Behavior of Listeria monocytogenes during fabrication and storage of experimentally contaminated smoked salmon

Experiments were carried out to examine the behavior of Listeria monocytogenes in the course of fabrication and storage of smoked salmon. In three trials, raw salmon fillets were surface inoculated with L. monocytogenes, marinated, smoked at 26 to 30 degrees C, and stored at 4 or 10 degrees C for up to 30 days. At different times during the fabrication and storage, samples were taken and, by means of the three-tube most probable number (MPN) method, quantitatively analyzed for the concentration of L. monocytogenes. The initial Listeria levels in the raw fillets were 10(4) MPN/g in trial 1, 10(1) MPN/g in trial 2, and 10(2) MPN/g in trial 3. During the fabrication, neither an increase nor a decrease of the inoculated quantities was observed. During the storage, however, a significant growth was measured in two of three trials; in trial 1, a 2.5 log10 MPN/g increase and in trial 3, an increase of even 4.5 log10 MPN/g. In the second trial, the Listeria level remained about the same. The results indicate the importance of preventing pre- and postprocessing contamination of L. monocytogenes in raw and smoked salmon. Because a significant increase of L. monocytogenes was measured during storage, there might be an increasing risk of infection for the consumer by storing such fish for a long time.