Pretreatments to enhance the digestibility of lignocellulosic biomass

Trends in biotechnological production of fuel ethanol from different feedstocks.

Plant litter and the microbial biomass are the major parent materials for soil organic matter (SOM) formation Plant litter is composed of complex mixtures of organic components, mainly polysaccharides and lignin, but also aliphatic biopolymers and tannins The composition and relative abundance of these components vary widely among plant species and tissue type Whereas some components, such as lignin, are exclusively found in plant residues, specific products are formed by microorganisms, eg amino sugars A wide variety of chemical methods is available for characterizing the chemical composition of these materials, especially the chemolytic methods, which determine individual degradation products and solid-state 13C NMR spectroscopy, that gives an overview of the total organic chemical composition of the litter material With the development of these techniques, an increasing number of studies are being carried out to investigate the changes during decay and the formation of humic substances An overview is given on the amount of litter input, the proportion of various plant parts and their distribution (below-ground/above-ground), as well as the relative proportion of the different plant tissues Major emphasis is on the organic chemical composition of the parent material for SOM formation and thus this paper provides information that will help to identify the changes occurring during biodegradation of plant litter in soils

The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

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.

Substrate pretreatment: the key to effective enzymatic hydrolysis of lignocellulosics?

Effective utilization of the lignin by-product is a prerequisite to the commercial viability of ethanol production from softwood wastes using a steam explosion (SE)/enzymatic hydrolysis (EH)/fermentation process. Changes in the chemical composition of Douglas fir wood on SO 2 -catalyzed SE followed by EH were assessed using conventional analytical methods and new halogen-probe techniques. A significant solubilization of hemicelluloses was observed in the SE stage, the severity of which affected subsequent fermentation of cellulose and sorption of enzymes. SE of softwood resulted in dramatic changes in the chemical structure of lignin in the residual material involving chemical reactions via the benzyl cation. This leads to a more condensed lignin with partly blocked α-reaction centres. Possible uses for this lignin are discussed. Index Entries: Lignin; steam explosion; enzymatic hydrolysis; softwood.

The nature of lignin from steam explosion/ enzymatic hydrolysis of softwood: structural features and possible uses: scientific note.

In the last decade, application of modern degradative and nondegradative analysis techniques to both lignin of living plants and humic substances of soil has demonstrated characteristic similarities in the structures of these two types of natural polymers. Recognition of the similarities resulted in a revival of an earlier hypothesis concerning the genesis of soil organic matter from the aromatic parts of wood and nonwoody plants. The hypothesis assumes functionalization and restructuring but not complete depolymerization of lignin during its biotransformation into humic and fulvic acids in the environment. The biotransformation process results in the preservation of certain structural features during the humification of dead plants. A genetic approach is useful in the analyses of structure, morphology, and chemical reactivity of humic substances.

Life after death: Lignin‐humic relationships reexamined

Effective utilization of the lignin by-product is a prerequisite to the commercial viability of ethanol production from softwood wastes using a steam explosion (SE)Jenzymatic hydrolysis (EH)Jfermentation process. Changes in the chemical composition of Douglas fir wood on SO2-catalyzed SE followed by EH were assessed using conventional analytical methods and new halogen-probe techniques. A significant solubilization of hemicelluloses was observed in the SE stage, the severity of which affected subsequent fermentation of cellulose and sorption of enzymes. SE of softwood resulted in dramatic changes in the chemical structure of lignin in the residual material involving chemical reactions via the benzyl cation. This leads to a more condensed lignin with partly blocked α-reaction centres. Possible uses for this lignin are discussed.

The nature of lignin from steam explosion/enzymatic hydrolysis of softwood

Effective utilization of lignin is a prerequisite for the commercial viability of a softwood-to-ethanol process. Changes in the chemical composition of Douglas-fir wood after SO 2 -catalyzed steam explosion, followed by enzymatic hydrolysis, were assessed using 13 C CP/MAS NMR, FTIR, differential scanning calorimetry and lignin analysis. Mild prenratment conditions produced an almost complete solubilization of the hemicellulose sugars, while more severe conditions led to further solubilization of cellulose. The severity of the pretreatment affected both the structure of lignin and subsequent enzymatic hydrolysis of cellulose. Steam explosion increased the cellulose crystallinity and altered the chemical connectivity of lignin. Suggested lignin transformations involve condensation reactions via the benzyl cation and increase the carbonyl content.

Structure and properties of lignin in softwoods after SO2- catalyzed steam explosion and enzymatic hydrolysis

Citric acid (CA) is a model for soil humic substances and an important participant In many biochemical processes. Humic and humic-like materials play crucial roles In the bioavailability and mobility of organic and inorganic pollutants in aquatic and terrestrial ecosystems. scanning tunneling microscopy (STM) was used to image and analyze the morphological character of CA aggregates on graphite. Nano-scale images of CA also were simulated and Interpreted using a molecular mechanics technique. The effects of intermolecuiar association and hydration on the electronic properties and chemical reactivity of CA were analyzed in relation to the behavior of similar compounds in natural processes. Computer-assisted STM proved to be a powerful technique to investigate structure, morphology, and chemical reactivity of soil organic matter components and their aggregates. The CA molecule aggregation and hydration do not affect the electronic properties significantly. However, electrostatic potential is highly sensitive to the conformation of the molecule and the morphology of the ensuing molecular aggregate.

Sergey M. Shevchenko

Papers

The nature of lignin from steam explosion/ enzymatic hydrolysis of softwood: structural features and possible uses: scientific note.

Life after death: Lignin‐humic relationships reexamined

The nature of lignin from steam explosion/enzymatic hydrolysis of softwood

Structure and properties of lignin in softwoods after SO2- catalyzed steam explosion and enzymatic hydrolysis

Combining scanning tunneling microscopy and computer simulation of humic substances: citric acid, a model.