Metabolism of lactose and citrate by mutants of Lactococcus lactis producing excess carbon dioxide

Mutants of Lactococcus lactis producing excess carbon dioxide could be isol ated on LDHA-20 agar (described by El Attar et al. Journal of Dairy Researc h 67 641-646 2000). The use of these mutants in the manufacture of Roquefor t cheese has the potential to improve the formation of openings in this che ese. The aim of this work was to examine the stability of these mutants, th eir enzymic activities and their metabolism of lactose and citrate during g rowth in milk. They produced less L-lactate than the parent strain and thei r lactate dehydrogenase activity was lower. Nevertheless none of the mutant s produced no L-lactate at all and the most active gas generators among the m generally produced 30-50 mM-L-lactate. Unexpectedly, all the strains prod uced some D-lactate, some >10 mM. We found that carbon dioxide production b y the mutants could be determined indirectly by assaying acetoin, citrate a nd 2,3-butanediol by high-performance liquid chromatography. Generally, spo ntaneous mutants were more stable than those obtained after treating with n itrosoguanidine or u.v. irradiation.