The incidence of Listeria monocytogenes in Karashi-Mentaiko (Red-Pepper Seasoned Cod Roe) was investigated and the method of controlling L. monocytogenes during the manufacturing process of Karashi-Mentaiko was studied. Of the 144 samples investigated, 16 (11.1%) were found to contain L. monocytogenes but the concentration of L. monocytogenes was low; most of the samples contained less than 1/g. Karashi-Mentaiko which is ordinarily manufactured under a low temperature using a seasoning liquid with aw 0.89-0.93 does not support the growth of L. monocytogenes; therefore the concentration detected seemed to be a low risk to consumers. However, the ingredients of the seasoning liquids differ with each manufacturer. When a high level of contamination by L. monocytogenes in raw materials such as cod roes or salted cod roes is caused by improper food handling, the number of L. monocytogenes might not decrease during the manufacturing process with high-aw seasoning liquid. To reduce this risk, the bactericidal effects of polylysine, sodium lactate, two kinds of sodium acetate products (product names: “KTN” and “NSK”), sodium acetate-glycine-lysozyme product and Nisin A on L. monocytogenes FCI-Lis10 strain (serovar 1/2a) were investigated by a model manufacturing experiment: salting process at 18°C for 20 hours and seasoning process at 6°C for 14 days using a special home-made seasoning liquid of aw 0.96. The sodium acetate products and Nisin A reduced the number of inoculated FCI-Lis 10 strain by less than 1/102-1/103 and less than 1/103-1/104, respectively, although their bactericidal effects were excellent on the other L. monocytogenes strains: FCI-Lis 12 strain (serovar 1/2b), FCI-Lis29 strain (serovar 3a) and LM17 strain (serovar 4b). These findings suggest that the addition of sodium acetate products or Nisin A would be effective for L. monocytogenes control in the manufacturing process of Karashi-Mentaiko.
We investigated the Campylobacter isolation method from retail imported chicken samples. Campylobacter spp. was isolated from 26 of 100 samples (26.0%) using Preston enrichment broth, whereas 13 of 100 samples (13.0%) using Bolton broth. It is suggested that Bolton broth is not suitable for the isolation of Campylobacter spp. when meat remains in the culture medium, because of its weakness in sustaining the growth of other bacterium. Isolation rate of Campylobacter spp. was increased (p<0.05) to 42.0% when 1 ml of Preston culture medium was inoculated to another sample of Preston broth (10 ml) after incubation for 24 hours, and similarly, isolation rate was raised (p<0.05) to 33.0% when 1 ml of Bolton culture medium was inoculated to Preston broth. Two-stage enrichment that is inoculation of Bolton culture medium to Preston broth following pre-incubation (for 24 hours) of whole emulsion samples, is useful for frozen chicken samples, such as imported chickens, in which the bacterial number of Campylobacter spp. is supposed to be reduced during the process of freezing or thawing.