Abstract: The diacetyl and acetoin production characteristics of eight lactic acid bacteria (LAB) strains isolated from Tibetan kefir and Inner Mongolia dairy tofu were studied. For this purpose, the six key genes related to the production of the flavor compounds including diacetyl and acetoin in the 8 strains were determined using genome-wide annotation analysis. The amounts of diacetyl produced by Streptococcus thermophilus GST-6, Lactobacillus rhamnosus 5-1, and L. paracasei N1115 in fermented milk were 0.72, 0.53 and 0.47 μg/mL, respectively, significantly higher than those produced by five other strains (P < 0.05), and were increased up to 6.23, 5.28 and 4.47 μg/mL, respectively, when citric acid was added to the milk. Correlation analysis showed that the oxaloacetate decarboxylase (OAD) gene was strongly correlated with the yield of diacetyl with Pearson correlation coefficient of 0.898 (P < 0.01), while the lactate dehydrogenase (LDH) and acetyl lactate synthase (ALS) genes were not significantly correlated with the yield of diacetyl (P > 0.05). The relationship between the yield of acetoin and each of the key genes was not significant (P > 0.05). Using gas chromatography-mass spectrometry (GC-MS), a total of 24 volatile compounds were detected in all the fermented milk samples. Principal component analysis showed that organic acids, acetoin and diacetyl were the characteristic flavor compounds of the strains. Heptanone and 2-nonanone were also important volatile flavor compounds.

Key words: lactic acid bacteria, diacetyl, acetoin, functional gene annotation, volatile compounds