Page 20 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-61 May 19, 2001 19:33

Biomass, Bioengineering of 145

FIGURE 2 Cost of fossil vs. biomass feedstocks.

nutrition. Sugar cane is grown largely in the tropics while Microbial and enzymatic conversion of these sugars in
sugar beets are grown mostly in temperate zones. hemicellulose and cellulose to biobased products is also
Approximately 10 dry tons of a fibrous residue (called significantly limited for the same reasons cellulose and
bagasse) are produced for every ton of cane sugar while hemicellulose are not easily digested by animals. The
about 0.5 dry tons of residue are produced for every ton of resistance of cellulose to conversion to simple sugars is
beet sugar. Worldwide the total production of sugarcane a key limitation in biomass processing that must be over-
bagasse is approximately 800 million metric tons per year. come if biomass processing is to attain its full potential.
These residues have few other uses and represent a very To put the potential of cellulose and hemicellulose in
large potential resource for bioprocessing to biobased in- perspective, the potential sugar that might be produced
dustrial products. from cellulose and hemicellulose in sugar cane bagasse
Sugar cane bagasse consists of approximately 40% alone is approximately six times the total sugar produced
cellulose, 30% hemicellulose and 20% lignin with small worldwide by both sugar cane and sugar beets.
amounts of minerals, sugars, proteins and other com-
pounds. The composition of bagasse is, however, variable
2. Starch Crops
depending on growing conditions, harvesting practices
and processing methods. Beet sugar residue consists of A wide variety of starch crops (mostly grains) are grown
approximately equal portions of cellulose, hemicellulose worldwide, including corn (maize), rice, wheat, manioc
and pectins, with a few percent of lignin. Cellulose and (cassava), barley, rye, potatoes and many more. At least
hemicellulose are polymers of glucose and other sugars. 2 billion tons per year of these crops are produced world-
However, the sugars in cellulose, and to a lesser degree wide. While most sugar is used to feed human beings,
those in hemicellulose, are not very good animal feeds much grain is used to feed animals, particularly in the
and they are essentially indigestible as human foods. moredevelopedcountries.Onekeyindicatorofacountry’s