Effect of a Lactobacillus sakei (LAK-23) culture on growth of Listeria monocytogenes intentionally inoculated into fillets of cold smoked sea bass vacuum packed and stored at 6°C

The aim of the work was to monitor the presence of Listeria monocytogenes in cold smoked fish products (trout, salmon and sea bass) marketed in Italy. Cold smoked sea bass represent a new product, not yet commercialized and was collected in a production facility. Furthermore, considering the ability of L. monocytogenes to grow in these products, due to their physico-chemical characteristics (pH > 6.0, Aw> 0.97) and their long shelf-life at 4°C, it was verified the activity of a bioprotective starter, LAK-23 (Sacco srl, Via Alessandro Manzoni 29/A, 22071 Cadorago, CO, Italy). In this regard, smoked sea bass were intentionally inoculated with a mixture of different strains of L. monocytogenes and LAK-23, consisting of Lactobacillus sakei. After inoculation, the smoked sea bass were vacuum-packed and stored at 6/7 °C for 60 days. At 0.15,30,45 and 60 days the samples of sea bass were analyzed to evaluate the effectiveness of the starter against L. monocytogenes. The data of the monitoring showed that the cold smoked products sold in the supermarkets can be contaminated by this microorganism, the presence of which has been highlighted mainly through enrichment culture (presence in 25 g). The isolated listeria have been serotyped and belong mainly to low virulent serotypes (1/2c), following serotypes 1/2a, 1/2b and 4b. The addition of the starter, consisting of lactic bacteria, prevented its growth and at the end of the shelf-life, the amount of L. monocytogenes observed was similar to that of the inoculum. The use of the tested starter can allow to include cold smoked sea bass or smoked fish products in category 1.3 (Reg. EC 2073/05) among the products that do not support the growth of this microorganism. Finally the activity of the starter did not produce off-flavor or odor of the smoked sea bass.