阅读说明：本技术 一种汽爆植物纤维原料制备低聚木糖的方法 (Method for preparing xylo-oligosaccharide from steam-exploded plant fiber raw material ) 是由 张民 苏昊 隋文杰 周晓丹 刘锐 吴涛 王帅 于 2019-10-22 设计创作，主要内容包括：本发明涉及一种汽爆植物纤维原料制备低聚木糖的方法,步骤为：将植物纤维原料进行破碎除杂后,加入水/酸/碱进行预浸处理,处理后物料进行汽爆处理,再进行水浸提,固液分离后,对水提液进行纯化精制,向固体渣加入内切木聚糖酶进行酶解反应得到酶解液,对酶解液进行纯化精制,将纯化后水提液和酶解液进行真空浓缩或喷雾干燥,制得高浓度低聚木糖糖浆或糖粉。本方法利用汽爆水热反应脱除部分半纤维素组分,联合水提处理获得含低聚木糖降解液,联合酶解处理进一步转化剩余半纤维素为低聚木糖,继而最大程度地完全利用半纤维素组分,有效提高低聚木糖溶出率,降低酶用量,生产过程处理量大,流程简单,易于实现工业化清洁生产。(The invention relates to a method for preparing xylo-oligosaccharide from steam-exploded plant fiber raw materials, which comprises the following steps: crushing plant fiber raw materials, removing impurities, adding water/acid/alkali for pre-dipping, performing steam explosion treatment on the treated materials, performing water extraction, performing solid-liquid separation, purifying and refining a water extract, adding endo-xylanase into solid residues for enzymolysis reaction to obtain an enzymolysis solution, purifying and refining the enzymolysis solution, and performing vacuum concentration or spray drying on the purified water extract and the enzymolysis solution to obtain high-concentration xylo-oligosaccharide syrup or powdered sugar. The method utilizes steam explosion hydrothermal reaction to remove partial hemicellulose components, combines water extraction treatment to obtain degradation liquid containing xylo-oligosaccharide, combines enzymolysis treatment to further convert residual hemicellulose into xylo-oligosaccharide, then fully utilizes the hemicellulose components to the maximum extent, effectively improves the dissolution rate of the xylo-oligosaccharide, reduces the enzyme consumption, has large treatment capacity in the production process, has simple flow and is easy to realize industrial clean production.)

1. A method for preparing xylo-oligosaccharide from steam exploded plant fiber raw materials is characterized by comprising the following steps: the method comprises the following steps:

crushing plant fiber raw materials, removing impurities, adding water/acid/alkali for pre-dipping, placing the treated materials in a steam explosion device for steam explosion treatment, collecting the steam explosion treated materials, performing water extraction, performing solid-liquid separation, purifying and refining the water extract by adopting ion exchange resin, macroporous resin and a nanofiltration membrane, adding endo-xylanase into solid residues for enzymolysis reaction to obtain an enzymolysis solution, treating the enzymolysis solution by adopting a microfiltration membrane, an ultrafiltration membrane and a nanofiltration membrane for purification and refining, and performing vacuum concentration or spray drying on the purified water extract and the enzymolysis solution to prepare the high-concentration xylo-oligosaccharide syrup or powdered sugar.

2. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 1, characterized in that: the acid used for the pre-dipping treatment is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid, and the alkali used is NaOH or KOH or NH₃Or H₂O₂(ii) a The steam explosion conditions during the steam explosion treatment are as follows: maintaining the temperature at 180-224 deg.C or the pressure at 1.0-2.5 MPa for 5-30 min.

3. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 1, characterized in that: the leaching conditions during water leaching are that the leaching temperature is 20-95 ℃, the extraction frequency is 1-3 times, and the solid-to-liquid ratio is 1: 5-20 (w/w).

4. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 1, characterized in that: the water extract adopts ion exchange resin to remove small molecular organic acids in the water extract; removing saccharide degradation products in the water extract by adopting macroporous resin; removing micromolecular monosaccharide substances by using a nanofiltration membrane; and (3) carrying out reduced pressure vacuum concentration or spray drying on the water extract to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

5. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 4, wherein: the small molecular organic acid is formic acid and acetic acid; the saccharide degradation products are furfural and 5-hydroxymethyl furfural; the micromolecular monosaccharide substances are glucose, xylose and arabinose.

6. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 1, characterized in that: the enzymolysis conditions of the enzymolysis reaction are as follows: adding water to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, and carrying out enzymolysis for 3-12 h.

7. The process for preparing xylo-oligosaccharides from steam exploded plant fiber raw material according to any of claims 1 to 6, characterized in that: removing impurities and decoloring the enzymolysis liquid by using a microfiltration membrane; ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide; and removing small molecular monosaccharides and salts by using a nanofiltration membrane.

8. The method for preparing xylo-oligosaccharide from steam exploded plant fiber raw material as claimed in claim 1, characterized in that: the method comprises the following specific steps:

pretreatment:

dedusting and mechanically crushing the raw materials, and adding water, dilute acid or dilute alkali solution for pre-dipping;

the acid is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid; the base used is NaOH or KOH or NH₃Or H₂O₂；

Secondly, the steam explosion treatment:

putting the raw materials into a steam explosion device, introducing saturated steam to heat the materials until the maintaining temperature reaches 180-224 ℃ or the maintaining pressure reaches 1.0-2.5 MPa, maintaining for 5-30 min, then instantaneously exploding the materials, and collecting the materials in a buffer tank;

thirdly, leaching treatment:

extracting the steam-exploded material with water at the temperature of 20-95 ℃ for 1-3 times at a solid-liquid ratio of 1: 5-20 (w/w), and performing solid-liquid separation on the extracted material to obtain water extract and solid residue;

purifying and refining an extracting solution:

purifying: removing small molecular organic acids in the water extract by using ion exchange resin, and removing saccharide degradation products in the water extract by using macroporous resin;

and (4) nanofiltration: removing small molecule monosaccharide substances;

concentration or drying: vacuum concentrating or spray drying the water extract to obtain high-concentration xylooligosaccharide syrup or sugar powder;

carrying out fifth enzymolysis treatment:

adding water into the solid residues to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, carrying out enzymolysis reaction for 3-12 h, and removing enzymolysis residues through centrifugation or filtration after enzymolysis to obtain an enzymolysis solution;

sixthly, purifying and refining enzymatic hydrolysate:

purifying: removing impurities and decoloring by using a microfiltration membrane;

and (3) ultrafiltration: ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide;

and (4) nanofiltration: removing small molecular monosaccharides and salts by using a nanofiltration membrane;

concentration or drying: and (4) carrying out reduced pressure vacuum concentration or spray drying on the enzymolysis liquid to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

Technical Field

The invention belongs to the technical field of xylo-oligosaccharide production, and particularly relates to a method for preparing xylo-oligosaccharide from steam-exploded plant fiber raw materials.

Background

Xylo-oligosaccharide is also called xylo-oligosaccharide, is a functional polymeric sugar formed by combining 2-7 xylose molecules by beta-1, 4 glycosidic bonds, and also contains arabinouronic acid and glucuronic acid side chains, which are mainly xylobiose and xylotriose. The xylo-oligosaccharide solid is milk white to light yellow powder, has unique advantages compared with the soybean oligosaccharide, fructo-oligosaccharide, isomalto-oligosaccharide and the like which are commonly used by people, and can selectively promote the proliferation activity of intestinal bifidobacteria. Xylo-oligosaccharide has good functional properties, processing properties and storage stability, and is widely applied to various fields such as food, feed and medicine.

In recent years, xylo-oligosaccharide is mainly prepared by separating hemicellulose from plant fiber raw materials such as corncobs and straws and hydrolyzing, and the following two processes are mainly adopted. Firstly, a dilute acid hydrolysis method is adopted, and xylan is partially hydrolyzed by dilute acid such as sulfuric acid to prepare xylo-oligosaccharide, but the product of the method is not easy to control, a large amount of xylose is often generated along with generation of harmful substances, and the refining process is relatively complicated; the method has high requirements on equipment, and the produced acid wastewater needs environmental protection treatment. The enzyme hydrolysis method, namely, the microorganism is used to generate endo-xylanase to decompose xylan, and then xylo-oligosaccharide is prepared by separation and purification; the microorganism producing xylanase includes bacteria, streptomycete, aspergillus, penicillium, trichoderma, chaetomium and the like; the endoxylanase can be used for directionally hydrolyzing xylan to obtain a xylooligosaccharide mixture taking xylobiose and xylotriose as main components, so that fewer byproducts are generated, and the separation and purification of xylooligosaccharide in the subsequent process are facilitated; however, the method has low purity and activity of industrial enzyme, and causes higher production cost.

