Environmental effects on ethanol production by immobilized Zymomonas mobilis in a trickle-flow fermentor

A continuous trickle-flow fermentor was successfully used to produce ethanol from glucose. Ethanol productivity of Z. mobilis ATCC 31821 was shown to produce ethanol about 2.5-fold faster and 38% higher than S. cerevisiae. A weighted equation was used to combine five parameters for the selection of a highly productive Z. mobilis NRRL 14022 which was used in the following experiment. The effect of environmental conditions (e.g., temperature, nutrient, initial pH, feed flow rate, ethanol and water activity) on fermentation and cell growth was investigated with the immobilized cell reactors. Natural sponge and cotton towel strip were used as a support to immobilize Z. mobilis. Temperatures over 35$\sp\circ$C decreased cell growth and fermentation activity remarkably. A semi-synthetic medium was developed to decrease cell growth without decreasing ethanol productivity. An uncoupling phenomenon in which ethanol fermentation was independent of cell growth was found when pH of a medium was greater than 5.45 or K$\sb2$HPO$\sb4$ concentration was lower than 0.1%. Ethanol productivity remained constant, but cell growth decreased with pH greater than 5.45 or K$\sb2$HPO$\sb4$ concentration less than 0.1%. Increasing feed flow rate increased ethanol productivity and cell growth. Ten percent glucose feed at a flow rate of 80.78 ml/h produced 3.04 g ethanol/h with a 80.7% conversion. Ethanol was found to be less inhibitory to fermentation activity than cell growth. Decreasing water activity reduced cell growth, but ethanol productivity remained constant in the range of 0.98 to 0.95. The conditions with uncoupling growth were the most effective ways to obtain high ethanol productivity and to decrease cell growth in a trickle-flow fermentor. Based on reactor volume, ethanol productivity as high as 40.6 g/l/h with 90.7% conversion in an immobilized Z. mobilis reactor-separator (type I) has been obtained with 10% glucose and at 30$\sp\circ$C. Ethanol productivity of this system was found to be 71% higher than that of the immobilized cell reactor with no simultaneous ethanol separation. Cotton towel strip was used as a support to construct another immobilized cell reactor-separator (type II). Reactor productivity was a linear function of inlet glucose concentration. This reactor was fed with 15% glucose and successfully operated for 41 days at 40$\sp\circ$C with no clogging problem as that occurred in the type I system