Acetic Acid Production from Fructose by Clostridium formicoaceticum Immobilized in a Fibrous‐Bed Bioreactor

Abstract The fermentation kinetics of acetic acid production from fructose by Clostridium formicoaceticum was studied at pH 7.6 and 37 °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 ∼1.0 g/g, with a final acetate concentration of ∼78 g/L and the overall reactor productivity (based on the fibrous bed bioreactor volume) of ∼0.95 g/(L·h) in the fed‐batch fermentation. For a similar fed‐batch fermentation with free cells, acetic acid yield was ∼0.9 g/g, the highest final acetate concentration was ∼46 g/L, and the overall productivity was ∼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.