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

Marielle Gay , Albert Amgar

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.

Facteurs ralentissant la croissance de Listeria monocytogenes dans le lait cru et dans les fromages à pâte molle au lait cru. Six souches de L. monocytogenes isolées de lait cru en 1995 et 1996 (Normandie) sont utilisées. L'effet du système lactopéroxydase (LPS), inhibiteur de croissance microbienne naturellement présent dans le lait cru, sur L. monocytogenes, est évalué. Dans le lait pasteurisé incubé 65 h à 15 °C, la population de L. monocytogenes, augmente de 2 à 3,8 log selon les souches. Dans le lait cru, la population augmente de 0,8 à 2,3 log. L'ajout de thiocyanate et de péroxyde d'hydrogène au lait cru (SRM) accroît son effet inhibiteur. Dans le SRM, trois souches ne se développent pas ; pour les autres, la population augmente de 0,7 à 1,3 log. L'effet inhibiteur du lait cru sur L. monocytogenes est lié à la présence de LPS, et probablement à la composition microbiologique du lait cru. La croissance de L. monocytogenes dans le Camembert est également étudiée. Les résultats montrent que L. monocytogenes se développe environ deux fois plus lentement dans le Camembert au lait cru (RMC) que dans celui au lait pasteurisé (PMC). Les durées moyennes des phases de latence sont de 15 et 34 j, respectivement dans le PMC et le RMC. L'effet inhibiteur du RMC est principalement lié à la composition microbiologique du lait cru, en particulier la présence de lactobacilles thermophiles et de levures.

Mots clés: Listeria monocytogenes; inhibition; raw milk; lactoperoxidase system<br>---<br>Listeria monocytogenes; lait cru; lactopéroxydase

Domaines: Alimentation et Nutrition; Ingénierie des aliments

Marielle Gay, Albert Amgar. Factors moderating Listeria monocytogenes growth in raw milk and in soft cheese made from raw milk. Le Lait, 2005, 85 (3), pp.153-170. ⟨hal-00895558⟩