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10 Enzyme Technology for the Degradation of Lignocellulosic Waste

D-galacturonic acids and α-1,2-linked L-rhamnose. L-arabinose and D-galactose can

form side chains by linking to the rhamnose residue. Substituted phenylpropane

units joined by carbon–carbon linkages form the backbone of lignin. Lignin forms

a cross-linked network within the cell wall which gives mechanical strength and

protection against osmotic lysis to the cell wall [7]. The four bonds viz. ether bonds,

ester bonds, carbon–carbon bonds, and hydrogen bonds provide intramolecular and

intermolecular linkages within and between various components of lignocellulose.

Degradation of lignocellulosic waste is difficult since it is hard to dissolve lignin

and its subunits. Lignin is also linked to cellulose and hemicellulose. Because of the

crystalline nature, cellulose is hard to degrade [8]. Fungi and other microorganisms

which are able to break the lignocellulosic wastes do so by secreting various enzymes

that act synergistically. Some monomers are released during the process which are

utilized by the microorganisms as their energy source [3]. The lignocellulosic wastes

can be utilized to develop various value-added products like animal feed, composites,

enzymes, biofuels, pulp, and paper.

10.2

Enzymes Required for the Degradation

of Lignocellulosic Waste

10.2.1

Degradation of Cellulose

Cellulose is a linear chain polysaccharide comprising D-glucose monomers linked

by β-1,4-glycosidic bonds [1]. Adjacent polysaccharide chains are linked by van der

Waals forces and hydrogen bonds. Cellulose is crystalline in nature, and the different

forms are cellulose I, II, III, and IV [9]. Cellulose is insoluble in water, but the solu-

bility increases under specific conditions. Acid and alkali treatment can break down

cellulose into glucose units. Cellulose forms aggregates with the rise in temperature

and/or becomes recalcitrant with dehydration [10].

10.2.1.1

Microbial Production of Cellulase

Both solid-state and submerged fermentations are used for the production of cellu-

lase from substrates like corn cob, wheat bran, and wheat and rice straws [11, 12].

Solid-state fermentation is more economical than submerged fermentation. The

commonly used and most efficient fungus for cellulase production is Trichoderma

reesei. Other fungi viz. Trichoderma longibrachiatum and Aspergillus niger are

also used for bioprocessing [13]. Water hyacinth can also produce cellulase, and

maximum enzymatic activity was observed at pH 5.0 and 40 ^∘C temperature [14].

The substrates used by fungi for cellulase production are shown in Table 10.1. The

Aspergillus japonicus C03 produces endocellulase via solid-state fermentation [15].

Certain bacteria are also capable of producing cellulase. The Clostridium,

Ruminococcus, Streptomyces, Pseudomonas fluorescens, Bacillus subtilis CEL PTK1,

Acidothermus sp., Rhodothermus marinus, and B. subtilis AS3 are some of the

bacteria commonly used in cellulase production [1]. The Cellulomonas bioazotea,

Cellulomonas fimi, and Streptomyces sp. are some of the actinobacteria also used for

the same purpose [16].