Due to the existence of the degradation-resistant barrier in the plant fiber raw material, the raw material needs to be pretreated before the enzyme method treatment, so that the structural constraint is broken, and the accessibility and the enzymolysis efficiency of the enzyme are improved. Steam explosion technology, hereinafter referred to as steam explosion technology, is considered to be one of the most economical and efficient pretreatment technologies for plant fiber raw materials. The method is a process of instantly reducing the solid material to normal pressure after the solid material is treated by saturated steam or high-pressure gas for a certain time. Under the actions of steam explosion process acid hydrolysis and thermal degradation, hemicellulose is partially hydrolyzed automatically to generate oligosaccharide and monosaccharide, and acetyl groups hydrolyzed from hemicellulose chains generate acetic acid to aggravate hydrolysis of hemicellulose glycosidic bonds, so that the higher the treatment strength in a certain range, the higher the hydrolysis degree of the hemicellulose. And then the steam exploded material is simply washed by water at normal temperature or leached by hot water to obtain the degraded hemicellulose-converted xylo-oligosaccharide extracting solution. Under the mechanical fracture action in the steam explosion process, the plant fiber raw material is torn to a certain degree, the porous structure of the plant fiber raw material is damaged from the cell wall, the cell to the tissue level, and the accessibility and the enzymolysis efficiency of subsequent enzymolysis conversion are improved. Therefore, xylo-oligosaccharide enzymatic hydrolysate converted from residual hemicellulose can be obtained by directionally carrying out enzymolysis on the leached steam-exploded solid material by using endo-xylanase. Based on the combined treatment of steam explosion-extraction and enzymolysis of plant fiber raw materials, the hemicellulose components can be fully utilized to the maximum extent, and the high-yield xylo-oligosaccharide product is finally obtained through a series of purification and refining treatments of the extracting solution and the enzymolysis liquid.

Through searching, the following patent publication related to the present patent application is found.

A method (CN102080116A) for preparing xylo-oligosaccharide by steam explosion-directional enzymolysis comprises: mechanical crushing of plant fiber raw materials, steam explosion, oriented hydrolysis of endo-xylanase, filter pressing of an enzymolysis system, microfiltration, ultrafiltration separation and purification, refining of anion exchange resin, refining of macroporous phenolic adsorption resin and reduced pressure evaporation and concentration. The method for preparing xylo-oligosaccharide organically combines steam explosion and directional enzymolysis to replace the existing acid and alkali extraction method or single steam explosion method, not only effectively degrades and dissolves out xylan in raw materials, but also overcomes the defects that other processes consume acid and alkali chemicals or have a lot of impurities, especially have overhigh harmful substance content and expensive subsequent separation and refining cost, can realize no discharge of wastewater in the production process, greatly simplifies the extraction process of xylan, saves the operation time, and obviously improves the environmental affinity and the economic efficiency of xylo-oligosaccharide production.

