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Four samples of commercially manufactured yogurts (plain, drinking type) were purchased and evaluated their physico-chemical properties, buffering capacity. And the survival rate of lactic acid bacteria and their β-galactosidase activity under the acidic conditions (in vitro) were investigated. The values of pH, titratable acidity, viscosity and viable cell counts of yogurts were 3.71∼4.08, 0.990∼1.045%, 256∼3164 cps. and 10^8∼10^9 cfu/㎖, respectively. The volume of 1.0 M-HCl required to reduce the pH of yogurt (50 ㎖) to minus 2 value was 3.58w4.33 ㎖. When commercial yogurts were incubated at 37℃ for 120 minutes under the acidic conditions (pH 3.5, 2.5, 1.5), the survival rates of lactic acid bacteria in yogurt were 3.5×10^(-2)∼3.6×10^(-1)% at pH 2.5, 8.3×10^(-5)∼4.2×10^(-3)% at pH 1.5, respectively, but there was no significant difference at pH 3.5. The remaining activities of β-galactosidase were 9.4∼36.2% at pH 2.5, 4.2∼19.0% at pH 1.5. respectively. These results suggested that a significant number of lactic acid bacteria in yogurt might be destroyed in the hostile environment of the stomach, but β-galactosidase activity from yogurt might be somewhat maintained probably due to the protecting effect by its cell wall and membrane.
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