CONTINUOUS-CULTURE STUDIES OF XYLOSE-FERMENTING ZYMOMONAS-MOBILIS

The continuous cofermentation performance of xylose-fermenting Zymomon as mobilis at 30 degrees C and pH 5.5 was characterized using a pure-s ugar feed solution that contained 8 g/L glucose and 40 g/L xylose. Suc cessful chemostat start up resulted in complete utilization of glucose and greater than 85% utilization of xylose, but was only reproducibly achieved using initial dilution rates at or less than 0.04/h; once in itiated, cofermentation could be maintained at dilution rates of 0.04 to 0.10/h. Whereas xylose and cell-mass concentrations increased gradu ally with increasing dilution rate, ethanol concentrations and ethanol yields on available sugars remained approximately constant at 20-22 g /L and 80-90% of theoretical, respectively. Volumetric and specific et hanol productivities increased Linearly with increasing dilution rate, rising from approx 1.0 each (g/L/h or g/g/h) at a dilution rate of 0. 04/h to approx 2.0 each (g/L/h or g/g/h) at a dilution rate of 0.10/h. Similarly, specific sugar-utilization rates increased from approx 2.0 g/g/h at dilution rate 0.04/h to approx 3.5 g/g/h at dilution rate of 0.10/h. The estimated values of 0.042 g/g for the maximum Z, mobilis cell-mass yield on substrate and 1.13 g/g/h for the minimum specific s ubstrate utilization rate required for cellular maintenance energy are within the range of values reported in the Literature. Results are al so presented which suggest that long-term adaptation in continuous cul ture is a powerful technique for developing strains with higher tolera nce to inhibitory hemicellulose hydrolyzates.