Expanded models for the non-thermal inactivation of Listeria monocytogenes

Previously developed four-variable response surface models for describing the effects of temperature, pH/lactic acid, sodium chloride and sodium nitrite on the time to achieve a 4-log, non-thermal inactivation (t4D)of Listeria monocytogenes in aerobic, acidic environments were expanded to five-variable models that distinguish the effects of pH and acidulant concentration. A total of 18 new variable combinations were evaluated and the inactivation kinetics data appended onto a consolidation of two data sets from earlier studies. The consolidated data set, which included 315 inactivation curves representing 209 unique combinations of the five variables, was analysed by response surface analysis. The quadratic model without backward elimination regression was selected for further evaluation. Three additional quadratic models were generated using the concentrations of undissociated lactic and/or nitrous acids as variables in place of percentage lactic acid and sodium nitrite concentration. Comparison of predicted t4D values against literature values for various food systems indicated that the models provide reasonable initial estimates of the inactivation of L. monocytogenes. The models based on the concentration of undissociated lactic and nitrous acids support the hypothesis that antimicrobial activity is associated with this form of the compounds. Evaluation of several examples suggests that these models may be useful for predicting the equivalent of the compounds' 'minimal inhibitory concentrations' for accelerating inactivation under various conditions.