Inactivation of Cronobacter sakazakii by ultrasonic waves under pressure in buffer and foods

The objective of this research was to characterize the resistance of Cronobacter sakazakii to ultrasonic waves under pressure (manosonication, MS). The DMS value (decimal reduction time value) of C. sakazakii in standard conditions (35 °C, 117 μm, 200 kPa, citrate–phosphate buffer pH 7.0) was 0.41 min. This value was higher than that of Yersinia enterocolitica (DMS = 0.19 min) and lower than those of Salmonella enterica serovar Enteritidis (DMS = 0.61 min), Listeria monocytogenes (DMS = 0.86 min), and Enterococcus faecium (DMS = 1.2 min). Strain studied (ATCC 29544, NCTC 8155, 9238, and 9529), growth temperature (10, 20, 30, and 37 °C), and pH of the treatment media (4.0, 5.0, 6.0, and 7.0) did not significantly change C. sakazakii MS resistance. Conversely, entry into stationary growth phase, decreasing water activity of the treatment media (0.98, 0.96, and 0.94), and treatment in food products (apple and orange juices, chicken and vegetable soups, and rehydrated powdered milk) resulted in up to a 1.6-, 3.9-, and 2.5-fold maximum change in DMS values, respectively. Whereas an exponential relationship between the amplitude of ultrasonic waves and DMS values was found, the relationship between static pressure and DMS values was better described by a quadratic equation. The energy transferred into the medium determined the lethality of the ultrasonic waves regardless of the combination of pressure (0, 50, 100, 200 and 300 kPa) and amplitude (34, 62, 90, 117 and 145 μm) applied. There was an exponential relationship between DMS values and the power input: an increase of 134 W increased the inactivation rate ten times regardless of the treatment medium. No C. sakazakii cells with sublethally injured cytoplasmic membrane or with sublethal oxidative damage occurred after MS treatments, but the results indicated that damage to the outer membrane preceded microbial death.