Method for producing l-lactic acid by simultaneous saccharification and fermentation of bamboo cellulose

Abstract

The application relates to the technical field of bamboo cellulose processing, and discloses a method for producing L-lactic acid through synchronous saccharification and fermentation of bamboo cellulose, which takes bamboo waste as raw material, adopts a pretreatment process combining mechanical crushing, composite chemical treatment and steam explosion on the bamboo, can cooperatively destroy the structure of the bamboo cellulose, realizes high-value utilization of the bamboo resources, and has no strong acid and strong alkali residues in the pretreatment process, is small in environmental pollution, conforms to the green production concept, selects suitable Pediococcus acidilactici and composite cellulase, realizes efficient cooperation of saccharification and fermentation, can meet the industrialized production demand of polylactic acid, adopts a separation and purification process to treat the fermentation liquor, avoids the influence of ions on the quality of L-lactic acid, and further purifies through a concentration, acidification and crystallization process, so that high-purity L-lactic acid meeting the industrial application standard can be obtained and can be directly used for polylactic acid synthesis and high-end food and medicine fields, and the application range of the bamboo cellulose fermentation L-lactic acid is greatly expanded.

Description

Technical Field

[0001] This invention relates to the field of bamboo cellulose processing technology, specifically a method for the simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid. Background Technology

[0002] L-lactic acid, as the core monomer for the production of polylactic acid (PLA), has wide applications in biodegradable materials, food, and pharmaceuticals. Traditional L-lactic acid production relies on starchy raw materials such as corn and cassava, which presents problems such as competition with human and animal food, high raw material costs, and poor sustainability, making it difficult to meet the market demand of millions of tons.

[0003] Bamboo cellulose, as an important source of lignocellulose, boasts advantages such as wide availability, high renewability, and low cost, making it an ideal alternative to grain raw materials. However, the dense structure of bamboo cellulose, with significant encapsulation effects of lignin and hemicellulose, leads to low enzymatic hydrolysis efficiency. Furthermore, the simultaneous saccharification and fermentation process faces technical bottlenecks such as inhibitory interference and insufficient strain tolerance, hindering the industrial utilization of bamboo cellulose. In existing technologies, bamboo cellulose pretreatment often employs single physical or chemical methods, resulting in poor pretreatment effects, significant environmental pollution, and high energy consumption. Simultaneous saccharification and fermentation processes often suffer from long fermentation cycles and low L-lactic acid yields due to mismatches between enzymatic hydrolysis and fermentation temperatures and poor strain synergy, making it difficult to achieve efficient industrial production. Therefore, this paper proposes a method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid. Summary of the Invention

[0004] The purpose of this invention is to provide a method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid, addressing the technical bottlenecks mentioned in the background art, such as the dense structure of bamboo cellulose, the significant encapsulation effect of lignin and hemicellulose leading to low enzymatic hydrolysis efficiency, and the interference of inhibitors and insufficient strain tolerance during simultaneous saccharification and fermentation, which restrict the industrial utilization of bamboo cellulose. Furthermore, in existing technologies, bamboo cellulose pretreatment often employs single physical or chemical methods, resulting in poor pretreatment effects, significant environmental pollution, and high energy consumption. Simultaneous saccharification and fermentation processes often suffer from long fermentation cycles and low L-lactic acid yields due to mismatches between enzymatic hydrolysis and fermentation temperatures and poor strain synergy, making it difficult to achieve efficient industrial production.

[0005] To achieve the above objectives, the present invention provides the following technical solution: 1. A method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid, comprising the following steps: S1. Pretreatment of bamboo cellulose raw materials: Select bamboo materials and sequentially perform mechanical crushing, compound chemical pretreatment and steam explosion treatment to improve the enzymatic hydrolysis of cellulose and create favorable conditions for subsequent simultaneous saccharification and fermentation. S2. Preparation of fermentation strains and enzyme systems: Select fermentation strains and activate and seed culture of Pietrococcus lactis, and select complex cellulase secreted by Trichoderma reesei. S3, Simultaneous saccharification and fermentation: The pretreated materials, fermentation medium, seed liquid and cellulase are mixed, pre-saccharified and then fermented simultaneously, and the pH is kept stable during the fermentation process; S4. Separation and purification of fermentation products: The fermentation products are processed by solid-liquid separation, decolorization, concentration and crystallization to obtain L-lactic acid products.

