Lignocellulosic Biotechnology

The prevailing energy and environmental crises have forced us, to re-evaluate the efficient utilization or finding alternative uses for natural, renewable resources, especially organic waste, using clean technologies. Meeting the massive energy-shortage demands, food security and developing technological solutions in the agriculture, agro-processing and other related manufacturing sectors are issues of pressing relevance. Ligno-cellulose biotechnology addresses some of these issues, since most of the technologies are based on the utilization of readily available residual plant biomass considered as waste to produce numerous value-added products.

Xylan and cellulose together constitute the most abundant organic carbon resource on the planet (Uffen, 1997). They are products of photosynthesis and constitute an inexhaustible renewable resource, offering an alternative natural source of chemical feedstock with a replacement cycle short enough to meet the demand in the world fuel market. Nature is abound with bacteria and fungi that can produce cell wall degrading enzymes to solubilize complex polysacchrides of plant cell wall to simple sugar molecules. The hydrolysis of cellulose is accomplished by components of cellulase including randomly acting endoglucanase (EC 3.2.1.4) that cleaves the internal β 1, 4 glycosidic bonds; cellobiohydrolase (EC 3.2.1.91), which releases cellobiose from reducing and non-reducing ends and β glucosidase (EC 3.2.1.21) that cleaves the cellobiose into glucose units. Endoxylanase (EC 3.2.1.8), primarily cleaves β 1,4 linked xylan back bone and β xylosidase (EC 3.2.1.37) hydrolyses xylo-oligomers.In addition, different debranching enzymes, e.g., α-L-arabinofuranosidase (EC 3.2.1.55), α-D- glucouronidase (EC 3.2.1.139), acetyl xylan esterases (EC 3.2.1.72) and ferulic or p-coumaric acid esterase (EC 3.2.1.73) are also required for co-operative and effective hydrolysis of hemicellulosic fraction.

With the environmental and cost issues surrounding conventional chemical processes, industrial enzymes are gaining ground rapidly due to the various advantages that they offer over conventional technologies. Hydrolases constitute approximately 75 % of the markets for industrial enzymes, with the glycosidases, including cellulases, amylases and hemicellulases, constituting the second largest group after proteases. Xylanases, that constitute the major commercial proportion of hemicellulases are now being employed in various industrial applications, including prebleaching of kraft pulp to reduce the use of harsh chemicals in the subsequent chemical bleaching stages , in feed formulations and in the food industry. In combination with pectinases and other enzymes, xylanases have also been used in other processes such as clarification of juices, extraction of coffee, and extraction of plant oils and starch. Other potential applications include the conversion of agricultural waste and the production of fuel ethanol. However, physicochemical properties of the xylanases required for each of the applications differ. To elaborate the point it may be mentioned that   technically pulp and paper industry requires a cellulase free xylanase preparation that must be able to withstand high temperature (55–70^o C) at alkaline pH of the pulp for efficient biobleaching. Thermo-alkaliphilic or even thermo-acidophilic xylanases may be of use in bioconversion processes where a variety of treatments, including hot water and  steam explosion, alkaline, solvent or acidic pretreatments may be used prior to or simultaneous to enzyme treatment. Alkaliphilic xylanases would be required as detergent applications where high pHs are typically used, while, a thermostable xylanase would be beneficial in animal feeds if added to the feeds before the pelleting process (typically carried out at 70–95^o C). Cold adapted xylanases, which are most active at low and intermediate temperatures, could be useful in the baking industry as dough preparation and proofing is generally carried out at temperatures below 35^o C. In view of the wide industrial applications of cell wall degrading enzymes (cellulases and hemicellulases), in cellulose to ethanol program, paper and pulp, textile and food industry, screening of natural ecosystems for isolation of novel microbial strains for studying their production capabilities as well as characterization of catalytic versatility, their regulation and applications is an area of  important research interest.