Lignocellulose (LC) is the most abundantly available biomass in plant community. This can potentially be used as cheap substrate for the production of fermentable sugar via microbial enzymatic hydrolysis. Lignocellulosic biomass (LCB) includes; mainly, agricultural wastes e.g. wheat straw, rice husk, sugarcane baggage, maize and corn stalks. These LCBs contains 35- 50% cellulosic fraction, 25 - 30% hemicelluloses and 20 - 25% lignin. The cellulosic biomass present inside LCBs, could be converted into different value added products such as; Bio-fuel (Ethanol, hydrogen and methane), Butanol and organic acids (e.g. Citric acid and lactic acid).
LC is building block of plant cell wall that consists following:
Lignin; polymer of phenyl-propane unit,
Hemicelluloses; consists pentose and hexose sugar residue (Hetero-polymer and branched), and
Cellulose; consists glucose unit (Homo-polymer and un-branched).
Structurally cellulosic fraction closely bound within lignin net. This Lignin contains three cross-linked aromatic alcohols namely; coniferyl, sinapyl and p-coumaryl. These aromatic polymers prevent microbial enzymatic attack on cellulosic fraction of LC. Thus lignin considered as main obstacle for the efficient bio-utilization of cellulose from lignocellulosic biomass.
To overcome this limitation, Pretreatment is an option to remove lignin fraction from LCB. This results the significant availability of cellulosic fraction for microbial enzymatic attack. When microorganism grows on cellulose (supplemented as a substrate), they release cellulase enzyme to breakdown cellulose to its monomer (D-Glucose). This sugar further used for the production of value added products via fermentation.
Cellulose is crystalline, insoluble and fibrous polysaccharide composed of repeating (1, 4)-D-glucopyranose (simply we can say Glucose) units, attached by Β-1,4 linkages. Most of Bacterial and fungal consortia produced enzyme cellulase that potentially degrade cellulose into its monomer sugar. Cellulase enzymatic system include three different enzyme, (1) Exo-Β-1, 4-glucanases (EC 18.104.22.168), (2) Endo-Β-1, 4-glucanases (EC 22.214.171.124), and (3) Β-1, 4-glucosidase (EC 126.96.36.199). These enzymes synergistically participate on sequential breakdown of cellulose to an utilizable energy form (like glucose or fructose). The endo-Β-1,4-glucanases randomly hydrolyzes the Β-1,4 bonds in the cellulose molecule, and the exo- Β-1,4-glucanases produce a cellobiose (small chain of 1,4-D-glucopyranose units) unit. Finally, the cellobiose is converted to glucose by Β-1, 4-glucosidase.
Research review revealed that wide variety of microbial strains; especially, fungus and bacterial, as potential cellulose degradation. Fungal strain Trichoderma Reesei, one of the potential cellulase producing strain and for this reason, T. reesei have been used for the commercial production of cellulases. Aspergillus niger, Phanerochaete chrysosporium and Gloeophyllum trabeum were also reported for efficient cellulase production. Among bacterial consortia, Cellulomonas, Pseudomonas, Baccilus and the Actinomycetes were studied for cellulase production and optimization. Aforementioned microorganisms reported as potential strains that can potentially be able to convert pretreated LCBs to utilizable sugars. It could be further used for production of value added products (e.g. Bio-fuel, alcohols and acids).
Beside this, there are still lots of barriers towards effective utilization of LCBs, like;
1. Enhancement cellulose accessibility for microbial strain via pretreatment,
2. Enhancement of saccharification (The hydrolysis of complex polysaccharides to fermentable sugars) efficiency,
3. Dark fermentation/Photo fermentation optimization by redirecting metabolic pathway, metabolic engineering of microbial strains, stimulate enzyme activity involved in biomass conversion of desired value added products.
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About Author / Additional Info:
Department of Biotechnology,
Research Interest: Bio-fuels