The effect of temperature, pH, and monolaurin on the growth and survival of Listeria monocytogenes in tryptic soy broth with yeast extract (TSBYE) was investigated. TSBYE medium containing 5, 6, 7, 8, or 9 μg/ml monolaurin was adjusted to pH 5.0, 5.5, or 7.0, inoculated with L. monocytogenes, and incubated at 7, 15, or 35°C. The effect of sublethal concentrations of monolaurin was measured by calculating generation times. Results indicated that temperature and pH were very important factors influencing the efficacy of monolaurin against L. monocytogenes. Bactericidal effects were observed at pH 5.0 but not observed at pH 5.5 or pH 7.0. Further, lethal effects of monolaurin increased as temperature increased at constant pH. Conversely, bacteriostatic effects on growth increased as temperature decreased at constant pH. At constant temperature the bactericidal and bacteriostatic effects of monolaurin increased as the pH of the medium decreased. Generation time of the organism as significantly (P < 0.05) influenced by pH, temperature, concentration of monolaurin, or their combined interactions. Thus, the results indicate that pH and temperature interact to affect the antimicrobial potential of monolaurin against L. monocytogenes.
1Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript No. 92-21-6373.