[0006] Preferably, S1 above includes the following sub-steps: S11. Mechanical crushing: Dry bamboo waste is put into a high-speed crusher and crushed to a particle size of 40-60 mesh to obtain bamboo cellulose coarse material; S12. Composite chemical pretreatment: The crushed bamboo cellulose coarse material is mixed with the pretreatment reagent at a solid-liquid ratio of 1:8-1:12 (w / v). The reagent is a composite solution of 2%-4% dilute sulfuric acid and 8%-12% ammonia water. The mixture is stirred at 100-120℃ for 2-3 hours. After treatment, the mixture is filtered, washed with deionized water until the pH value is neutral, and dried for later use. S13. Steam explosion treatment: The bamboo cellulose material after composite chemical pretreatment is put into a steam explosion device, saturated steam is introduced, and the pressure is maintained at 1.5-2.0MPa and temperature at 180-200℃ for 20-30 minutes. Then the pressure is released instantly, the explosion material is collected, and dried to a moisture content of 10%-15%.

[0007] Preferably, S2 above includes the following sub-steps: S21. Strain activation and seed culture: Select fermentation strains and activate and culture them for seed proliferation. S22. Selection of cellulase system: A complex cellulase secreted by Trichoderma reesei is selected, with an enzyme activity of 80-100 FPIU / g, including endoglucanase, exoglucanase and β-glucosidase, and the enzyme system ratio is adapted to the degradation requirements of bamboo cellulose.

[0008] Preferably, S21 above includes the following sub-steps: S211. Strain activation: Select *Pediococcus lactis*, which is tolerant to high concentrations of L-lactate, as the fermentation strain. Inoculate the strain into fresh MRS liquid medium and culture it at 37-45℃ and 150-200 rpm with constant temperature shaking until the OD600 is greater than 3.5 to complete the strain activation. S212, Seed Culture: The activated culture solution is inoculated and transferred to fresh MRS liquid medium, and cultured under the same conditions to obtain seed culture solution.

[0009] Preferably, in S211 above, the lactic acid cocci is Pediococcus acidilactici NL1, preservation number CCTCC M 2023152, and in S212, the inoculum amount for seed culture is 5%-10% (v / v), and the culture is carried out until OD600 is greater than 3.5.

[0010] Preferably, S3 above includes the following sub-steps: S31. Preparation of fermentation medium: Mix the pretreated bamboo cellulose with the fermentation medium; S32. Synchronous saccharification and fermentation operation: Seed culture medium is inoculated into the fermentation medium, and synchronous saccharification and fermentation treatment is carried out under specific conditions.

[0011] Preferably, in S31 above, the fermentation medium consists of: pretreated bamboo cellulose material (solid content 25%-35%, w / w), sodium acetate 2-10 g / L, manganese sulfate monohydrate 0.1-0.5 g / L, ammonium sulfate 2-15 g / L, hydrolyzed plant protein 10-30 g / L, and the remainder is deionized water. Preferably, S32 above includes the following sub-steps: S321. Fermentation broth preparation: Add the prepared fermentation medium to the fermenter, inoculate with the above seed culture medium at an inoculation rate of 10%-15% (v / v), and add cellulase at the same time. The amount of cellulase is 4-5 mg protein / g bamboo cellulose material (dry basis). S322, Simultaneous saccharification and fermentation: First, pre-saccharify at 45-50℃ and 150-200rpm for 6-8h. After pre-saccharification, lower the temperature to 42℃, adjust the pH to 5.5, maintain the rotation speed at 150-200rpm, and continue fermentation for 60-72h. During the fermentation process, maintain pH stability by adding sodium hydroxide or calcium hydroxide solution.

[0012] Preferably, S4 above includes the following sub-steps: S41. Solid-liquid separation and preliminary purification: After fermentation, the fermentation broth is separated into solid and liquid components by a plate and frame filter press to remove undegraded bamboo cellulose residue and obtain a clear fermentation broth containing L-lactic acid. Activated carbon is added to the clear broth and the mixture is stirred and decolorized at 50-60℃ for 30-60 minutes. The activated carbon is then removed by filtration to obtain a decolorized broth. S42. Refining and Crystallization: The decolorizing solution is adsorbed with a cation exchange resin to remove metal ions, then concentrated, acidified and crystallized. After centrifugation and drying, high-purity L-lactic acid crystals are obtained.

[0013] Preferably, in the above S42, the purity of the L-lactic acid obtained by separation and purification is not less than 99%, and the chirality is not less than 99.5%.