By contrast, the method fully utilizes the removing effect on hemicellulose in the steam explosion process after the plant fiber raw material is subjected to steam explosion treatment, adopts a combined water extraction mode to extract a hemicellulose degradation product rich in xylo-oligosaccharide, and then purifies and refines the extract; aiming at a small part of hemicellulose components contained in the residual solid residue after leaching, the part of hemicellulose is converted into xylo-oligosaccharide by further combining an enzyme hydrolysis mode. The complete conversion of the components of the hemicellulose can be promoted by combining steam explosion with water extraction and enzymolysis treatment, so that the yield of the xylo-oligosaccharide is obviously improved. Therefore, the method utilizes the similar acidity and thermal degradation of the hemicellulose component of the plant fiber raw material by steam explosion treatment, innovatively adopts a one-step water extraction mode for the material after steam explosion, can fully extract xylo-oligosaccharide from hemicellulose degradation, greatly reduces the enzyme consumption of the enzymolysis treatment of the residual hemicellulose component, has simple, convenient and feasible process, and has essential difference from the patent publications.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide the method for preparing the xylo-oligosaccharide from the high-tolerance steam-exploded plant fiber raw material, so that the hemicellulose component in the plant fiber raw material is fully utilized, and the xylo-oligosaccharide is produced with low cost and high efficiency.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a method for preparing xylo-oligosaccharide from steam exploded plant fiber raw materials comprises the following steps:

crushing plant fiber raw materials, removing impurities, adding water/acid/alkali for pre-dipping, placing the treated materials in a steam explosion device for steam explosion treatment, collecting the steam explosion treated materials, performing water extraction, performing solid-liquid separation, purifying and refining the water extract by adopting ion exchange resin, macroporous resin and a nanofiltration membrane, adding endo-xylanase into solid residues for enzymolysis reaction to obtain an enzymolysis solution, treating the enzymolysis solution by adopting a microfiltration membrane, an ultrafiltration membrane and a nanofiltration membrane for purification and refining, and performing vacuum concentration or spray drying on the purified water extract and the enzymolysis solution to prepare the high-concentration xylo-oligosaccharide syrup or powdered sugar.

Furthermore, the acid used for the pre-dipping is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid, and the alkali used is NaOH or KOH or NH₃Or H₂O₂(ii) a The steam explosion conditions during the steam explosion treatment are as follows: maintaining the temperature at 180-224 deg.C or the pressure at 1.0-2.5 MPa for 5-30 min.

And the leaching conditions during water leaching are that the leaching temperature is 20-95 ℃, the extraction times are 1-3 times, and the solid-to-liquid ratio is 1: 5-20 (w/w).

Moreover, the aqueous extract adopts ion exchange resin to remove small molecular organic acids in the aqueous extract; removing saccharide degradation products in the water extract by adopting macroporous resin; removing micromolecular monosaccharide substances by using a nanofiltration membrane; and (3) carrying out reduced pressure vacuum concentration or spray drying on the water extract to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

Moreover, the small molecular organic acid is formic acid and acetic acid; the saccharide degradation products are furfural and 5-hydroxymethyl furfural; the micromolecular monosaccharide substances are glucose, xylose and arabinose.

Moreover, the enzymolysis conditions of the enzymolysis reaction are as follows: adding water to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, and carrying out enzymolysis for 3-12 h.

Moreover, the enzymolysis liquid adopts a microfiltration membrane to remove impurities and decolor; ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide; and removing small molecular monosaccharides and salts by using a nanofiltration membrane.

The method comprises the following specific steps:

pretreatment:

dedusting and mechanically crushing the raw materials, and adding water, dilute acid or dilute alkali solution for pre-dipping;

the acid is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid; the base used is NaOH or KOH or NH₃Or H₂O₂；

Secondly, the steam explosion treatment:

putting the raw materials into a steam explosion device, introducing saturated steam to heat the materials until the maintaining temperature reaches 180-224 ℃ or the maintaining pressure reaches 1.0-2.5 MPa, maintaining for 5-30 min, then instantaneously exploding the materials, and collecting the materials in a buffer tank;

thirdly, leaching treatment:

extracting the steam-exploded material with water at the temperature of 20-95 ℃ for 1-3 times at a solid-liquid ratio of 1: 5-20 (w/w), and performing solid-liquid separation on the extracted material to obtain water extract and solid residue;

purifying and refining an extracting solution:

purifying: removing small molecular organic acids in the water extract by using ion exchange resin, and removing saccharide degradation products in the water extract by using macroporous resin;

and (4) nanofiltration: removing small molecule monosaccharide substances;

concentration or drying: vacuum concentrating or spray drying the water extract to obtain high-concentration xylooligosaccharide syrup or sugar powder;

carrying out fifth enzymolysis treatment:

adding water into the solid residues to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, carrying out enzymolysis reaction for 3-12 h, and removing enzymolysis residues through centrifugation or filtration after enzymolysis to obtain an enzymolysis solution;

sixthly, purifying and refining enzymatic hydrolysate:

purifying: removing impurities and decoloring by using a microfiltration membrane;

and (3) ultrafiltration: ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide;

and (4) nanofiltration: removing small molecular monosaccharides and salts by using a nanofiltration membrane;

concentration or drying: and (4) carrying out reduced pressure vacuum concentration or spray drying on the enzymolysis liquid to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

