ABSTRACT
Streptococcus lectic was exposed to different doses of gamma- irradiation [10, 50, 100 and 150 Kr]. The results of acidity production in sterilized milk inoculated by isolates from radiation treatments and control could be summarized in the following: The mean acidity produced by S. lactic isolates after irradiation at 10 Kr increased to be 0.66% than that of control isolates [0.62%]. The acidity produced by the isolates of the 50 Kr treatment showed more increment to reach the peak [0.7%]. Thereafter, acidity production decreased by isolates of each 100 Kr [0.53%] and 150 Kr [0.51%] treatments. Hence, the 50 Kr treatment could be considered activation dose to S. Lactic starter for acid production. Two mutants were selected. Acidity production by mutant I [from 10 Kr treatment] was 0.95%, and that of mutant II [from 50 Kr treatment] was 1.0%, while acid production by the parent S. lactic culture was 0.62%. Concerning the stability of isolates, it was found that the acid production by mutant I and mutant II slightly decreased by time. The mutants were re-irradiated after 37 and 60 days at doses 10, 25 and 50 Kr. Acid production in milk by isolates of the radiation treatments was determined