Production of carbon dioxide by Lactococcus lactis strains with attenuated lactate dehydrogenase activity, in pure cultures and in mixed cultures with an acidifying strain

Abstract

-Lactic acid bacteria that produce CO 2 are used in the dairy industry to favor the formation of openings in some types of cheeses. This work was undertaken to characterize the gas producing activity of Lactococcus lactis strains that over-produce CO 2 as a result of their attenuated lactate dehydrogenase activity. The method used involved following the concentration of dissolved CO 2 and the quantity of CO 2 released into the gas phase in standard conditions. Five mutants with attenuated lactate dehydrogenase activity were tested in pure culture in milk. Total CO 2 production after 24 h of culture varied between 47 and 88 mmol . L -1 , while that of the parental strain was only 17 mmol . L -1 . CO 2 was released into the gas phase only after the concentration of dissolved CO 2 reached the saturation point in the culture medium, i.e. about 29 mmol . L -1 . Its release was then linear vs. pH. Co-culturing these mutants with an acidifying strain of L. lactis increased the rate of acidification of cultures and reduced CO 2 production and the pH below which CO 2 was released into the gas phase. This effect increased with the size of the inoculum of the acidifying strain. These results show that very different changes in pH and CO 2 production can be obtained by using strains L. lactis strains with attenuated lactate dehydrogenase activity and by the conditions of combining these strains with an acidifying Lactococcus. For a given cheese technology, this could lead to a more precise determination of the optimal characteristics of acidification and CO 2 production for the formation of openings. caractériser l'activité gazogène de souches de Lactococcus lactis capables de surproduire du CO 2 du fait de l'atténuation de leur activité lactate déshydrogénase. Pour cela, une méthode permettant de suivre, dans des conditions standardisées, la concentration en CO 2 dissous et la quantité de CO 2 libéré en phase gazeuse, a été utilisée. Cinq mutants ayant une activité lactate déshydrogénase atténuée ont été testés en culture pure dans du lait. Leur production totale de CO 2 après 24 h de culture était variable, et comprise entre 47 et 88 mmol . L -1 , alors que celle de la souche parentale n'était que de 17 mmol . L -1 . Le CO 2 n'était libéré dans la phase gazeuse qu'à partir du moment où la concentration en CO 2 dissous atteignait la saturation du milieu de culture, soit approximativement 29 mmol . L -1 . Sa libération évoluait ensuite de façon linéaire par rapport au pH. L'association de ces mutants avec une souche de L. lactis acidifiante augmentait la vitesse d'acidification des cultures et diminuait la production de CO 2 ainsi que le pH à partir duquel le CO 2 est libéré dans la phase gazeuse. Cet effet était d'autant plus important que le niveau d'ensemencement de la souche acidifiante était élevé. Ces ré-sultats montrent qu'en agissant sur le choix des souches de L. lactis ayant une activité lactate déshy-drogénase atténuée, ainsi que sur les conditions d'association de ces souches avec un lactocoque acidifia...

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