Straw-based sugar solution and process for preparing lactic acid from straw-based sugar solution

Abstract

The invention belongs to the field of biological fermentation, and particularly relates to a straw-based sugar solution and a process for preparing lactic acid from the straw-based sugar solution. A carrier containing a hydrogen peroxide-producing catalyst is prepared, lignin peroxidase is immobilized, and an enzyme-containing pretreatment agent is prepared; the method comprises the following steps: crushing straws, and pretreating with ionic liquid to obtain a precursor; and treating the precursor by using an enzyme-containing treating agent, and decomposing phenols to obtain the straw-based sugar solution. According to the method, various lignocellulose degradation modes are combined, the lignocellulose can be fully and thoroughly degraded, and the yield of sugar in the sugar liquid is high. Particularly, the activity of phenolic compounds in the liquid glucose can be reduced, the toxicity of the liquid glucose during fermentation and acid production is reduced, and the yield of lactic acid is high.

Description

Technical Field

[0001] The present invention belongs to the field of biological fermentation, and particularly relates to a straw-based sugar solution and a process for preparing lactic acid therefrom. Background Art

[0002] The methods for producing lactic acid mainly include chemical synthesis method, biocatalysis method and microbial fermentation method. The existing processes for producing L-lactic acid generally use grains, and the production capacity is difficult to meet the demand of the biodegradable plastic products market. Lignocellulose is the most abundant and renewable organic matter in nature. The cellulose and hemicellulose contained therein can be converted into fermentable sugars that can be utilized by microorganisms through certain technologies, and then can be fermented by microorganisms to produce bioenergy and chemicals. Therefore, using lignocellulose as a raw material to replace grains for producing lactic acid can relieve the pressure on grain supply and meet the huge demand for lactic acid products.

[0003] During the enzymatic hydrolysis and saccharification of lignocellulosic raw materials, hydrolysis occurs, mainly producing carbohydrate degradation products and phenolic lignin degradation products. The characteristics of feedback inhibition of these products on hydrolases restrict the increase of substrate concentration and enzymatic hydrolysis efficiency. Moreover, both of these two types of compounds will also have a certain inhibitory effect on the fermentation of L-lactic acid. In particular, during the hydrolysis process, lignin mainly generates a wide variety of phenolic compounds through degradation and the condensation of degradation products, mainly including monophenols and polyphenols. Although the content of phenolic degradation products derived from lignin is very low, it will seriously affect the subsequent fermentation process. White rot fungi can degrade lignin. However, phenolic compounds can penetrate the microbial cell membrane and damage the integrity of the cell membrane structure, thus affecting the normal growth of the cells and further reducing the fermentation efficiency. The esters in lignin also have an inhibitory effect on the growth of fungi.

[0004] Enzymatic hydrolysis is a key step in the fermentation conversion of lignocellulose into lactic acid. The complex structure of lignocellulosic biomass leads to low enzymatic hydrolysis efficiency, so appropriate pretreatment methods are needed to break its complex structure. Among them, steam explosion pretreatment is a commonly used pretreatment method, but during the pretreatment process, substances that inhibit cellulase and microbial fermentation will be produced, affecting the efficiency of subsequent enzymatic hydrolysis and fermentation processes. On the other hand, in the process of bioconversion, a high sugar concentration in the enzymatic hydrolysate is beneficial to increasing the concentration of the final product and reducing costs such as product separation and purification, but this means that enzymatic hydrolysis needs to be carried out at a high substrate concentration, and phenomena such as the decline in mass transfer efficiency and product inhibition caused by high substrate concentration will lead to a decrease in enzymatic hydrolysis efficiency. Ionic liquids are salts composed of anions and cations, and the limitations of using ionic liquids for the degradation of lignocellulose are much smaller than other methods. The ionic liquids for pretreating lignocellulose are mostly imidazole-based and pyridine-based ionic liquids, and the research on such ionic liquids is relatively in-depth, but the synthesis cost of such ionic liquids is relatively high and they have certain toxicity, which is not conducive to the subsequent microbial fermentation process. Choline-based ionic liquids with relatively low toxicity also have the problem of relatively high cost. Summary of the Invention

