Validation of radio-frequency dielectric heating system for destruction of Cronobacter sakazakii and Salmonella species in nonfat dry milk

Cronobacter sakazakii and Salmonella species have been associated with human illnesses from consumption of contaminated nonfat dry milk (NDM), a key ingredient in powdered infant formula and many other foods. Cronobacter sakazakii and Salmonella spp. can survive the spray-drying process if milk is contaminated after pasteurization, and the dried product can be contaminated from environmental sources. Compared with conventional heating, radio-frequency dielectric heating (RFDH) is a faster and more uniform process for heating low-moisture foods. The objective of this study was to design an RFDH process to achieve target destruction (log reductions) of C. sakazakii and Salmonella spp. The thermal destruction (decimal reduction time; D-value) of C. sakazakii and Salmonella spp. in NDM (high-heat, HH; and low-heat, LH) was determined at 75, 80, 85, or 90 °C using a thermal-death-time (TDT) disk method, and the z-values (the temperature increase required to obtain a decimal reduction of the D-value) were calculated. Time and temperature requirements to achieve specific destruction of the pathogens were calculated from the thermal destruction parameters, and the efficacy of the RFDH process was validated by heating NDM using RFDH to achieve the target temperatures and holding the product in a convection oven for the required period. Linear regression was used to determine the D-values and z-values. The D-values of C. sakazakii in HH- and LH-NDM were 24.86 and 23.0 min at 75 °C, 13.75 and 7.52 min at 80 °C, 8.0 and 6.03 min at 85 °C, and 5.57 and 5.37 min at 90 °C, respectively. The D-values of Salmonella spp. in HH- and LH-NDM were 23.02 and 24.94 min at 75 °C, 10.45 and 12.54 min at 80 °C, 8.63 and 8.68 min at 85 °C, and 5.82 and 4.55 min at 90 °C, respectively. The predicted and observed destruction of C. sakazakii and Salmonella spp. were in agreement, indicating that the behavior of the organisms was similar regardless of the heating system (conventional vs. RFDH). Radio-frequency dielectric heating can be used as a faster and more uniform heating method for NDM to achieve target temperatures for a postprocess lethality treatment of NDM before packaging.