SYNERGISTIC EFFECT OF NISIN AND THE LACTOPEROXIDASE SYSTEM ON LISTERIA-MONOCYTOGENES IN SKIM MILK

Nisin added at 10 or 100 IU/ml to ultra-high temperature processed (UH T) skim milli had no effect on counts of Listeria monocytogenes after 34 h at 30 degrees C, whereas addition of the lactoperoxidase system ( LPS) resulted in counts of viable cells three log units lower than tho se of control milk after 24 h at 30 degrees C. Addition of nisin and L PS showed a synergistic effect and resulted in counts up to 5.6 log un its lower than the control milk. When the two preservatives were added to actively growing cells of L. monocytogenes in two steps with a 2 h interval, their synergistic effect was enhanced. Counts of L. monocyt ogenes Ohio after 24 h at 30 degrees C in milk with nisin and LPS adde d together after 3 h of growth were 5.7 log units lower than the contr ol milk. The difference in counts increased to 7.4 log units if LPS wa s added after 3 h and nisin after 5 h of growth. Similar but less pron ounced effects were observed for the more resistant strain L. monocyto genes Scott A. (C) 1998 Elsevier Science B.V.