In this study, real-time quantitative polymerase chain reaction (q-PCR) was used to compare and quantitatively analyze the predominant species Lactobacillus helveticus, Leuconostoc mesenteroides and Lactococcus lactis subsp. lactis isolated from 28 samples of traditional fermented dairy products（including 7 samples of fermented sheep milk, 8 samples of fermented goat milk and 13 samples of fermented cow milk）from Ordos in Inner Mongolia. The results showed that the order of the number of three predominant species in fermented goat milk and fermented cow milk was Leu.mesenteroides＞L.helveticus＞Lac. lactis subsp. lactis whereas that of fermented sheep milk was L.helveticus > Leu.mesenteroides > Lac. lactis subsp. lactis.

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.