5. Polyoses (Hemicelluloses)

Abstract

In addition to cellulose a number of various polysaccharides called polyoses or hemicelluloses are present in wood as well as in other plant tissues. The name hemicelluloses can be traced back to E. Schulze (1891), and it is based on the assumption that these polysaccharides are precursors of cellulose. Though in the scientific sector the term hemicelluloses is well defined, in the technical sector misunderstandings still exist. Thus the extractives in the alkalizing liquor from chemical grade pulp consisting of polyoses and short-chain cellulose are called hemicelluloses, or the term "hemicelluloses" is explained as "low molecular cellulose" (Bauer 1970). To avoid ambiguity in this book the non-cellulosic polysaccharides are called polyoses. This term ("Holzpolyosen") was first used by Staudinger and Reinecke (1939). (Fig. 5-1). The main chain of a polyose can consist of only one unit (homopolymer), e.g. xylans, or of two or more units (heteropolymer), e.g. glucomannans. Some of the units are HO HO, The polyoses differ from cellulose by a composition of various sugar units, by much shorter molecular chains, and by a branching of the chain molecules. The sugar units (anhydro-sugars) making up the polyoses can be subdivided into groups such as pentoses, hexoses, hexuronic acids and deoxy-hexoses Penhises | HexosesHexuronic acids Deoxy-hexoses 0 K OH CH 2 0H C00H HO. J-0 HO J 0 108 Polyoses (Hemicelluloses) useful for many years. In this system the polyoses are classified as xylans, mannans, galactans etc.A general classification comprising all plant carbohydrates was presented by Aspinall (1973). His system consists of the following groups: -cellulose; -hemicelluloses: xylans, glucomannans; -pectic substances: galacturonans, arabinans, galactans and/or arabinogalactans I (essentially linear chains) -other polysaccharides: arabinogalactans II (highly branched chains), fuco-(or galacto-)xyloglucans; -glycoproteins.Softwoods and hardwoods differ not only in the percentages of total polyoses but also in the percentages of individual polyoses and composition of these polyoses. Regarding the non-glucosic sugar units present in wood it can be noticed that softwoods have a high proportion of mannose units and more galactose units than hardwoods, and hardwoods have a high proportion of xylose units and more acetyl groups than softwoods (Table 5-1). Previous results in the field of polyoses chemistry have been summarized by