Thermal inactivation of Cronobacter sakazakii ATCC 29544 in powdered infant formula milk using thermostatic radio frequency

Abstract A 27.12 MHz thermostatic radio frequency (RF) system was employed to inactivate Cronobacter sakazakii in powdered infant formula milk. A proportional-integral-derivative controller achieved constant material temperature during the holding period. Hot air and similar dielectric material assistance were used to improve RF heating uniformity. The results showed that the microbial thermal resistance was reduced by increasing temperature (55–70 °C) and water activity (0.2–0.4 at 25 °C). Combined RF and hot air pasteurization (RF-HA) was more effective than single RF and material assistance due to better temperature uniformity. D70oC-value (23.3 min) of Cronobacter sakazakii at water activity of 0.2 from RF-HA was not significantly (P > 0.05) higher than that under hot water treatment (20.8 min), indicating that thermal effect led to microbial inactivation. Flow cytometry and transmission electron microscopy further confirmed that RF had no additional damage to cell membrane. The findings contribute to better understanding of RF pasteurization mechanism.