[0005] The present invention mainly provides a method for decomposing straw into sugar solution without toxicity and at low cost, and a straw-based sugar solution with little inhibition to fermentation prepared by this method and its process for preparing lactic acid. The technical solutions are as follows:

[0006] A straw-based sugar solution, prepare a carrier containing a hydrogen peroxide-producing catalyst, immobilize lignin peroxidase, and prepare an enzyme-containing pretreatment agent; crush the straw and pretreat it with an ionic liquid to obtain a precursor; then use the enzyme-containing treatment agent to treat the precursor to decompose phenols and obtain a straw-based sugar solution.

[0007] Further, the mass ratio with the ionic liquid is 1:10 - 20; the mass ratio of the precursor to the enzyme-containing treatment agent is 2 - 20:1; the ionic liquid includes alkanolamine, carboxylic acid and sulfonic acid; the molar ratio of the carboxylic acid to the sulfonic acid is 3 - 10:1.

[0008] Further, the alkanolamine is one or more of ethanolamine, diethanolamine, 2-methylaminoethanol, 3-aminopropanol; the carboxylic acid is one or more of acetic acid, propionic acid, butyric acid, lactic acid; the sulfonic acid is one or two of methylsulfonic acid and ethylsulfonic acid.

[0009] Further, the preparation method includes the following steps:

[0010] a. Prepare a carrier containing a hydrogen peroxide-producing catalyst, immobilize the enzyme, and prepare an enzyme-containing pretreatment agent;

[0011] b. The straw is crushed and placed in the ionic liquid, fully mixed and dispersed, and then stirred at 100-150°C for 1-3 hours for pretreatment; after the pretreatment, water 2-5 times the mass of the ionic liquid is added to the system, mixed evenly, and the precipitate is separated and collected, and the precipitate is fully washed with water to obtain a precursor;

[0012] c. Place the enzyme-containing treatment agent in a weakly acidic buffer solution, then add the precursor, mix evenly, stir at a low speed and react under light for 12 to 48 hours, take out the enzyme-containing treatment agent, and obtain the straw-based sugar solution.

[0013] Furthermore, the preparation method of the enzyme-containing pretreatment agent comprises the following steps:

[0014] Take copper nitrate and trimesic acid, place them in N, N-dimethylformamide and mix them evenly, add g-C3N4 and fully disperse them, and react at 80-110°C for 8-12 hours; collect the precipitate, dry it, and heat treat it at 400-520°C for 4-8 hours to obtain a catalyst precursor;

[0015] The ferric chloride is dissolved in an aqueous solution of ethanol to obtain a ferric chloride solution; a catalyst precursor and lignin peroxidase are dispersed in a buffer solution, the ferric chloride solution is added at 0-5°C, and after being fully dispersed, pyrrole is added dropwise to react for 4-8 hours; the solvent is evaporated below 40°C, the obtained solid is immersed in a glutaraldehyde solution for 1-30 minutes, fully washed with water, and then freeze-dried to obtain an enzyme-containing pretreatment agent.

[0016] Furthermore, the mass ratio of the copper nitrate to trimesic acid is 1.5-2:1; the mass ratio of the copper nitrate to g-C3N4 is 10-20:1.

[0017] Furthermore, the mass ratio of the catalyst precursor to lignin peroxidase is 3-10:1; the mass ratio of the pyrrole to lignin peroxidase is 0.2-1:1; the concentration of ethanol in the ethanol aqueous solution is 30-50%; and the molar ratio of the pyrrole to ferric chloride is 1:1-1.5.

