Dynamic Modelling of Listeria monocytogenes Growth in a Milk Model Medium as Affected by pH and Selected Lactic Acid Bacteria Strains

Previous research showed that three strains of lactic acid bacteria (LAB)-<i>Leuconostoc mesenteroides</i>, <i>Lacticaseibacillus paracasei</i> and <i>Loigolactobacillus coryniformis</i>-exhibited in vitro anti-listerial activity. The objectives of the present study was: (1) to determine whether the initial pH of heat-treated reconstituted milk affects the growth of <i>Listeria monocytogenes</i>, in monoculture and coculture with each of the LAB strains; and (2) to compare the capacity of the LAB strains to inhibit <i>L. monocytogenes</i> in the milk model. Monoculture and coculture challenge tests were conducted in milk adjusted to three initial pH levels of 5.5, 6.0, and 6.5. A pH-driven model and a Jameson-effect model were fitted to the growth curves. The former provided more precise estimates than the latter. In monoculture, <i>L. mesenteroides</i> exhibited the fastest development at all initial pH levels whereas, in coculture, this strain more effectively controlled <i>L. monocytogenes</i> by reducing its growth rates. As the initial milk pH increased, the maximum concentration of <i>L. monocytogenes</i> in monoculture and in coculture also increased, although the LAB strains were all able to reduce the pathogen's maximum concentration. These findings demonstrated that adjusting milk to a more acidic pH before fermentation provides an additional barrier against the development of <i>L. monocytogenes</i>.