Effect of the combined treatments of high hydrostatic pressure and temperature on Zygosaccharomyces bailii and Listeria monocytogenes in smoothies

In the present study, the combined treatments of high hydrostatic pressure (100 to 300 MPa) and temperature (−5 to 45 °C) were performed according to a central composite design to inactivate Listeria monocytogenes LGB and two strains of Zygosaccharomyces bailii that were artificially added to commercial smoothies (6-7 Log cfu/mL). The response surfaces showed a synergistic effect of temperature and pressure for both species, resulting in the highest cell lethality at 300 MPa and at either 45 °C or −5 °C and lowest cell lethality when the temperature was closer to room temperature. For some combinations, the L. monocytogenes LGB inactivation predicted in this study is in compliance with the FDA requirement of a 5 Log reduction of microorganisms in fruit juices and beverages. The appropriate selection of temperature values in the treatment of smoothies with high hydrostatic pressure permits cold pasteurization at pressure levels well below 300 MPa to be obtained, thus enabling cost reduction to manufacturing plants and making the industrialization of the process favourable. Selective and recovery media counts indicated that the content of injured L. monocytogenes LGB cells in the survivor population reached a maximum value after the most severe treatments. Regarding such a cell fraction, microscopy observations of LIVE/DEAD BacLight and JC-1 staining appear to indicate a widespread presence of plasmolysis in cells with a partially depolarized membrane, providing additional information about their physiological state.