Extractive Isolation of Xylan

Classically, the procedure for the isolation of xylan consists in the extraction of plant material with alkaline solutions. Because of the 

Aqueous alkaline extraction of wood was employed by Poumarede and Figuier48 in 1846 for the removal of a substance called wood gum. Similar very crude xylan or hemicellulose preparations were made by other workers of the early period. Extractions were made not only from wood sawdust, but from annual plant materials such as wheat straw, corn cob, etc.49-66 

Usually it is considered that xylan has been effectively removed if the extracted residue compares in composition to a-cellulose. Yet, even a-cellulose contains small amounts of xylan as well as other hemicelluloses such as mannan.67 69 Beta and 7-cellulose are mixtures which contain pentosans along with other alkali-soluble extractives.1M0 

For the most part alkaline extractions are conducted at room temperature because hot alkaline solutions possess a marked degradative effect on pentose constituents.61 Previously, little or no attempt was made to exclude oxygen although, at present, extractions are often performed in an atmosphere of nitrogen. In view of the ease with which 

Many substances besides xylan may be extracted from plant material and, hence, if this polysaccharide alone is desired one or preferably more purification steps must be employed. 

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Potential resources of xylans are by-products produced in forestry and the pulp and paper industries (forest chips, wood meal and shavings), where GX and AGX comprise 25-35% of the biomass as well as annual crops (straw, stalks, husk, hulls, bran, etc.), which consist of 25-50% AX, AGX, GAX, and CHX [4]. New results were reported for xylans isolated from flax fiber [16,68], abaca fiber [69], wheat straw [70,71], sugar beet pulp [21,72], sugarcane bagasse [73], rice straw [74], wheat bran [35,75], and jute bast fiber [18]. Recently, about 39% hemicelluloses were extracted from vetiver grasses [76]. 

The late discovery of acetyl xylan and feruloyl esterases has been partly due to the lack of suitable substrates. Xylans are often isolated by alkaline extraction, in which ester groups are saponified. Treatment of plant materials under mildly acidic conditions, as in steaming or aqueous-phase thermomechanical treatment, leaves most of the ester groups intact. These methods, however, partly hydrolyze xylan to shorter fragments (63,69). Polymeric acetylated xylan can be isolated from delignified materials by dimethyl sulfoxide extraction (70). The choice of substrate is especially important in studies of esterases for deacetylation of xylans. The use of small chromophoric substrates (p-nitrophenyl acetate, a-naphthyl acetate, and methylumbelliferyl acetate) analogously to the assays of disaccharidases may lead to the monitoring of esterases unable to deacetylate xylan (33, 63, 64). 

For isolation of hardwood xylans by way of holocellulose, a choice has to be made between the different methods available, according to the objectives to be attained. If molecular properties are to be investigated, the modified chlorine-2-aminoethanol method and, possibly, also the method of Klauditz are suitable. For chemical studies, especially if the location of the 0-acetyl groups is to be established, only chlorine dioxide or chlorous acid can be used. The subsequent extraction of the holocellulose will also have to be adapted to the desired ends. For isolation of a pure (4-0-methylglucurono)xylan, aqueous potassium hydroxide is excellent, since it gives almost quantitative yields and seldom causes removal of the gluco-mannan. If necessary, further purification can be effected by precipitation with Fehling solution. - ... 

As mentioned previously (see Part I, p. 255), only minor quantities of hemicelluloses can be removed from softwood by direct extraction with alkali. Very likely, the hemicelluloses are protected by the lignin, especially by that portion which surrounds the polysaccharides in the secondary wall. Isolation of softwood xylans by direct, alkaline extraction of tbe wood seems to have been used on only two occasions. Saarnio, using wood shavings of Norway spruce Picea abies), was able to obtain a relatively pure arabino-(4-0-methylglucurono)xylan in a yield of 3%, simply by extracting with 5% aqueous potassium hydroxide, a remarkable result. With wood of loblolly pine, Jones and Painter obtained a mixture of hemicelluloses, in a yield of about 1%, on direct extraction with 25% aqueous potassium hydroxide solution containing 4% of borate. 

The total alkaline charge required for xylan extraction was of major importance for the feasibility of the isolation process. These factors were dependant on the alkalinity of the extraction liquor and the consistency used in the process. In Figure 1 the effect of alkalinity on the yield of xylan and extraction residue is depicted for 10% and 25% consistency. The upper picture for 10% consistency demonstrates that an increase of NaOH concentration from 2.2 to 5% resulted in a continuous increase of xylan yield. Totalling the yield of xylan and residue it became apparent that up to 20% of the starting materials were not recovered. This material remained in the precipitation liquid and should consist of extractives, lignin fragmentation products, and low molar mass carbohydrates. A further analysis of those components was not pursued at this stage of the project. 

Various Bacillus have been studied for 2,3-BD production. Much of the focus has been on Bacillus polymyxa, as besides Klebsiella, it is considered to have industrial potential. It produces primarily pure L-isomer. A number of isolates of B. polymyxa were checked on media that contained xylan. One strain especially, B. polymyxa NRCC 9035, produced a considerable amount of diol on xylose. Yeast extract appeared to have a beneficial effect on the xylose fermentation, and diol concentrations of 4.2 g/1 were seen. Bacillus polymyxa also... 

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