[0018] Furthermore, the preparation method of g-C3N4 includes the following steps: calcining melamine at 500-600°C for 3-5h, taking it out after cooling, and grinding it to obtain g-C3N4.

[0019] A process for preparing lactic acid using the above-mentioned straw-based sugar liquid comprises the following steps: inoculating fermentation bacteria into the straw-based sugar liquid, fermenting at a speed of 200-220 rpm for 48-72 hours; separating the fermentation liquid, and purifying the upper liquid to obtain lactic acid.

[0020] Furthermore, the inoculation amount of the fermentation bacteria is 5-10%; and the fermentation bacteria is white rot fungi.

[0021] Adopting the above solution, the method of the present invention has the following advantages:

[0022] 1. The present invention combines various ways of degrading lignocellulose, can fully and thoroughly degrade lignocellulose, and has a high sugar yield in the sugar solution. In particular, it can reduce the activity of phenolic compounds in the sugar solution, reduce the toxicity during fermentation to produce acid, and improve the lactic acid yield.

[0023] 2. The present invention uses ionic liquid to degrade straw. The free hydroxyl groups are beneficial to effectively break the linkage between lignin and carbohydrates, deconstruct the plant cellulose cell wall, can combine with the hydrogen bond acceptor on cellulose to dissolve cellulose, pretreat lignocellulose, and improve the enzymatic hydrolysis effect.

[0024] 3. The ionic liquid of the present invention contains amino and sulfonic acid groups, which can improve the dissolution efficiency of lignocellulose, increase the solubility of oxygen, promote the oxidative depolymerization of lignin, and can achieve good degradation of lignin and cellulose without heating and pressurization. Moreover, the raw materials are cheap and easy to obtain, easy to separate, and the production cost is low.

[0025] 4. The present invention uses lignin peroxidase to enzymatically hydrolyze the precursor pretreated with ionic liquid, especially to decompose lignin and phenols, crack the aromatic ring, reduce the activity of phenolic compounds produced by lignin decomposition, reduce the inhibition of such products on the later lactic acid fermentation process, and improve the fermentation efficiency.

[0026] 5. The present invention adds a catalyst for photocatalytic production of hydrogen peroxide on the basis of peroxidase. The hydrogen peroxide generated by light irradiation can drive lignin peroxidase, and hydrogen peroxide can also act on copper oxide in the skeleton and residual iron to achieve catalytic depolymerization of lignin, reduce the molecular weight of lignin, and is beneficial to the further enzymatic hydrolysis of phenolic compounds.

[0027] 6. The residual iron ions in the enzyme-containing pretreatment agent of the present invention can combine with hydrogen peroxide to produce the Fenton effect, generate reactive oxygen species with stronger oxidation activity, promote the decomposition of lignin-like phenolic compounds, improve the efficiency of enzymatic hydrolysis of phenolic compounds, and catalytic production of hydrogen peroxide can save the technical cost and labor cost of adding hydrogen peroxide additionally.

[0028] 7. The present invention uses polypyrrole to compound the enzyme and the catalytic precursor, which is beneficial to electron transfer. While fixing the enzyme, it enhances the connection between the enzyme and the substance, improves the electron transfer ability between the enzyme active center and the substance, and promotes the progress of the catalytic process.

[0029] 8. The ionic liquid and the enzyme-containing pretreatment agent of the present invention can be well separated and recovered from the product, can be reused repeatedly, and the activity of the enzyme is well maintained, and can be applied to cyclic industrial production. Detailed implementation mode

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] Example 1: (1) Acetic acid and methanesulfonic acid were taken in a molar ratio of 6:1, mixed, neutralized with ethanolamine, and then the excess acid and water were evaporated to obtain an ionic liquid; 10 g of straw was crushed and placed in 20 mL of the ionic liquid, fully mixed and dispersed, and then stirred and reacted at 120 °C for 3 h for pretreatment; after the pretreatment was completed, water three times the mass of the ionic liquid was added to the system, mixed evenly, the precipitate was separated and collected, and the precipitate was washed thoroughly with water to obtain a precursor.

