Response Surface Methodology, an approach to predict the effects of a lactoperoxidase system, Nisin, alone or in combination, on Listeria monocytogenes in skim milk

Experimental designs using Response Surface Methodology (RSM) were used to determine effects and interactions of Nisin (0-200 IU ml(-1)) values (5.4-6 .6), incubation time (0 36 h or 0-144 h) and the lactoperoxidase-thiocyanat e-hydrogen peroxide system (LPS) on Listeria monocytogenes CIP 82110 in ski m milk, at 25 degrees C. The LPS varied from level 0-2; LPS at level 1 cons isted of lactoperoxidase (35 mg l(-1)), thiocyanate (25 mg l(-1)) and H2O2 which was supplied exogenously by glucose-oxidase (1 mg l(-1)) and glucose (0.2 g l(-1)); LPS activity was dependent on LPS level and incubation time. In the presence of LPS at level 1, a bacteriostatic phase was followed by growth, whereas at a higher level, a bactericidic phase was observed. Nisin response was time- and pH-dependent. Nisin was bactericidic at acidic pH v alues and for a short incubation time (12 h) only; then, a re-growth phase was observed. Nisin and LPS in combination gave an original response which lacked the transitory bactericidal effect of Nisin and had a continuously b actericidal affect, leading to 10 cfu ml(-1) of L. monocytogenes at 144 h; the response was greatly affected by incubation time. Predicted values were in good agreement with experimental values. Response Surface Methodology i s a useful experimental approach for rapid testing of the effects Of inhibi tors.