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

Lactic acid bacteria have a profoundly negative influence on the fermentation performance of Zymomonas mobilis. Lactic acid bacteria are an important part of the steeping process in starch wet-milling. Their prevalence and acid pH optimum for growth make these organisms opportunistic contaminants of continuous ethanol fermentations, where the feedstock is enzymatic starch hydrolysate. At concentrations of lactic acid in the range of 3-10 g/L, cell density, product yield, and productivity are reduced. However, under similar operating conditions, a chemostat culture fed a synthetic glucose medium with added DL-lactic acid (range 0-13 g/L) did not exhibit a decrease in either yield or productivity. In pH-stat (5.0) batch cultures, with about 11 g/L lactic acid added to the medium, growth and conversion efficiency were only very slightly inhibited. At pH 4.5, this amount of lactic acid caused a 23% decrease in growth yield, but only a 3% reduction in ethanol yield. Acetic acid is a minor metabolic byproduct of lactic cultures, and at pH 5, the growth rate and growth yield of Z. mobilis were inhibited 17 and 23% respectively, by the addition of 3.37 g/L lactic acid. Ethanol yield was decreased only 3.5% by this amount of acetic acid. It is concluded that lactic acid per se is not the causative inhibitory agent. Also, although a possible role for acetic acid and unidentified natural antibiotic substances is implied, the mechanism whereby lactic acid bacteria negatively affect Zymomonas remains unresolved.