The invention has the advantages and positive effects that:

1. according to the method, partial thermal degradation and physical structure crushing of hemicellulose components are realized through steam explosion treatment on the plant fiber raw material, water extraction and enzymolysis treatment are combined, complete conversion of the hemicellulose can be achieved to the maximum extent in the process, the dissolution rate of xylooligosaccharide is effectively improved, and a xylooligosaccharide product with low polymerization degree is prepared through purification and refining treatment on water extract and enzymolysis liquid after steam explosion, so that full utilization of the hemicellulose components in the plant fiber raw material and low-cost and high-efficiency production of the xylooligosaccharide are realized.

2. The method can break the anti-degradation barrier of the plant fiber raw material through steam explosion treatment, improve the accessibility of subsequent enzymolysis, reduce the enzyme consumption, improve the product purity and further reduce the production cost.

3. The method has the advantages of large treatment capacity, simple flow, no pollutant emission in the treatment process, high product quality and yield, easy realization of industrial clean production and good social and economic benefits.

4. The method utilizes steam explosion hydrothermal reaction to remove partial hemicellulose components, combines water extraction treatment to obtain degradation liquid containing xylo-oligosaccharide, combines enzymolysis treatment to further convert residual hemicellulose into xylo-oligosaccharide, then fully utilizes the hemicellulose components to the maximum extent, effectively improves the dissolution rate of the xylo-oligosaccharide, reduces the enzyme consumption, has large treatment capacity in the production process, has simple flow and is easy to realize industrial clean production.

Detailed Description

The following detailed description of the embodiments of the present invention is provided for the purpose of illustration and not limitation, and should not be construed as limiting the scope of the invention.

The raw materials used in the invention are conventional commercial products unless otherwise specified; the methods used in the present invention are conventional in the art unless otherwise specified.

A method for preparing xylo-oligosaccharide from steam exploded plant fiber raw materials comprises the following steps:

crushing plant fiber raw materials, removing impurities, adding water/acid/alkali for pre-dipping, placing the treated materials in a steam explosion device for steam explosion treatment, collecting the steam explosion treated materials, performing water extraction, performing solid-liquid separation, purifying and refining the water extract by adopting ion exchange resin, macroporous resin and a nanofiltration membrane, adding endo-xylanase into solid residues for enzymolysis reaction to obtain an enzymolysis solution, treating the enzymolysis solution by adopting a microfiltration membrane, an ultrafiltration membrane and a nanofiltration membrane for purification and refining, and performing vacuum concentration or spray drying on the purified water extract and the enzymolysis solution to prepare the high-concentration xylo-oligosaccharide syrup or powdered sugar.

Preferably, the acid used for the pre-dipping is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid, and the base used is NaOH or KOH or NH₃Or H₂O₂(ii) a The steam explosion conditions during the steam explosion treatment are as follows: maintaining the temperature at 180-224 deg.C or the pressure at 1.0-2.5 MPa for 5-30 min.

Preferably, the leaching conditions during water leaching are that the leaching temperature is 20-95 ℃, the extraction frequency is 1-3 times, and the solid-to-liquid ratio is 1: 5-20 (w/w).

Preferably, the water extract adopts ion exchange resin to remove small molecular organic acids in the water extract; removing saccharide degradation products in the water extract by adopting macroporous resin; removing micromolecular monosaccharide substances by using a nanofiltration membrane; and (3) carrying out reduced pressure vacuum concentration or spray drying on the water extract to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

Preferably, the small molecular organic acid is formic acid or acetic acid; the saccharide degradation products are furfural and 5-hydroxymethyl furfural; the micromolecular monosaccharide substances are glucose, xylose and arabinose.

