THE PHYSIOLOGY OF LACTOCOCCUS-LACTIS SUBSP LACTIS BIOVAR DIACETYLACTIS IMMOBILIZED IN HOLLOW-FIBER BIOREACTORS - GLUCOSE, LACTOSE AND CITRATE METABOLISM AT HIGH CELL DENSITIES

Lactococcus lactis subsp. lactis biovar. diacetylactis was selected to study the physiological influences of immobilization and growth to high cell densities. Cells were cultivated on glucose or lactose medium in the presence and absence of citrate. With excess glucose the cells produced mainly lactate as the fermentation product (homofermentative) providing that not all of the substrate was consumed. The population so cultivated was exposed to extreme gradients of pH and lactate concentrations. When the glucose concentration was reduced the population showed a mixed product profile with half of the glucose being fermented to lactate, the remainder to formate, acetate, ethanol and 2,3-butanediol. Inclusion of citrate in the medium shifted the population to homofermentation, with respect to the amount of glucose or lactose consumed. The citrate was metabolized via the pyruvate-formate lyase and alpha-acetolactate synthase routes. The pH of the medium was shown to strongly influence the product profile from citrate, presumably by affecting the activity of the key enzymes of pyruvate metabolism. The lactococci immobilized at high cell densities show product profiles typical of carbohydrate limitation at low dilution rates.