Effect of alpha-acetolactate decarboxylase Inactivation on alpha-acetolactate and diacetyl production by Lactococcus lactis subsp lactis biovar diacetylactis

Strains of Lactococcus lactis subsp. lactis biovar diacetylactis deficient in alpha-acetolactate decarboxylase produce alpha-acetolactate. This unstab le compound is a precursor of acetoin and an aromatic compound, diacetyl. F ollowing random mutagenesis of strain CNRZ 483, alpha-acetolactate decarbox ylase-negative mutant 483 M1 was selected. When grown in milk, its growth a nd acidification characteristics were similar to those of the parental stra in. In anaerobic conditions, the parental strain produced 2.10 mM acetoin a nd less than 0.05 mM diacetyl. The mutant accumulated up to 2.11 mM alpha-a cetolactate, which spontaneously degraded to acetoin and diacetyl. After 24 h of culture, the alpha-acetolactate concentration was only 0.49 mM and th e acetoin and diacetyl concentrations reached 1.50 mM and 0.26 mM, respecti vely. Diacetyl production by both strains increased in aerobic conditions, as well as when citrate was added. In contrast to cultures of the parental strain, however, diacetyl and acetoin concentrations in mutant cultures con tinued to increase without reaching a plateau. The results also showed that diacetyl production by wild type L. lactis subsp. lactis biovar diacetylac tis strains cannot be explained uniquely by the spontaneous decarboxylation of the alpha-acetolactate produced in the culture medium.