Alkalis, holocellulose extraction

Prior to xylan removal it is possible to extract from the plant material those hemicelluloses soluble in dilute alkali. Soluble hemicelluloses of this type are the principal impurities extracted with xylan and consist for the most part of low molecular weight polysaccharides and polyuronides. Corn cob holocellulose may be freed of these compounds (called hemicellulose-B fraction, see Figure 2) by extraction with potas-... 

Fig. 2.—Yield of hemicellulose-A and -B and residue obtained by alkali extraction of corn cob holocellulose. O Residue, A-fraotion, 3 B-fraction.

For the study of soluble polysaccharides, a treatment with diluted TFA is sufficient and the reaction time can be kept short (7). Soluble polysaccharides of wood are separated from holocellulose by extraction with alkali. Wise et al. (10) term the extract with 5% KOH polyoses (hemicelluloses) A. Polyoses A can be hydrolyzed completely with 2N TFA within 1 hr. The chromatograms of the hydrolysates of polyoses A from spruce and beech holocelluloses recorded with a sugar analyzer (Biotronik ZA 5100) are shown in Figure 1. 

The hemicellulosic materials extracted by alkali are, in part, precipitated on neutralization or mild acidification, and, in further part, by the subsequent addition of an excess of acetone or ethanol.29 Under these conditions, part of the hemicellulosic material remains in solution, and is commonly not recovered. An oat-leaf holocellulose was treated with aqueous acid, and the precipitated hemicelluloses were recovered after the addition of an excess of acetone. The solvents were removed from the remaining solution and, after dissolving the... 

It should be noted that, during methylation, there is a selective loss of hemicellulose rich in galactose residues.148 In most studies on hemicelluloses, the materials extracted from holocellulose by alkali are recovered by mild acidification, followed by addition of an excess... 

The phenomena which have been observed on extraction of wood with aqueous sodium hydroxide - also occur, albeit to a smaller extent, with holocelluloses the restriction to dissolution is, in this case, mostly due to the fiber structure itself. In a study of the extraction of a chlorine holocellulose from European beech with sodium hydroxide solutions of various concentrations, Corbett and Ewart found that 87% of the (4-0-methyl-glucurono) xylan could be removed under mild conditions, and another 10% by extraction with hot alkali, but that 1.3% remained associated with the cellulose. The same authors have also described an apparatus suitable for the alkaline extraction of wood or holocellulose in an atmosphere of nitrogen. 

As the chlorine-2-aminoethanol method offers no obvious advantages in the case of softwoods, most investigators have used chlorite holocelluloses for isolating the hemicelluloses. The most common procedure has been to subject the holocellulose to a preliminary extraction with aqueous potassium hydroxide or sodium hydroxide solution. Low concentrations of alkali favor removal of the acidic xylan, as does the use of potassium... 

Hemicelluloses can be partly extracted from the holocellulose by aqueous solutions of alkali, which may saponify any hemicellulosic ester linkages... 

The residue, holocellulose, consisting of cellulose and h., is extracted by alkaline solutions xylans are dissolved in S-10% aqueous alkali solution mannans are soluble in 17-20% alkali or in alkali-borate solutions (24% 4%). Herbage and wood h. are precipitated by acid (pH 4.5-5) to yield the rich-in-xylose, higher-molecular, water-insoluble Hemicellulose A . 

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