Freezing effects on survival of Listeria monocytogenes in artificially contaminated cold fresh-salmon

Abstract

The effects of freezing at -20 °C were examined on surviving of Listeria monocytogenes contaminating food during freezing. Slices of fresh salmon were inoculated with three strains of Listeria monocytogenes (10 8 CFU/g). The inoculated pieces of fish were dried for 1 h at 24 °C and then stored in closed containers at -20 °C for ten months. The identification of atypical cells found after freezing was achieved by PCR. This technique was based on the amplification of two specific genes of L. monocytogenes hlyA and iap. The results showed that after ten months of frozen storage, the concentration of L. monocytogenes determined on Trypticase Soya agar-Yeast Extract had declined by 2.39 ± 0.01 log CFU/g and 2.22 ± 0.01 log CFU/g for the strains isolated from meat, S1 and S2 respectively, and by 3.69 ± 0.03 log CFU/g for the reference strain. The effect of frozen storage of L. monocytogenes in salmon did not decrease the potential survival of food-borne pathogens' over a period of ten months.