Preferably, the enzymolysis conditions of the enzymolysis reaction are as follows: adding water to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, and carrying out enzymolysis for 3-12 h.

Preferably, the enzymolysis liquid adopts a microfiltration membrane to remove impurities and decolor; ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide; and removing small molecular monosaccharides and salts by using a nanofiltration membrane.

Preferably, the specific steps are as follows:

pretreatment:

dedusting and mechanically crushing the raw materials, and adding water, dilute acid or dilute alkali solution for pre-dipping;

the acid is hydrochloric acid or sulfuric acid or phosphoric acid or acetic acid; the base used is NaOH or KOH or NH₃Or H₂O₂；

Secondly, the steam explosion treatment:

putting the raw materials into a steam explosion device, introducing saturated steam to heat the materials until the maintaining temperature reaches 180-224 ℃ or the maintaining pressure reaches 1.0-2.5 MPa, maintaining for 5-30 min, then instantaneously exploding the materials, and collecting the materials in a buffer tank;

thirdly, leaching treatment:

extracting the steam-exploded material with water at the temperature of 20-95 ℃ for 1-3 times at a solid-liquid ratio of 1: 5-20 (w/w), and performing solid-liquid separation on the extracted material to obtain water extract and solid residue;

purifying and refining an extracting solution:

purifying: removing small molecular organic acids in the water extract by using ion exchange resin, and removing saccharide degradation products in the water extract by using macroporous resin;

and (4) nanofiltration: removing small molecule monosaccharide substances;

concentration or drying: vacuum concentrating or spray drying the water extract to obtain high-concentration xylooligosaccharide syrup or sugar powder;

carrying out fifth enzymolysis treatment:

adding water into the solid residues to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0% (w/w), adjusting the pH to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, carrying out enzymolysis reaction for 3-12 h, and removing enzymolysis residues through centrifugation or filtration after enzymolysis to obtain an enzymolysis solution;

sixthly, purifying and refining enzymatic hydrolysate:

purifying: removing impurities and decoloring by using a microfiltration membrane;

and (3) ultrafiltration: ultrafiltering the purified enzymolysis liquid by using an ultrafiltration membrane to obtain ultrafiltrate rich in xylo-oligosaccharide and ultrafiltration macromolecule trapped fluid rich in macromolecule protein and polysaccharide;

and (4) nanofiltration: removing small molecular monosaccharides and salts by using a nanofiltration membrane;

concentration or drying: and (4) carrying out reduced pressure vacuum concentration or spray drying on the enzymolysis liquid to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

More specifically, the method for preparing xylo-oligosaccharide from steam exploded plant fiber raw materials comprises the following steps:

after the corn straws are subjected to dust removal and mechanical crushing treatment, water is added according to the solid-liquid ratio of 1: 0.5-1.5 (w/w) for pre-soaking treatment. And (3) placing the pre-soaked materials in a steam explosion device, introducing saturated steam to heat the materials until the maintaining temperature reaches 170-207 ℃ (the maintaining pressure reaches 0.8-1.8 MPa), maintaining for 5-30 min, then instantaneously exploding the materials, and collecting the materials in a buffer tank. And (3) extracting the steam-exploded material with water at normal temperature for 1-3 times at a solid-liquid ratio of 1: 5-20 (w/w), and performing solid-liquid separation on the extracted material to obtain water extract and solid residues.

The xylooligosaccharide content in the aqueous extract of steam exploded corn stover and the hemicellulose content in the solid residue obtained were determined according to the Laboratory Analytical Procedure (LAP) -Determination of Structural Carbohydrates and Lignin in Biomass method established by National Renewable Energy Laboratory (NREL) of the United states, as shown in tables 1 and 2.

TABLE 1 comparison of xylooligosaccharide content (%) in aqueous extract of corn stover before and after steam explosion treatment

TABLE 2 comparison of hemicellulose content (%) in corn stalk solid residue obtained by steam explosion

Data of the content of xylo-oligosaccharide in the corn straw water extract before and after steam explosion treatment in table 1 show that the hydrothermal effect in the steam explosion process promotes the degradation and conversion of hemicellulose into xylo-oligosaccharide components in the water extract, and the content of xylo-oligosaccharide increases as the steam explosion maintaining temperature and time increase, but the degradation of xylo-oligosaccharide can be caused by overhigh temperature and time.

