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 CO2 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 CO2 as a result of their attenuated lactate dehydrogenase activity. The method used involved following the concentration of dissolved CO2 and the quantity of CO2 released into the gas phase in standard conditions. Five mutants with attenuated lactate dehydrogenase activity were tested in pure culture in milk. Total CO2 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. CO2 was released into the gas phase only after the concentration of dissolved CO2 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 CO2 production and the pH below which CO2 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 CO2 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 CO2 production for the formation of openings.