摘要：In this study, thirty-one Lactobacillus delbrueckii subsp. bulgaricus and thirty-four Streptococcus thermophilus were screened for β-galactosidase enzyme activities and specific activities. The β-galactosidase enzyme activities were determined by using o-nitrophenyl-beta-D-galactopyranoside (o-NPG) as a substrate. L. delbrueckii bulgaricus strains showed specific activity between 0.186-6.500 U/mg, while S. thermophilus strains exhibited specific activity between 0.172-5.064 U/mg. The highest specific enzyme activities among bacteria cultures were determined at L. delbrueckii subsp. bulgaricus ZN541 (6.500 U/mg) and S. thermophilus Z1052 (5.064 U/mg) strains. The ZN541 and Z1052 strains that showed high specific β-galactosidase activity were selected and the effects of different conditions (pH, temperature, lactose concentration, and fermentation time) on the activities of β-galactosidase enzymes of these strains were determined. It was determined that the pH required for optimum enzyme activity in ZN541 strain was 6.2, the temperature was 42°C, the lactose concentration was 2% and the fermentation time was 24 hours. In the Z1052 strain, the pH required for optimum enzyme activity was 6.8, the temperature was 42°C, the lactose concentration was 4%, and the fermentation time was 24 hours.