The data of the content of hemicellulose in the corn straw solid residue after steam explosion treatment in table 2 show that steam explosion has a significant hemicellulose removal effect, and the hemicellulose is more significantly degraded as the steam explosion maintains the temperature and time, and the temperature affects the hemicellulose for more than time.

For the water extract, ion exchange resin, macroporous resin and nanofiltration are respectively adopted to remove micromolecule monosaccharide substances and degradation products thereof. And (3) carrying out vacuum concentration or spray drying on the purified water extract to prepare high-concentration xylo-oligosaccharide syrup or sugar powder.

Adding water into the solid residues to adjust the solid concentration to 5-10% (w/w), adding endo-xylanase according to the enzyme dosage of 0.5-2.0%, adjusting the pH value to 4.8-5.2, carrying out enzymolysis at the temperature of 45-55 ℃, carrying out enzymolysis reaction for 3-12 h, and filtering to remove enzymolysis residues after enzymolysis to obtain an enzymolysis solution.

The results of measuring the xylo-oligosaccharide content in the enzymatic hydrolysate under different steam explosion conditions, the steam explosion temperature of 180 ℃ and the explosion pressure of 2.0MPa, and the influence of different enzyme types and dosages on the xylo-oligosaccharide yield and the average polymerization degree are shown in tables 3 and 4. The measurement conditions were as follows: respectively preparing standard solutions of xylose, xylobiose, xylotriose, xylotetraose and xylopentaose reference substances of 0.2mg/mL, 0.4mg/mL, 0.6mg/mL, 0.8mg/mL and 1.0mg/mL by using a mobile phase, and filtering the standard solutions through a microfiltration membrane of 0.45 mu m for later use; the chromatographic conditions were Shodex KS-802 column (8.0X 300mm, packed with polyethylene divinylbenzene) and the mobile phase was ultrapure water, flow rate 0.8ml/min, column temperature 60 ℃, injection volume 20. mu.L, differential refractive index detector.

TABLE 3 content of xylooligosaccharide in corn stalk enzymatic hydrolysate under different steam explosion conditions (maintenance time 5min)

Temperature of steam explosion	170℃	180℃	188℃	198℃	207℃
						Content of xylooligosaccharide	4.76	5.47	5.35	8.42	8.35

TABLE 4 influence of different enzyme types and dosages on the yield and average polymerization degree of xylo-oligosaccharide from corn stalks under steam explosion temperature of 180 ℃, explosion pressure of 2.0MPa and holding time of 20min

Table 3 shows that the data of the xylooligosaccharide content in the enzymatic hydrolysate under different steam explosion conditions indicate that as the steam explosion temperature increases, the hydrothermal chemical action and the physical tearing action of the steam explosion treatment on the corn stalks are enhanced, and the breaking of the degradation-resistant barrier of the corn stalks promotes the enzymatic conversion behavior, so that the xylooligosaccharide product in the enzymatic hydrolysate is improved.

Table 4 compares the influence of different endo-xylanases and enzyme dosages on the xylo-oligosaccharide content and polymerization degree of steam-exploded corn straw conversion, and provides experimental data support for the selection of xylanase types and dosages in the invention. The data show that the content of xylooligosaccharide in the enzymatic hydrolysate is increased along with the increase of enzyme consumption, and the polymerization degree of xylooligosaccharide is mainly between 2 and 3, namely the product is mainly xylobiose and xylotriose.

And (3) removing impurities and decoloring the enzymatic hydrolysate by using a microfiltration membrane, and removing macromolecular proteins and micromolecular monosaccharides by ultrafiltration and nanofiltration. And (3) carrying out reduced pressure vacuum concentration or spray drying on the purified enzymolysis liquid to prepare xylo-oligosaccharide syrup or powdered sugar with the purity of more than 90 percent and taking xylobiose and xylotriose as main components.

Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the embodiments disclosed.