The importance of slicing equipment as a vector for L. monocytogenes
Slicing and skinning are the final process stages for the manufacture of many smoked fish products. Slicing is a particularly important process stage because several studies have reported L. monocytogenes contamination of previously-uncontaminated fish fillets during slicing. Genetically indistinguishable L. monocytogenes were repeatedly isolated from slicing equipment surfaces for more than three years of a six year monitoring study inside a single smoked-salmon processing plant (Di Ciccio et al. 2012). 37% of the slicing machines surface samples taken during the study duration tested positive for a small number of L. monocytogenes biotypes. The Di Ciccio study observations of isolations from slicing equipment were typical of previous surveillance studies. Dass et al. (2010) also reported a small number of L. monocytogenes biotypes isolated from the slicer and skinning machines in an Irish fish processor over a one year surveillance period. However, in contrast to the Di Ciccio study, the L. monocytogenes prevalence was much lower the Irish slicing equipment with only 6% of samples testing positive. Johansson et al (1999) also determined that L. monocytogenes contamination of fish could occur during the slicing and skinning process stages.
More generally, slicing has been shown to reduce the shelf life of cold smoked fish because unsanitary slicing equipment can also transfer spoilage bacteria in addition to L. monocytogenes. Whole cold-smoked fillets were reported to have a significantly longer shelf-life compared with sliced cold smoked salmon (Hansen et al. 1998).
Aarnisalo et al. (2007) undertook practical and modelling work to estimate the numbers of L. monocytogenes transmitted from a slicer blade to fish and from contaminated fish to another slicing machine. The amounts of L. monocytogenes transferred between blade and fish, and fish and blade were similar. A slicing blade that was contaminated with 6-9 log cfu/g of L. monocytogenes initially transferred high numbers of cells up to 5.3 log cfu/g to the first few fish slices. However, after these four slices, the numbers of cells transferred dropped rapidly. A predictive mathematical model extrapolating from the numbers of cells transferred initially, determined that after 39 slices had been cut; the number of L. monocytogenes cell transferred would typically be around 1.6 log cfu/g of slice. When fillets were contaminated with 7.6 log cfu/fillet were cut using an uncontaminated slicer and uncontaminated fillets were subsequently cut, 1.5 log CFU/g slice was predicted to be present on the un-inoculated fillets after 39 slices. Aarnisalo et al. (2007) advised that to minimise contamination from unsanitary slicer blades, processors could discard the first few slices of fish from each new fillet. From a food safety viewpoint, a much better approach is to aim to process using only sanitary equipment. Consequently, Aarnisalo et al. (2007) advised that slicing equipment and blades should be cleaned and sanitised regularly (i.e. every few hours) to prevent the attachment and establishment of L. monocytogenes biofilms on the slicer surfaces (Aarnisalo et al. 2007). Strategies for the effective decontamination of equipment harbouring L. monocytogenes biofilms are available here and here.
Prevention of biofilm formation on all food contact surfaces in fish processing plants is important. Hansen and Vogel (2011) investigated how long L. monocytogenes could survive in fish processing. Under typical conditions of temperature, salt and humidity and on food grade stainless steel, the authors reported survival of up to 49 days. There was significant extended survival if the L. monocytogenes were allowed to form biofilms on the steel surfaces.
Di Ciccio,P., Meloni,D., Festino,A.R., Conter,M., Zanardi,E., Ghidini,S., Vergara,A., Mazzette,R. and Ianieri,A. (2012) Longitudinal study on the sources of Listeria monocytogenes contamination in cold-smoked salmon and its processing environment in Italy. International Journal of Food Microbiology 158, 79-84.
Hansen,L.T. and Vogel,B.F. (2011) Desiccation of adhering and biofilm Listeria monocytogenes on stainless steel: Survival and transfer to salmon products. International Journal of Food Microbiology 146, 88-93
Johansson,T., Rantala,L., Palmu,L. and Honkanen-Buzalski,T. (1999) Occurrence and typing of Listeria monocytogenes strains in retail vacuum-packed fish products and in a production plant. International Journal of Food Microbiology 47, 111-119.
Dass,S.C., Abu-Ghannam,N., Antony-Babu,S. and Cummins,E.J. (2010) Ecology and molecular typing of L. monocytogenes in a processing plant for cold-smoked salmon in the Republic of Ireland. Food Research International 43, 1529-1536.