Sugar recovery and fermentability of hemicellulose hydrolysates from steam-exploded softwoods containing bark

The hemicellulose sugar recovery and ethanol production obtained from SO2-c atalyzed steam explosion of a mixed white fir (70%) and ponderosa pine (30% ) feedstock containing bark (9% dry weight/dry weight) was assessed. More t han 90% of the available hemicellulose sugars could be recovered in the hyd rolysate obtained after steam explosion at 195 degreesC, 2.38 min, and 3.91 % SO2, with 59% of the original hemicellulose sugars detected in a monomeri c form. Despite this high sugar recovery, this hydrolysate showed low ethan ol yield (64% of theoretical yield) when fermented with a spent sulfite liq uor-adapted strain of Saccharomyces cerevisiae. In contrast, most hydrolysa tes prepared at higher steam explosion severity showed comparable or higher ethanol yields. Furthermore, the hydrolysates prepared from bark-free feed stock showed better fermentability (87% of theoretical yield) despite conta ining higher concentration of known inhibitors. The ethanol yield from the hydrolysate prepared from a bark-containing wood sample could be improved t o 81% by an extra stage acid hydrolysis (121 degreesC for 1 h in 3% sulfuri c acid). This extra stage acid hydrolysis and steam explosion at higher sev erity conditions seem to improve the fermentability of the hydrolysates by transforming certain inhibitory compounds present in the hydrolysates prepa red from the bark-containing feedstock and thus lowering their inhibitory e ffect on the yeast used for the ethanol fermentation.