Lignocellulose hemicellulose

One of the favored organisms for study of cellulolysis by Trichoderma is T. reesei. Consequently, many mutant strains which hyperproduce cellulase have been obtained by treatment with ultraviolet light, gamma irradiation, the linear accelerator, diethyl sulphate and N-methyl-N -nitro-N-nitroso-guanidine (7). Whereas much of the study of T. reesei has been with cellulose as substrate, it is relevant to consider the other fractions of natural lignocelluloses hemicellulose and holocellulose (the combined cellulose and hemicellulose fraction). 

Butyric acid-producing microorganisms can utilize a large variety of substrates (aldo hex-ose, pentose, polysaccharides, oligosaccharides, lignocellulose, hemicellulose, cellulose, etc.) to produce butyrate. The production of butyric from the complex substrate is low due to low sugar utilization. In order to enhance the complex sugar utilization, different pretreatment... 

Lu and Pizzi [83] showed that lignocellulosic substrates have a distinct influence on the hardening behavior of PF-resins, whereby the activation energy of the hardening process is much lower than for the resin alone [84]. The reason is a catalytic activation of the PF-condensation by carbohydrates like crystalline and amorphous cellulose and hemicellulose. Covalent bonding between the PF-resin and the wood, especially lignin, does not play any role [84]. 

Lignocellulose is the fibrous material that forms the cell wall of a plants architecture . It consists of three major components (Fig. 2.1) cellulose, hemicellulose and lignin [3, 14-16]. It contrasts with the green parts of the plants and the seeds, which are rich in proteins, starch and/or oil. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

Levulinic acid and formic acid are end products of the acidic and thermal decomposition of lignocellulosic material, their multistep formation from the hexoses contained therein proceeding through hydroxymethylfurfural (HMF) as the key intermediate, while the hemicellulosic part, mostly xylans, produces furfural.A commercially viable fractionation technology for the specific... 

Pretreatment of Substrate. Several different lignocelluloses were pretreated with NaOH. This pretreatment partially solubilizes the hemicelluloses and lignin and swells the cellulose so that the organism can utilize it for its growth and for production of a cellulase system in SSF. The treated lignocelluloses were not washed. The NaOH treatment is done with a minimum amount of water so that, after the addition of nutrient solution and inoculum, the moisture content is less than 80% wt/wt and there is no free water in the medium. More water was added to make suspensions of different lignocellulosic substrates of the desired concentration (1% or 5%) for liquid-state (submerged) fermentation (LSF). 

Cellulose is found in nature in combination with various other substances, the nature and composition of which depend on the source and previous history of the sample. In most plants, there are three major components cellulose, hemicelluloses, and lignin. Efficient utilization of all three components would greatly help the economics of any scheme to obtain fuel from biomass. Hemicelluloses, lignocellulose and lignin remaining after enzymatic degradation of the cellulose in wood would require chemical or thermal treatment - as distinct from biochemical - to produce a liquid fuel. 

When the hyper-cellulolytic mutant T. reesei QM9414 was grown on various substrates derived from wheat straw in stirred batch culture, the highest specific growth rates were on holocellulose and the hemicellulose-A defined by O Dwyer (8) it grew poorly on lignocellulose. By contrast,... 

Table I. Growth of Trichoderma reesei QM9414 (Tr) and T. harzianum IMI275950 (Th) on wheat straw lignocellulose (lignocell.) or derived cellulosic materials from straw (cell., cellulose hemicell-A, hemicellulose-A holocell., holocellulose)...

As far as the ethylene glycol lignin is concerned, it has been shown to be a native-like lignin which can be produced and recovered by direct solvolytic treatment of the initial lignocellulosic substrate. It would also be possible to remove the hemicelluloses via an aqueous/steam treatment prior to solvolytic separation of the lignin and cellulose. Such an option would facilitate the recovery of the three main constitutive fractions of lignocellulosics in significant yields. Work in this direction is now underway. 

Lignocellulosic materials have a common basic structure, but vary greatly in chemical composition and physical structure.4 Typically, these materials contain 30 percent to 60 percent cellulose, 10 percent to 30 percent hemicellulose (polyoses), and 10 percent to 20 percent ligmn. Cellulose provides strength and flexibility, while lignin supports and protects the cellulose from biological and chemical attack. Hemicellulose bonds lignin to cellulose. 

Levulinic acid is formed by the treatment of six-carbon sugar carbohydrates from starch or lignocellulosics with acids, or by add treatment plus a reductive step of five-carbon sugars derived from hemicellulose. Levulinic add can serve as a building block for the synthesis of many derivatives of interest may be the selective oxidation to succinic and acrylic add. [i-Acetylacrylic add could be used in the production of new acrylate polymers. 

Lignocellulosic biomass is a valuable and plentiful feedstock commodity and its high cellulose and hemicellulose content (about 80% of total) provides considerable potential for inexpensive sugars production. However, enzymatic deconstruction of these polysaccharides remains a costly prospect. Strides in cellulase cost reduction have been made, yet further improvements are needed to reach the goal of 0.10/gal of EtOH expected to enable this new industry. Strategies to reach this goal will combine reduction in the cost to produce the needed enzymes as well as efforts to increase enzyme efficiency (specific activity). As this work proceeds, the more easily attained achievements will be made first, and thus the overall difficulty increases with time. 

Corn stover, a well-known example of lignocellulosic biomass, is a potential renewable feed for bioethanol production. Dilute sulfuric acid pretreatment removes hemicellulose and makes the cellulose more susceptible to bacterial digestion. The rheologic properties of corn stover pretreated in such a manner were studied. The Power Law parameters were sensitive to corn stover suspension concentration becoming more non-Newtonian with slope n, ranging from 0.92 to 0.05 between 5 and 30% solids. The Casson and the Power Law models described the experimental data with correlation coefficients ranging from 0.90 to 0.99 and 0.85 to 0.99, respectively. The yield stress predicted by direct data extrapolation and by the Herschel-Bulkley model was similar for each concentration of corn stover tested. 

The production of fuel ethanol from renewable lignocellulosic material ("bioethanol") has the potential to reduce world dependence on petroleum and to decrease net emissions of carbon dioxide. The lignin-hemicellulose network of biomass retards cellulose biodegradationby cellulolytic enzymes. To remove the protecting shield of lignin-hemicellulose and make the cellulose more readily available for enzymatic hydrolysis, biomass must be pretreated (1). 

The breakdown of furan aldehydes leads to the formation of formic and levulinic acid. Moreover, acetic acid is formed during the degradation of hemicellulose. Partial breakdown of lignin can generate a variety of phenolic compounds (23), which also inhibit S. cerevisiae (14,15). In contrast to furan aldehydes and aliphatic acids, the toxic effect of specific phenolic compounds is highly variable (15). Different raw materials and different approaches to prepare lignocellulose hydrolysates will result in different concentrations of the fermentation inhibitors (16,17). 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

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