Hardwoods polyoses

The main polyoses of hardwoods are 4-O-methylglucuronoxylan, with an average acetyl content of 4% (based on wood), glucomannan, with an average ratio of glucose to mannose of 1 2, and arabinogalactan. 

The empirical nature of this fractionation may be appreciated by comparing the composition of a number of different preparations of alpha-cellulose. The alpha-cellulose from such coniferous woods as spruce, pine, and hemlock may contain appreciable amounts of mannan, although the xylan content is usually low. Similarly, the alpha-cellulose from hardwoods may contain appreciable quantities of non-cellulosic polyoses, although, in this case, the chief polyose is xylan, not mannan. 

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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