Prevalence and growth of Listeria on naturally contaminated smoked salmon over 28 days of storage at 4 degrees C

Only limited data are available on the growth characteristics of Listeria in naturally contaminated ready-to-eat foods. To evaluate Listeria contamination patterns and growth in smoked salmon, 72 smoked salmon product samples from two processing plants were tested for Listeria spp. and L. monocytogenes. Samples were divided into four approximately equal portions: one portion was tested on receipt, and the other three were vacuum sealed and stored at 4 degrees C for 7, 14, and 28 days. Listeria testing was performed using both an enrichment procedure and direct plating to enumerate Listeria in samples that contained >2 to 10 CFU/g. Five samples were positive for Listeria spp., including one sample that was positive for L. monocytogenes. Most samples yielded only sporadic positive results among the portions tested on days 0, 7, 14, and 28. Only one sample contained Listeria spp. in numbers above the detection limit for enumeration. For this sample, the portions tested on days 7 and 28 contained 46 and 52 CFU/g, respectively, whereas the portion tested on day 14 was negative. Overall, our data indicate that there is considerable heterogeneity in Listeria spp. distribution within a single positive smoked fish sample. Even with refrigerated storage for 28 days, none of the naturally contaminated samples reached Listeria spp. numbers >100 CFU/g, which indicates that Listeria growth was limited within a 4-week storage period. However, because of the apparent heterogeneity of Listeria distribution within samples, the interpretation of growth data collected on naturally contaminated samples is difficult.