Inactivation of Salmonella enterica and Enterococcus faecium NRRL B-2354 in cumin seeds by radiofrequency heating

Radiofrequency (RF) heating was evaluated as a novel spice decontamination technology of cumin seeds. A plastic pouch with 20 g inoculated cumin seeds was placed at the top center of a tray (16.5 cm length x 13 cm width at the top tapering down to 14 x 10 cm at the bottom with 4 cm height) containing 430 g of uninoculated cumin seeds in the same tray. The top center of the tray containing cumin seeds was found to be the cold spot by monitoring temperature profiles at six different locations using fiber optic sensors in a 6 kW 27.12 MHz RF system. Either Salmonella enterica cocktail or Enterococcus faecium was inoculated into cumin seeds for microbial challenge studies in RF heating. The whole tray was subjected to various processing times until 5-log reduction of Salmonella was achieved. After the same RF heating times, E. faecium consistently showed higher survivals than those of Salmonella in all three batches of cumin samples, demonstrating the suitability of E. faecium as a surrogate of Salmonella during RF heating. There were no significant differences in cumin quality before and after RF treatment. Particle sizes and bulk densities of all three batches samples were measured. Batch 1 had a much larger particle size and lower bulk density than those of batch 2 and batch 3. These differences in material properties influenced RF heating rate. Therefore, production lot (batch) variation due to these different material properties requires stricter process control parameters for RF microbial inactivation. Cold spot temperature and moisture content of cumin seeds are two critical process control parameters for RF microbial inactivation, in addition to electrode gap in a certain RF system. RF heating is suitable for decontaminating cumin seeds with no significant changes in relative composition of all volatile components.