Effect of Incubation Temperature on Repair of Heat-Injured Listeria in Milk

Heat-injured Listeria species were examined for their ability to repair in pasteurized whole and 2% (fat) bovine milk. Listeria monocytogenes F5069 (serotype 4B) and F5027 (serotype 1/2a) and Listeria innocua CWD139 were heated at 55°C. After 20 min, 99% of the surviving population was injured as determined by their inability to grow in the presence of 4% NaCl. Bacterial cells were immediately suspended in sterile milk at a concentration of 10² to 10³ per ml and incubated at 4, 10, 26 and 37°C. For all of the Listeria tested, repair at 4°C was initiated between days 8 and 10 and was complete between days 16 and 19; at 10°C, repair began immediately and was complete in 4 d; at 26 and 37°C, repair was complete by 13 and 9 h, respectively. The kinetics of repair were similar in whole and 2% (fat) milk. The relationship between the time required for repair and increasing temperature was nonlinear and indicated that repair of heat-injured Listeria in milk is highly sensitive to minor increases in temperature. Current Listeria detection techniques are not adequate for the detection of injured organisms. The public health consequences associated with failure to detect injured L. monocytogenes which subsequently repair in milk may be significant.