Xylan extraction from wheat straw

However, to characterize the solubilized hemicelluloses of the wheat-straw samples, the four extracted fiactions obtained and the commercial hemieellulose of birch wood (Sigma ) were hydrolyzed to determine their sugar constituents, and the results are given in Table 2. The major monosaccharide obtained was xylose of birch wood (48.1%), indicating the presence of a xylan. In the hemicelluloses of wheat straw, the major monosaccharide was arabinose (36.5%), indicating the presence of an arabinoxylan. However, these sugar concentrations were very low which can likely be due to the methodology used to determined the chemical composition of the hemieellulose extracted from wheat straw. 

The aim of this work was therefore to develop a process for direct atomization of the extract by considering twin-screw extrusion and ultrafiltration. Twin-screw extrusion is very efficient for alkali impregnation of wheat bran, but xylan extraction is achieved only with large L/S ratios. As xylan extraction from wheat bran alone is not feasible by twin-screw extmsion widi a low L/S, the coextrusion of wheat bran and wheat straw was investigated. 

The best results for both the extraction yield in the hemicellulosic powder and the sugar purity in the case of xylan extraction from wheat bran in a twin-screw extmder are obtained for a low NaOH ratio (bran/soda = 7) and a low straw content. 

Xylans can be coextracted from wheat straw and bran in a twin-screw extruder. The best results for both the production yield and the extract properties are obtained with low alkali content, as the majority of the xylans conies from bran. The desired concentration of the extract solution can be achieved by ultrafiltration. The membrane configuration and molecular weight cut off (MWCO) must be adapted to each solution to limit fouling and concentration polarisation. A permeate flux of 20 dmVh.nr was obtained at a final concentration ratio of 2. Ultrafiltration allows for a partial demineralization of the solutions but does not change the properties of the final powder. 

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]. 

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... 

The FTIR spectra of hemicelluloses extracted from birch wood (commercial xylan, line 1) and wheat straw hemieellulose extracted with 0.25 mol/1 NaOFI/30 °C (line 2),... 

Fig. 5 FTIR spectra of commercial xylan from birch wood (line 1) and FTIR spectra of wheat straw hemicellulose extracted with 0.25 mol/1 NaOH/30 °C (tine 2), 0.50 mol/1 NaOH/30 °C (line 3), 0.25 mol/ 1 NaOH/55 °C (line 4), and 0.50 mol/1 NaOH/55 °C (line 5) all the treatments for 2 h...

The twin-screw extruder has proved to be a versatile tool for continuous treatment of vegetable matter, either for food or non-food applications. Trials for the direct alkaline extraction of xylans from wheat bran in a twin-screw extruder were unsuccessful. Bran impregnation with sodium hydroxide in the twin-screw extruder was very efficient, but it was necessaiy to make the separation between the hemicellulosic gel and the lignocellulosic matrix in another apparatus and remained difficult without a dilution to a L/S ratio of 50. Bran and straw co-extrusion was therefore investigated to be able to reduce the L/S ratio. Straw fibres form a dynamic plug in the restrictive elements of the screw profile just after the filtration zone. The pressure induced in the extruder sheath by the cellulosic fibres enabled the liquid/solid separation. 

Xylans from batch extraction are also good film formers even though the tensile properties of these films differ slightly from the twin-screw extracts. The difference might be a result of the compostion in minor compounds liable to oxidative cross linking in the twin-screw extract, thus leading to a more rigid film. Combined extrusion of wheat straw and bran leads to an extract witli a... 

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