Although the structure and function of cellulase systems continue to be the subject of intense research, it is widely acknowledged that the rate and extent of the cellulolytic hydrolysis of lignocellulosic substrates is influenced not only by the effectiveness of the enzymes but also by the chemical, physical and morphological characteristics of the heterogeneous lignocellulosic substrates. Although strategies such as site-directed mutagenesis or directed evolution have been successfully employed to improve cellulase properties such as binding affinity, catalytic activity and thermostability, complementary goals that we and other groups have studied have been the determination of which substrate characteristics are responsible for limiting hydrolysis and the development of pretreatment methods that maximize substrate accessibility to the cellulase complex. Over the last few years we have looked at the various lignocellulosic substrate characteristics at the fiber, fibril and microfibril level that have been modified during pretreatment and subsequent hydrolysis. The initial characteristics of the woody biomass and the effect of subsequent pretreatment play a significant role on the development of substrate properties, which in turn govern the efficacy of enzymatic hydrolysis. Focusing particularly on steam pretreatment, this review examines the influence that pretreatment conditions have on substrate characteristics such as lignin and hemicellulose content, crystallinity, degree of polymerization and specific surface, and the resulting implications for effective hydrolysis by cellulases.

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in Wiley InterScience (www.interscience.wiley.com).The focus of this study was to alter the xylan content of corn stover and poplar using SO 2 -catalyzed steam pretreatment to determine the effect on subsequent hydrolysis by commercial cellulase preparations supplemented with or without xylanases. Steam pretreated solids with xylan contents ranging from $1 to 19% (w/w) were produced. Higher xylan contents and improved hemicellulose recoveries were obtained with solids pretreated at lower severities or without SO 2 -addition prior to pretreatment. The pretreated solids with low xylan content (\4% (w/w)) were characterized by fast and complete cellulose to glucose conversion when utilizing cellulases. Commercial cellulases required xylanase supplementation for effective hydrolysis of pretreated substrates containing higher amounts of xylan. It was apparent that the xylan content influenced both the enzyme requirements for hydrolysis and the recovery of sugars during the pretreatment process.

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The precise role of extracellular polymeric substances (EPS) in relation to the formation and physicochemical properties of microbial floc in wastewater treatment systems is not well known. Studies were undertaken to provide more comprehensive descriptions of EPS and properties of microbial floc. Acidic polysaccharides and DNA were relatively labile components of the EPS when biomass was stored at 4°C or at −20°C, and significant losses of these components were observed within 24 hours. The composition and properties of activated sludge were found to vary between different full-scale treatment systems reflecting the importance of wastewater composition and operation conditions on microbial communities and the response to environmental conditions. The COD:N:P ratio was found to influence hydrophobicity, surface charge and the EPS composition of microbial flocs in well-controlled bench-scale sequencing batch reactors. Phosphorus depleted and P-limited conditions resulted in a decrease in surface charge but increases in acidic polysaccharides which corresponded to a strong carboxyl stretch at 1740 cm−1 when the biomass was analysed by FTIR-spectroscopy. Electron dense particles, identified by energy-dispersive spectroscopy as containing iron, phosphorus and sulfur, were observed in the fibrils of the floc matrix by transmission electron microscopy.

Economic barriers preventing commercialization of lignocellulose-to-ethanol bioconversion processes include the high cost of hydrolytic enzymes. One strategy for cost reduction is to improve the specific activities of cellulases by genetic engineering. However, screening for improved activity typically uses "ideal" cellulosic substrates, and results are not necessarily applicable to more realistic substrates such as pretreated hardwoods and softwoods. For lignocellulosic substrates, nonproductive binding and inactivation of enzymes by the lignin component appear to be important factors limiting catalytic efficiency. A better understanding of these factors could allow engineering of cellulases with improved activity based on reduced enzyme-lignin interaction ("weak lignin-binding cellulases"). To prove this concept, we have shown that naturally occurring cellulases with similar catalytic activity on a model cellulosic substrate can differ significantly in their affinities for lignin. Moreover, although cellulose-binding domains (CBDs) are hydrophobic and probably participate in lignin binding, we show that cellulases lacking CBDs also have a high affinity for lignin, indicating the presence of lignin-binding sites on the catalytic domain.

Utilization of ethanol produced from biomass has the potential to offset the use of gasoline and reduce CO(2) emissions. This could reduce the effects of global warming, one of which is the current outbreak of epidemic proportions of the mountain pine beetle (MPB) in British Columbia (BC), Canada. The result of this is increasing volumes of dead lodgepole pine with increasingly limited commercial uses. Bioconversion of lodgepole pine to ethanol using SO(2)-catalyzed steam explosion was investigated. The optimum pretreatment condition for this feedstock was determined to be 200 degrees C, 5 min, and 4% SO(2) (w/w). Simultaneous saccharification and fermentation (SSF) of this material provided an overall ethanol yield of 77% of the theoretical yield from raw material based on starting glucan, mannan, and galactan, which corresponds to 244 g ethanol/kg raw material within 30 h. Three conditions representing low (L), medium (M), and high (H) severity were also applied to healthy lodgepole pine. Although the M severity conditions of 200 degrees C, 5 min, and 4% SO(2) were sufficiently robust to pretreat healthy wood, the substrate produced from beetle-killed (BK) wood provided consistently higher ethanol yields after SSF than the other substrates tested. BK lodgepole pine appears to be an excellent candidate for efficient and productive bioconversion to ethanol.