Hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively. The main hemicelluloses of hardwood are glucuronoxy — lans (O-acetyl-4-O-methylglucurono-b-(1,4)-D-xylan;

GXs), which can also contain small amounts of GMs. In hardwoods, GXs represent 15—30% of their dry mass and consist of a linear backbone of b-(1,4)-D-xylo- pyranosyl units. Some xylose units are acetylated at C2 and C3 and 1 in 10 molecules has an uronic acid group (4-O-methylglucuronic acid) attached by a-(1,2) linkages (Table 17.2). The percentage of acetyl groups ranges between 8% and 17% of total xylan (about 3.5—7 seven acetyl residues per 10 xylose units). The xylosidic bonds

between the xylose units are easily hydrolyzed by acids, but the linkages between the uronic acid groups and xylose are very resistant. Acetyl groups are easily cleaved by alkali, and the acetate formed during kraft (alkaline) pulping of wood mainly originates from these groups. Besides these main structural units, GXs may also contain small amounts of L-rhamnose and galactur — onic acid. The latter increases the polymer resistance to alkaline agents. The average degree of polymerization (DP) of GXs is in the range of 100—200 (Peng et al., 2012; Girio et al., 2010).