Predictive thermal inactivation model for Listeria monocytogenes with temperature, pH, NaCl, and sodium pyrophosphate as controlling factors

The effects and interactions of heating temperature (55 to 65 degrees C), pH (4 to 8), salt (NaCl; 0 to 6%, wt/vol), and sodium pyrophosphate (SPP; 0 to 0.3%, wt/vol) on the heat inactivation of a four-strain mixture of Listeria monocytogenes in beef gravy were examined. A factorial experimental design comparing 48 combinations of heating temperature, salt concentration, pH value, and SPP content was used. Heating was carried out using a submerged-coil heating apparatus. The recovery medium was plate count agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. Decimal reduction times (D-values) were calculated by fitting a survival model to the data with a curve-fitting program. The D-values were analyzed by second-order response surface regression for temperature, pH, NaCl, and SPP levels. Whereas increasing the NaCl concentration protected L. monocytogenes against the lethal effect of heat, high SPP concentrations increased heat sensitivity. Also, low pH values increased heat sensitivity of L. monocytogenes. The four variables interacted to affect the inactivation of the pathogen. Thermal resistance of L. monocytogenes can be lowered by combining these intrinsic factors. A predictive model that described the combined effect of temperature, pH, NaCl, and SPP levels on thermal resistance of L. monocytogenes was developed. The model can predict D-values for any combination of temperature, pH, NaCl, and SPP that are within the range of those tested. Using this predictive model, food processors should be able to design adequate thermal regimes to eliminate L. monocytogenes in thermally processed foods.