In Vitro Studies on the Synergistic Bacteriostatic Effects of Nisin and Chitosan Against Selected Pathogenic and Food Spoilage Bacteria, and a Subsequent Challenge Study with Listeria monocytogenes in Vacuum-Packaged Cold Smoked Salmon

Summary

This study investigated combination technologies with nisin and chitosan as a potential broad-spectrum hurdle technology for the preservation of vacuum-packaged cold smoked salmon (CSS). First, the in vitro effects of nisin and chitosan, alone and in combination, were assayed against Listeria monocytogenes, Staphylococcus aureus, Escherichia coli, Vibrio cholerae, and Photobacterium sp. The minimum inhibitory concentration (MIC) of nisin was 4.0 to 8.0 µg/mL in Gram-positive (G +) bacteria and ≥ 512 µg/mL in Gram-negative (G-) bacteria, except for Photobacterium sp., which displayed an atypically low MIC for G-bacteria of 2.0 µg/mL. The MIC for chitosan was 0.3 to 0.6 mg/mL for L. monocytogenes, 0.8 mg/mL for S. aureus, 0.6 mg/mL for Photobacterium sp., and 1.3 mg/mL for V. cholerae and E. coli. Interactive effects of nisin and chitosan combined were additive for G + bacteria and Photobacterium sp., and synergistic for V. cholerae and E. coli. Second, challenge studies with L. monocytogenes cocktails inoculated on vacuum-packaged CSS were performed to demonstrate the inhibitory effects of nisin and chitosan alone and in combination during storage at 4 °C. Both nisin and chitosan independently displayed an inhibitory effect on L. monocytogenes growth, and even more so when combined. During the storage period, an increase in texture parameters and a decrease in redness and yellowness were observed across samples. Samples treated with nisin and chitosan appeared darker than the others, but this was only evident on Day 1. No significant differences in breaking force, redness, or yellowness were found among the treatments.