Yeasts and Listeria monocytogenes Inactivation After Ultra High Pressure Treatments of Smoothies at Various Temperatures

Common microbial contaminants of fruit are yeasts and moulds; however, the thermoduric and psychrotrophic traits of Listeria monocytogenes make its presence in smoothies a microbiological risk also at refrigeration temperatures. Non-thermal pasteurization methods as High Hydrostatic Pressure (HHP) could fulfil the need of retaining smoothies freshness and healthy properties. In the present study, combined treatments of HHP (100 to 300 MPa) and temperature (-5°C to 50°C) were performed to inactivate mixed strains of Pichia ssp. and L. monocytogenes added to commercial smoothies (108 CFU ml-1). An inactivation model was developed according to a CCD. Counts of survivors by selective (SM), as well as recovery (RM) media, permitted to estimate the occurrence of Viable but Not Culturable (VNC) cells. The response surfaces showed a synergistic effect of T and HHP, giving the highest cell lethality at 300MPa either at 50°C or -5°C (>8 and 5.0 Log CFU ml-1 for Yeasts, 6.0 and 4.5 Log ml-1 for L. monocytogenes respectively); interestingly, the model showed a reduction of the treatment effectiveness (to < 1 Log CFU ml-1) as the T approaches the optimum growth temperature of the target species, regardless the HHP. Also, SM and RM counts indicated that VNC cells reached their maximum level at these temperatures for all the HHP employed.