| Abstract: | Cold shock at 0 to 15°C for 1 to 3 h increased the thermal sensitivity of Listeria monocytogenes. In a model broth system, thermal death time at 60°C was reduced by up to 45% after L. monocytogenes Scott A was cold shocked for 3 h. The duration of the cold shock affected thermal tolerance more than did the magnitude of the temperature downshift. The Z values were 8.8°C for controls and 7.7°C for cold-shocked cells. The D values of cold-shocked cells did not return to control levels after incubation for 3 h at 28°C followed by heating at 60°C. Nine L. monocytogenes strains that were cold shocked for 3 h exhibited D60 values that were reduced by 13 to 37%. The D-value reduction was greatest in cold-shocked stationary-phase cells compared to cells from cultures in either the lag or exponential phases of growth. In addition, cold-shocked cells were more likely to be inactivated by a given heat treatment than nonshocked cells, which were more likely to experience sublethal injury. The D values of chloramphenicol-treated control cells and chloramphenicol-treated cold-shocked cells were no different from those of untreated cold-shocked cells, suggesting that cold shock suppresses synthesis of proteins responsible for heat protection. In related experiments, the D values of L. monocytogenes Scott A were decreased 25% on frankfurter skins and 15% in ultra-high temperature milk if the inoculated products were first cold shocked. Induction of increased thermal sensitivity in L. monocytogenes by thermal flux shows potential to become a practical and efficacious preventative control method. |
