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

Abstract

Previous research showed that three strains of lactic acid bacteria (LAB)—Leuconostoc mesenteroides, Lacticaseibacillus paracasei and Loigolactobacillus coryniformis—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 Listeria monocytogenes, in monoculture and coculture with each of the LAB strains; and (2) to compare the capacity of the LAB strains to inhibit L. monocytogenes 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, L. mesenteroides exhibited the fastest development at all initial pH levels whereas, in coculture, this strain more effectively controlled L. monocytogenes by reducing its growth rates. As the initial milk pH increased, the maximum concentration of L. monocytogenes 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 L. monocytogenes.