COMBINED EFFECT OF NISIN AND MODERATE HEAT ON DESTRUCTION OF LISTERIA-MONOCYTOGENES IN MILK

The kinetics of destruction of two strains of L monocytogenes in skim milk heated at 60 degrees C with and without addition of 25 or 50 IU/m l of nisin were studied. The survival curves displayed an initial lag phase followed by an accelerating killing phase. As both the length of the lag phase and the destruction rate depended on the temperature an d the presence of nisin, a mathematical model was necessary to compare the survival curves and to determine the time to achieve a given redu ction of the Listeria counts. Two models were compared, each allowing satisfactory goodness of fit. However, as they gave very diverging D-v alues, the time to achieve a given reduction (3 and 6 log(10)) of the numbers of Listeria in milk was calculated using one model. Addition o f 25 or 50 IU/ml of nisin to milk heated between 54 and 65 degrees C c onsiderably reduced the heal resistance of one of the two strains of L isteria so that the time needed to achieve a 3 and 6 log(10) reduction of populations of this bacterium was substantially diminished. For in stance, 16 min at 54 degrees C was sufficient to achieve a 10(3)-fold reduction of the number of Listeria in milk containing 25 IU/ml nisin, compared to 77 min in absence of nisin. The combined effect of heat a nd nisin was somewhat enhanced if the bacteria were previously grown i n milk at low temperatures. Factors that affect the heat resistance of Listeria are discussed as well as the possible mode of action of nisi n and heat on destruction of bacterial cells.