DIFFERENCES IN RESPONSE OF ZYMOMONAS-MOBILIS AND SACCHAROMYCES-CEREVISIAE TO CHANGE IN EXTRACELLULAR ETHANOL CONCENTRATION

In high cell density batch fermentations, Zymomonas mobilis produced 9 1 g L-1 ethanol in 90 min but culture viability fell significantly. Si milar viability losses in rapid fermentations by yeast have recently b een shown to be attributable in part to the high rate of change of the extracellular ethanol concentration. However, in simulated rapid ferm entations in which ethanol was pumped continuously to low cell density Z. mobilis suspensions, increases in the rate of change of ethanol co ncentration in the range 21-83 g L-1 h-1 did not lead to accelerated v iability losses. The lag phase of Zymomonas cultures exposed to a 30-g L-1 step change in ethanol concentration was much shorter than that o f Saccharomyces cerevisiae, providing evidence that the comparative in sensitivity of Zymomonas to high rates of change of ethanol concentrat ion is due to its ability to adapt to changes in ethanol concentration more rapidly than yeast. (C) 1994 John Wiley & Sons, Inc.