Co-culture of Zymomonas mobilis and Scheffersomyces stipitis immobilized in polymeric membranes for fermentation of glucose and xylose to ethanol

Submerged hollow fiber membrane bioreactor (SHFMB) was fabricated and operated to co-culture Zymomonas mobilis and Scheffersomyces stipitis for fermentation of glucose and xylose mixture. Suspended Z. mobilis and S. stipitis in co-culture exhibited poor xylose fermentation due to substrate/product inhibition and catabolite repression. Immobilization in polymeric membranes was effective in alleviating these inhibitions in the SHFMB, which resulted in concomitant fermentation of glucose and xylose at initial glucose and xylose concentrations of 20-80 g L-1 and 10-40 g L-1, respectively. Selective aeration in SHFMB resulted in 100% glucose and > 70% xylose fermentation within 48 h, producing 36.7 g L-1 ethanol from 90 g L-1 sugar mixture. Partial immobilization in the membranes allowed Z. mobilis to diffuse in suspension under benign condition, whereas S. stipitis remained inside membrane pores throughout the operation. Xylose fermentation in the SHFMB could be enhanced further by increasing the number of S. stipitis immobilized membranes. Cell immobilization in the SHFMB was stable during 12 consecutive batch runs over 24 days, and exhibited improved xylose fermentation due to gradual increase in S. stipitis accumulation within the membranes. These results indicate that SHFMB can be promising for co-culture of Z. mobilis and S. stipitis in fermentation of mixed sugars from lignocellulosic hydrolysate.