Effect of salting and cold-smoking process on the culturability, viability, and virulence of Listeria monocytogenes strain Scott A

The aim of the present study was to determine the effect of the different steps of the cold-smoking process and vacuum storage on the culturability and viability of Listeria monocyrogenes strain Scott A inoculated in sterile salmon samples. Additionally, the virulence of L. monocytogenes cells was assessed by intravenous inoculation of immunocompetent mice. Salmon (Salmo salar) portions were inoculated with L. monocytogenes at a level of 6 log CFU/2 and were then dry salted (5.9%), smoked (0.74 mg phenol per 100 g), partially frozen (-7degreesC), vacuum packed, and stored for 10 days at 4degreesC followed by 18 days at 8degreesC. Salting represented the only step of the process with a weak but significant listericidal effect (0.6 log reduction). Although the other processing steps had no immediate reduction effect on L monocytogenes. the combination of steps significantly lowered by 1.6 log CFU/g the number of L. monocytogenes. The culturable count remained less than 7 log CFU/g until the end of the storage period, whereas in unprocessed samples (control) the culturable counts reached values up to 9 log CFU/g. To mimic a postprocess contamination, salmon portions were also inoculated with L monocytogenes after being cold-smoke processed. A reduction of the culturable count during the 2 first weeks of storage was observed, but then growth occurred and identical values observed for preprocess contamination were reached at the end Of the storage. A viable but nonculturable state transition of strain Scott A was not observed, and the cold-smoking process did not affect the virulence of bacteria isolated at the be-inning and end of the storage.