In general, a symbiotic relationship between S. thermophilus and L. delbrueckii subsp. bulgaricus is valid during the processing of yogurt (Lourens‐Hattingh & Viljoen, 2001). During fermentation, L. delbrueckii subsp. bulgaricus produces essential amino acids which are necessary for the growth of S. thermophilus. Streptococcus thermophilus produces lactic acid simultaneously, which stimulates the growth of L. delbrueckii subsp. bulgaricus. As shown in Table 1, absence of L. delbrueckii subsp. bulgaricus in the ABT‐4 culture decreased the rate of postacidification. The stimulated growth of streptococci was responsible for the increased rate of postacidification, which can be attributed to L. delbrueckii subsp. bulgaricus presented in the ABY‐2 and YC‐X11, and fermentation period was lasted at the IFpH of 4.80. It was also stated that S. thermophilus acting as an oxygen scavenger creates an anaerobic environment and may enhance growth and survival of Bifidobacterium when used together in starter cultures (Lourens‐Hattingh & Viljoen, 2001). However, the similar trend was not observed in this study when the fermentation period was lasted at the IFpH of 4.55. It is well known that streptococci are inhibited at pH values of 4.2–4.4. At the low IFpH, lactobacilli are mainly responsible for the increase in acidity by producing excessive amounts of lactic acid. Differences occurred in kinetic behaviors of different starter cultures at different IFpH values may also depend on survival of probiotic bacteria in yogurt samples. Lourens‐Hattingh and Viljoen (2001) reported that final acidity and interaction between species present affected the survival of probiotic strains primarily during cold storage. It has also been reported that L. acidophilus survived better than the traditional yogurt culture organisms, L. delbrueckii subsp. bulgaricus and S. thermophilus, in yogurt under acidic conditions. On the other hand, most strains of Bifidobacteria are sensitive to pH values below 4.6. Lourens‐Hattingh and Viljoen (2001) stated that L. delbrueckii subsp. bulgaricus causes “overacidification” during storage and it can be prevented by using modified or ABT yogurt starter cultures. The kinetic results of the present study supported this statement when the fermentation period lasted at the IFpH of 4.80.