KINETICS AND STABILITY OF A FIBROUS-BED BIOREACTOR FOR CONTINUOUS PRODUCTION OF LACTIC-ACID FROM UNSUPPLEMENTED ACID WHEY

The kinetics and long-term stability of the fibrous-bed bioreactor were investigated for continuous lactic acid production from unsupplemented acid whey containing similar to 3.7% (w/v) lactose and similar to 0.8% (w/v) lactic acid, using immobilized cells of Lactobacillus helveticus at pH 5.5 and 42 degrees C. Depending on dilution rate and lactate concentration, reactor volumetric productivities ranged from 2.6 g l(-1) h(-1) to 7 g l(-1) h(-1). These productivities were approx. 10-times higher than those obtained in batch cultivations with free cells. The product inhibition followed noncompetitive kinetics, with the inhibition constant, K-p, equal to 7.9 g l(-1) and the maximum volumetric productivity, q(max)', equal to 20 g l(-1) h(-1). Lactate yield from lactose was > 95% (w/w). Reactor performance was stable for continuous, long-term operation for both sterile and nonsterile unsupplemented acid whey feeds for the 6-month period studied. Concentrated acid whey was also found to be an appropriate feedstock to this bioreactor. SEM micrographs showed a dense biofilm covering the fibrous reactor packing. Atypical cell morphology was also observed with the immobilized cells. Total cell density, including suspended cells, within the reactor exceeded 60 g l(-1). The specific cell productivity was lower than for free cells, indicating a relatively low cell efficiency, perhaps due to diffusion limitation.