Control of Listeria monocytogenes in raw-milk cheeses

The development of Listeria monocytogenes in cheeses made with raw-milk originating from six different farms and according to the Saint-Nectaire cheesemaking technology was studied. Milk was inoculated with two strains of L. monocytogenes at 5 to 10 CFU/25 ml. Microbial and chemical analyses were carried out at appropriate intervals during ripening. L. monocytogenes did not grow in the cores of cheeses prepared with milk originating from three farms. That inhibition could be partially attributed to the pH values and l-lactate content. There was no growth in cheeses with pH below 5.2 and lactate content around 14 mg/g. In all cheeses, L. monocytogenes stopped growing in the cores of cheeses after eight days and some other factors may be involved in the inhibition. No relation was found between L. monocytogenes count and other microbial counts. Growth occurred on cheese surfaces between eight and eighteen days, when the pH significantly increased. The lowest L. monocytogenes growth was found on the surface of cheeses with the lowest pH and without any core growth. Further studies will be performed to clarify the involvement of the microbial community in L. monocytogenes inhibition, in particular during the ripening period.