CELLULOSIC FEEDSTOCKS
Like sugar materials, starchy materials are also in the human food chain and are thus expensive. Fortunately, a third alternative exists—cellulosic materials. Examples of cellulosic materials are paper, cardboard, wood, and other fibrous plant material.
Cellulosic resources are in general very widespread and abundant. For example, forests comprise about 80% of the world’s biomass. Being abundant and outside the human food chain makes cellulosic materials relatively inexpensive feedstocks for ethanol production.
Cellulosic materials are comprised of lignin, hemicellulose, and cellulose and are thus sometimes called lignocellulosic materials. One of the primary functions of lignin is to provide structural support for the plant. Thus, in general, trees have higher lignin contents then grasses. Unfortunately, lignin which contains no sugars, encloses the cellulose and hemicellulose molecules, making them difficult to reach.
Cellulose molecules consist of long chains of glucose molecules as do starch molecules, but have a different structural configuration. These structural characteristics plus the encapsulation by lignin makes cellulosic materials more difficult to hydrolyze than starchy materials.
Hemicellulose is also comprised of long chains of sugar molecules; but contains, in addition to glucose (a 6-carbon or hexose sugar), contains pentoses (5-carbon sugars). To complicate matters, the exact sugar composition of hemicellulose can vary depending on the type of plant.
Since 5-carbon sugars comprise a high percentage of the available sugars, the ability to recover and ferment them into ethanol is important for the efficiency and economics of the process. Recently, special microorganisms have been genetically engineered which can ferment 5-carbon sugars into ethanol with relatively high efficiency.
One example is a genetically engineered microorganism developed by the University of Florida that has the ability to ferment both 5- and 6-carbon sugars. This microorganism was issued US patent 5,000,000. Other researchers have developed microorganisms with the ability to efficiently ferment at least part of the sugars present.
Bacteria have drawn special attention from researchers because of their speed of fermentation. In general, bacteria can ferment in minutes as compared to hours for yeast.
