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

ABSTRACT

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 1x10[4] 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[degree sign] C] storage