Longitudinal studies on Listeria monocytogenes and other Listeria species in two salmon processing plants

Abstract

Two plants processing salmon fillets and cold smoked salmon were investigated for occurrence of Listeria in products and the environment. Analyses were conducted for a period of 31 weeks. At plant A, 252 samples were examined of which 97 were from unprocessed fish and 155 from cold-smoked fish. At plant B, 189 samples of unprocessed fish were investigated. The first examination of unprocessed fish at plant A showed a presence of L. monocytogenes and L. spp. in 81% and 19% of the samples respectively. For cold-smoked fish the figures were 43% and 23%. At plant B, L. monocytogenes was isolated in 63% of the samples. During the test period, management at the processing plant initiated various hygiene precautions to improve the sanitary situation. The last batch of analyses of unprocessed fish at plant A showed a presence of L. monocytogenes and L. spp. in 42% and. 33% of the samples respectively. For cold-smoked fish, the figures were 6% and 11%. The isolation figures at plant B for L. monocytogenes and L. spp. were 50% and 17% respectively. The hygienic precautions did not have a significant effect on the presence of L. monocytogenes and L. spp. We suggest that Listeria bacteria are a part of the resident flora and are not eliminated by current cleaning and sanitation programmes. Cold-smoking, however, gave a significant reduction in the isolation of L. monocytogenes (P= 0.0082), while the isolation of L. spp. did not decrease after this process.