184 PLANT BIOCHEMISTRY 



from less than 0.5 jier cent in cotton fiber to 40 per cent in hybrid 

 corncobs. Table 8-1 lists reported cellulose, hemicellulose, and lignin 

 contents of some of the agricultural sources of cellulose. 



TABLE 8-1. Cellulose, Hemicellulose, and Lignin Contents 



of Plant Tissues 



Tissue Cellulose, % Hemicellulose, % Lignin, % 



By far the most common polysaccharides in the hemicellulose mix- 

 ture are the xylans. These pentosans are composed of D-xylose units 

 coupled by 1-^4^ linkages with or without branching. Xylans occur 

 in the secondary cell walls of practically all land plants. In some of 

 them, xylans may contain, or be closely associated with, L-arabinose. 

 Pentosans of esparto grass are reported to contain 1 arabinose unit per 

 18 xylose residues. 



In addition to xylans and arabans, the hemicellulose fraction con- 

 tains polysaccharides composed of D-glucuronic or D-galacturonic acids 

 in combination with other pentoses and hexoses. Some of the hard- 

 wood hemicelluloses are reported to contain as high as 18 per cent 

 uronic acid. 



The hemicelluloses of oat hulls or corncobs are the point of origin 

 of the important industrial chemical, furfural. Development of produc- 

 tion methods for furfural can be attributed to an accumulation of oat 

 hulls and the resulting disposal problem. Chemists at Quaker Oats 

 Company found that they could produce furfural in commercial quan- 

 tities by digesting the hulls with sulfuric acid followed by distillation. 

 After developing the production methods, it was then necessary to find 

 a market for the product. Today furfural is an important industrial 

 solvent, particularly in petroleum refining, as well as an intermediate 

 in chemical reactions. Its production may be pictured as follows: 



