STUDY OF THE CITRATE METABOLISM OF LACTOCOCCUS-LACTIS SUBSP LACTIS BIOVAR DIACETYLACTIS BY MEANS OF C-13 NUCLEAR-MAGNETIC-RESONANCE

The metabolic fate of citrate and pyruvate in four strains of Lactococcus lactis subsp. lactis biovar diacetylactis has been studied by means of C-13 nuclear magnetic resonance, using as a substrate either [3-C-13]pyruvic acid or custom-synthesized citric acid that is C-13 labeled either at carbons 2 and 4 or at carbon 3. The fermentations were carried out batchwise in modified M17 broth. For the actual conversions of the C-13-labeled substrates, cells at the end of their logarithmic growth phase were used to minimize the conversion to lactic acid. A mass balance of the main citric acid metabolites was obtained; the four strains produced from 50 to 70% (on a molar basis) lactic acid from either citrate or pyruvate. The remaining 50 to 30% was converted mainly to either alpha-acetolactic acid (for one strain) or acetoin (for the other three strains). One of the strains produced an exceptionally high concentration of the diacetyl precursor alpha-acetolactic acid. Another strain (SDC6) also produced alpha-acetolactic acid, but this was decarboxylated to acetoin at a high rate. The C-13 nuclear magnetic resonance method confirmed that the biosynthesis of alpha-acetolactic acid occurs via condensation of pyruvate and "active" acetaldehyde. Diacetyl was not found as a direct metabolite of citrate or pyruvate metabolism.