-The growth of propionic acid bacteria was directly dependent on the concentration of free amino acids in cheese, regardless of the strains of lactobacilli contained in the raw mixed cultures, suggesting that the growth of these bacteria was mainly dependent on the proteolytic activity of lactic acid bacteria. The stimulating effect of S. thermophilus was poor probably because this species produces only low amounts of amino acids in milk or cheese. Lactobacilli, which undergo cell Iysis shortly after cheese manufacture, produce large amounts of amino acids. Facultatively heterofermentative lactobacilli were shown to reduce the risk of secondary fermentation in Emmental cheese. This interaction was not due to their production of diacetyl, formate or acetate. Propionibacteria able to grow at 14 "C have a greater tendency to induce secondary fermentation compared to those unable to grow at this temperature. © InralElsevier, Paris.propionic acid bacteria / lactic acid bacteria / facultatively heterofermentative Iactobacilli / interaction / secondary fermentation Résumé -Interaction entre les bactéries propioniques et les bactéries lactiques thermophiles. L'étude présentée a porté sur l'interaction entre les bactéries propioniques (BP) et 23 levains lactiques, 30 de leurs souches isolées, 36 souches isolées de produits laitiers fabriqués hors de la Suisse, une culture de S. thermophilus ainsi que la flore naturelle de deux laits de mélange. La croissance des bactéries lactiques a été réalisée dans des expériences de simulation de fabrication fromagère dans du lait UHT, dont le pH a été maintenu au niveau de celui du fromage par l'ajout de carbonate de calcium. Les lactosérums obtenus ont été utilisés comme source d'azote pour la croissance des BP. Les résul-tats ont montré que l'intensité de la croissance des BP variait en fonction du lactosérum utilisé. Toutefois, les différences observées ne dépendaient pas spécifiquement de la souche des bactéries lactiques, mais uniquement de la concentration en acides aminés. Les différences entre les intensités de croissance ont pu être supprimées en uniformisant la concentration des acides aminés ajoutés, ce qui suggère que la croissance des BP dépend essentiellement de l'activité protéolytique des bactéries lactiques. L'effet stimulant de S. thermophilus sur la croissance des BP a été peu prononcé à cause de la faible concentration des acides aminés libérés. Par contre, les lactobacilles, dont la lyse cellulaire apparaît rapidement après la fabrication du fromage, produisent de grandes quantités d'acides aminés qui stimulent la croissance des BP. Il a été démontré que l'addition de lactobacilles hétérofermentaires Oral communication at the 2nd Symposium on Propionibacteria, Cork, Ireland, June 25-27, 1998.

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The heat denaturation of a-lactalbumin (a-la) in NaCl and KC1 solutions, milk ultrafiltrate and milk was studied using the method of micro complement fixation. It was established that this protein was very resistant to heat denaturation and that it was more stable in milk ultrafiltrate than in the other media studied at temperatures up to 70 °C. Of the various milk proteins added to a-la, only /?-lactoglobulin (/?-lg) formed a heat-induced complex with this protein. This complex was identical in milk ultrafiltrate or in milk and depended on the molar ratio between both proteins; it was not modified by any other milk proteins. The binding of a-la to yff-lg changed the ability of the latter protein to bind /c-casein.The heat-induced interactions between /Mactoglobulin (/?-lg) and casein have been one of the predominant subjects investigated in dairy chemistry during the last 20 years. In contrast, only a limited number of studies has been devoted to heatinduced complex formation between a-lactalbumin (a-la) and other milk proteins. Hunziker & Tarassuk (1965), however, were able to demonstrate that an interaction occurred between a-la and /?-lg when the proteins were heated together in a buffer at pH 6-7. It should be emphasized that most of the physico-chemical analyses performed on heat-treated milk proteins have been done in buffers, at protein concentrations often different from those found in milk; also it has been uncertain whether results so obtained could be compared to reactions in the original milk medium. Therefore, in the present study, we have compared the immunochemical behaviour of a-la in milk and milk ultrafiltrate, as well as in isolated systems after heat treatment. MATERIALS AND METHODS Reagentsa-Ladalbumin. a-Lactalbumin was prepared from bulk raw milk by gel filtration and ion-exchange chromatography by the method of Armstrong et al. (1970). Only the highest of the 3 peaks eluted from DEAE-Sephadex was used. The homogeneity of the preparation was checked by polyacrylamide-gel (PAG) disc electrophoresis and by immunoelectrophoresis using a pool of antisera against the whey proteins.

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