Radio frequency tempering uniformity investigation of frozen beef with various shapes and sizes

Radio frequency (RF) energy generates fast and volumetric heating as it penetrates food materials and converts electromagnetic energy to heat. With these advantages, RF heating is considered as a promising technology for tempering and thawing processes in the meat and fishery products industry. However, non-uniform heating problems hinder its further application to meat products due to their various sizes and irregular shapes. This study utilized representative frozen beef samples to investigate the parameters of varying sample thickness (40 mm; 50 mm; 60 mm), base area (small:160 x 102 x 60 mm(3); medium: 220 x 140 x 60 mm(3); large: 285 x 190 x 60 mm(3)) and shape (cuboid; trapezoidal prism; step) and their influence on tempering uniformity in a parallel-plate RF system. A computer simulation model was established, verified by experiments and then was utilized to evaluate the volumetric temperature distribution in food samples. Results show that the heating rate increases and heating uniformity decreases with increasing sample thickness and decreasing sample base area. As sample thickness increased from 4 cm, 5 cm to 6 cm, the simulated temperature uniformity index (STUI) increased from 0.093, 0.117 to 0.194. Sample base area increases from small to large decreased the STUI from 0.229 to 0.194 and 0.090. Among all three shapes, the cuboid shape has the best heating uniformity (STUI 0.194), followed by the trapezoidal prism (STUI 0.209) and the step shape (STUI 0.282). The step shape has the worst tempering uniformity because the RF energy focuses mainly on the vertical section and results in severe regional heating. Strategies to improve the step-shape frozen beef tempering uniformity by decreasing the input power to 1/3 and enlarging the electrode gap by 40 mm only reduced the hot spot temperature from 88 to 78 degrees C. Further research is needed in order to develop methodologies or suitable equipment for irregular shape food RF tempering in the future.