Characterization of metabolic pathways for biosynthesis of the flavor compound 3-methylbutanal by Lactococcus lactis.

3-Methylbutanal is a key volatile compound that imparts a nutty flavor to Cheddar cheese. Lactococcus lactis has been successfully applied as a starter to increase the level of 3-methylbutanal produced during the ripening of cheese. However, the mechanism of action and genetic diversity of this bacterium for 3-methylbutanal biosynthesis remains unclear. In this study, we investigated the association between the L. lactis genotype and phenotype in the biosynthesis of 3-methylbutanal via both direct and indirect pathways. Fourteen strains of L. lactis were screened for the capacity to produce 3-methylbutanal, and strain 408 (>140 μ M) produced the highest among all tested strains, which exhibited both α-keto acid decarboxylase and α-ketoacid dehydrogenase activities. Furthermore, the results of a sodium meta-arsenite inhibition experiment showed that the 3-methylbutanal–producing capacities of each strain declined to various degrees. The kdcA gene, which encodes the direct pathway component α-ketoacid decarboxylase, was detected in 4 of the 14 strains, of which only strain 408 contained the full-length gene. We then characterized the genes associated with the indirect pathway by detecting the expression levels of the pdh gene cluster, ack , and pta , which were expressed at relatively higher levels in a high-yield strain than in a low-yield strain. As a result, these L. lactis strains were divided into 3 categories according to gene diversity, gene expression, and 3-methylbutanal production. The results of this study refine our knowledge of the genetic determinants of 3-methylbutanal biosynthesis in L. lactis and explain the effect of both synthesis pathways on 3-methylbutanal production. [Display omitted] [ABSTRACT FROM AUTHOR]