The importance of including hemicellulosic sugars in the conversion of lignocellulosic feedstocks to ethanol to ensure process efficiency and an economic base for biofuel production has often been emphasized.134135 Given that hemicellulosic sugars constitute a fermentable resource equal to approximately 50% of the glucose residues present in the cellulose in most plant species (table 1.5), this is an unsurprising conclusion.

Thermochemical and acid-catalyzed pretreatments of lignocellulosic biomass materials extensively degrade hemicelluloses (see above, section 2.3.2); depending

on the pretreatment method and the feedstock. However, hemicellulose solubilization may be as high as 100% or as low as 10%, and the hemicellulose sugars may be pres­ent primarily as monomers (xylose, arabinose, etc.) or as oligomers.60 A detailed investigation of destarched corn fiber as a starting material, initially extracted with hot water, showed that 75% of the total hemicellulose could be easily extracted; if enzyme preparations generated by growing Hypocrea jecorina and Aspergillus niger (containing xylanase, P-xylosidase, and feruloyl esterase as well as cellulase) were then used, the recovery of total xylan as xylose increased greatly (from <15% to >50%), and the arabinose yield also improved, although from a much higher starting point of more than 50%; total recoveries of hemicellulosic sugars such as arabinose and xylose reached 80% when supplementations with commercial glucoamylase, P-glucosidase, and feruloyl esterase were included.136 Similarly, a xylose yield of 88% of the theoretical — as the free monosaccharide xylose — was achieved from pretreated corn stover by supplementing cellulase with xylanase, pectinase, and P-glucosidase activities.122 Commercial cellulase preparations contain variable but often high activities of hemicellulases; this may have adventitiously contributed to the production of hemicellulose sugars in lignocellulosic materials processed for ethanol production.137,138