Fermentability of the hemicellulose-derived sugars from steam-exploded softwood (Douglas Fir)

Steam explosion of Douglas fir wood chips under low-severity conditions (log Ro = 3.08 corresponding to 175 degrees C, 7.5 min, and 4.5% SO2) resulted in the recovery of around 87% of the original hemicellulose component in the water-soluble stream. More than 80% of the recovered hemicellulose was in a monomeric form. As the pretreatment severity increased from 3.08 to 3.76, hemicellulose recovery dropped to 43% of the original hemicellulose found in Douglas fir chips while the concentration of glucose originating from cellulose hydrolysis increased along with the concentration of sugar degradation products such as furfural and hydroxymethylfurfural. Despite containing a higher concentration of hexose monomers (mainly glucose originating from cellulose degradation), the water-soluble fraction prepared under high-severity conditions (log Ro = 3.73 corresponding to 215 degrees C, 2.38 min, and 2.38% SO2) was not readily fermented. Only the two hydrolyzates obtained at low and medium (195 degrees C, 4.5 min, and 4.5% SO2) severities were fermented to ethanol using a spent sulfur liquor adapted strain of Saccharomyces cerevisiae. High ethanol yields were obtained for these two hydrolyzates with 0.44 g of ethanol produced per gram of hexose utilized (86% of theoretical). However, the best results of hemicellulose recovery and fermentability were obtained for the low-severity water-soluble fraction which was fermented significantly faster than the fraction obtained after medium-severity treatment probably because it contained higher amounts of fermentation inhibitors.