Abstract: L. delbrueckii subsp. bulgaricus ZLB was irradiated for 60s with UV (lethal effect was about 99%), and mid-cultivated and counterselected with penicillin. After the spread cultivation, forty strains that grew slowly were cultured in RSM (Reconstituted Skimmed Milk). Twelve strains which coagulated RSM were stored at 12℃, and a mutant (ZU05), with the slowest lactic acid producing rate, was selected for further use. ZU05 produced the same amount of lactic acid as the parent strain at 42℃. However, during the storage at 12℃. It is significantly different between ZU05 and ZLB in their lactic acid producing capability (p<0.01), and the postacidification of ZU05 was delayed four days. Also, the sensitivity to streptomycin of ZU05 was different with ZLB, and ZU05 could be genetically stable.

Key words: postacidification, Lactobacillus bulgaricus, UV mutagenesis