Document Type

Theses, Ph.D


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Publication Details

Successfully submitted for the award of Doctor of Philosophy (Ph.D) to the Dublin Institute of Technology, April, 2011.


Contamination of cold-smoked salmon with Listeria monocytogenes, a bacterium causing listeriosis, presents a risk to consumer health. The overall aim of this thesis was to investigate the prevalence and source of L. monocytogenes in different stages of vacuum packed cold-smoked salmon production chain/retail market and to develop a risk assessment model to quantitatively assess the risk of human listeriosis upon consumption of vacuum packed cold-smoked salmon. The study necessitated the identification of novel isolation techniques for the isolation and quantification of L. monocytogenes in vacuum packed cold-smoked salmon. The techniques currently used in isolation of L. monocytogenes from ready-to-eat food (EN/ISO 11290-01 and -02) were evaluated and were found to be 64 % effective in isolating L. monocytogenes. Use of 16S rRNA gene sequence analysis and molecular fingerprinting method multi-locus variable number tandem repeat analysis (MLVA) in combination with EN/ISO 11290-01 and -02 was found to be more effective (98 %) in quantification of L. monocytogenes in vacuum packed cold-smoked salmon. The prevalence of L. monocytogenes in five brands of vacuum packed cold-smoked salmon (n = 120) marketed in different retail outlets in the Republic of Ireland was 21.60%.The prevalence of L. monocytogenes surveyed in a vacuum packed cold-smoked salmon factory (n = 444) was 24.54 %. The final product (vacuum packed cold-smoked salmon) was contaminated with three major types of L. monocytogenes; one type originating from the raw material and the others colonising the production line. To validate these routes of contamination, 60 raw salmon were tagged and sampled after each stage of processing, the results showed that the final product was contaminated with 3 strain types of L. monocytogenes isolated from raw, curing and filleting stages of cold-smoking respectively. The prevalence and tagging results indicate the current ubiquitous nature of L. monocytogenes in vacuum packed cold-smoked salmon A product specific model was developed and validated under dynamic temperature conditions to predict the growth of L. monocytogenes in cold-smoked salmon taking into account the retail and consumer phases of the food pathways. The values of bias factor and accuracy factorof the modelwere close to unity, indicating good agreement between observations and predictions of the model. Finally a quantitative Monte Carlo risk assessment model was developed to assess likely human exposure and the probability of human illness by L. monocytogenes on cold-smoked iii salmon in Ireland. A surveillance study conducted at the retail level served as the starting point for the model with a mean prevalence of L. monocytogenes in vacuum packed cold-smoked salmon of 21.60 % and a mean count on contaminated vacuum packed cold-smoked salmon of 2.60 log10 CFU/g (95 % confidence interval 0.00 – 4.53 log10 CFU/g). The model predicted the annual log probability of illness by consuming contaminated vacuum packed cold-smoked salmon in a low risk and high risk population, with mean values -5.76 and -1.63, respectively (assuming weekly consumption). The model sensitivity analysis highlights the importance of reducing the initial contamination levels of L. monocytogenes on raw fish and the maintenance of proper storage conditions. Various ‗what-if‘ scenarios were studied to assess the likely impact on the log probability of illness per serving. Careful control of consumer storage temperature and time were identified as the best strategies to decrease the probability of illness. In conclusion, the results from this study indicated that sub-typing of the different strains using MLVA implicated a possible carryover of L. monocytogenes from the raw fish and in-house strain to the final product. Therefore, suitable processing parameters and pre- processing handling practices should be treated as important control measures to minimise the exposure to this pathogen. The product specific dynamic model developed in this study provides the sea food industry with a useful tool for effective management and optimization of product safety and may contribute to more realistic estimations of safety risks related to vacuum packed cold-smoked salmon. The results from the quantitative risk assessment developed in this thesis may help risk managers to make informed decisions with regard to possible control measures for L. monocytogenes in cold smoked salmon and therefore improve food safety.



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