Inactivation and Injury of Listeria monocytogenes under Combined Effect of Pressure and Temperature in UHT Whole Milk

Abstract

AbstractInactivation 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.Practical ApplicationsListeria monocytogenes is responsible for many foodborne illnesses. Because of its psychrotrophic nature, L. monocytogenes is a potential threat to milk and dairy products as it has the ability to grow under refrigeration temperatures. Inactivation of L. monocytogenes in milk under high pressure has been studied widely. However, few studies have been done to evaluate the injury of the microorganisms, which were limited to low pressure and temperature. Quantification of injured population of L. monocytogenes is important; i.e., after high pressure treatment the sublethally injured cells can recover in the milk medium during the storage period. Hence, there is a great need of quantification of injured population of L. monocytogenes in addition to the inactivation over a wide range of pressure and temperature conditions. The kinetic data on both inactivation and injury of microorganisms will help to establish the safe processing conditions for milk during high pressure and temperature treatment.