0-Xylans

The pentosan polysaccharides, xylan and arabinan, commonly known as hemiceUulose, are the principal precursors of furfural and are always found together with lignin and cellulose in plant materials. 

The plant is harvested by hand sickle and, after defoUation, is stripped and scraped by hand or machine decorticated. Because of the high gum (xylan and araban) content of up to 35%, retting is not possible. The fibers are separated chemically by boiling in an alkaline solution in open vats or under pressure, then washed, bleached with hypochlorite, neutralized, oiled to facUitate spinning, and dried. 

The common hemiceUulose components of arborescent plants are listed in Table 3. Xylans, arabinogalactans, and pectic substances are common to all while only traces (if at all) of glucomaimans are found in the cell walls of bamboo. Other polysaccharides are found in trace amounts in wood as well as in bark, growing tissues, and other specialized parts of trees. 

Fig. 1. A schematic representation of the xylan backbone of arborescent plants, where Xylp = xylopyranosyl unit ...

Table 4. A Comparison of Isolated Xylans from Different Sources...

The amount of softwood xylan sorbed from alkati at kraft cooking temperatures (100—170°C) is proportional to the quantity of hemiceUulose present and inversely to the extent of branching (57). At neutral pH, the presence of carboxyl groups inhibits sorption compared to a control, but no difference is observed when ionization is suppressed (58). 

Both types of xyloglucans exhibit monolayer sorption onto cellulose (116) and tamarind xyloglucan exhibits maximum specific sorption onto cellulose less than that of coniferous xylan. By inference with other data, this is also less than that of glucomannan and hardwood xylan, but similar to many additives used in the paper industry. 

Derivatives of hemicellulose components have properties similar to the ceUulosic equivalents but modified by the effects of thek lower molecular weight, more extensive branching, labile constituents, and more heterogeneous nature. Acetates, ethers, carboxymethylxylan (184), and xylan—poly(sodium acrylate) (185) have been prepared. 

Xyhtol is synthesized by reduction of D-xylose catalyticahy (40), electrolyticahy (41), and by sodium amalgam (42). D-Xylose is obtained by hydrolysis of xylan and other hemiceUulosic substances obtained from such sources as wood, com cobs (43), almond shells, hazelnuts, or oHve waste (44). Isolation of xylose is not necessary xyhtol results from hydrogenation of the solution obtained by acid hydrolysis of cottonseed hulls (45). 

Scientific name/Common name Glucan Xylan Galactan Arabinan Man nan Uronic anhydride Acetyl Lignin Ash... 

The L-arabino-(4-0-methyl-D-glucurono)xylans are found in softwoods and annual plants. The L-arabinose is present primarily as a-L-arabinofuranosyl units, although P-L-arabinopyranosyl units may also be present. In either case, the arabinosyl units are often, but not always, present as single-unit side chains, as are the uronic acid units. 

P -D-xylan with 3-hnked a-L-arabiaose branches (1 4)-P -D-xylan with 2-Unked 4-0-methyl-a-D-glucuronic acid and arabiaose branches... 

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