Synergistic combination of heat and ultrasonic waves under pressure for Cronobacter sakazakii inactivation in apple juice

The combined effect of the simultaneous application of heat and ultrasonic waves under pressure (manothermosonication, MTS) on the survival of a strain of Cronobacter sakazakii was studied in apple juice. Below 45 °C, the inactivation by ultrasound under pressure was independent of temperature. Above 64 °C, the lethal effect of ultrasound under pressure was negligible when compared to the lethality of the heat treatment at the same temperature. Between 45 °C and 64 °C, the lethality of the combined process (MTS) was higher than expected if heat and ultrasound under pressure processes acted simultaneously but independently, that is, a synergistic effect was observed. The maximum synergistic effect (38.2%) was found at 54 °C. Recovery on selective media – with sodium chloride or bile salts – revealed that a certain proportion of the survivors after MTS treatments were sublethally injured. It was also observed that survivors after MTS treatments progressively died during refrigerated storage (up to 96 h at 4 °C) in the apple juice. The practical implication of these findings is discussed.