Hemicelluloses extraction from holocellulose

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... 

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-... 

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. 

Woods contain a number of acyl (mainly acetyl) groups, and it is probable that these are associated with the xylan components of the hemicellulose fraction. Evidence for the original esterification of aspen-wood xylan comes from the observation that the isolated xylan (obtained by alkaline extraction) is readily cleaved by periodate, whereas only a small proportion of the D-xylose residues in the wood itself are oxidized under similar conditions.66 Hemicelluloses still containing acyl groups may be extracted from wood holocelluloses by means of dimethyl sulfoxide, and further quantities... 

A new approach to the isolation of hemicelluloses is through the production of holocellulose by chlorination of the wood followed by extraction of the holocellulose with a 3% solution of ethanolamine in alcohol. The holocellulose contains the carbohydrate portion of the wood freed of lignin. Hemicelluloses are then extracted from this product in the regular way. " ... 

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

However, there are some methods for extraction of cellulose from plants. Han and Rowell s (1996) method describes a procedure for extraction of holocellulose, hemicelluloses, cellulose and lignin [18]. The method consists of four principle steps (1) preparation of sample (grinding of the wood), (2) removal of extractives, (3) preparation of holocellulose (removal of lignin), (4) preparation of a-cellulose (removal of hemicellulose). Several other procedures for a-cellulose extraction from wood samples have already been described during the last decades. Older methods used benzene-methanol instead of toluene-ethanol as organic solvent for the second step. Toluene-ethanol works as well as benzene-methanol mixture, and reduces health risks associated with the use of benzene and methanol. 

Holocellulose Cellulosic material obtainable from wood after removal of lignin. The term therefore means total carbohydrates (cellulose and hemicellulose) present in the wood (Refs 1 2) Ritter (Ref 3) succeded in isolating holocellulose from wood pulp by repeated chlori-nations, followed by extraction with alcohol containing 3% of monoethanolamine. The resulting product was white but changed color on standing... 

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... 

Hemicellulose Samples. Two water-soluble hemicellulosic samples were used (1) acetyl-4-O-methylglucuronoxylan (molecular weight = 30,000) from white birchwood, isolated by the extraction of chlorine holocellulose with dimethyl sulfoxide and (2) arabinogalactan, Stractan from St. Regis Paper Company (arabinose galactose = 1 6 molecular weight = 60,000), prepared by the water extraction of heartwood of western larch. Both samples were kindly provided by T. E. Timell (SUNY). 

Holocellulose. Holocellulose is the total polysaccharide (cellulose and hemicelluloses) content of wood, and methods for its determination seek to remove all of the lignin from wood without disturbing the carbohydrates. The procedure generally used (34) was adopted as Tappi Standard T9m (now useful method 249), and as ASTM Standard D 1104. Extracted wood meal is treated alternately with chlorine gas and 2-aminoethanol until a white residue (holocellulose) remains. The acid chlorite method is also used (3). The... 

The holocellulose, lignin, and pentosans from Ref. 58 are percent extractive-free wood. f Cross and Bevan cellulose is largely pure cellulose but contains some hemicelluloses. 

Different separation and purification procedures have been proposed to isolate hemicelluloses from different raw materials. Extraction of hemicelluloses from different resources has been studied since a long time ago. Isolation of hemicelluloses using cost efficient extraction methods would be beneficial to increase the utilization of hemicelluloses. Alkaline aqueous solutions and organic solvents have been used to extract hemicelluloses from the original or delignified wood [holocellulose] [1,9,21,25,31,33,34,5 7]. 

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