In the present study, 5 strains of Streptococcus thermophilus which were isolated from traditional fermented dairy products were selected to perform fermentation experiments. During the fermentation process and 504 h of storage at 4 ℃, some indexes such as fermentation time, titratable acidity (TA), pH value and viable cell counts were determined, and the composition of organic acids was tested by reverse phase high performance liquid chromatography(RP-HPLC). Fermentation and acid-producing properties of Streptococcus thermophilus were evaluated; meanwhile changes in organic acid composition and post-acidification characteristics during storage were determined. After 2 h of fermentation at 42 ℃, the pH of fermented milk by each of the five strains declined rapidly. The fermentation reached its end-point at 5–9 h, and the shortest fermentation time was required for strains IMAU10632 and IMAU20772. Over the 504 h storage period, the pH of milks fermented by Streptococcus thermophilus was reduced by about 0.5 and finally reached a stable level of about 4.2, and acids were produced at lower rates in all fermented milks until the acidity values reached levels below 90 oT , suggesting the weak post-acidification ability of the 5 strains. Furthermore, IMAU10632 was selected from these strains, which could shorten milk coagulation time, increase acid-producing rate, enhance post-acidification ability and reach a viable cell count exceeding 108 CFU/mL which remained unchanged over the entire storage period.

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.