Inhibition, Injury, and Inactivation of Four Psychrotrophic Foodborne Bacteria by the Preservatives Methyl ρ-Hydroxybenzoate and Potassium Sorbate.

The minimum inhibitory concentrations (MICs) of methyl ρ-hydroxybenzoate (methyl paraben) and potassium sorbate for four psychrotrophic bacteria were compared at pH 5 and 6 and at 5 and 30°C. The bacteria tested were Listeria monocytogenes , Pseudomonas putida , Yersinia enterocolitica , and Aeromonas hydrophila . L. monocytogenes was generally the most resistant and A. hydrophila the least resistant to the preservatives. The differences between the bacteria were substantial. The MICs of the two preservatives were similar at pH 5, but at pH 6 the MICs of paraben were well below those of sorbate, except in the case of A. hydrophila . The MICs at 5°C were much lower than those observed at 30°C for all of the bacteria except P. putida . All four bacteria were inhibited by 1000 mg methyl paraben per L at 5°C. Exposure of the bacteria to concentrations of preservative that permitted growth at 30°C did not lead to adaptation to the preservative. The death rates of the bacteria in media containing 1000 mg methyl paraben per L varied over a wide range. At 5°C, a 3 log decrease in viable counts of L. monocytogenes and A. hydrophila took >4 months and a few days, respectively. Injury of L. monocytogenes , Y. enterocolitica , and A. hydrophila was detected under these conditions. Repair of the injury was demonstrated, with up to 24 h required for complete recovery. The type of buffer in which the test medium was prepared affected the preservative MICs and rate of injury of L. monocytogenes .