RUENDEFISV

Atlas

Hemicelluloses

Hemicelluloses are the younger relatives of cellulose. Their chains are shorter, branched and built from several sugars. They coat the cellulose threads and tie them to lignin.

Hemicelluloses are cell wall heteropolysaccharides with a degree of polymerisation around 100-200. Unlike cellulose they are branched and amorphous, hydrolyse more easily and dissolve in alkali. Softwoods and hardwoods carry fundamentally different hemicelluloses.

Schematic structure of the main hemicelluloses. Unit ratios are approximate.

Which sugars

Cellulose is made of glucose only. Hemicelluloses contain several sugars: glucose, mannose, galactose, xylose, arabinose, plus sugar acids and acetic acid residues.

Hemicelluloses contain hexoses (D-glucose, D-mannose, D-galactose), pentoses (D-xylose, L-arabinose), uronic acids (mainly 4-O-methyl-D-glucuronic acid) and acetyl groups. By the main chain they are divided mainly into mannans and xylans; larch wood also contains much water-soluble arabinogalactan.

Softwood hemicelluloses

In pine and spruce the main hemicellulose is galactoglucomannan: a chain of mannose and glucose with occasional galactose side units. Second comes a xylan with arabinose and acid side groups.

Galactoglucomannans (about 20% of softwood) have a backbone of β-(1→4)-linked D-mannopyranose and D-glucopyranose units with α-(1→6)-linked galactose side units; some hydroxyls are acetylated. Low- and high-galactose fractions are distinguished. Arabino-4-O-methylglucuronoxylan (5-10%) has a β-(1→4)-xylose backbone with 4-O-methylglucuronic acid and arabinofuranose side units and is not acetylated.

Hardwood hemicelluloses

In birch and aspen almost all hemicellulose is xylan: a xylose chain carrying acetic acid residues and occasional acid groups. There is very little glucomannan.

O-acetyl-4-O-methylglucuronoxylan makes up 15-30% of hardwood. Per 10 xylose units there are about 7 acetyl groups at C2 or C3 and about one 4-O-methylglucuronic acid unit at C2. Glucomannan (2-5%) consists of glucose and mannose in a ratio between 1:2 and 1:1.

Role in the cell wall

Hemicelluloses act as an interlayer. They stick to cellulose threads and are chemically bound to lignin. They also attract water most strongly, so wood swelling depends largely on them.

Hemicelluloses adsorb onto microfibril surfaces and form covalent lignin-carbohydrate bonds (ether, ester, glycosidic). Being amorphous with many accessible hydroxyls, they make the main contribution to water sorption by the cell wall.

In pulping and biorefining

In pulping, hemicelluloses suffer first: part of them dissolves and fibre yield drops. On the other hand they can be turned into valuable products: xylitol, furfural, acetic acid.

In alkali, polysaccharides depolymerise from the reducing end (the peeling reaction); glucomannans are lost more than xylans, and part of the dissolved xylan re-adsorbs onto the fibres. Acid hydrolysis turns xylans into xylose, which gives xylitol by hydrogenation and furfural by dehydration; acetyl groups are released as acetic acid.

Key facts

DP
~100-200
softwood: GGM
about 20%
softwood: xylan
5-10%
hardwood: xylan
15-30%
acetyl in xylan
~7 per 10 Xyl
backbone
β-(1→4)

Sources for this chapter

  1. Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
  2. Ek M., Gellerstedt G., Henriksson G. (eds.) Wood Chemistry and Wood Biotechnology. Pulp and Paper Chemistry and Technology, vol. 1. Berlin: De Gruyter, 2009.
  3. Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
  4. Fengel D., Wegener G. Wood: Chemistry, Ultrastructure, Reactions. Berlin; New York: De Gruyter, 1984.