Developing a square container with ideal dielectric constant for improving radio frequency heating uniformity in low-moisture agricultural products

Radio frequency (RF) treatment is considered as an efficient and promising physical method for disinfesting and pasteurizing low-moisture agricultural products (LMAP) due to the rapid and volumetric heating. However, non-uniform RF heating is a major obstacle for industrial applications. A square container with ideal dielectric constant was developed for improving RF heating uniformity in LMAP by changing volume ratio of TiO2 in low-density polyethylene (LDPE)/TiO2. A 6 kW, 27.12 MHz pilot-scale RF system was used to evaluate effects of LDPE/TiO2 containers with different dielectric constants on heating uniformity of LMAP. Then, the linear relationship of the dielectric constant between container and samples was established when the better RF heating uniformity of samples was obtained. The result showed that the dielectric constants of the LDPE/TiO2 composites were 2.41 ± 0.00, 3.38 ± 0.01, 4.45 ± 0.00, 5.84 ± 0.01, and 7.78 ± 0.01 at 25 °C and 27.12 MHz when the volume percentages of TiO2 were 0, 10, 20, 30, and 40% in LDPE/TiO2, respectively. The best heating uniformity of soybeans, mung beans, rice, wheat flour, and peanuts with the dielectric constant of 3.60 ± 0.00, 3.88 ± 0.05, 4.68 ± 0.00, 4.80 ± 0.01, and 6.37 ± 0.01 was achieved when they were heated in the LDPE/TiO2 containers with the TiO2 ratio of 10%, 10%, 20%, 20% and 30%, respectively. To obtain best RF heating uniformity, the relationship of the dielectric constant between the container (ε՛c) and sample (ε՛s) was described by the linear equation ε՛c = (0.75–1.22) × ε՛s (R2 ≥ 0.97). The results of this research provided a useful information on ideal dielectric constants of the packaging materials for LMAP to improve the RF heating uniformity and will promote the industrial applications of RF treatment.