In this study, 12 strains of Lactobacillus delbrueckii subsp. bulgaricus which were isolated from fermented dairy products in different regions of Mongolia were selected to perform fermentation experiments in comparison with one commercial strain. The fermentation time required by each strain was recorded, and the titritable acidity, pH and organic acid content (by reverse phase high-performance liquid chromatography, RP-HPLC) in fermented milks were determined after different periods (0-48 h, 7 days, 14 days and 21 days) of low-temperature storage. Strain IMAU20458 that provided weak post-acidification was selected from the tested strains based on acid-producing capacity and the changes in acid components during post-fermentation storage.

The aim of this study was to analyze the acetaldehyde-producing characteristics of Streptococcus thermophilus and Lactobacillus bulgaricus in fermented milk during post-ripening and to investigate the relationship between the expression of key regulatory genes and acetaldehyde synthesis. Methods: Fermented milk was obtained from single strain fermentation of reconstituted whole milk by potent strains of S. thermophilus and L. bulgaricus isolated from traditional fermented milk products. After the end of fermentation (pH 4.5-4.6), the fermented milk was ripened by storage at 4 ℃ for 48 h. During the storage period, the change in acetaldehyde content was determined, and the expression characteristics of the key regulatory genes pdc, pdh, ald, and ldh involved in acetaldehyde synthesis were examined by reverse transcriptionqPCR. Results: The amounts of acetaldehyde produced by 6 S. thermophilus strains were 2.59–14.53 μg/g and 9.17–39.45 μg/g by 6 Lactobacillus bulgaricus strains. The synthesis of acetaldehyde was positively correlated with the expression levels of pdc, ald and pdh but negatively correlated with the expression level of ldh. Conclusion: Different bacterial species and different strains have significantly different capacities to produce acetaldehyde during post-ripening, and one strain produces significantly different amounts of acetaldehyde at different post-ripening times, suggesting strain and temporal specificity of acetaldehyde production. The regulatory genes pdc, ald, and pdh promote the synthesis of acetaldehyde, while the gene ldh has a negative effect on acetaldehyde production.