Lignocellulose consists of three polymer types, cellulose, hemicellulose and lignin, which are the main constituents of plant cell walls (Fig. 6.4). The primary cell wall consists of cellulose fibres embedded in a polysaccharide matrix of hemicellulose and pectin. Cellulose is the most abundant plant compound and the second most abundant is lignin which provides mechanical support and protection in plants. The composition of some lignocellulose sources is given in Table 6.7 in terms of lignin, cellulose and hemicellulose. Cellulose is a polymer of glucose linked together by 1,4-glycosidic bonds. Hemicellulose is heterogeneous polymers with a backbone of 1,4-linked xylose residues but contains short side chains containing other sugars such as galactose, ara — binose and mannose. Xylose is the predominant sugar in hardwoods and arabinose in agricultural residues.

Lignin is a highly branched polymer of phenyl-propanoid groups such as con — iferyl alcohol (Fig. 6.4). Cellulose represents 40-50% of dry wood, hemicellulose 25-35% and lignin 20-40% depending on the plant type. Although lignocellulose is abundant, it cannot be metabolized by yeast and therefore needs to be broken down to its constituent sugars before it can be used. In addition not all yeast can metabolize the pentose sugars derived from the hemicellulose. The composition of agricultural lignocellulose sugars is shown in Table 6.8. Saccharomyces cerevisiae can ferment

glucose, mannose and galactose, but not the other sugars. Depending on the source of lignocellulose the sugar produced will change. Glucose and xylose are the main sugars in the agricultural lignocellulose except in switchgrass. Investigations are under way to find organisms which can ferment these other sugars and produce etha­nol or to genetically manipulate yeasts to be able to metabolize these sugars.

Lignocellulose is difficult to break down into sugars, but a number of technolo­gies are under investigation including enzymes. Because of the presence of hemicel — lulose and lignin and the crystalline nature of cellulose in lignocellulose some form of pretreatment is required before enzymatic or chemical hydrolysis. These pretreat­ments are shown in Fig. 6.10, and include carbon dioxide, steam and ammonia explo­sion, mechanical grinding, acid, white rot fungi treatment and ozonolysis.