[0032] (2) Melamine was calcined at 550 °C for 4 h, taken out after cooling, and ground to obtain g-C3N4; 4 g of copper nitrate and 2 g of trimesic acid were placed in N,N-dimethylformamide and mixed evenly, 0.25 g of g-C3N4 was added and fully dispersed, and then reacted at 100 °C for 10 h; the precipitate was collected, dried and heat-treated at 480 °C for 8 h to obtain a catalyst precursor.

[0033] (3) Ferric chloride was dissolved in an aqueous solution of ethanol to obtain a ferric chloride solution; 10 g of the catalyst precursor and 2 g of lignin peroxidase were dispersed in a phosphate buffer solution, 3.5 g of the ferric chloride solution was added at 3 °C, fully dispersed, 2 g of pyrrole was added dropwise, and the reaction was carried out for 6 h; the solvent was evaporated below 40 °C, the obtained solid was immersed in a 5% glutaraldehyde solution for 20 min, washed thoroughly with water, and then freeze-dried to obtain an enzyme-containing pretreatment agent.

[0034] (4) 1 g of the enzyme-containing pretreatment agent was placed in a weakly acidic lactic acid-sodium lactate buffer solution, and then 10 g of the precursor was added. After mixing evenly, it was stirred at a low speed and irradiated for 24 h, and the enzyme-containing pretreatment agent was taken out to obtain a straw-based sugar solution.

[0035] Example 2: The difference from Example 1 is that:

[0036] (1) Acetic acid and methanesulfonic acid were taken in a molar ratio of 3:1, mixed, neutralized with ethanolamine, and then the excess acid and water were evaporated to obtain an ionic liquid; 10 g of straw was crushed and placed in 20 mL of the ionic liquid, fully mixed and dispersed, and then stirred and reacted at 120 °C for 3 h for pretreatment; after the pretreatment was completed, water three times the mass of the ionic liquid was added to the system, mixed evenly, the precipitate was separated and collected, and the precipitate was washed thoroughly with water to obtain a precursor.

[0037] Example 3: The difference from Example 1 is as follows:

[0038] (1) Acetic acid and methanesulfonic acid were taken in a molar ratio of 3:1, mixed, neutralized with ethanolamine, and then the excess acid and water were removed by evaporation to obtain an ionic liquid. 10 g of straw was crushed and placed in 10 mL of the ionic liquid, fully mixed and dispersed, and then stirred and reacted at 120 °C for 3 h for pretreatment. After the pretreatment was completed, water three times the mass of the ionic liquid was added to the system, mixed evenly, the precipitate was separated and collected, and the precipitate was washed thoroughly with water to obtain a precursor.

[0039] Example 4: The difference from Example 1 is as follows:

[0040] (2) Melamine was calcined at 550 °C for 4 h, taken out after cooling, and ground to obtain g-C3N4. 4 g of copper nitrate and 2 g of trimesic acid were placed in N,N-dimethylformamide and mixed evenly. After adding 0.25 g of g-C3N4 and fully dispersing, the mixture was reacted at 100 °C for 10 h. The precipitate was collected, dried, and heat-treated at 400 °C for 8 h to obtain a catalyst precursor.

[0041] Example 5: The difference from Example 1 is as follows:

[0042] (2) Melamine was calcined at 550 °C for 4 h, taken out after cooling, and ground to obtain g-C3N4. 4 g of copper nitrate and 2 g of trimesic acid were placed in N,N-dimethylformamide and mixed evenly. After adding 0.25 g of g-C3N4 and fully dispersing, the mixture was reacted at 100 °C for 10 h. The precipitate was collected, dried, and heat-treated at 480 °C for 4 h to obtain a catalyst precursor.

