Factors moderating Listeria monocytogenes growth in raw milk and in soft cheese made from raw milk

Abstract

Listeria monocytogenes is a foodborne pathogenic bacterium sometimes found in raw milk. Raw milk contains natural bacterial inhibitors such as the lactoperoxidase system (LPS) and specific microflora. Six strains of L. monocytogenes isolated from raw milk in 1995 and 1996 in Normandy (France) were tested. The aim of the first part of this work was to evaluate the inhibitory effect of LPS on L. monocytogenes in milk. Kept at 15 °C for 65 h, in static conditions, populations of L. monocytogenes in pasteurized milk increased by 2 to 3.8 log depending on the strain. In raw milk, in the same conditions, populations increased by 0.8 to 2.3 log. Adding thiocyanate and hydrogen peroxide to raw milk (supplemented raw milk, SRM) enhanced its inhibitory effect. In SRM, three strains were unable to grow and the populations of the other strains increased by 0.7 to 1.3 log. The inhibitory effect of the LPS in milk was clearly demonstrated. The inhibitory effect of raw milk on L. monocytogenes was due to LPS, probably combined with the microbiological composition of raw milk. The aim of the second part of this work was to evaluate the inhibitory effect of using raw milk for making Camembert cheese (RMC). The results show that the growth of L. monocytogenes was about twice as slow in RMC as in Camembert made from pasteurized milk (PMC). The average lag phase (Lag) was 15 d in PMC and 34 d in RMC. Statistical analysis showed that the inhibitory effect of RMC on the growth of L. monocytogenes was mainly related to the microbiological composition of the raw milk, in terms of thermophilic Lactobacillus and yeast. Although our results did not clearly demonstrate an inhibitory effect of chemical composition of raw milk, inhibition of L. monocytogenes in RMC is probably due to the interrelationship between microbiological and chemical factors.