Ultraviolet Light-Induced Mutants of Streptococcus lactis Subspecies diacetylactis with Enhanced Acid- or Flavor-Producing Abilities

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Abstract
A strain of S. lactis ssp. diacetylactis S1 isolated from fresh milk was exposed to 7200 ergs/mm2 of UV radiation. Over 8100 colonies surviving from 7.4 .times. 106 cells exposed to radiation were screened on citrate agar for detection and isolation of mutants with increased flavor and/or acid production. Of the survivors, 960 were type-I mutants that exhibited clear zone on citrate agar after 18 h (presumed to be high diacetyl producers) and 288 were type-II mutants which did not exhibit clear zones on citrate agar for up to 72 h (high acid producers). Type-II mutants produced an average .93% titratable acidity which was 34% more than the .69% of the parent. Reduction in titratable acidity (56% less) was considerable in type-I mutants, compared with the parent culture. Diacetyl + acetoin production by type-I mutants was 137.9 ppm which has 4.5 times more than that of the parental strain. Acetaldehyde production in the mutants varied from 1.5-34.5 ppm (parent culture 3.0 ppm). The mutants with increased acid and high acetoin plus diacetyl production were stable after 50 subcultures in milk.