Interactions between pH and malic acid concentration on the inactivation of Listeria monocytogenes

The effects and interactions of malic acid concentration and pH on the inactivation kinetics of a three strain mixture of Listeria monocytogenes was studied in brain heart infusion broth (BHI). The medium was supplemented with malic acid and monosodium malate to achieve pH levels of 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 or 7.0 in conjunction with concentrations of 0.0, 0.1, 0.5, 1.0 and 2.0 M. Duplicate 20-mL portions of each pH/malate level were inoculated (ca 10(8) cfu/mL), stored aerobically at 28C, and assayed periodically for viable counts by plating on BHI agar. Survivor curves were generated by fitting data to a linear model that includes a lag term and used to calculate D-values and "times to 4-D inactivation". Inactivation rates were dependent on both the pH and malic acid concentration. At the higher pH levels, malic acid appeared to provide some degree of protection compared to control cultures where the pH was adjusted with HCl. At lower pH values and at higher malic acid concentrations, a concentration-dependent anion effect was observed The results indicate that malic acid is a relatively benign organic acid. Its antimicrobial characteristics are similar to those of citric acid and is substantially less bactericidal than lactic or acetic acids.