ignin is an aromatic and rigid biopolymer with a molecular
weight of 10,000 Da bonded via covalent bonds to xylans (hemicellulose portion) conferring rigidity and high level of compactness
to the plant cell wall [66]. Lignin is composed of three phenolic
monomers of phenyl propionic alcohol namely, coumaryl, coniferyl
and sinapyl alcohol. Forest woody biomass is primarily composed
of cellulose and lignin polymers. Softwood barks have the highest
level of lignin (30e60%) followed by the hardwood barks (30e55%)
while grasses and agricultural residues contain the lowest level of
lignin (10e30% and 3e15%, respectively)[55,64]. Conversely, crop
residues such as corn stover, rice and wheat straws are comprised
mostly of a hemicellulosic heteropolymer that includes a large
number of 5-carbon pentose sugars of primarily xylose [74].
Previously, little interest has been given to lignin chemistry
potential on hydrolysis. However, lignin components are gaining
importance because of their dilution effect on the process once
solids are added to a fed batch hydrolytic or fermentation bioreactor in addition to their structure and concentration effects that
would affect potential hydrolysis[75]. For instance, the adsorption
of lignin to cellulases requires a higher enzyme loading because this
binding generates a non-productive enzyme attachment and limits
the accessibility of cellulose to cellulase[76]. Furthermore, phenolic
groups are formed from the degradation of lignin.
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