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A Microbiological Survey of Retail Smoked Fish with Particular Reference to the Presence of Listeria Monocytogenes

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The Food Standards Agency’s survey of Listeria monocytogenes in smoked fish was carried out over a period of 4 months between July and November 2006. The main objective was to measure the prevalence of L. monocytogenes in ready-to-eat smoked fish sold in retail premises in the United Kingdom. Tests were also performed to investigate the presence of Listeria spp., Salmonella, Escherichia coli, coagulase positive staphylococci and Enterobacteriaceae. Physico-chemical testing was also carried out to measure salt, pH and water activity.
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Results: A total of 3,226 samples of ready-to-eat hot and cold smoked fish were sampled from over 1,000 retail premises within the UK, of which 3,222 samples were acceptable for testing. The samples were tested for the presence of Listeria spp. and L. monocytogenes. Overall Listeria spp. was detected in 378 samples with a weighted prevalence of 10.5%. Of these L. monocytogenes was detected in 302 samples with a weighted prevalence of 8.3%, of which 99.3% were satisfactory according to the Microbiological Criteria Regulations.

Of the 1,344 samples of cold smoked fish tested, 282 (20.5%) contained Listeria spp.; 236 (17.4%) were L. monocytogenes which was present below the 100 cfu/g legal limit. Of the 1,878 samples of hot smoked fish tested, 96 (5.2%) contained Listeria spp.; 66 (3.4%) were L. monocytogenes. Three of these samples (0.06%) were in breach of the limits (>100 cfu/g) for L. monocytogenes as laid down in the Microbiological Criteria Regulations. To protect public health, the Agency took appropriate action as soon as the results for these samples were reported and the products were withdrawn from retail sale.

Salmonella was not detected in any of the samples tested. Only 7 samples contained E. coli (threshold for detection was 10 cfu/g); 13 samples contained coagulase positive staphylococci (threshold for detection was 10 cfu/g).

Of the 3,222 samples taken, 587 samples contained levels of Enterobacteriaceae ranging from 10cfu/g to 106cfu/g. There was a significant correlation between the detection of Enterobacteriaceae and detection of Listeria (p<0.001). Where Enterobacteriaceae was detected, 21.2% (CI: 17.7% - 24.9%) of samples also contained Listeria spp. compared with 8.6% (CI: 7.6 – 9.7%) of those not contaminated with Enterobacteriaceae being positive for Listeria spp.

All smoked fish samples taken were tested to define levels of salt, water activity as these properties can affect the ability of bacteria to grow during the course of their shelf life. The ranges for salt indicated a lower concentration of salt in some hot smoked fish sampled (0.1g - 6.8 g/100g) compared to the cold smoked (0.6g – 6.4g/100g). The pH range for cold smoked fish (pH 3.1 - 7.2) indicated a slightly lower pH compared to hot smoked (pH 4.1 – 8.4). The water activity for both types of smoked fish covered the same ranges (0.89 – 0.99).

Find more about this project and other FSA food safety-related projects at the Food Standards Agency Research webpage.

Funding Source
Food Standards Agency
Project number
Bacterial Pathogens