Acid-Producing Ability of Streptococcus thermophilus and Organic Acid Composition in Fermented Milk

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), p H 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 p H 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 p H 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/m L which remained unchanged over the entire storage period.