Fermentable Sugars

Fermentable sugars are the largest feedstock available to support a bio-based chemicals industry in the United States. A wide range of fermentable sugars can be found in crops and wastes from agriculture and forestry. Major feedstocks include corn, wheat, sorghum, potato, sugarbeet, and sugarcane; other sources include potato-processing residues, sugarbeet and cane molasses, and apple pomace (Polman 1994). Sugars can be produced directly or derived from polysaccharides such as cellulose and starch and then, via microbial fermentation, used to produce a wide range of commodity and specialty chemicals. Existing commercial fermentation primarily utilizes glucose to produce ethanol, acetic acid, amino acids, antibiotics, and other chemicals. Over the long term, new sources of glucose will be required to meet the demands of a bio-based industry. Growth of a bio-based chemicals industry will depend on production of cellulose-rich crops, including those currently under production such as corn and alfalfa and others that presently are not grown commercially, such as switchgrass and hybrid poplar.

Significant increases in glucose reserves are available from lignocellulosic substances found in most plants, crop residues, and waste paper. Cellulose can be hydrolyzed by acid to glucose, although much of the glucose is destroyed during this process. The second most abundant sugar found in hardwood and agricultural residues is xylose, which is derived from xylan hemicelluloses. Xylose is relatively easily recovered by acid or enzymatic hydrolysis and can be fermented to ethanol by naturally occurring organisms or recombinant microbes. The practical sugar yield from lignocellulosics would increase significantly if commercial fermentations could utilize xylose, a 5-carbon sugar or pentose, as well as glucose, a 6-carbon sugar or hexose. Novel genetically engineered microorganisms will play a key role in the direct conversion of cellulose oligomers and 5- and 6-carbon sugars to ethanol.

Enxymatic hydrolysis using mixtures of enzymes, such as cellulase and hemicellulaces, is used to avoid the destruction of sugars associated with acid treatments (hydrolysis) of lignocellulosic material. These enzymes, when combined with effective pretreatment of lignocellulosics, provide high yields of glucose, xylose, and other fermentable sugars with minimal sugar losses. However, these enzymes are currently too costly to use in large-scale conversion of lignocellulosic materials to fermentation substrates.