Effect of nisin on growth boundaries of Listeria monocytogenes Scott A, at various temperatures, pH and water activities

The effect of nisin on growth boundaries of Listeria monocytogenes Scott A in Tryptone Soy Broth (TSB) under different aws, pH, and temperatures was studied. Growth/no growth turbidity data was modeled using logistic regression. Combinations of various temperatures (5–35 °C), pH (4.05–6.70) adjusted with HCl, aws (0.937–0.998) NaCl (0.5–10.5%) and nisin (0–100 IU/ml) were used to monitor the growth/no growth response of L. monocytogenes Scott A for 60 days. The concordance of the logistic regression model was 99.4%, indicating successful data fitting. The minimum pH at which growth was observed was 4.81 at the temperature range of 25–35 °C and at aw as high as 0.992. Growth was observed at aw as low as 0.937, at pH 6.7, at the temperature range of 25–35 °C. Increasing nisin concentrations above 25 IU/ml resulted in a more inhibitory environment for L. monocytogenes. Presence of 100 IU/ml resulted in a minimum pH for growth at 5.20, and a minimum aw at 0.967 at the temperature range of 25–35 °C. It was remarkable that low to medium salt concentrations (2.5–4.5 NaCl% w/v) provided a protective effect against inhibition of L. monocytogenes by nisin. The present study points out the applicability of growth/no growth modeling in order to study any interactions between various factors affecting initiation of growth of micro-organisms, in which its turn helps the understudying of microbe–food ecosystem relations and the development of safer food.