DIACETYL PRODUCTION IN MILK BY AN ALPHA-ACETOLACTIC ACID ACCUMULATING STRAIN OF LACTOCOCCUS-LACTIS SSP LACTIS BIOVAR DIACETYLACTIS

When grown in skim milk, Lactococcus lactis ssp. lactis biovar. diacetylactis D1 accumulated alpha-acetolactic acid at 1.7 mM because of the absence of alpha-acetolactate decarboxylase. The alpha-acetolactic acid produced was unstable and underwent a chemical decarboxylation. At the beginning of the culture, the redox potential was high, and the decarboxylation of alpha-acetolactic acid produced diacetyl and acetoin. At the end of the culture, the redox potential was low, and alpha-acetolactic acid was converted into acetoin only. When synthetic alpha-acetolactic acid was added to skim milk or fermented milk, the chemical decarboxylation of alpha-acetolactic acid into diacetyl occurred only at high redox potential. The strain D1 has been cultivated under constant agitation, permitting high redox potential to be maintained. In these conditions, diacetyl production was threefold higher than in an unagitated culture. When the strain D1 was cultivated in association with an acidifying Lactococcus lactis ssp. lactis strain, the amount of diacetyl was <.02 mM because of the rapid drop of the redox potential at the beginning of the fermentation, which prevented the formation of diacetyl from the chemical oxidative decarboxylation of alpha-acetolactic acid.