Search

This study focused on the ecology of Listeria monocytogenes in a fish farm by following the changes in its occurrence in different types of samples for a three year period. In addition, L. monocytogenes isolates from different seafood industry areas were compared with pulsed field gel electrophoresis (PFGE) typing to discover possible associations between primary production, further processing and final products. Weather conditions were found to have a strong influence on the probability of finding Listeria spp. in a fish farm environment. The number of samples contaminated with Listeria spp. was typically bigger after rainy periods. Brook and river waters as well as other runoff waters seemed to be the main contamination source at the farm studied. The farmed fish originally found to carry L. monocytogenes become gradually Listeria free. The time needed for the purification of the fish was several months. The sea bottom soil samples were the ones that preserved the L. monocytogenes contamination the longest time. It can be stated that the fish and fish farm equipment studied did not spread listeria contamination. On the contrary, they were found to suffer from listeria contamination coming from outside sources like the brook water. There was a wide range of different L. monocytogenes PFGE-pulsotypes (30) found at 15 Finnish fish farms and fish processing factories. L. monocytogenes isolates from the final products often belonged to the same pulsotypes as did the isolates from the processing environment as well as from the raw fish. This suggests that, in addition to the fish processing factory environment, the fish raw materials are important sources of L. monocytogenes contamination in final products.

D- and z-values for a mixture of four Listeria monocytogenes strains originating from the roe of different fish species were determined in rainbow trout (Oncorhynchus mykiss) roe. The D60- and D63-values obtained were 1.60 and 0.44 min, respectively, and the z-value was 5.36 degrees C accordingly. In pilot-scale experiments, rainbow trout roe (100 g) was vacuum packaged into glass jars and pasteurized both at 62 and 65 degrees C for 10 min. These treatments were enough to destroy 10(8) CFU/ g of L. monocytogenes cells, which was the highest possible Listeria cell count to grow in roe. On the basis of the determined z-value and calculation of pasteurization values, these experimental pasteurizations were found to theoretically destroy at least 45 log units of L. monocytogenes cells in rainbow trout roe. In addition, these pasteurization treatments did not significantly affect the sensory quality of the roe. The sensory quality of pasteurized vacuum-packaged rainbow trout roe stored at 3 degrees C was evaluated as good after 6 months of storage and not statistically different from the control that was frozen from the same roe lot as the pasteurized roe samples. Pasteurization of rainbow trout roe was proven to be an appropriate method for ensuring product safety with regard to L. monocytogenes and to stabilizing the sensory and microbial quality of roe. However, the safety risk caused by spore-forming bacteria still exists in pasteurized roe. Therefore, it has to be stored below 3 degrees C.

The prevalence of Listeria monocytogenes in retail roe, as well as the microbiological and sensory qualities of the roe, were studied for three fish species under three different storage conditions. A total of 147 Finnish rainbow trout (Oncorhynchus mykiss), whitefish (Coregonus lavaretus), vendace (Coregonus albula), and burbot (Lota lota) roe samples were bought fresh, frozen, or frozen-thawed from Finnish retail markets. The overall prevalence of L. monocytogenes was 5%; however, the prevalence of the pathogen in fresh roe was 18%. Fresh-bought roe tested positive for Listeria spp. and for L. monocytogenes, respectively, 5 and 20 times as often as did frozen and frozen-thawed roe products combined. The microbiological quality (analyzed as total aerobic heterotrophic bacteria and coliform bacteria) of 78% of the roe samples was unacceptable. Frozen roe samples were found to have the best microbiological quality. According to the results of a sensory evaluation, at least one sensory attribute (appearance, odor freshness, texture, and freshness of taste) was unacceptable for 29% of the roe samples studied. The sensory quality of roe samples bought fresh was better than that of roe samples bought frozen or frozen-thawed. From the results of this study, it is concluded that both the microbiological and the sensory qualities of roe at the retail level need to be improved.

Abstract: A total of 510 rainbow trout originating from fish farms in lakes and sea areas around Finland were studied for the presence of Listeria monocytogenes. Samples were studied as pools from five fish. Gill, viscera, and skin from the pooled samples were analysed separately. The individual samples were analysed later if the pooled sample was found to be Listeria positive. The prevalence of Listeria spp. and L. monocytogenes in pooled unprocessed fresh rainbow trout was on average 35.0% and 14.6%, respectively. On the other hand, the prevalence of Listeria spp. and L. monocytogenes in individual thawed fish was found to be 14.3% and 8.8%, respectively. These numbers tend to overestimate and underestimate the real situation because not all fish in pooled samples were necessarily contaminated and in some of the Listeria positive pooled samples all individual samples turned out to be Listeria free. The prevalence of L. monocytogenes varied greatly between different fish farms from zero to 100% in pooled samples and from zero to 75% according to individually studied fish samples. Some indications of the influence of weather conditions and seasonal variations that strongly affected the Listeria contamination of fish were also noticed. The location of Listeria spp. and L. monocytogenes in different parts of the fish differed with statistical significance in rainbow trout. Up to 95.6% of the L. monocytogenes and 84.5% of Listeria spp. positive samples were gill samples. Only 4.4% (2/45) of the L. monocytogenes positive samples were obtained from skin or viscera. Closer study at one fish farm revealed that there was only one L. monocytogenes ribotype present in the contaminated fish, although water and surfaces were heavily contaminated with six other L. monocytogenes ribotypes. [Copyright &y& Elsevier]