Effects of nisin and thyme essential oil, individually and in combination, on inoculated populations of Listeria monocytogenes in minced silver carp

Food poisoning caused by Listeriamonocytogenes results in death in 30% of the cases. Considering the high probability of L. monocytogenes contamination of local fish, the present study aimed at investigating the effects of thyme (Zataria multiflora Boiss) essential oil (EO) and nisin, individually and in combination, on the growth of L. monocytogenes in minced silver carp during refrigerated storage. Minced fish samples were inoculated with 1×104 cfu/g of L. monocytogenes. A total of 11 samples were inoculaetd with thyme EO at a concentration (weight/volume)of 0.3 %, 0.8 % or 1.2%, nisin at a concentration of 500 or 1000 IU/g, or a combination of the two. The treated and control samples were packaged in plastic bags and kept at refrigerator temperature for 12 days.. Samples were cultured on CHROMagarTM Listeria every 2 days and the bacteri counted. Nisin at two different levels (500 and 1000 IU/g)could not inhibit the growth of L.monocytogenes to a level below the acceptable level in raw food (100 cells/g). The antibacterial activity of nisin decreased during the storage period, while simultaneous use of nisin and thyme EO at a concentartion of 0.8 and 1.2% reduced L. monocytogenes viable count to a level below the acceptable limit during 12 days. A combination of 0.8% thyme (weight/volume)and 1000 IU/g nisin has the best inhibitory effect on growth of L. monocytogenes in minced silver carp during cold (4C) storage.