Fractionation of hemicelluloses

Kinetic pattern of hemicellulose hydrolysis Hj refers to the fraction of hemicellulose easy to hydrolyze, and H2 refers to the fraction difficult to hydrolyze. 

The encrusting of cellulose fibrils by lignin-carbohydrate complexes within the cell wall is still a possible mechanism for lowered cellulose availability. The nutritive availability of the different fractions of hemicellulose and cellulose is quite different as shown by the data of Gaillard (26) in Table IV. Xylan is generally less digestible than other fractions (26, 30), and Lyford et ah (36) have shown that the micellular pentosans have a low digestibility. 

A sample of 5 g dry ground straw was treated with 200 ml NaOH solution (concentrations of 0.25 and 0.50 mol/l) for 2 h at different temperatures (30 and 55 °C). After the indicated period of treatment, the sample was acidified to pH 5 with glacial acetic acid, concentrated to about 30 ml under reduced pressure, and then filtered. The alkali-soluble hemicelluloses were then precipitated by pouring the concentrated supernatant fluid with four volumes of 95% ethanol (20 °C, 24 h). The precipitates were recovered by filtration, washed with 70% ethanol, and air dried. The scheme for fractionation of hemicelluloses from wheat straw is illustrated in Fig. 3. 

Data and kinetic models of dilute acid pretreatment ate vital to provide a foundation for undostanding hemicellulose hydrolysis and the cause of enhanced performance by flow system systems. Initial hemicellulose hydrolysis models were adapted from Saeman s first-order homogeneous kinetic model of cellulose hydrolysis in a dilute acid batch system (30) and later modified to include two different fractions of hemicellulose, one of which is more easily... 

Several methods for the fractionation of hemicellulose from lignocellulose have been described in the literature. These include physical methods such as steam treatment (7-73), and chemical methods such as for exanq>le organosolv processes (d, 14, 75). In addition, microwave irradiation of lignocellulose in the presence of water has been investigated for the purpose of enzymatic saccharification and ethanol production (7d-7S) and within our project for the extraction of hemicellulose (7). 

During the past decade, MALDI-TOF MS has proven to be an effective tool for the analysis of oligo- and polymeric mannoglucans (for extensive reviews see [222,223]). SEC/MALDI mass spectrometry was employed in the analysis of hemicelluloses isolated by microwave heat-fractionation from spruce and aspen wood [94]. These methods allowed the separation and characterization of the oligo- and polysaccharide fractions derived from the xylan and mannan components of both woods [224]. 

The carbohydrates in rice bran are made up of the cell wall components such as polysaccharides, oligosaccharides, hemicelluloses, starch and some sugars. The health benefits of rice bran polysaccharides and hemicelluloses are discussed under phytochemicals of rice bran below. It is lactose-free and gluten-free. The water-soluble non-starchy polysaccharides, oligosaccharides and hemicelluloses are concentrated in the water-soluble fraction of rice... 

Rha, Ara and Gal are the neutral sugar components from all the fractions. Xyl is not present in Fla and is significantly present in the hemicellulose fractions, indicating that this monosaccharide is component of hemicellulosic polymers. Chemical composition of the water fractions were determined (Table V). High protein contents and the presence of O-acetyl-groups were observed in four aqueous fractions. Neutral sugar and uronic acid composition points to inclusion of these polymers in the class of pectic polysaccharides. 

Polysaccharides from which 2,4,6-trimethyl-D-glucose can be prepared by methylation and hydrolysis include a polyglucose isolated from the cell wall of brewer s yeast,166 laminarin168 and the hemicellulose fraction of Iceland moss.188... 

Fig. 1.—Yield of hemicellulose-A and -B and residue obtained by alkali extraction of raw corn cob. O Residue, A-fraction, 3 B-fraction.

Yundt64 obtains a crystalline xylan from the xylan-rich fraction of straw or birchwood hemicellulose. The fraction is hydrolyzed with 0.2% oxalic acid solution for five hours at 100°. Upon autoclaving the insoluble residue approximately 0.2% dissolves and precipitates as hexagonal platelets when the filtrate is cooled to 60-70°. Very likely this crystalline material is of low molecular weight. 

The basic structure of all wood and woody biomass consists of cellnlose, hemicelluloses, lignin and extractives. Their relative composition is shown in Table 2.4. Softwoods and hardwoods differ greatly in wood stmctnie and composition. Hardwoods contain a greater fraction of vessels and parenchyma cells. Hardwoods have a higher proportion of cellulose, hemicelluloses and extractives than softwoods, but softwoods have a higher proportion of lignin. Hardwoods ate denser than softwoods. 

Biorefinery includes fractionation for separation of primary refinery products. The fractionation refers to the conversion of wood into its constituent components (cellulose, hemicelluloses and lignin). Processes include steam explosion, aqueous separation and hot water systems. Commercial products of biomass fractionation include levulinic acid, xylitol and alcohols. Figure 3.3 shows the fractionation of wood and chemicals from wood. 

Xyloisosaccharinic acid [2,4-dihydroxy-2-(hydroxymethyl)butanoic acid] is one of the major, alkaline-degradation products of wood xylan, in particular, that of birch. The disaccharide, 2-O-D-xylopyranosyl-L-arabinose, which was isolated as a hydrolysis product of corn-cob hemicellulose, is readily degraded at 100° in 15 mM Ca(OH)2 to acidic products, primarily saccharinic acids. Xylan oligosaccharides from corn-cob hemicellulose produced 2,4-dihydroxy-2-(hydroxymethyl)buta-noic acid when exposed to 0.02 M Ca(OH)2 at 25°. However, it was noted that the xylan, itself, was stable at 100° in Af NaOH. The major acidic component of the hemicellulose fraction of slash pine Pinus el-liotti) after acid hydrolysis was identified as 4-O-methyl-D-glucuronic... 

The apparent difference between viscosity average (M ) and number average molecular weight (M ) for unfractionated triacetate samples and high molecular weight fractions may be attributed to the presence of hemicellulose and polydispersity effect in these materials as shown in the table in question and in Figure 3. 

The partial blocking of the GPC column with 5 x 10 A exclusion limit in both dichloromethane and H.-methylpyrrolidone may be attributed to the presence of hemicellulose in both unfractionated triacetate samples and high molecular weight fractions used in this work. The blocking of the column in question was Indicated... 

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