Effect of growth and recovery temperatures on pressure resistance of Listeria monocytogenes

Experimental conditions can affect the outcome of bacterial stress-tolerance assays. Growth conditions that optimize microbial recovery should be established to help evaluate the effectiveness of treatment conditions for food safety. The objectives of this study were to determine the effects of growth and recovery temperatures on pressure resistance of early stationary-phase Listeria monocytogenes in milk. The tested conditions were the following: (1) L. monocytogenes was grown at various temperatures (10, 15, 20, 25, 30, 35, 40 and 43 °C), suspended in ultra-high temperature (UHT) -processed whole milk, pressure-treated at 400 MPa for 2 min at 21 °C and recovered on Tryptic Soy Agar supplemented with 0.6% yeast extract (TSAYE) at 35 °C; (2) L. monocytogenes was grown at 35 and 43 °C, pressure treated in milk (400 and 500 MPa, respectively, for 2 min at 21 °C) and recovered on TSAYE at various temperatures (4, 10, 15, 20, 25, 30, 35 and 40 °C); (3) L. monocytogenes originally grown at 35 °C, was pressure treated in milk (400 or 450 MPa for 2 min at 21 °C), and recovered on TSAYE at 10 °C for various time intervals (1, 2, 3, 6, 9 and 12 days) then at 35 °C for 5 days. There was no significant difference (P > 0.05) in pressure-resistance of L. monocytogenes grown at 10 to 25 °C with approximately 6.5-log CFU/ml population reductions. At growth temperatures greater than 25 °C, pressure resistance increased with less than 1-log CFU/ml reduction observed for L. monocytogenes originally grown at 43 °C. After pressure treatment, regardless of growth temperature and pressure treatment, the greatest recovery of L. monocytogenes was within the 4 to 20 °C range; maximum recovery at 10 °C required approximately 24 days. The time for comparable post-pressure treatment recovery could be reduced by incubation at 10 °C for at least 2 days followed by incubation at 35 °C for 5 days. The findings of the present study indicate that growth and recovery temperatures affect the pressure resistance of L. monocytogenes and should, therefore, be taken into account when assessing the adequacy of inactivation treatments.