Resistance of Listeria monocytogenes to ultrasonic waves under pressure at sublethal (manosonication) and lethal (manothermosonication) temperatures

The influence of amplitude of ultrasonic waves, static pressure and temperature on the inactivation rate of Listeria monocytogenes by ultrasonic waves under pressure (manosonication; MS) has been investigated. Also the influence of growth temperature, pH and composition of treatment medium, and the presence of sodium chloride in the recovery medium on the MS resistance of L. monocytogenes has been investigated and compared with its heat resistance. The inactivation of L. monocytogenes by high power ultrasonic waves (20 kHz, 117 mu m) at ambient temperature and pressure was low (D-UW = 4.3 min). The increase of relative pressure (MS) to 200 kPa decreased D-MS to 1.5 min and the increase to 400 kPa to 1.0 min. inactivation rate by MS increased exponentially with the amplitude of ultrasonic waves in the range of 62-150 mu m. An amplitude increase of 100 mu m decreased the MS resistance of approximately six times. The inactivation rate by MS was not influenced by treatment temperature up to 50 degrees C. However at higher temperatures the lethality of this combined process (MTS) increased considerably. inactivation by MTS was the result of the inactivation by heat in addition to that by ultrasound under pressure. Cells grown at 37 degrees C were twice more heat resistant than those grown at 4 degrees C but no differences were found between decimal reduction time values to MS (117 mu m, 200 kPa, 40 degrees C) of both suspensions. While the inactivation rate by heat in pH4 buffer was two- and five-fold higher than in pH7 buffer and skimmed milk respectively, the differences among D-MS-value in these media were lower than 60%. The addition of 57% of sucrose to the treatment medium increased D-61 from 0.22 to 5.7min and D-MS from 1.5 to 3.1 min. The addition of 3% sodium chloride to the recovery medium decreased D-60 by 1/3 but did not modify D-MS-values. (C) 1999 Academic Press.