KINETIC-STUDIES ON ALPHA-ACETOLACTIC ACID EXTRA AND INTRACELLULAR OXIDATIVE DECARBOXYLATION TO DIACETYL BY LACTOCOCCUS-LACTIS SSP. LACTIS VAR. DIACETYLACTIS SD-933

alpha-acetolactic acid (ALA) oxidative decarboxylation is thought to b e the dominant mechanism for the dairy flavour diacetyl production by lactic acid bacteria of Leuconostoc and Lactococcus genera. However, k inetic data on that particular reaction are lacking due to the conjunc tion of the high reactivity of ALA in presence of oxygen and the diace tyl and ALA specific measurement difficulties. The use of a gas chroma tographic technique coupled with a gas membrane sensor equipped biorea ctor with a controlled atmosphere nonetheless allows the on-line acqui sition of diacetyl production kinetics by lactococci cultures or direc tly from ALA. Thus, this paper provides kinetic data on both diacetyl microbiological production and extra-and intracellular ALA oxidative d ecarboxylation. The comparison of these kinetics shows that oxidative decarboxylation can account for diacetyl biological production and tha t this reaction is located at the intracellular level. Moreover, the e volution of the pattern of intracellular kinetics with and without hea t treatment demonstrates the action of a thermolabile compound in oxid ative decarboxylation. (C) Inra/Elsevier, Paris.