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

Non-uniform heating is a major challenge for using radio frequency (RF) treatment in continuous-flow pasteurization, enzyme inactivation, extraction or other applications for fluid/semi-fluid foods. The objective of this study was to designed an auxiliary device for continuous-flow treatment of fluid/semi-fluid samples in Strayfield SO6B RF heating system produced in UK. This device mainly composed of peristaltic pumps, electromotor, sample trough, and control system etc. The sample trough was equipped with a set of cut and folded flight screw to realize the mixing and stirring of samples. The sample trough was connected with the feed tank by a peristaltic pump to realize continuous-flow treatment. The control system mainly realized the opening and closing of the motor and peristaltic pump, as well as temperature measurement of the outlet and inlet. The parts of the device placed between the RF electrode plates were made of polytetrafluoroethylene (PTFE) and polyetheretherketone (PEEK) with low dielectric loss factor to avoid absorbing significant amount of RF energy causing heating and distortion. To determine the internal temperature distribution of sample in the trough, a custom-designed multi-point temperature measurement platform which consisted of a temperature recorder connected to 9 K-type thermocouples was used to accurately measure the 9 representative points in the three different longitudinal sections in the sample trough. The heating uniformity index (λ) inside the sample can be calculated by using temperature values of the above 9 points to evaluate the RF heating uniformity. Three samples include raw cow's milk, carboxymethylcellulose (CMC) solution, and apple pomace-citric acid mixed solution were selected for continuous-flow RF treatment. The results showed that under the same RF heating conditions (electrode gap 140 mm), the heating rate of samples from high to low was: raw cow's milk, CMC solution, apple pomace-citric acid mixed solution, and all three samples can be reached target temperature (70 ℃) within 20 min. It was also confirmed that the device could well assisted RF continuous and uniform heating of fluid/semi-fluid samples with different properties. By using above samples in the RF heating with or without screw agitation under stationary condition, the results showed that screw agitation can be improve the RF heating uniformity of all samples, among which CMC solution was more significant, and its average λ value of 9 points can be reduced from 0.139 to 0.068. By further using CMC solution and apple pomace-citric acid mixed solution (PS5) with poorer heating uniformity in the continuous-flow RF treatment mode, the results showed that in the absence of without 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, which may be due to the edge/corners overheating caused by electromagnetic field deflection and accumulation. Screw agitation could also significantly improve the heating uniformity of the two samples under flowing condition, and the maximum temperature difference can be decreased by 13.4 and 11.4℃, respectively, while the floating of solid particles in the solid-liquid mixed samples would reduce the heating uniformity at the lower stirring speeds. Overall, this device can realize continuous and uniform RF treatment of fluid/semi-fluid samples, and provide technical and theoretical support for the RF continuous heating study and industrialized treatment.