Vicinal diketone formation in yogurt: C-13 precursors and effect of branched-chain amino acids

Addition of branched-chain amino acids (BCAA) or an inhibitor of the BCAA biochemical pathways during fermentation of milk with a lac(-) mutant of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus strongly influenced the formation of two aroma-impact compounds, 2,3-butanedione and 2,3-pentanedione, as well as their direct precursors 2-acetolactate and 2-acetohydroxybutyrate. This suggests a connection between vicinal diketone formation and BCAA biosynthesis in yogurt bacteria. A recently developed static-and-trapped headspace technique combined with gas chromatography-mass spectrometry demonstrated incorporation of C-13 from [U-C-13(6)]-D-glucose and [U-C-13(4)]-L-threonine into both vicinal diketones. For 2,3-butanedione, glucose is the major precursor via pyruvate and activated acetaldehyde. For 2,3-pentanedione, L-threonine is a precursor via 2-ketobutyrate, but glucose is the major contributor tia activated acetaldehyde and, possibly, also tia 2-ketobutyrate, which is a degradation product of 3-methylaspartate, an intermediate in glutamate synthesis.