Hemicelluloses from holocellulose

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. 

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... 

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]. 

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-... 

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)...

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 experiments were repeated under similar conditions with the same and another mannanase fraction of Yamazaki s, with sprucewood holocellulose prepared in a slightly different way (19). After three weeks incubation, about 19% of the mannan was dissolved away, and the loci of hemicellulose removal were revealed by electron microscopy. In these experiments, the bulk of hemicelluloses was obviously removed by the buffer solution. In addition to mannose, glucose and galactose derived from mannan, xylose and glucose were found in the reaction solutions in appreciable amounts. 

The foregoing observations confirm the conclusions derived from former experiments with beechwood holocellulose (10) (1) A partial degradation of the hemicelluloses is imperative before the cellulose fibrils can be attacked. (2) The hemicelluloses seem to be deposited between the cellulose fibrils or even to be encrusting them. (3) The enzymatic hydrolysis of the cellulose is governed by the porosity of the tissue (enzyme diffusion), the impediment of the hemicelluloses, and the properties of the cellulose (e.g., crystallinity). 

The current observations confirm previous studies on beechwood and sprucewood holocellulose (7,10,19). The attack of the hemicellulose proceeds from the primary wall/Si as well as from the tertiary wall into S2 the pit chambers constitute preferred paths of enzyme diffusion into the walls. Also, substances of the middle lamella, especially in the cell corners, are removed by the xylanase and the mannanase treatments. Parallel to the removal of hemicelluloses, the fibrillar structure of the cellulose and its lamellar arrangement in transections of cell walls became obvious. In samples treated with cellulases, the cellulose fibrils were often completely hydrolyzed in the Si layer, occasionally accompanied by complete dissolution of cell-wall portions. This is also in conformity with the previous conclusion that the cellulases hydrolyze highly ordered zones of cellulose and remove hemicelluloses by hydrolysis or by detachment. 

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... 

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). 

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