Neutral hemicelluloses

Some hemicelluloses are partiaUy extractable with water, but they are usuaUy extracted with alkaline solutions foUowing removal of Upids and lignin. DeUgnifted plant material is termed, holoceUulose. Neutralization of the alkaline extract effects precipitation of the more linear and less acidic hemicelluloses, termed the hemiceUulose A [63100-39-0] fraction. The more acidic and more branched material, termed hemiceUulose B [63100-40-3], is precipitated with ethanol (70%). HemiceUulose B types type are usuaUy water-soluble after extraction. 

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. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

Pectic polysaccharides are classified as those polymers composed mainly of galactosyluronic residues, together with covalently or nonco-valently bound, neutral fractions. Hemicelluloses are polymers that are probably covalently linked to pectin, and noncovalently associated with cellulose it has been proposed that they are capable of strong hydrogen-... 

Index Entries Pretreatment dilute acid hemicellulose hydrolysis bisulfate neutralization. 

Both Eqs. 3 and 4 show that hydrogen ion concentration should affect the rate of hemicellulose hydrolysis, but the neutralization capacity of biomass is not always taken into consideration in models reported in the literature. Neutralization is caused by basic minerals containing potassium, sodium, calcium, iron, and other cations present in biomass reacting with sulfuric acid and reducing available hydrogen ions stoichiometrically (17) ... 

A second effect on hydrogen ion activity not previously considered in dilute-acid hydrolysis of hemicellulose is the shift to bisulfate that occurs with neutralization. Neutralization products are water and mineral sulfates (Eq. 5). These mineral sulfates, if not removed from the system, will form bisulfates from some of the remaining hydronium ions to reestablish equilibrium as follows (Eq. 7) ... 

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