17.5 Fermentation

277

with arabinose, xylose, galactose, fucose, mannose, glucose, or glucuronic acid [32].

Hemicellulose and the cellulose microfibrils fashion the hydrogen bonds, providing

the auxiliary spine to the plant cell divider [33].

17.4.1

Hydrolysis of Polysaccharides

17.4.1.1

Cellulose and Hemicellulose Degrading Enzymes and Mechanisms

Cellulases catalyze the breakdown of β-(1,4) linkages within cellulose. Conversion

of cellulose to glucose requires three separate catalysts – cellobiohydrolase, endoglu-

canase, and β-glucosidase. While the endoglucanases hydrolyze the β-(1,4) glyco-

sidic linkages present in the cellulose chain, cellobiose is converted to glucose by

the β-glucosidase, leaving the cellobiohydrolase to remove the individual cellobiose

units from the terminal end of the chain [36]. Cellulases have a sugar-attaching

region which is associated with the enzymatic area by an adaptable linker. All these

modules play a vital role in associating the enzymes with cellulose, thereby enhanc-

ing the enzymatic activity of cellulase [38].

Hemicellulose being a branched polymer is made up of a mixture of monomeric

units of sugar and glucose. The most prevailant hemicelluloses are xylan, which is

made up of pentose units, such as xylose. Enzymes called xylanases catalyze the

hydrolysis of xylan. For the complete hydrolysis of xylan, the activity of different

xylanases, each with different specifications and activities, is essential [31]. Soft-

wood hemicelluloses are essentially made out of arabinogalactans, glucomannans,

xyloglucans, and arabinoglucuronoxylans; hard woods are mostly made out of

xylans and glucomannans [39].

Microorganisms used for the commercial production of xylanase on a large scale

include Bacillus sp., A. niger, Humicola insolens, and T. reesei.

17.5

Fermentation

17.5.1

Microorganisms Involved in Fermentation

The most well-known and broadly utilized microorganism for bioethanol formation

is a yeast (S. cerevisiae), appropriate for LCB fermentation. It can effectively ferment

hexa carbon sugars, be that as it may, barely pentoses because of the absence of

proteins that transform xylose to xylulose. The basic bacterial species utilized

for bioethanol formation is Zymomonas mobilis. Some thermophilic anaerobic

microscopic organisms, for example, Clostridium thermohydrosulfuricum, Ther-

moanaerobacter ethanolicus, Thermoanaerobium brockii, Thermoanaerobacter

mathranii, and Clostridium thermosaccharolyticum, have been examined for

bioethanol production. Despite the fact that most microscopic organisms have a

wide substrate run, ethanol is once in a while the single item of their digestion that

makes downstream processing challenging of ethanol product recovery.