Hemicelluloses are, after cellulose, the second most abundant biomass polysaccharide in the world and they comprise about one third of wood material (see Table I). Other chemical components in the wood material are lignin, extractives, inorganics, and polysaccharides other than cellulose and hemicelluloses. (Sjöström 1993)
Table I General chemical composition of softwood and hardwood (Alén 2000) and more detailed chemical composition of pine (Pinus sylvestris) and birch (Betula pendula) as described in Sjöström (1993).
Wood species
Cellulose,
%
Hemicelluloses,
% Other
polysaccharides,
%
Lignin,
%
Extractives,
%
Inorganics, GGM Xylan %
Soft-wood 40 25-30 0-5 25-30 4-6 <0.5
Pinus
sylvestris 40 16.0 8.9 3.6 27.7 3.5 0.3
Hard-wood 40 30-35 0-4 20-25 2-4 <0.5
Betula
pendula 41 2.3 27.5 3.4 22.0 3.2 1.4
The term hemicellulose was originally proposed by Schulze (1891) to designate the polysaccharides extractable from plants by aqueous alkali. Initially, these polysaccharides were mistakenly believed to represent an intermediate material of the biosynthesis of cellulose. However, it turned out that these polysaccharides represent a distinct and separate group of plant polysaccharides and later in the 20th century many other terms, such as polyoses, cellulosans, polyuronides, and non-cellulosic carbohydrates were proposed for hemicelluloses by several researchers (Wise 1949). However, the term hemicellulose is still valid today and several reviews on the chemistry and biochemistry of hemicelluloses are available (see e.g., Scheller and Ulvskov 2010; Pauly et al. 2013).
Hemicelluloses designate the cell-wall polysaccharides of land plants, excluding cellulose, other miscellaneous polysaccharides (starch, callose, laricinan, xyloglucan, fucoxyloglucan, and rhamnoarabinogalactan), and pectin components (galactouronans, galactans, and arabinan). Hemicelluloses in their natural form are generally water-insoluble, alkali-soluble
substances that are more readily hydrolyzed by acid than cellulose. However, some hemicelluloses, such as fragments of hardwood xylan and the arabinogalactan found especially in larch species are partly or even totally water-soluble. Structurally, hemicelluloses differ from cellulose in that they are branched and have much lower molecular weights, i.e., degree of polymerization (DP). For hemicelluloses the DP is on average between 100 and 200, whereas for native cellulose it is around 10000. Unlike cellulose, hemicelluloses have a random and amorphous structure. Thus, the chemical and thermal stability of hemicelluloses is lower than that of cellulose. (Sjöström 1993; Alén 2000)
In general, the hemicellulose fraction of woods comprises a collection of polysaccharide polymers, polymers consisting of several different kinds of pentoses (D-xylose and arabinose), hexoses (D-mannose, D-glucose, and D-galactose), and deoxyhexoses (e.g., L-rhamnose). Small amounts of sugar acids (4-O-methyl-D-glucuronic acid, D-galacturonic acid, and D-glucuronic acid) can also be present. Hemicelluloses consist of more than one type of these sugar units and are sometimes referred to in terms of the sugars they contain, for example, galactoglucomannan (GGM), arabinoglucuronoxylan (AGX), arabinogalactan (AG), glucuronoxylan (GX), and glucomannan (GM). The proportion and chemical composition of hemicelluloses differ in softwoods and hardwoods, while cellulose is a relatively uniform component of all wood species. (Sjöström 1993; Alén 2000)
2.1.1 Softwood hemicelluloses
The major constituents of the hemicelluloses in softwoods are GGMs (15-20% odw), and AGX (5-10% odw). The former contain a backbone polymer of D-glucose and D-mannose.
