KINETIC-MODEL OF GROWTH AND LACTIC-ACID PRODUCTION FROM LACTOSE BY LACTOBACILLUS-BULGARICUS

Batch and continuous fermentation kinetics of Lactobacillus bulgaricus were examined in detail. The batch kinetic model includes the effect of pH, lactate and substrate. The effect of pH on the enzyme beta-galactosidase is reflected in the value of mu(max), the Monod's growth constant. Empirical expressions to estimate model parameters based on pH were established. These were further used to estimate the cell mass and lactic acid concentration for the fermentation proceeded with no pH control. At high pH lactic acid was mostly formed by growth associated mechanisms and at low pH it was due to the cell maintenance. The relationship between ATP concentration and cell mass in a batch fermentation could be approximated by a Leudeking-Piret like relationship. The continuous fermentation was studied with varying pH and initial substrate conditions. A maximum productivity of 6.2 g/litre per hour was obtained at pH 5.6 for the CSTR system, which was three times the productivity obtained for a batch system at the same pH. The CSTR system had a high productivity over a very small range of retention times. Three stages were found to be sufficient to utilize completely the lactose, but at the cost of decreasing the productivity to 4.8 g/litre per hour.