INTERACTION OF CITRIC-ACID CONCENTRATION AND PH ON THE KINETICS OF LISTERIA-MONOCYTOGENES INACTIVATION

The effects and interactions between pH and citric acid concentration on the inactivation of Listeria monocytogenes was determined using a three-strain mixture. Citric acid/sodium citrate combinations were added to brain heart infusion (BHI) broth to achieve concentrations of 0.1, 0.5, 1.0 and 2.0 M in conjunction with pH values of 4, 5, 6 and 7. The media were dispensed in 20-ml portions in dilution bottles, inoculated to approximately 10(8) CFU/ml, and incubated at 28-degrees-C. Survivor curves were generated using a linear model incorporating a lag term, and D-values and ''time to 4-D inactivation'' values were calculated. The results were compared against control cultures in which the pH was modified using hydrochloric acid (HCl). The rate of inactivation was dependent on both the pH and concentration of citric acid. Low levels of citric acid were protective, particularly at pH 5 and 6. At higher concentrations, a distinct anion effect was observed as compared to the HCl controls, with inactivation rates being correlated with the completely undissociated form of the acid. Comparison of the kinetic data with earlier results with lactic and acetic acids suggests that citric acid has both protective and bactericidal activity against L. monocytogenes, which involve different modes of action.