The backbone of GGM is a linear or slightly branched chain of β-(1→4)-linked D-mannopyranose (β-D-Manp) and D-glucopyranose (β-D-Glcp) units. D-galactopyranose (α-D-Galp) residues are linked as single-unit side chains by α-(1→6) bonds. GGM in softwood can be roughly divided into two types: one with low galactose content (10-15% odw), and the other with high galactose content (5-8% odw). The low-galactose fraction has a ratio of galactose:glucose:mannose of about 0.1:1:4 and the high-galactose fraction has a ratio of 1:1:3. In both cases, the acetyl content is about 6% of the total GGM, i.e., the C2-OH and C3 -OH positions of the backbone polymer have acetyl groups substituted on them an average of every three to four hexose units. Often, the two forms of GGM are simply called glucomannan. (Sjöström 1993; Alén 2000)
Softwood AGX (5-10% odw) has a backbone of β-(1→4)-linked xylopyranose (β-D-Xylp) units. Single-unit side chains are pyranoid 4-O-methyl-α-D-glucuronic acid (4-O-Me-α-D-GlcpA) units attached by α-(1→2) bonds on an average of every two to ten xylose units and L-arabinofuranose (α-L-Araf) units attached by α-(1→3) bonds on average every 1.3 xylose units. The typical ratio arabinose:glucuronic acid:xylose is 1:2:8. Another softwood hemicellulose is arabinogalactan, AG. However, AG occurs significantly only in the heartwood of larches (10-20% odw), whereas in other softwoods the concentration is generally less than 1% odw. Figure 2 depicts the structure of the major softwood hemicelluloses, GGM and AGX. (Sjöström 1993; Alén 2000)
Figure 2 Partial chemical structure with average molar ratios of the major softwood hemicelluloses GGM and AGX according to Alén (2000) and Teleman (2009).
2.1.2 Hardwood hemicelluloses
In the case of hardwoods, the predominant components of the hemicelluloses are O-acetyl-(4-O-methylglucurono)xylan, or simply GX (15-30% odw), together with small amounts of GM (2-5% odw). As in softwood AGX, the backbone of hardwood GX consists of β-(1→4)-linked
β-D-Xylp units with acetyl groups at C2-OH and C3-OH of the xylose units on an average of 3.5-7 acetyl groups per ten xylose units, corresponding to an acetyl content of 8-17%. The GX is substituted with side chains of 4-O-Me-α-D-GlcpA units α-(1→2) linked to the GX backbone with an average frequency of approximately one uronic acid group per ten xylose units. In addition to these main structural units, GX contains a small amount of L-rhamnose (α-L-Rhap) and galacturonic acid (α-D-GalpA). Rhamnose is present mainly in the reducing end-groups of glucuronoxylan and galacturonic acid in the side groups. Hardwood GM consists of β-(1→4)-linked β-D-Manp and β-D-Glcp units with no side chains attached to it.
However, recent findings show that native hardwood GM is partially O-acetylated to the C2 -OH and C3-OH position of some of the mannose residues (random distribution), with a degree of acetylation of approximately 0.3 (Teleman et al. 2003; Pawar et al. 2013). The glucose to mannose ratio of hardwood GM varies between 1:2 and 1:1 depending on the wood species.
Figure 3 depicts the structure of the major hardwood hemicelluloses, GX and GM. (Sjöström 1993; Alén 2000; Teleman 2009)
Figure 3 Partial chemical structure with average molar ratios of the major hardwood hemicelluloses GX and GM according to Alén (2000) and Teleman (2009).
2.1.3 Differences between hardwood and softwood hemicelluloses
Although the hemicelluloses in softwoods and hardwoods are fairly similar, there are distinct structural differences. Softwood hemicelluloses have a high proportion of mannose units and more galactose units than hardwood hemicelluloses, whilst hardwood hemicelluloses have a high proportion of xylose units and more acetyl groups than softwoods. Furthermore, softwood xylans differ from hardwood xylans by the lack of acetyl groups and, unlike hardwood xylans, softwood xylans contain α-L-Araf side units. In addition to the aforementioned, hemicelluloses from both softwoods and hardwoods also contain other polysaccharide groups, usually present in minor quantities. These might be important components for the living tree, but are of little interest when considering the technical applications. Table II summarizes the main structural features of hemicelluloses appearing in both softwoods and hardwoods. (Sjöström 1993; Alén 2000)
Table II The major hemicellulose components in softwood (SW) and hardwood (HW).
Modified from Sjöström (1993).
1Approximate values, 2On average, 3Partial solubility
SW=softwood, HW=hardwood, p=pyranose, f=furanose, Me=methyl, Man=mannose, Glc=glucose, Gal=galactose, Ara=arabinose, Xyl=xylose, and GlcpA=glucuronic acid.