Evolution de la flore microbienne à la surface des Camemberts fabriqués avec du lait cru

Abstract

Evolution of the microbial flora of the surface of Camembert cheese made from raw milk The microbial flora of the surface of Camembert made from raw milk in five factories has been studied 5 times over one year. As a rule, the manufacturing milk was prepared with at least 50 per cent of milk not cooled at the farn. In addition, an attempt of making cheese with 100 per cent refrigerated milk was carried out in one factory. Salt tolerant bacteria (Mannitol Salt Agar), coliform bacteria, yeasts and lactic acid bacteria were enumerated at taking out from moulds and at approximatively one week intervals up to the end of the ripening period (6th weeks). About 600 orange opaque colonies were isolated from Mannitol Salt Agar and characterized using 10 tests : cell morphology after one day of growth in Brain Heart Infusion Broth, colony colour change in presence of 5N NaOH, utilization of 4 amino acids (Ser, Met, Phe, Orn) and resistance to 4 antibiotics. A collection of coliform bacteria representative of all stages of ripening were submitted to 12 tests (growth at 7, 41 and 45°C ; acid production from lactose, sorbitol, adonitol, salicine, rhamnose and inositol ; indol production ; hydrolysis of tributyrin and casein) for presumptive identification. In addition, the evolution of the microbial flora was followed by scanning electron microscopy. The largest differences between factories were observed in nature and / or level of salt tolerant bacteria. In one factory, the growth of this flora started earlier and attained a higher level on cheese than on those made in the other factories. Comparatively, smaller difference in nature or level of the flora was observed between seasons. The lower level of lactic acid bacteria were observed when 100 per cent refrigerated milk was used. Usually, the orange pigmented colonies were coryneform bacteria able to utilize the amino acids and particularly methionin (to produce methanethiol, a precursor of sulfur compounds which play an important role in the flavour of this type of cheese). In one factory, orange pigmented cocci (Staphylococcus saprophyticus group) were sometimes dominant. These bacteria were generally unable to utilize amino acids. Coliform bacteria attained the highest level on cheese surface at taking out from moulds (E. coli, and lactose-positive strains of Hafnia alvei dominating) and at the end of the ripening period (lactose-positive and lactose-negative strains of H. alvei dominating) with a minimum after 1 week. Scanning electron microscopy revealed the nature of the association between micro-organisms and theyr physiological state. Conditions of growth of bacteria and possible interaction between the micro-organisms found on the surface of Camembert are discussed.