One of natures most important biological processes is the degradation of lignocellulosic materials to carbon dioxide, water and humic substances. This implies possibilities to use biotechnology in the pulp and paper industry and consequently, the use of microorganisms and their enzymes to replace or supplement chemical methods is gaining interest. This chapter describes the structure of wood and the main wood components, cellulose, hemicelluloses and lignins. The enzyme and enzyme mechanisms used by fungi and bacteria to modify and degrade these components are described in detail. Techniques for how to assay for these enzyme activities are also described. The possibilities for biotechnology in the pulp and paper industry and other fiber utilizing industries based on these enzymes are discussed.

Microorganisms and enzymes involved in the degradation of plant fiber cell walls

Fractional purification and bioconversion of hemicelluloses.

Past technoeconomic modeling work has identified the relatively large contribution that enzymatic hydrolysis adds to the total cost of producing ethanol from lignocellulosic substrates. This cost was primarily due to the high concentration of enzyme and long incubation time that was required to obtain complete hydrolysis. Although enzyme and substrate concentration and end-product inhibition influenced the rate of hydrolysis, the effect was less pronounced during the initial stages of hydrolysis. During this time most of the cellulases were adsorbed onto the unhydrolyzed residue. By recycling the cellulases adsorbed to the residual substrate remaining after an initial 24 h, a high rate of hydrolysis, with low overall residence time and minimal cellulase input, could be achieved for several rounds of enzyme recycle. A comparison of the front end (pretreatment, fractionation, and hydrolysis) of a softwood/hardwood to ethanol process indicated that the lignin associated with the softwood-derived cellulose stream limited the number of times the cellulose containing residue could be recycled. (c) 1996 John Wiley & Sons, Inc.

Factors affecting cellulose hydrolysis and the potential of enzyme recycle to enhance the efficiency of an integrated wood to ethanol process

Production of ethanol from sugar (hexoses) and starchis carried out by a well developed technology. Production of ethanol from lignocellulosic materials is more difficult, requires several process steps, and is as yet in the development stage.

Production of ethanol from lignocellulosic materials: State of the art

The State of Hawaii is interested in converting the large volume of agricultural residues, principally sugarcane bagasse, that is generated in the state into transportation fuels. One of the technologies that is currently being evaluated is steam explosion as a pretreatment for conversion of the bagasse into ethanol. In order to identify the optimum conditions of the steam explosion cycle, a range of operating temperatures (188–243°C) and residence times (0.5–44 min) were used to pretreat bagasse in a 10 l batch reactor. The analytical results were also used to evaluate the “reaction ordinate” concept. The exploded bagasse samples were examined as to total mass recovery, weight loss by water extraction, composition of water extracts, total sugar recovery, and conversion of the polysaccharide of the exploded biomass to monosaccharide by a cellulosic enzyme mixture. Steam explosion followed by enzymatic saccharification was found to be an effective pretreatment for converting biomass into monosaccharides. However, the total sugar recovery, and thus the ethanol “potential”, of the process was relatively insensitive to changes in reaction conditions due to the trade-off between xylose recovery and glucose recovery. Furthermore, it was found that the reaction ordinate concept was not universally valid for the variety of sample characteristics examined in this study.

Steam explosion of sugarcane bagasse as a pretreatment for conversion to ethanol

Autohydrolysis explosion pretreatment of hardwood (Eucalyptus regnans) sawdust at 200°C and 6.9 MPa gas pressure (steam + nitrogen) for 5 min solubilized 85% of the total hemicellulose components and produced a pulp that was highly accessible to attack by cellulases from Trichoderma reesei C-30 and by a commercial preparation, Meicelase. The autohydrolysis liquor, representing 15% of the original weight of the sawdust on a solids basis, consisted mainly of xylose, xylose oligomers and minor amounts of galactose, mannose, arabinose, glucose and uronic acids. Enzymic hydrolysis of pretreated E. regnans pulps using Trichodermal cellulases resulted in saccharification yields of <50% within 24 h from 10% (w/v) substrate slurries and 20 cellulase (FPU) units per g of pretreated pulp. The cellulose-to-glucose conversions were lower and this was attributable to the production of a compound(s) during enzymic hydrolysis that was inhibitory to the β-glucosidase component, but not the cellulases, in the Trichodermal ...

H. Karageorge

Papers

Pretreatment of hardwood (eucalyptus regnans) sawdust by autohydrolysis explosion and its saccharification by trichodermal cellulases