Resistance of heat-shocked cells of Listeria monocytogenes to mano-sonication and mano-thermo-sonication.

Heat shocks did not increase the resistance of Listeria monocytogenes to an ultrasonication treatment under pressure (Mano-Sonication; MS). While heat-shocked cells (180 min, 45 degrees C) became sixfold more heat resistant than native cells (D62 = 1.8 min vs D62 = 0.24 min), the resistance of native and heat-shocked cells to MS (200 kPa, 117 microns) was the same (DMS = 1.6 min). The inactivation rate of non-heat-shocked cells of L monocytogenes by a combined heat/ultrasonication treatment under pressure (Mano-Thermo-Sonication; MTS) was an additive effect. On the contrary, on heat-shocked cells, the inactivation rate of MTS was greater than that of heat added to the inactivation rate of MS (synergistic effect) in the range 62-68 degrees C.