Acetic acid production from fructose by Clostridium formicoaceticum immobilized in a fibrous-bed bioreactor

The fermentation kinetics of acetic acid production from fructose by Clostridium formicoaceticum was studied at pH 7.6 and 37 degrees C. Recycle batch, fed-batch, and continuous fermentations using immobilized cells in a fibrous-bed bioreactor were studied for their potential application in producing acetic acid from fructose, a fermentable sugar commonly found in corn steep liquor and many other food processing wastes. For the immobilized cell fermentation, acetic acid yield from fructose was similar to 1.0 g/g, with a final acetate concentration of similar to 78 g/L and the overall reactor productivity (based on the fibrous bed bioreactor volume) of similar to 0.95 g/L.h) in the fed-batch fermentation. For a similar fed-batch fermentation with free cells, acetic acid yield was similar to 0.9 g/g, the highest final acetate concentration was similar to 46 g/L, and the overall productivity was similar to 0.12 g/(L.h). In the continuous fermentation with immobilized cells, the reactor productivity decreased from 3.2 to 1.3 g/L.h) as retention time increased from 16 to 72 h to reach 100% conversion. Compared to free-cell fermentations, the superior performance of the fibrous-bed bioreactor can be attributed to the high density (>30 g/L) of viable cells immobilized in the fibrous bed. The fermentation product, acetic acid, was found to be a noncompetitive inhibitor to the cells. However, the immobilized cells had a higher maximum production rate (p(max)) and a higher value for the inhibition rate constant (K-p) than those for the free cells, suggesting that the immobilized cells in the fibrous-bed bioreactor were less sensitive to acetic acid inhibition than the free cells. This improvement in kinetic behaviors for immobilized cells confirms that the fibrous-bed bioreactor can be used as an effective tool for adapting and screening for acetate-tolerant strains.