Inhibition of Listeria monocytogenes on cold-smoked salmon by nisin and carbon dioxide atmosphere.

The bacteriostatic and bacteriocidal effect of nisin in combination with carbon dioxide, NaCl and low temperature on the survival of Listeria monocytogenes was investigated in in vitro model studies and in trials with cold-smoked salmon. Addition of nisin caused various degrees of inhibition and sometimes death of L. monocytogenes in model experiments performed at 10 degrees C. The antilisterial effect of nisin was improved in the presence of 100% CO2 and increasing NaCl concentrations (0.5 to 5.0% w/v). Minimal bactericidal concentrations (MBC) of nisin varied from 30 to more than 500 IU/ml. The most pronounced effect of nisin was found when 10(2) cfu/ml was grown in media with 5.0% NaCl and incubated in CO2 atmosphere (MBC = 30 IU/ml). The bactericidal effect of nisin was reduced in air and vacuum, and did not increase systematically with increasing NaCl concentrations. In general, nisin concentration < or = 50 IU/ml resulted in the survival and growth of L. monocytogenes in all combinations with other preservatives (NaCl, CO2). Addition of nisin (500 or 1000 IU/g) to cold-smoked salmon inoculated with L. monocytogenes and stored at 5 degrees C delayed, but did not prevent growth of L. monocytogenes in vacuum-packs. Numbers of L. monocytogenes increased to 10(8) cfu/g in vacuum packed cold-smoked salmon in 8 days, whereas CO2 packing of cold-smoked salmon resulted in an 8-day lag phase of L. monocytogenes, with numbers eventually reaching 10(6) cfu/g in 27 days. Addition of nisin to CO2 packed cold-smoked salmon resulted in a 1 to 2 log reduction of L. monocytogenes followed by a lag phase of 8 and 20 days in salmon with 500 and 1000 IU nisin/g, respectively. The levels of L. monocytogenes remained below 10(3) cfu/g during 27 days of storage at both concentrations of nisin.