Diacetyl and alpha-acetolactate overproduction by Lactococcus lactis subsplactis biovar diacetylactis mutants that are deficient in alpha-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 diacet yl or its precursor alpha -acetolactate. Using random mutagenesis with nitr osoguanidine, we selected mutants that were deficient in alpha -acetolactat e decarboxylase and had low lactate dehydrogenase activity. The mutants pro duced large amounts of alpha -acetolactate in anaerobic milk cultures but n ot in aerobic cultures, except when the medium was supplemented with catala se, yeast extract, or hemoglobin.