Modelling and Predicting the Simultaneous Growth of Listeria Monocytogenes and Spoilage Micro-organisms in Cold-smoked Salmon

Aims: To evaluate and model the simultaneous growth of Listeria monocytogenes and spoilage micro-organisms in cold-smoked salmon.

Methods And Results: Growth kinetics of L. monocytogenes, lactic acid bacteria (LAB), Enterobacteriaceae, enterococci and Photobacterium phosphoreum were determined in two series of challenge tests with sliced and vacuum-packed cold-smoked salmon (SVP-CSS). The product contained a high level of smoke components and at 2 degrees C levels of L. monocytogenes increased <100-fold in 193 days. Without the addition of spoilage micro-organisms, L. monocytogenes reached ca 108 CFU g-1 at 5, 10, 17.5 and 25 degrees C. Inoculation with spoilage micro-organisms reduced this level to 102-104 CFU g-1. LAB dominated the spoilage microfora of SVP-CSS and competition between LAB and L. monocytogenes in SVP-CSS was appropriately described by a simple expansion of the Logistic model. This interaction model aided in predicting the growth of L. monocytogenes in naturally contaminated SVP-CSS when it was used in combination with expanded versions of existing secondary models for L. monocytogenes and LAB.

Conclusions: Temperature, water activity/NaCl, simultaneous growth of LAB, smoke components and to a lesser extent lactate and pH control growth of L. monocytogenes in SVP-CSS. These factors must be included in mathematical models to predict growth of the pathogen in this product.

Significance And Impact Of The Study: The suggested predictive model can be used to support assessment and management of the human health risk due to L. monocytogenes in SVP-CSS.