Amino acid catabolism by Lactococcus lactis during milk fermentation

Two wild-type Lactococcus lactis strains isolated from naturally Tunisian fermented milk (Leben), and one laboratory strain, were used to investigate the ability of L. lactis to transform amino acids into aroma compounds during milk fermentation. The α-ketoacid acceptor used for leucine transamination, the first step of catabolism, was identified by gas chromatography/mass spectrometry analysis of the 15N-labelled amino acids that formed from 15N-labelled leucine in fermented milk. Furthermore, the amino acids produced or catabolized by the laboratory strain via transamination were identified by comparing the free amino acids in milk fermented with the wild-type strain and the double mutant for aromatic and branched-chain aminotransferases, which cannot transaminate amino acids. The three L. lactis strains strongly catabolized leucine and valine during milk fermentation. The principal amino acid formed via leucine and valine transamination was glutamate indicating that α-ketoglutarate was the principal α-ketoacid acceptor and was generated during milk fermentation.