[0043] Example 6: The difference from Example 1 is as follows:

[0044] (3) Ferric chloride was dissolved in an aqueous solution of ethanol to obtain a ferric chloride solution. 20 g of the catalyst precursor and 2 g of lignin peroxidase were dispersed in a phosphate buffer solution. At 3 °C, 3.5 g of the ferric chloride solution was added, and after full dispersion, 2 g of pyrrole was added dropwise, and the reaction was carried out for 6 h. The solvent was evaporated below 40 °C, and the obtained solid was immersed in a 5% glutaraldehyde solution for 20 min, washed thoroughly with water, and then freeze-dried to obtain an enzyme-containing pretreatment agent.

[0045] Example 7: The difference from Example 1 is as follows:

[0046] (3) Dissolve ferric chloride in an aqueous solution of ethanol to obtain a ferric chloride solution; disperse 10 g of the catalyst precursor and 2 g of lignin peroxidase in a phosphate buffer solution, add 3.5 g of the ferric chloride solution at 3 °C, after sufficient dispersion, add 0.5 g of pyrrole dropwise, and react for 6 h; evaporate the solvent below 40 °C, immerse the obtained solid in a 5% glutaraldehyde solution for 20 min, wash thoroughly with water, and then freeze-dry to obtain the enzyme-containing pretreatment agent.

[0047] Example 8: The difference from Example 1 is that:

[0048] (4) Place 1 g of the enzyme-containing treatment agent in a weakly acidic phosphate buffer solution, then add 20 g of the precursor, mix evenly, stir at a low speed and carry out a light reaction for 24 h, take out the enzyme-containing treatment agent to obtain a straw-based sugar solution.

[0049] Example 9: The difference from Example 1 is that:

[0050] (4) Place 1 g of the enzyme-containing treatment agent in a weakly acidic phosphate buffer solution, then add 2 g of the precursor, mix evenly, stir at a low speed and carry out a light reaction for 24 h, take out the enzyme-containing treatment agent to obtain a straw-based sugar solution.

[0051] Comparative Example 1: The difference from Example 1 is that:

[0052] (2) Disperse 0.5 g of lignin peroxidase in a weakly acidic phosphate buffer solution, then add 10 g of the precursor, mix evenly, stir at a low speed and carry out a light reaction for 24 h to obtain a straw-based sugar solution.

[0053] Comparative Example 2: The difference from Example 1 is that:

[0054] (2) Dissolve ferric chloride in an aqueous solution of ethanol to obtain a ferric chloride solution; disperse 2 g of lignin peroxidase in a phosphate buffer solution, add 0.8 g of the ferric chloride solution at 3 °C, after sufficient dispersion, add 0.5 g of pyrrole dropwise, and react for 6 h; evaporate the solvent below 40 °C, immerse the obtained solid in a 5% glutaraldehyde solution for 20 min, wash thoroughly with water, and then freeze-dry to obtain the enzyme-containing pretreatment agent;

[0055] (3) Place 1 g of the enzyme-containing treatment agent in a weakly acidic phosphate buffer solution, then add 10 g of the precursor, mix evenly, stir at a low speed and carry out a light reaction for 24 h, take out the enzyme-containing treatment agent to obtain a straw-based sugar solution.

[0056] Comparative Example 3: The difference from Example 1 is that:

[0057] Do not use ionic liquid pretreatment, and directly use the enzyme-containing pretreatment agent to enzymatically hydrolyze the crushed straw.

[0058] Example sample test:

[0059] The Folin-ciocalteu colorimetric method was used to determine the content of phenolic compounds in the straw-based sugar solutions of each example and comparative example. The removal rate was evaluated by the ratio of the content difference between Comparative Example 1 and the other examples and comparative examples to that of Comparative Example 1. The formula is as follows: (the content of phenolic compounds in Comparative Example 1 - the content of phenolic compounds in the example) / the content of phenolic compounds in Comparative Example 1 × 100%.

