How safe from Listeria monocytogenes are frozen ready-to-eat fish products, either cooked or smoked, when stored under refrigeration after purchasing?

This research aimed to access the incidence of Listeria sp. and L. monocytogenes in frozen ready-to-eat (RTE) fish products immediately after purchasing, and after refrigerated storage. Out of 72 kani-kama samples, 17 (23,6%) and 16 out of 48 samples (33,3%) of smoked salmon samples, tested positive for Listeria sp. by the traditional culture method, whereas 45,8% and 56.2% were positive by PCR, for kani-kama and smoked salmon samples, respectively. Applying qPCR, 21 samples of kani-kama out of 72 (29.2%) and 23 out of 48 samples of smoked and sliced salmon (47.9%) were positive for L. monocytogenes. Quantification was performed by culture to account only for viable cells, and confirmation was carried out via PCR and qPCR for Listeria sp. and L. monocytogenes, respectively. The average counts of L. monocytogenes did not exceed 2 Log CFU/g, both in kani-kama or smoked salmon. However, 15.3% of individual kani-kama samples and 14,6% of smoked salmon samples showed counts of up to 3.2 Log CFU/g, some even immediately after purchasing. Higher number of positive samples was observed during refrigerated storage compared to the number of samples analysed immediately after purchasing, indicating that refrigeration for a long time can pose an additional health threat. Unlike kani-kama, which is a heat-processed product, a higher background microbiota count was detected in smoked salmon. However, this background microbiota apparently did not influence the growth of Listeria spp.