BEHAVIOR OF LISTERIA-MONOCYTOGENES IN THE PRESENCE OF SODIUM PROPIONATE TOGETHER WITH FOOD ACIDS

Tryptose broth (TB) containing 0.00, 0.05, 0.15, or 0.3% sodium propionate was adjusted to pH 5.0 or 5.6 with acetic, tartaric, lactic, or citric acid, inoculated to contain ca. 10(3) CFU of Listeria monocytogenes/ml and incubated at 13 or 35-degrees-C. The bacterium grew in all controls (free of propionate) under all conditions; however, only slight growth occurred at 13-degrees-C when the pH was adjusted to 5.0 with acetic or tartaric acid. Growth also occurred at 13 and 35-degrees-C when TB adjusted to pH 5.6 with acetic or tartaric acid contained 0.05 or 0.15% propionate, but 0.3% inhibited growth of the pathogen. When the pH of TB was adjusted to 5.0 with the same acids, L. monocytogenes was inhibited or inactivated by 0.15 or 0.3% propionate. The pathogen grew at 13 or 35-degrees-C in TB that contained 0.05 or 0.15% propionate and was adjusted to pH 5.6 with lactic or citric acid although the lag phase was prolonged as the concentration of propionate was increased. Under these conditions, propionate at 0.3% occasionally inhibited growth of L. monocytogenes. Growth was reduced and sometimes inhibited completely by 0.15 or 0.3% propionate when the pH of TB was adjusted to 5.0 with the same acids.