Growth of Listeria monocytogenes ATCC 49594 inoculated onto cold smoked salmon and cold seasoned salmon roe, and the growth inhibition by sodium lactate (NaL) were investigated. The growth of L. monocytogenes inoculated onto cold smoked salmon (103 CFU/g) was investigated at 25, 10, and 5°C. The level of L. monocytogenes increased to 105CFU/g after 2 days of storage at 25°C, 106CFU/g for 10 days of storage at 10°C, and 105CFU/g for 21 days of storage at 5°C. When L. monocytogenes was inoculated (103 CFU/g) onto cold seasoned salmon roe, the level increased to 107 CFU/g after 1day of storage at 25°C, 6days of storage at 10°C, and 8 days of storage at 5°C. The growth of L. monocytogenesinoculated into cold smoked salmon or cold seasoned salmon roe was inhibited at lower storage temperatures; higher inhibition was observed at 10°C than at 25°C and at 5°C than at 10°C. When 3% NaL was added to cold smoked salmon, the growth of inoculated L. monocytogenes was inhibited at all storage temperatures. When 3% NaL was added to cold seasoned salmon roe, no growth of inoculated L. monocytogenes was observed at 10 and 5°C, except for at 25°C. These results indicated that the addition of 3% NaL to cold smoked salmon or cold seasoned salmon roe was effective for inhibiting the growth of L. monocytogenes at 5 and 10°C.
Acidic hypochlorite (prepared by addition of NaOCl and HCl to water) and electrolyzed acidic water (prepared by electrolysis of 0.1% NaCl, recognized as a food additive) were compared with regard to their disinfectant activities against food-borne pathogens and their mechanisms of disinfectant activities were examined. The both disinfectants reduced colony forming units of vegetative cells by more than 6 log units in 10 seconds. Spores of Clostridium botulinum and Bacillus subtilis were reduced by 5.0 and 1.8 log units, respectively, in 10 minute treatments by these disinfectants. The extents of reductions were significantly more than that by sodium hypochlorite at a same free chlorine concentration. Normal flora in bean sprout was reduced by either of the acidic hypochlorite and electrolyzed acidic water more effectively than sodium hypochlorite. Electron microscopic observations revealed that numerous structures that appeared like membrane vesicles formed on the surface of gram negative rods exposed to the two disinfectants. The exposed bacteria did not show the plasmolysis when subjected to hypotonic treatment. Propidium iodide which does not penetrate through the intact cytoplasmic membrane invaded into the cytoplasm of the disinfectant exposed bacterial cells. These observations suggested that the disinfectants killed bacteria by destruction of their cell wall and cytoplasmic membrane. No difference was found between the two disinfectants, indicating that acidic hypochlorite, which is prepared quite inexpensively by just an addition of HC1 to sodium hypochlorite solution, is as effective, and has the same mechanism of action, as electrolyzed acidic water.
The behavior of Listeria monocytogenes in various seasoning liquids, salted cod roe samples and the manufacturing process of Karashi-Mentaiko (red-pepper seasoned cod roe) was examined. A proliferative test on L. monocytogenes FCI-Lis 10 strain (serovar 1/2a) in three home-made seasoning liquids (pH 5.9, 6.5 and 7.0; Aw 0.95) and four commercial seasoning liquids (pH 5.7-6.1;Aw 0.89-0.93) was performed at 6 and 15°C. The number of FCI-Lis 10strains decreased as the preservation period increased, except of 15°C preservation using home-made seasoning liquid (pH 7.0). A proliferative test on the FCI-Lis10 strain in salted cod roe samples (pH 6.0, 6.5 and 7.0; Aw 0.97) was also performed. The number of FCI-Lis 10 strains increased at 6 and 15°C in the order of pH 7.0>6.5>6.0;however, in the test of 6°C preservation at pH 6.0, the number on the 14th day was almost the same as the inoculated bacterial count. The behavior of the FCI-Lis10 strain was examined in the manufacturing process of Karashi-Mentaiko using the above home-made seasoning liquids and two commercial seasoning liquids (pH 5.7 and 6.0; Aw 0.93). The number of FCI-Lis 10 strains decreased in the manufacturing process regardless of the kind of seasoning liquid . The behavior of the FCI-Lis 12 strain (serovar 1/2b), FCI-Lis29 strain (serovar 3 a) and LM 17 strain (serovar 4b) was examined in the manufacturing process of a home-made seasoning liquid (pH 5.9; Aw 0.95) and two commercial seasoning liquids (pH 5.7 and 6.0; Aw 0.93). The number of all three strains also decreased in the manufacturing process. These results suggest that the growth of L. monocytogenes at 6°C in seasoning liquid, salted cod roe sample and during the manufacturing process is controlled by pH, Aw and temperature, and the control point of L. monocytogenes in the manufacturing process is to use a seasoning liquid with low pH and low Aw, and manufacture under low temperature refrigeration.