3.2.2 Sugar Content of Biological Materials

Figure 3.3 shows a much generalized view of plant cell wall composition. The base molecules that give plants their structure can be processed to produce sugars, which can be subsequently fermented to ethanol. As such, feedstock that can generate sugars more readily and cost-effectively automatically become prime candidates for ethanol fermentation.

The principal components of most plant materials are commonly described as lignocellulosic biomass. This type of biomass is mainly composed of the compounds called cellulose, hemicellulose, and lignin. Cellulose is a pri­mary component of most plant cell walls and is made up of long chains of

the 6-carbon sugar, glucose, that are arranged in bundles (often described as crystalline bundles). The cellulose molecules in the plant cell wall are interconnected by another molecule called hemicellulose. The hemicellulose is primarily composed of the 5-carbon sugar, xylose. In addition to cellu­lose and hemicellulose, another macromolecule called lignin is also present in significant amounts and provides the structural strength for the plant. As also explained in Chapter 4, lignin is not easily converted into sugars or starches by current technology and therefore, has not been a target for alcohol fermentation. However, economically beneficial utilization of lignin is quite important in enhancing the overall process economics of ethanol production as well as minimizing process wastes. Technological develop­ments have recently introduced a variety of processes of extracting and dis­solving the cellulose and hemicellulose to produce sugars in such a form that can be readily fermented to ethanol. Generally speaking, efficient pretreat­ment can liberate the cellulose and hemicellulose from the plant material. Further treatment using chemicals, enzymes, or micro-organisms can also be applied to liberate simple sugars from the cellulose and hemicellulose, thus making them available to micro-organisms for fermentation to ethanol.