Comparative study of separated fermentations and cofermentation processes to produce ethanol from hardwood derived hydrolysates

As a part of the alcoholic conversion of lignocellulosic sugars, the co-culture process was studied in continuous conditions using a medium resulting from complete hydrolysis of aspen wood and compared with the separated fermentations process performing continuously on hemicellulosic and cellulosic aspen wood derived hydrolysates. The complete conversion of mixture of glucose and xylose was obtained using a respiratory deficient mutant of Saccharomyces diastaticus cocultivated with Pichia stipitis in continuous culture. At the dilution rate of 0.125 h−1, ethanol (13.5 g l−1) was produced with a yield of 0.25 g g−1, a volumetric productivity of 1.6 g l−1 h−1 and a substrate conversion rate of 100%. In the separated fermentations scheme, ethanol (39 g l−1) was produced by Zymomonas mobilis from cellulosic hydrolysate with a yield of 0.35 g g−1 and a productivity of 7.8 g l−1 h−1, whereas Pichia stipitis grown on hemicellulosic hydrolysate produced ethanol (14 g l−1) with a yield of 0.37 g g−1 and a productivity of 0.56 g l−1 h−1.