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Effects of sonication, survival, and p-galactosidase activity of four lactic cultures were investigated in pH 1.5-3.5 range. Lactobacillur delbruekii subsp. bulgaricus and Streptococcus thermophilus exhibited the highest P-galactosidase activity in skim milk and broth systems, respectively. The P-galactosidases from L. delbruekii subsp. bulgaricus, S. thennophilus, and L. acidophilus showed optimum activity in the neutral pH range and 55°C. Viable count of all four cultures decreased most rapidly at pH 1.5, but L. acidophilus and L. detbruekii subsp. bzd~uricus survived better than the other organisms. The decrease of enz$ne activity of unsonicated cultures withpH was slight. esneciallv at DH 3.5. However. acidification of sonicated cultures to PH 3.5 or'lower resulted in rapid and permanent loss of enzyme activity.

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