A method for determining beta-galactosidase activity of yogurt cultures in skim milk

A method was developed for determining the specific activity of bacterial beta-galactosidase (EC 3.2.1.23) during growth of Streptococcus thermophilus and Lactobacillus bulgaricus in skim milk. Individual and mixed strain cultures of S. thermophilus (St 3642, St14485) and L. bulgaricus (Lb11842, Lb880) were examined for growth (OD at 600 nm and viable cell counts), acid production, and beta-galactosidase activity (expressed as a function of recoverable TCA-precipitable cellular protein). Cultures were inoculated into 10% skim milk (2% inoculum) and incubated at 40 degrees C for 12 h. Aliquots were removed at 2-h intervals and diluted with ice cold EDTA, pH 12. The EDTA chelates calcium and solubilizes milk protein, allowing separation of the bacteria by centrifugation. Cells were then washed twice with 20 mM phosphate buffer and disrupted by sonication. Cell debris and intact cells were removed by centrifugation and the cell-free extract evaluated for beta-galactosidase activity using o-nitrophenyl-beta-D-galactopyranoside as substrate. Specific activities ranged from 0 to 6 units/mg protein. This simple and reproducible method is applicable for enzyme assays and measurement of cellular components where contamination by milk proteins is a potential problem.