Combined effect of high pressure and temperature on inactivation and injury of Listeria monocytogenes in UHT Whole milk

Listeria monocytogenes is a Gram positive, psychrotrophic microorganism that can grow even at -0.4°C. As L. monocytogenes is capable of growing at refrigeration temperature, it is a potential threat for stored foods such as milk and dairy products. The microorganism shows barotolerance when treated with high pressure in milk. In the present study, inactivation and injury of L. monocytogenes was evaluated under the combined effect of high pressure and temperature in UHT whole milk. The milk samples were treated at two pressure levels (500 and 600 MPa) and 43°C for different treatment times. Stationary phase cells grown at 43°C, documented to be most resistant, were used for experiments. After the high pressure treatment, the milk samples were serially diluted and spiral plated on Tryptic Soy Yeast Extract Agar (TSAYE) and Modified Oxford Agar (MOX). After incubating at 35°C for 3-5 days, the log reduction values were calculated for both TSAYE and MOX plates. Approximately 6-log reduction was obtained when milk samples were treated at 500 MPa and 43°C for 6 minutes and at 600 MPa and 43°C for 80 s. Percent injured cells was calculated. Approximately, 99.9999% injury was obtained at 500 MPa, 43°C for six and seven minutes of treatment time and at 600 MPa, 43°C for 80,100, and 120 s.