Softwoods polyoses

The main polyoses of softwoods are galactoglucomannan, with an average ratio of glucose to mannose of 1 3 and an average acetyl content of 1.3% (based on wood), and arabino-4-O-methylglucuronoxylan. 

On the other hand, certain types of wood pulp, prepared by se((uences involving both acidic and alkaline treatments, show a linear relationship between gamma-cellulose and non-cellulosic polyoses. This may be illus-strated from the studies of White, Steinman and Work on acetylation pulps. Acetylation-grade pulps are usually prepared from softwoods by sulfite cooking followed by hot-caustic refining. When the total polyose content of a number of such pulps was compared with their gamma-cellulose content, a linear relationship was obtained (see Fig. 3). In spite of this excellent correlation, it cannot be assumed that all of the non-cellulosic polyoses are contained in the gamma fraction of these pulps (see below). 

Figure 4.2.24 Typical structures of hemicelluloses or polyoses hardwood D-xylan and glucomannan and softwood galactan.

Even if, in the first instance, it might seem that this material does not vary a lot, the different amounts and differing chemical composition of the polyoses (see Section 9.4.2.3) and the lignins (see Section 9.4.2.4) in hardwood and softwood strongly affect the manufacturing processes. This applies for the extractives too, which due to their large chemical diversity influence some of the properties of wood and behave respectively different with regard to the chemical utilization of the wood. 

As already mentioned, there are differences in the content and the composition of polyoses in hardwood and softwood. While xylans (pentosans) are dominant in hardwood, mannans (hexosans) predominate in softwood. The xylans are so-called homopolymers, as the main chain consists of xylose only (Scheme 9.4.2), whereas the mannans present a heteropolymer consisting of glucose and mannose (Scheme 9.4.3). Furthermore, softwood has a higher content of polyoses as well as acetyl groups compared to softwood. The higher content of acetyl groups in hardwood can cause elimination of acetic acid in certain procedures e.g. steaming of beech or thermal modification. Section 9.4.5). 

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