Abstract
In this study, the growth characteristics of Listeria monocytogenes in commercially supplied ready-to-eat foods (RTEFs) were investigated. The tested RTEFs were a seafood product, Kanifumi-kamaboko (imitation crab surimi product; KK), and a vegetable product, Asazuke-Hakusai (lightly pickled Chinese cabbage; AH), both these foods are available in Japan. Four strains were used for inoculation in these RTEFs: L. monocytogenes ATCC 19115, 99023, LC-8, and LC-21. These strains produced upto 107-109 CFU/mℓ in tryptic soy broth at 20, 10, and 4 ℃. The viable cell count and L. monocytogenes cell count in KK were more than 107 CFU/g at 4, 10, and 20 ℃ in 20, 6, and 2days, respectively. The viable cell count in AH were more than 107 CFU/mℓ in 4 days at 20 ℃ and in 8days at 10 ℃, but there was no significant change in the viable cell count at 4 ℃. In contrast, the viable L. monocytogenes counts in AH decreased to about 2 log CFU/g after 4 days at 20 ℃, and to about 1 log CFU/g after 10 and 20 days at 10 ℃ and 4 ℃, respectively. These results show that while viable counts increased during preservation, L. monocytogenes count decreased. These findings indicate that lactic acid bacteria proliferate during preservation and they produce organic acids that repress the growth of the Listeria in AH.
