Production Of Xylo-oligosaccharides And Monosaccharides From Poplar By Lactic Acid Pretreatment Combined With Enzymatic Hydrolysis

Poplar is one of the most promising lignocelluloses for the production of biofuels and biochemicals due to its fast growth rate,short nutritional cycle,and wide distribution.This study aimed at the problems of the traditional xylo-oligosaccharide（XOS）production process,which is complicated,low yield and serious environmental pollution.The lactic acid（LA）pretreatment combined with enzymatic hydrolysis of poplar was used to produce XOS,and the techniques of delignification by hydrogen peroxide-acetic acid（HPAA）and p H regulation were proposed to increase the yield of XOS.This work provided a scientific basis and theoretical evidence for the efficient and clean production of XOS from poplar.The main research results are as follows:（1）The process on XOS production from poplar by LA pretreatment combined with xylanase hydrolysis was obtained.Single-factor experiments showed that the optimum process for XOS production was 2%LA pretreated poplar at 170℃for 30 min.Under this condition,30.3%XOS yield was obtained with a xylose/XOS ratio of 0.85 and total by-products of 6.4 g/L.Based on the optimal process for XOS production by LA pretreatment,the hydrolysis of poplar residues by 18.4mg xylanase protein/g DM for 24 h resulted in 7.9%XOS yield.In combination with lactic acid pretreatment,a maximum yield of 38.2%XOS was obtained from poplar.The obtained poplar residues were hydrolyzed at an enzyme dose of 10.0 mg cellulase protein/g DM for 72 h yielding55.6%of glucose and 13.9%of xylose.The above results indicated that LA can selectively and efficiently hydrolyze xylan in poplar to XOS with less by-products.（2）Aiming at the spatial barrier effect of the lignin in poplar to organic acid pretreatment,the effect and pattern of LA pretreatment on the production of XOS from delignification of poplar was investigated.The results showed that the HPAA pretreatment with 1%H₂SO₄ as catalyst had a strong selective delignification ability for poplar,and HPAA-pretreated poplar obtained had not only a high xylan retention（>95.0%）but also a high lignin removal（83.2%）.Not only was the lignin removal positively correlated with the yield of XOS（R²=0.92）,but the xylan removal was also linearly correlated with the yield of XOS（R²=0.72）.After removing 83.2%of poplar lignin,the XOS yield increased from 30.3%to 42.7%,and the xylose/XOS ratio decreased from 0.85 to0.16.The glucose yield of 88.9%was obtained after hydrolysis of poplar solid residue by cellulase.The above results showed that delignification significantly improved the yield of XOS from poplar by LA pretreatment,substantially reduced the production of by-products,and improved the effectiveness of solids in the production of monosaccharides.（3）The technique of regulating the p H value of LA aqueous solution to enhance the production of XOS by LA pretreatment from poplar was proposed.The results showed that regulating the p H value of LA solution could improve the yield of XOS when delignified poplar was used as substrate.At p H 2.0-2.5 of LA solution,the yield of XOS increased with increasing p H value of（2%）LA aqueous solution.The highest XOS yield of 47.1%was obtained when the p H of the LA solution was 2.5,which was a 1.55-fold increase compared to the XOS yield produced by poplar without delignification（30.3%）.However,when the p H value of the LA solution increased to 5.0,the yield of XOS decreased with the increase of the p H value of the LA aqueous solution.In addition,after hydrolysis of the XOS-rich hydrolysate by xylanase,part of the XOS with high polymerization degree（DP）was degraded to XOS with low DP（DP 2-6）.The final yield of XOS was up to 61.0%,which was a 1.43-fold increase compared to the optimum XOS yield（42.7%）by delignification.A 95.4%glucose yield was obtained from poplar residues after LA hydrolysis by cellulase,indicating that regulating the p H value of the LA solution can improve the yield of XOS and reduce the production of xylose as a by-product.In this study,the yield of XOS was substantially increased and the by-products were reduced from poplar by means of delignification,p H regulation and xylanase hydrolysis,which avoided the xylan separation process required for the traditional process of producing XOS.Therefore,this method had the advantages of high yield,high purity,low by-products,simple steps and low energy consumption.