THE EFFECT OF ACETIC-ACID ON FUEL ETHANOL-PRODUCTION BY ZYMOMONAS

Acetic acid acts to promote uncoupling with Zymomonas. At pH 5, 36% of acetic acid is in the uncharged and undissociated form (HAc), which is able to permeate the plasma membrane. The transmembrane DELTApH drives the accumulation of acetic acid, which results in the acidification of the cytoplasm. The consequential increase in maintenance metabolism represents a diversion of energy that would otherwise be available for growth. At pH 5, the growth of Z. mobilis (ATCC 29191) was 50% inhibited with 8.3 g/L acetic acid (50 mM HAc) and completely inhibited by 11 g/L. Addition of 6 g/L acetic acid caused the glucose-to-ethanol conversion efficiency to decrease from 98 to 90% of theoretical maximum. The growth yield coefficient for glucose was 50% decreased by 2.3 g/L acetic acid (13.5 mM HAc) from 0.036 to 0.018 g cell/g glucose. However, the specific (ethanol) productivity of batch cultures was enhanced by < 5 g/L acetic acid (< 30 mM HAc). For continuous cultures, the acetic acid sensitivity depends on the growth rate (dilution rate), but an increase in specific productivity can be achieved at proportionately lower concentrations of acetic acid. At a growth rate of 0.112/h, the addition of 1.7 g/L acetic acid to the 5% glucose feed resulted in an increase in specific productivity from 2.68 to 5.87 g ethanol/g cell/h. The uncoupling effect of acetic acid could be beneficial in terms of improving the productivity in closed, continuous fermentations, such as cell recycle or immobilized cell reactors.