Growth of Listeria monocytogenes in Goat’s Pasteurized Milk Cheese During Maturation: Its Prediction from a Milk Model Medium

Previous research showed that a strain of Leuconostoc mesenteroides, isolated from goat’s raw milk cheese, was effective in slowing down the growth and reducing the maximum concentration of L. monocytogenes when evaluated in a milk model; and, furthermore, that the extent of inhibition was dependent on the milk initial pH. The objectives of this study were: (1) to determine whether the growth of L. monocytogenes in goat’s pasteurized milk cheese during maturation could be approximated from growth data obtained in the milk model medium, either in monoculture or in coculture with L. mesenteroides; and if so; (2) to model a milk-to-cheese conversion factor (Cf) for L. monocytogenes growth rate. Challenge tests were conducted by inoculating L. monocytogenes in monoculture and in coculture with L. mesenteroides in goat’s pasteurized milk adjusted at initial pH levels of 5.5, 6.0 and 6.5. The process of cheesemaking went on, and cheeses were ripened at 12 ºC during 12 days. Each experimental growth curve was adjusted to a pH-driven dynamic model where the microbial maximum growth rate is a function of pH. As observed in the milk model medium, in coculture with L. mesenteroides, the optimum growth rate (μ_opt) of L. monocytogenes in maturing cheese was affected by the initial pH of milk: the lowest rate of 0.863 ± 0.042 day^-1 was obtained at the initial pH 5.5, in comparison to 1.239 ± 0.208 and 1.038 ± 0.308 day-¹ at pH 6.0 and 6.5, respectively. Regardless of the milk initial pH, L. mesenteroides did not reduce the maximum load of L. monocytogenes in maturing cheeses, as it did in the milk medium. By contrary, at the milk initial pH of 5.5, 6.0, and 6.5, L. mesenteroides was able to decrease, on average, 2.2-fold, 1.5-fold and 1.9-fold the μ_opt of L. monocytogenes in both milk medium and cheese, without significant differences between matrices. Following such validation in goat’s cheese, the square-root of milk-to-cheese Cf for L. monocytogenes was estimated as 0.751 (SE=0.0108), and type of culture (monoculture, coculture) was not found to affect Cf (p=0.320). In conclusion, this work validated pre-acidification of milk as an efficient strategy that, when combined with the use of a protective culture, can synergically enhance the control of L. monocytogenes in cheese.