The isolates were incubated in M17 broth at 42 C for overnight
activation. 200mL batches of 10g/100 mL skimmed milk containing
1g/100 mL yeast extract were inoculated (1%) with the propagated
cultures, and incubated at 42 C for 6 h. The pH of the samples was
determined every hour using a pH meter (Metrohm 744 pH meter,
Switzerland). S. thermophilus isolates reaching to the pH range of
4.60e4.70 within 5 h of incubation were selected as promising
candidates for yoghurt fermentation. The isolates that could not
reduce the pH of skimmed milk to 4.6 within 6 h were further
incubated until the pH values reached to 4.6, and the times required
for this pH reduction were determined.