Water, xylan extraction

Pectin and pectic substances are sometimes removed prior to xylan extraction. This is especially desirable when dealing with plant material containing large amounts of pectic substances such as the cambium layer of wood28 or the leaves and stems of plants.29-80 Extraction for one or more separate periods with 0.5% solutions of ammonium oxalate at 90-100° removes water-insoluble pectin substances3182 83 that are not mechanically trapped or chemically bound in the plant struc-... 

Due to the location and degree of integration in the different tissues of the grain, a part of AX is water-extractable, but the bulk can only be released by alkali solutions. Both the water- and alkali-extracted xylans consist of a family of related polymers differing in DS by a-Ara/ residues and substitution pat-... 

Thermoplastic xylan derivatives have been prepared by in-hne modification with propylene oxide of the xylan present in the alkaline extract of barley husks [424,425]. Following peracetylation of the hydroxypropylated xylan in formamide solution yielded the water-insoluble acetoxypropyl xylan. The thermal properties of the derivative quahfy this material as a potential biodegradable and thermoplastic additive to melt-processed plastics. Xylan from oat spelts was oxidized to 2,3-dicarboxyhc derivatives in a two-step procedure using HI04/NaC102 as oxidants [426]. 

Hemicelluloses are quite difficult to extract from cell walls of softwoods (9,10) and are usually destroyed or depolymerized during the chemical pulping of these raw materials. However, other hemicelluloses, primarily xylans, can be extracted by cold, dilute sodium hydroxide from grasses and many hardwoods in very high yields (9,77). These xylans are deacetylated in an alkaline medium and are for the most part insoluble (hemicellulose A). A partially water soluble fraction (hemicellulose B) has also been... 

That not all of the xylans of wood are homogeneous polymers of anhydro-D-xylose units has been shown by the classical studies of O Dwyer (1923 to 1940) on hemicelluloses of American white oak. O Dwyer prepared a hemi-cellulose fraction from water-extracted, oakwood sawdust by extraction for two days with 4% aqueous sodium hydroxide solution. The polysaccharide material was obtained, after acidification, by the addition of ethanol. The product ([a]n —75° in 1 % sodium hydroxide), contained 70% of pentosan, and yielded n-xylose, n-maiinose, n-galactose, and L-arabinose on hydrolysis. 

The investigations of Mitchell and Ritter - on holocellulose isolated from sugar maple are of interest. These workers isolated 25% of the holocellulose ill four, levorotatory, soluble fractions by successive treatments with hot water, cold 2% sodium carbonate, cold 4% sodium hydroxide, and hot 10% sodium hydroxide. The fraction extracted with hot water contained acetyl gi oups. All fractions contained xylan, and uronic anhydride and methoxyl were found in them in equivalent proportions. 

As biomass samples, sugi Cryptomeria japonica D. Don) and buna (Fagus crenala Blume) woods were selected for supercritical water treatment. In addition, microciystalline cellulose (avicel), glucomannan extracted from softwood dissolving pulp, larch xylan (Shigma Co., Ltd.), and milled wood lignin (MWL) from buna wood were studied to understand reaction behaviors of wood components dunng supercritical water treatment. 

A water extraction of the hydrolyzed raw material to produce a crude xylose-xylan solution. 

The xylan of hardwoods (O-acetyl-4-O-methylglucuronoxylan) consists of at least 70 p-xylopyranose residues (average degree of polymerization between 150 and 200) linked by P-l,4-glycosidic bonds (Fig. 11.4-4) 139. Every tenth xylose residue carries a 4-O-methylglucuronic acid attached to the C-2 of xylose 131. In addition, hardwood xylans are highly acetylated e. g. birchwood xylan contains more than 1 mol of acetic acid per 2 mols of xylose 140. Acetylation occurs usually at the C-3 rather than the C-2 position of xylose. Acetylation at both positions has also been reported 141, 142. The presence of these acetyl groups is responsible for the partial solubility of xylan in water 133. The alkali extraction of xylan leads to the deacetylation of this substrate 140. ... 

In an attempt to isolate a water-soluble 0-acetyl-(4-0-methylglucurono)-xylan from a birch chlorite holocellulose, Marchessault and coworkers kept the time of contact between the ammonia and the holocellulose at a minimum. Successive extractions of the pretreated holocellulose with water and methyl sulfoxide removed the xylan in yields of 7.1 and 14.6%, respectively, based on the weight of the original wood. The products still contained most of the iironic acid, but only one third of the 0-acetyl groups of the native xylan. 

Bird and Ritter isolated, from wood of white oak, a chlorine holocellu-lose which contained all of the 0-acetyl groups present in the wood. Mitchell and Ritter later extracted a chlorine holocellulose from sugar maple with water and obtained a xylan in a yield of 3.4% of the wood. This polysaccharide contained 9.2% of 0-acetyl groups. A xylan which had been obtained in the same way, from aspen, by Wise and Jones, was, on treatment with periodate, oxidized almost to completion. When the wood itself was similarly treated, most of its xylan escaped oxidation. Although it appears evident that all of the xylan in the wood could not possibly have been accessible to the aqueous reagent, it was concluded that the lack of oxidation was most probably due to the fact that the native xylan was partly 0-acetylated. After treatment of wood from Eucalyptus regnans with methanol at 150°, Stewart and coworkers obtained, on extraction with water, a xylan (in a jdeld of 3.7%) which contained 5-6% of acetate... 

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