Induced mutants of Streptococcus lactis by gamma irradiation and its effect on milk acidity

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