Radio frequency heating of granular and powdered foods in aluminum, polypropylene and glass container: Heating rate and uniformity

Non-uniform heating is the primary challenge during large-scale radio frequency (RF) disinfestation and pasteurization applications. Packaging materials can absorb, transmit or reflect RF energy and can be used to overcome high-temperature differences throughout the product caused by overheating at the edges and corners. In this study, effects of three container materials (aluminum-AL, polypropylene-PP, and glass-GL), electrode gap, end-point temperature, and processing time on heating uniformity were investigated by determining the temperature distribution, heating uniformity index (λ), and heating rate (v) profile in the upper, middle, and bottom layers of RF (6 kW, 27 MHz) treated corn kernels associated with different sizes (d) and moisture content (MC). Results showed that corn kernels (d = 6 mm), powders (d = 2 mm) and flours (d = 0.2 mm) in the AL container (200 × 120 × 40 mm3) indicated a lower v (AL: 3.73 ± 0.21, 4.21 ± 0.21, and 6.17 ± 0.31, PP: 7.77 ± 0.14, 7.97 ± 0.40, and 8.98 ± 0.32, GL: 6.24 ± 0.19, 6.55 ± 0.19 and 7.16 ± 0.10 °C/min), but better heating uniformity (AL: 0.101 ± 0.005, 0.090 ± 0.004, and 0.082 ± 0.004, PP: 0.214 ± 0.010, 0.187 ± 0.009, and 0.172 ± 0.009). The shielding effect of AL container on RF wave was stronger than that of PP and GL. The corner overheating problem in the AL container was greatly improved, and the hot spot was shifted from the corner of the sample to the center. Meanwhile, the λ of corn kernels with different sizes in AL container first decreased and then increased with the increase of MC (from 10 to 16%, w.b.). When the λ reached a minimum value, in order to further increase the v of food in the AL container, the relationship between d, MC and v was finally established. The results can provide valuable information for improving the heating uniformity of food particles/powders and optimizing the shielding structure and performance of metallized RF packaging.