A model based on absorbance data on the growth rate of Listeria monocytogenes and including the effects of pH, NaCl, Na-lactate and Na-acetate.

A mathematical model was developed for predicting the growth of L. monocytogenes at 9 degrees C in the presence of 70 ppm sodium nitrite, and at different levels of pH (5.5-6.5), sodium chloride (1.0-4.0%), sodium lactate (0-0.5%) and sodium acetate (0-0.6%). Collection of the growth data was done using absorbance measurements in broth cultures and the absorbance measurement was evaluated. The model was compared to the Food MicroModel, and against the growth of L. monocytogenes in a vacuum-packed meat product stored at 9 degrees C. A linear relationship was obtained, for the absorbance data on different dilutions of the inoculum, in the absorbance interval studied. There was also a linear relationship between the values of the maximum specific growth rates derived from the absorbance and the ones derived from viable count measurements; and corrections were made accordingly. The statistical evaluation showed that all the main factors, i.e. pH, sodium chloride, sodium lactate and sodium acetate were statistically significant for the growth rate of L. monocytogenes. Comparison to the Food MicroModel (FMM) showed a slight underprediction for the developed model (bias = 0.84). The predictions were, on average, within 20% of the FMM predictions (n = 10). Validation against the observed growth of L. monocytogenes inoculated into an emulsion type of sausage (n = 4) also showed a slight underprediction by the model. The predictions were, on average, 16% below the observed values in the sausage (Bias 0.84, Accuracy 1.26).