In addition to xylan, hardwoods contain 2—5% of a GM, which is composed of b-glucopyranose and b-mannopyranose units linked by b-(1 / 4) bonds (Table 17.2). However, the mannose/glucose monomer ratio may vary depending on the original source of GM. The ratio of glucose to mannose varies between 1:2 and 1:1. Galactose is not present in hardwood mannan. The mannosic bonds between the mannose units are more rapidly hydrolyzed by acid than the cor­responding glycosidic bonds, and GM is easily depoly — merized under acidic conditions. There may be certain short side branches at the C3 position of the mannoses and acetyl groups randomly present at the C6 position of a sugar unit. The acetyl groups frequently range from 1 per 9 to 1 per 20 sugar units (Peng et al., 2012).

Xyloglucans

Besides xylan and GM, xyloglucans (XGs) are also present in the primary cell walls of some higher plants (mainly in hardwoods, and less in softwoods) (Peng et al., 2012). They can also appear in small amounts (2—5%) in grasses. XGs consist of b-1,4-linked D-glucose (cellulosic) backbone with 75% of these residues substituted at O-6 with D-xylose. L-Arabinose and D-galactose residues can be attached to the xylose resi­dues forming di — or triglycosyl side chains. Also L-fucose has been detected attached to galactose residues. In addition, XGs can contain O-linked acetyl groups. XGs interact with cellulose microfibrils by the formation of hydrogen bonds, thus contributing to the structural integrity of the cellulose network (Girio et al., 2010).

Galactoglucomannans

The major hemicelluloses in softwoods (gymno — sperms) are acetylated GGMs, accounting up to 20—25% of their dry mass (Girio et al., 2010). GGMs consist of a linear backbone of b-D-glucopyranosyl and b-D-mannopyranosyl units, linked by b-(1,4) glycosidic bonds, partially acetylated at C2 or C3 and substituted by a-D-galactopyranosyl units attached to glucose and mannose by a-(1,6) bonds. Acetyl groups content of GGM is around 6%, corresponding, on average, to one acetyl group per three to four hexose units (Girio et al.,

2010) (Table 17.2). Some GGMs are water soluble, presenting in that case higher galactose content than the insoluble GGMs. There are two main types of acetylgalactoglucomannans in softwoods, one being galactose-poor (5—8% of dry wood) and the other galactose-rich (10—15% of dry wood). The ratio of galactose:glucose:mannose is approximately 0.1:1:3 and 1:1:3, for the two woods, respectively (Peng et al.,

2012) . GGMs have an approximate DP between 100 and 150, which is equivalent to a molecular weight around 16,000—24,000 Da. GGMs are easily depolymer — ized by acids, especially the bonds between galactose and the main chain. The acetyl groups are much more easily cleaved by alkali and acid (Peng et al., 2012). GMs occur in minor amounts in the secondary wall of hardwoods (<5% of the dry wood mass) (Girio et al.,

2010) . As GGMs, they have a linear backbone of b-D-glucopyranosyl (Glcp) and b-D-mannopyranosyl (Manp) units but the ratio Glcp:Manp is lower. In GGMs and GMs the extent of galactosylation governs their association tendency to the cellulose microfibrils and hence their extractability from the cell wall matrix (Ebringerova et al., 2005).