Development And Optimization Of Rapid Enzymatic Process For Xylooligosaccharides Preparation

Xylooligosaccharide,the hydrolysates of xylan,is a functional oligosaccharide consisting of 2-7 xylose molecules linked by glucoside bonds.The main effective components of the xylooligosaccharides are xylobiose and xylotriose.At present,there are many ways to synthesize xylooligosaccharides,among which enzymatic hydrolysis by xylanase is the most effective and widely used method.Wheat bran is a byproduct of wheat processing,and it is versatility and low price necessitates the need to transform it into value-added products.In the present study,after the characterization of the biochemical properties of three xylanases from Aspergillus niger,the process for the preparation xylooligosaccharides was optimized and developed.Finally,the effects of many enzymes on the production of xylooligosaccharides were also investigated.The main results are as follows:After preparation,the enzymatic properties of xylanases XynA,XynB and XynC were studied.The optimum temperatures of these three xylanases were 30-55℃,30-45℃ and 25-50℃,and their pH optima were 3.5-4.0,2.0-4.0 and 2.0-3.0,respectively.K+ exerted different enhancing effects on the activities of XynA,XynB and XynC,whereas they were significantly inhibited by SDS.Beside,Mn2+,Fe3+,Cu2+,Sn2+,EDTA and SDS displayed different inhibitory effects on these three enzymes.According to HPLC analysis,the main products of xylan hydrolyzed by XynA and XynB were xylose and xylobiose,and the contents of xylobiose and xylotriose in their hydrolysates were 68.0%and 77.2%,respectively.But wheat bran was mainly hydrolyzed by XynC into xylobiose and xylotriose,and their total content was 81.7%.Therefore,XynC was chosen for the preparation of xylooligosaccharides.The process for the production of xylooligosaccharides was optimized by "change one factor at a time" and orthogonal experiments,and the optimal conditions obtained were as follows:the concentrations of wheat bran and xylanase,reaction time,pH and temperature were 5%(w/v),7 U/g,10 h,3.0 and 35℃,respectively.Based on the results of thin layer chromatography(TLC)and high performance liquid chromatography(HPLC),xylose,xylobiose,xylotriose and xylotetraose were found to be the three main products of wheat bran hydrolysates,and their concentrations were 1.07 g/L,1.15 g/L,1.79 g/L and 1.36g/L.Moreover,the conversion rate of wheat bran was 10.72%.The main components of wheat bran were hemicelluloses,starches,proteins and pectins,and their corresponding hydrolysis by amylases(thermostable a-amylase,amylase,pullulanase and glucoamylase),protease(ExpA),pectinase(PgaB)and cellulase(Eng3B)enhanced the yield of xylooligosaccharides.The process for the preparation of xylooligosaccharides by these enzymes was then optimized and established as follows:after adjusting the pH to 5.0,150 U/g of thermostable a-amylase was added into the reaction mixture,boiled for 10 min and cooled to 50℃.The mixture was then hydrolyzed for 8 h by 15 U of a-amylase,2 U of pullulanase and 150 U of glucoamylase to remove starches.After 8-h hydrolysis at 50℃ by 150 U/g of pectinase(PgaB),50U/g of cellulose(Eng3B),0.25 U/g of protease(ExpA)was added and reacted at 50℃ for 8 h to remove proteins.Under this optimized condition,the concentrations of xylose,xylobiose,xylotriose and xylotetraose were 2.47 g/L,2.66 g/L,4.10 g/L and 3.11 g/L,respectively.The conversion rate of wheat bran was 19.69%,which was nearly 1 times higher than before.These results lay a solid foundation for the high-efficiency preparation of xylooligosaccharides from wheat bran and provide new insights into the comprehensive utilization of wheat bran.