[0060] White rot fungi were inoculated into the straw-based sugar solutions of each example and comparative example at an inoculation amount of 6% and fermented at a speed of 200 - 220 rpm for 60 h; the fermentation broth was separated, and the upper liquid was purified to obtain lactic acid. The contents of lactic acid, glucose, and xylose in the fermented product were measured by a liquid chromatograph, and the yield of lactic acid and the sugar / acid conversion rate were calculated. Three parallels were set for the fermentation experiment of the straw-based sugar solution of each example, and the average value was taken. The results are as follows:

[0061]

[0062] As can be seen from the above table, compared with Example 1, the removal rate of phenolic compounds in Example 2 with a higher methylsulfonic acid content in the ionic liquid is higher, but its yield decreases significantly. It may be that after treatment with an ionic liquid with a higher content of methylsulfonic acid, lignin decomposes more thoroughly and the oxidation of phenols is also more thorough, but more methylsulfonic acid is likely to remain in the precursor, which affects the subsequent acid production by fermentation. In Example 3, the content of the ionic liquid participating in straw pretreatment is small, and both the removal rate of phenolic compounds and the acetic acid yield decrease significantly; combined with Comparative Example 3 without ionic liquid pretreatment, it can be seen that the ionic liquid of the present invention can significantly help promote the decomposition and degradation of phenolic compounds during the hydrolysis of lignocellulose and enzymatic hydrolysis, and can significantly increase the acetic acid yield.

[0063] Example 4 has a lower heat treatment temperature than Example 1 when preparing the catalyst precursor, while Example 5 has a shorter time. The removal rates of phenolic compounds, acetic acid yields, and conversion rates in Example 4 and Example 5 all decrease. In particular, the decrease in the removal rate of phenolic compounds is obvious, indicating that the heat treatment process here is beneficial to the activation of the catalyst and has an obvious promoting effect on reducing the activity of phenolic compounds for oxidation and enzymatic hydrolysis. In Comparative Example 2, there is no catalyst precursor in the enzyme-containing treatment agent, and both the removal rate of phenolic compounds and the acetic acid conversion rate decrease significantly, indicating that this catalyst precursor has an obvious impact on both enzymatic hydrolysis saccharification and sugar conversion to acetic acid. The enzyme-containing pretreatment agent in Example 6 has more catalyst precursors, but the removal rate of phenolic compounds decreases, and the acetic acid yield and conversion rate also decrease, indicating that an increase in the content of the catalyst precursor causes a decrease in the enzyme content, which will affect the saccharification efficiency, and the catalyst precursor and enzyme content need to maintain an appropriate ratio. Example 7 has a small pyrrole content, and the removal rate of phenolic compounds, as well as the acetic acid yield and conversion rate, also decrease. Example 8 uses a larger amount of sugar precursor for treatment, and the removal rate of phenolic compounds and the acetic acid yield decrease significantly, while the removal rate and yield in Example 9 with a smaller amount of sugar precursor increase, indicating that the content of the enzyme-containing treatment agent is positively correlated with the efficiency of lactic acid production from sugar solution. However, as can be seen from the increase degree in Example 9, the promoting effect of the enzyme-containing treatment agent also has an upper limit.

[0064] For those skilled in the art, according to the technical solutions and concepts described above, various corresponding changes and deformations can be made, and all these changes and deformations should fall within the protection scope of the claims of the present invention.

Claims

1. A straw-based sugar solution, characterized in that: A carrier containing a hydrogen peroxide-producing catalyst is prepared, and lignin peroxidase is fixed to prepare an enzyme-containing pretreatment agent; the straw is crushed and pretreated with an ionic liquid to obtain a precursor; and then the precursor is treated with an enzyme-containing treatment agent to decompose phenols to obtain a straw-based sugar solution.

