Inactivation and injury of listeria monocytogenes under combined effect of pressure and temperature in UHT whole milk

Inactivation and injury of Listeria monocytogenes was studied under the combined effect of high pressure and temperature. Three pressure levels (400, 500 and 600 MPa) and three temperature levels (27, 43 and 60C) were chosen. In total, seven combinations were tested: (1) 400 MPa at three temperature levels; and (2) 500 and 600 MPa at 27 and 43C, i.e., the treatment times at 60C were too short (<1 s) to obtain reliable data based on machine precision. Comparison of D-values on both media showed that (1) the D-values of treatment condition 400 MPa, 27C and 400 MPa, 43C were significantly different (P < 0.05), and from the rest of the five pressure and temperature combinations, (2) the remaining five combinations were not significantly different from each other (P > 0.05). Injury of L. monocytogenes was evaluated in terms of log reduction value, which was obtained from nonselective and selective media. Treatment temperature played an important role on injury of L. monocytogenes specifically at lower pressure level. At 400 MPa, the injured log reduction values obtained at 27 and 43C were 2.10 and 3.46, respectively, for 15-min treatment time, whereas at 60C, it was 4.58 for 1 min. This study showed that 6 log₁₀ reductions could be obtained in <2 min using low pressure (400 MPa) and high temperature (60C), which would have industrial relevance in reducing the cost related to high pressure processing of food.