Use of nisin-coated plastic films to control Listeria monocytogenes on vacuum-packaged cold-smoked salmon

Cold-smoked ( Salmo salar) salmon samples were surface-inoculated with a cocktail of three nisin-resistant strains of L. monocytogenes (PSU1, PSU2 and PSU21) to a level of approximately 5 × 10 2 or 5 × 10 5 CFU/cm 2 of salmon surface. The inoculated smoked salmon samples were vacuum-packaged with control film (no nisin) or nisin-coated plastic films and stored at either 4 or 10 °C. When the inoculated smoked salmon samples were packaged with film coated with 2000 IU/cm 2 of nisin, a reduction of 3.9 log CFU/cm 2 (compared with control) was achieved at either temperature for samples inoculated with 5 × 10 2 CFU/cm 2 of L. monocytogenes after 56 (4 °C) and 49 (10 °C) days of storage while reductions of 2.4 and 0.7 log CFU/cm 2 were achieved for samples inoculated with a high level of L. monocytogenes (5 × 10 5 CFU/cm 2) after 58 (4 °C) and 43 (10 °C) days, respectively. For samples packaged in film coated with 500 IU/cm 2 of nisin, reductions of 0.5 and 1.7 log CFU/cm 2 were achieved for samples inoculated with a low level of L. monocytogenes (5 × 10 2 CFU/cm 2) after 56 (4 °C) and 49 (10 °C) days of storage while reductions of 1.8 and 0.8 log CFU/cm 2 were achieved for samples inoculated with high level of L. monocytogenes after 58(4 °C) and 43 (10 °C) days, respectively. In addition, nisin inhibited the proliferation of background microbiota on smoked salmon in a concentration-dependent manner at both storage temperatures although the bacteriostatic effect was more pronounced at refrigeration temperature. This work highlights the potential for incorporating nisin into plastic films for enhancing the microbial safety of smoked salmon as well as controlling its microbial spoilage.