Enhancement of Diacetyl Production by a Diacetyl-Resistant Mutant of Citrate-Positive Lactococcus lactis ssp. lactis 3022 and by Aerobic Conditions of Growth

Abstract

Addition of Cu2+ and Fe3+ inhibited citrate uptake by citrate-positive Lactococcus lactis ssp. lactis 3022 and stimulated the production of diacetyl, probably because citrate uptake activity of the cells is lowered by diacetyl. A diacetyl-resistant mutant, HK-7, derived from citrate-positive L. lactis ssp. lactis 3022, in which diacetyl production was deregulated, produced a larger amount of diacetyl than the parent strain. The mutant also utilized a larger amount of citrate than the parent strain.However, citrate-positive L. lactis ssp. lactis 3022 cells grown aerobically produced more diacetyl than those grown under static conditions and continued to accumulate diacetyl even after all the citrate had been metabolized. A considerable amount of diacetyl accumulation was observed in cells grown aerobically in the absence of citrate, and diacetyl production was further increased by addition of Cu2+. The cells grown aerobically utilized more glucose with or without citrate than those grown under static conditions and produced less lactate and ethanol. From these results, enhancement of diacetyl production in shaken cultures appears to be a complicated phenomenon involving an increase in diacetyl synthase, activity limitation of the reduction of diacetyl to acetoin, and stimulation of diacetyl production from pyruvate formed from glycolysis.