Diacetyl and a-Acetolactate Overproduction by Lactococcus lactis subsp. lactis Biovar Diacetylactis Mutants That Are Deficient in a-Acetolactate Decarboxylase and Have a Low Lactate Dehydrogenase Activity

Lactococcus lactis subsp. lactis biovar diacetylactis strains are utilized in several industrial processes for producing the flavoring compound diacetyl or its precursor a-acetolactate. Using random mutagenesis with nitrosoguanidine, we selected mutants that were deficient in a-acetolactate decarboxylase and had low lactate dehydrogenase activity. The mutants produced large amounts of a-acetolactate in anaerobic milk cultures but not in aerobic cultures, except when the medium was supplemented with catalase, yeast extract, or hemoglobin.