Purpose : This study evaluated antimicrobial compounds nisin, lauric arginate (LAE), epsilon-polylysine (EPL), and chitosan for their effectiveness in reducing initial levels as well as inhibiting growth of L. monocytogenes in a laboratory medium and on cold-smoked salmon.
Methods : Brain Heart Infusion broth (4.65% wp-NaCl, pH 6.1) and cold-smoked salmon slices containing nisin (10 ppm), LAE (10 ppm), EPL (250 ppm), or chitosan (200 ppm) were inoculated with L. monocytogenes and incubated at 7°C. Viable cells were enumerated over time and growth parameters lag phase (λ), growth rate (µmax), initial cell density (N0), maximum cell density (Nmax), and maximum log reduction (NmaxR) were calculated.
Results : In broth, nisin and LAE caused significantly greater NmaxR than the rest of the treatments, achieving reductions of 5.13 log CFU/ml and 4.02 log CFU/ml, respectively (P < 0.05). Regrowth of LAE-treated L. monocytogenes was significantly delayed (23.42 days) (P < 0.05). In addition, µmax and Nmax of LAE-treated cells were significantly lower than most treatments (0.12 log CFU/ml/day and 6.33 log CFU/ml, respectively) (P < 0.05), with nisin being the only exception. Preliminary results in inoculated salmon found no evidence for antimicrobial activity of LAE against L. monocytogenes. The reduction in efficacy compared to broth is hypothesized to be due to partitioning of LAE molecules into the lipid phase of salmon.
Significance : Despite strong antimicrobial activities exhibited by LAE in broth, its application to foods that are high in fat may be limited. This highlights the importance of validating antimicrobial efficacy in the target food matrix.