Ethanol Production from Glucose and Xylose by Immobilized <i>Zymomonas mobilis</i> CP4(pZB5)

摘要

Fermentation of glucose-xylose mixtures to ethanol was investigated in batch and continuous experiments using immobilized recombinant &amp;lt;i&amp;gt;Zymomonas mobilis&amp;lt;/i&amp;gt; CP4(pZB5). This microorganism was immobilized by entrapment in k- carrageenan beads having a diameter of 1.5-2.5 mm. Batch experiments showed that the immobilized cells co-fermented glucose and xylose to ethanol and that the presence of glucose improved the xylose utilization rate. Batch fermentation of rice straw hydrolyzate containing 76 g/L glucose and 33.8 g/L xylose gave an ethanol concentration of 44.3 g/L after 24 hours, corresponding to a yeild of 0.46 g ethanol/g sugars. Comparable results were achieved with a synthetic sugar control. Continuous fermentation runs were performed in a laboratory scale fluidized-bed bioreactor (FBR). Glucose-xylose feed mixtures were run through the FBR at residence times of 2 to 4 hours. Glucose conversion to ethanol was maintained above 98% in all continuous runs. Xylose conversion to ethanol was highest at 91.5% for a feed containing 50 g/L glucose-13 g/L xylose at a dilution rate of 0.24 h^-1. The xylose conversion to ethanol decreased with increasing feed xylose concentration, dilution rate and age of the immobilized cells. Volumetric ethanol productivities in the range of 6.5 to 15.3 g/L-h were obtained.