement of the outlet and inlet. The components of the device were placed between the RF electrode plates. Polytetrafluoroethylene (PTFE) and polyetheretherketone (PEEK) with low dielectric loss factor were selected to avoid significant absorption of RF energy from heating and distortion. The internal temperature distribution of the sample in the trough was determined using the multi-point temperature measurement platform. A temperature recorder was connected with 9 K-type thermocouples. Nine representative points were accurately measured in the three longitudinal sections in the sample trough. The heating uniformity index (λ) inside the sample was then calculated to evaluate the RF heating uniformity, according to the temperature values of the above 9 points. Three samples were selected for the continuous-flow RF treatment, including raw cow's milk, carboxymethylcellulose (CMC) solution, and apple pomace-citric acid mixed solution. The results showed that the heating rate of samples was ranked in the descending order of the raw cow's milk, CMC solution, and apple pomace-citric acid mixed solution, under the same RF heating conditions (electrode gap 140 mm). All three samples reached the target temperature (70 °C) within 20 min. The device was well-assisted RF continuous and uniform heating of fluid/semi-fluid samples with different properties. The samples were performed on the RF heating with or without screw agitation under stationary conditions. Screw agitation was improved the RF heating uniformity of all samples. Among them, the average λ value of 9 points was reduced from 0.139 to 0.068 in the CMC solution, indicating a more significant performance. Furthermore, the CMC solution and apple pomace-citric acid mixed solution (PS5) were utilized with the lower heating uniformity in the continuous-flow RF treatment mode. In the absence of screw agitation, the temperature of the upper layer was significantly higher than that of the middle and lower layers for both samples, especially in the upper layer near the outlet. The edge/corners overheating was then attributed to the electromagnetic field deflection and accumulation. Screw agitation also significantly improved the heating uniformity of two samples under flowing conditions. The maximum difference in temperature decreased by 13.4 and 11.4 °C, respectively. While the floating of solid particles in the solid-liquid mixed samples reduced the heating uniformity at the lower stirring speeds. Overall, this device can be expected to realize the continuous and uniform RF treatment of fluid/semi-fluid samples. The finding can provide technical and theoretical support to the RF continuous heating and industrialized treatment.