Classification and Distribution of Lignin

Both Adler and Freudenberg have constructed structural schemes for spruce lignin and Nimz for beech lignin in which the main units and linkage types are represented in their approximate proportions. The scheme by Adler is depicted in Fig. 4-9. 

Many studies have indicated that covalent linkages must exist between lignin and wood polysaccharides. Separation and analysis of lignin -carbohydrate complexes (LCC) have led to the conclusion that the hemicel-lulose components (xylan and galactoglucomannans in softwood) are bound to lignin mainly through arabinose, xylose, and galactose moieties as shown in Fig. 4-10. 

The lignin concentration is high in the middle lamella and low in the secondary wall. Because of its thickness, at least 70% of the lignin in softwoods is, however, located in the secondary wall as shown by quantitative UV microscopy (Fig. 4-11 and Table 4-4). The picture is very similar for 

TABLE 4-4. Distribution of Lignin in Spruce (Black Spruce, Picea mariana) Tracheid  

Wood Morphological region6 Tissue volume (%) Lignin (% of total) Lignin concentration (%) 

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