2. The straw-based sugar liquid according to claim 1, characterized in that: The mass ratio of the straw to the ionic liquid is 1:10-20; the mass ratio of the precursor to the enzyme-containing treatment agent is 2-20:1; the ionic liquid includes alcohol amine, carboxylic acid and sulfonic acid; the molar ratio of the carboxylic acid to the sulfonic acid is 3-10:

1.

3. The straw-based sugar liquid according to claim 3, characterized in that: The alcoholamine is one or more of ethanolamine, diethanolamine, 2-methylaminoethanol, and 3-aminopropanol; the carboxylic acid is one or more of acetic acid, propionic acid, butyric acid, and lactic acid; and the sulfonic acid is one or both of methylsulfonic acid and ethylsulfonic acid.

4. The straw-based sugar liquid according to claim 1, characterized in that: The preparation method comprises the following steps: a. preparing a carrier containing a hydrogen peroxide-producing catalyst, immobilizing the enzyme, and preparing an enzyme-containing pretreatment agent; b. The straw is crushed and placed in the ionic liquid, fully mixed and dispersed, and then stirred at 100-150°C for 1-3 hours for pretreatment; after the pretreatment, water 2-5 times the mass of the ionic liquid is added to the system, mixed evenly, and the precipitate is separated and collected, and the precipitate is fully washed with water to obtain a precursor; c. Place the enzyme-containing treatment agent in a weakly acidic buffer solution, then add the precursor, mix evenly, stir at a low speed and react under light for 12 to 48 hours, take out the enzyme-containing treatment agent, and obtain the straw-based sugar solution.

5. The straw-based sugar liquid according to claim 1, characterized in that: The preparation method of the enzyme-containing pretreatment agent comprises the following steps: Take copper nitrate and trimesic acid, place them in N, N-dimethylformamide and mix them evenly, add g-C3N4 and fully disperse them, and react at 80-110°C for 8-12 hours; collect the precipitate, dry it, and heat treat it at 400-520°C for 4-8 hours to obtain a catalyst precursor; The ferric chloride is dissolved in an aqueous solution of ethanol to obtain a ferric chloride solution; a catalyst precursor and lignin peroxidase are dispersed in a buffer solution, the ferric chloride solution is added at 0-5°C, and after being fully dispersed, pyrrole is added dropwise to react for 4-8 hours; the solvent is evaporated below 40°C, the obtained solid is immersed in a glutaraldehyde solution for 1-30 minutes, fully washed with water, and then freeze-dried to obtain an enzyme-containing pretreatment agent.

6. The straw-based sugar liquid according to claim 5, characterized in that: The mass ratio of the copper nitrate to trimesic acid is 1.5 to 2:1; the mass ratio of the copper nitrate to g-C3N4 is 10 to 20:

1.

7. The straw-based sugar liquid according to claim 5, characterized in that: The mass ratio of the catalyst precursor to the lignin peroxidase is 3-10:1; the mass ratio of the pyrrole to the lignin peroxidase is 0.2-1:1; the concentration of ethanol in the ethanol aqueous solution is 30-50%; and the molar ratio of the pyrrole to ferric chloride is 1:1-1.

5.

8. The straw-based sugar liquid according to claim 5, characterized in that: The preparation method of g-C3N4 comprises the following steps: calcining melamine at 500-600°C for 3-5h, taking it out after cooling, and grinding it to obtain g-C3N4.

9. A process for preparing lactic acid using the straw-based sugar solution according to any one of claims 1 to 8, characterized in that: The following steps are involved: The fermentation bacteria are inoculated into the straw-based sugar solution, and the solution is fermented at a speed of 200 to 220 rpm for 48 to 72 hours; the fermentation solution is separated, and the upper layer of the solution is purified to obtain lactic acid.

10. The process for preparing lactic acid from straw-based sugar liquid according to claim 8, characterized in that: The inoculation amount of the fermentation bacteria is 5-10%; the fermentation bacteria is white rot fungus.

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