Microwave Assisted Short-Time Alkaline Extraction of Birch Xylan

Efficacy of microwave energy for the extraction of xylan from birch wood as an alternative to conventional method of extraction was investigated. Effect of irradiation time and microwave power input on the solubilization of wood and yield of extracted xylan was studied. The maximum yield of xylan obtained at the higher power level was significantly lesser compared to the lower power level indicating the molecular degradation of the polymer. The highest yield of xylan (60 % of the original xylan) was obtained at the lowest power level studied, 110 W, for an irradiation time of 10 min. Comparison with conventional extraction showed that 10 min of microwave extraction provided a similar wood dissolution to that at 90 °C for 1.5 h, but with a higher yield of xylan. Characterization of the precipitated xylan indicated that the extracted xylan contained 68–88 % of xylose with the major chemical structure consisting of a linear backbone of (1-4) β-d-xylopyransoyl residues. Molecular mass of the extracted xylan indicated that the xylan extracted using microwave contained 60–70 % of high molecular weight fraction, and about 30–40 % of low molecular weight fraction, whereas xylan extracted using conventional method showed a reverse trend. Molecular mass of non-aggregated xylan was reported to be 6,000 Da (in terms of dextran equivalents). Crystallinity of wood fibers increased irrespective of the method of extraction indicating no degradation of the strength of the fibers occurred during the extraction.