Study On Enzymatic Production And Refining Of Xylo-oligosaccharides From Steam-exploded Straw

The process for producing xylo-oligosaccharides (XOS) with enzymatic hydrolysis of steam-exploded wheat straw was conceived and tested. The research work included the enzymatic hydrolysis of steam-exploded straw, colorants analysis and decoloring of the hydrolysate. The following results were obtained:Satisfactory hydrolysis was achieved at 1:7.5 of ratio of steam-exploded straw to water,196U/g (dried solids) of enzyme addition, pH 6.0,53℃,and 12h of reaction time. The amount of total sugar and reducing sugar in the hydrolysate was 49.80mg/mL and 17.03mg/mL, respectively. The average degree of polymerization of xylan decreased to 3.10, and hydrolysis rate of xylan reached 63.77% (w/w, based on the xylan in raw material). The oligosaccharides in the hydrolysate were mainly composed of xylobiose and xylotriose, and content of xylo-oligosaccharides in the hydrolysate reached 50.80% (w/w ,based on dried solids).Different decoluring measures for hydrolysate were investigated. Results showed that : Acid-Flocculation was a good method for decoloring of xylo-oligosaccharides syrups. Under conditions of pH3.0 and 50℃, the color stripping rate was 40%;Among the test 9 types of activated carbons, activated carbon AC5 performed best. Under conditions of 80℃, pH3.0, 160r/min, 20% of activated carbon addition(based on dried solids), 30% dried solid,60min of reaction time, the color stripping rate was 81.87%; Among the test 6 types of anion exchange resins, anion exchange resin D301-G performed best. The optimum of static decolorizing was 55℃,3h; And the ultrafiltration with a membrane (molecular weight cut-off 3000) was studied. Results showed that ultrafiltration membrane performed best under conditions of 40℃, 0.05Mpa, 6.8% dried solid, and the color stripping rate was 99% . And content of xylo-oligosaccharides in the hydrolysate reached 84.08% (w/w, based on dried solids).The colorants removement through the XOS process was investigated, using Sephadex gel permeation chromatography and ultraviolet spectra. Results showed that: SuperdexTM Peptide 10/300GL was suitable. The colorants in XOS could be mainly divided into three main species presented as follows. One was the melanin with MW5000. The second were the caramel and acid decomposition of xylose or xylo-oligosaccharides (about MW500). The third was the lignin (about MW350). Acid-Flocculation was able to remove low molecular colorants (about MW350). Activated carbon was able to adsorb caramel and acid decomposition (about MW500). Ultrafiltration membrane (molecular weight cut-off 3000) was able to remove high molecular colorants.