The fermentability of concentrated softwood-derived hemicellulose fractions with and without supplemental cellulose hydrolysates

The hemicellulose-rich component recovered following the steam explosion of softwood residues typically contain very dilute sugar concentrations, which can impact the overall economics of the bioconversion process. In an attempt to address this concern, the water-soluble fractions derived from steam exploded Douglas-fir whitewood and whitewood/bark chips were subjected to a concentration step (rotary-evaporation or freeze-drying) prior to fermentation. The fermentability of the resultant streams was largely dependent on the accumulation of inhibitory material, with poor fermentation of the whitewood concentrates at as low as a twofold increase in concentration. The bark-containing concentrates were slightly easier to ferment, but above a 2.5-fold concentration, a significant “lag” in fermentation was observed. Concurrently, the water-soluble fractions were supplemented with the glucose-rich cellulose hydrolysate obtained after enzymatic hydrolysis of the water-insoluble, cellulose component. Supplementation of the original water-soluble fraction with the readily-fermentable cellulose hydrolysate resulted in significantly improved fermentation. Using this approach, it was possible to increase the hexose sugar concentration in the water-soluble fraction by more than 50%, while still achieving high yield ethanol production.