[0014] Compared with the prior art, the present invention, employing the above technical solution, has the following technical effects: 1. Using bamboo waste as raw material, which is widely available and inexpensive, can replace grain-based raw materials, avoiding the problem of "competing with humans for food". At the same time, it realizes the high-value utilization of bamboo waste, which has good economic and environmental benefits and is easy to promote industrialization. Furthermore, the pretreatment process of bamboo material using a combination of mechanical crushing, compound chemical treatment and steam explosion can synergistically destroy the structure of bamboo cellulose, significantly improving the accessibility of cellulose. The enzymatic hydrolysis efficiency of cellulose after pretreatment is more than 30% higher than that of existing treatment methods, realizing the high-value utilization of bamboo resources, reducing raw material costs, and the pretreatment process leaves no strong acid or alkali residues, resulting in less environmental pollution and conforming to the concept of green production. It has good prospects for industrial application.

[0015] 2. By screening suitable Pediococcus lactis and compound cellulase, and controlling the temperature of pre-saccharification and simultaneous fermentation, efficient synergy between saccharification and fermentation is achieved. The fermentation cycle is shortened by 40% compared with stepwise fermentation, and the L-lactic acid yield reaches 19-20 g / L, which is more than 20% higher than the existing technology. In addition, the product has high chirality, which can meet the industrial production needs of polylactic acid.

[0016] 3. The fermentation broth is treated using a separation and purification process. After solid-liquid separation, sodium, calcium, and other metal ions introduced during fermentation are removed using cation exchange resin to avoid affecting the quality of L-lactic acid. Then, it is further purified through concentration, acidification, and crystallization processes to obtain high-purity L-lactic acid that meets industrial application standards. The separation and purification process is simple and efficient, and the purity of the obtained L-lactic acid is ≥99%. It can be directly used in polylactic acid synthesis and high-end food and pharmaceutical fields, which greatly expands the application range of bamboo cellulose fermented L-lactic acid. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the main production steps of L-lactic acid according to the present invention. Figure 2 This is a schematic diagram of the specific production process of L-lactic acid according to the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that the structures, proportions, sizes, etc., shown in the accompanying drawings of this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the conditions under which this application can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size should still fall within the scope of the technical content disclosed in this application, provided that they do not affect the effects and purposes that this application can produce.

[0021] In existing technologies, bamboo cellulose has a dense structure, and lignin and hemicellulose significantly encapsulate it, resulting in low enzymatic hydrolysis efficiency. Furthermore, the simultaneous saccharification and fermentation process suffers from technical bottlenecks such as inhibitor interference and insufficient strain tolerance, hindering the industrial utilization of bamboo cellulose. Moreover, current bamboo cellulose pretreatment methods often employ single physical or chemical approaches, leading to poor pretreatment effects, significant environmental pollution, and high energy consumption. Simultaneous saccharification and fermentation processes often suffer from mismatched enzymatic hydrolysis and fermentation temperatures, as well as poor strain synergy, resulting in long fermentation cycles and low L-lactic acid yields, making it difficult to achieve efficient industrial production.

[0022] Example Please see Figure 1-2 This invention provides a technical solution: a method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid, comprising the following steps: S1. Pretreatment of Bamboo Cellulose Raw Materials: Bamboo is selected as the raw material, which is widely available, inexpensive, and can replace grain-based raw materials, avoiding the problem of "competing with humans for food." Simultaneously, it achieves high-value utilization of bamboo waste, resulting in good economic and environmental benefits and facilitating industrialization. The bamboo is subjected to mechanical crushing, compound chemical pretreatment, and steam explosion treatment in sequence to improve the enzymatic hydrolysis of cellulose, creating favorable conditions for subsequent simultaneous saccharification and fermentation. Specifically, S1 includes the following sub-steps: S11. Mechanical crushing: Dry bamboo waste is put into a high-speed crusher and crushed to a particle size of 50 mesh to obtain bamboo cellulose coarse material; The purpose of this step is to mechanically crush the macroscopic structure of bamboo, reduce the particle size of the raw material, increase the contact area between cellulose and subsequent treatment reagents and enzymes, lay the foundation for subsequent chemical or physical pretreatment, and improve the overall conversion efficiency. S12. Composite chemical pretreatment: The crushed bamboo cellulose coarse material is mixed with the pretreatment reagent at a solid-liquid ratio of 1:10 (w / v). The reagent is a composite solution of 2% dilute sulfuric acid and 10% ammonia water. The mixture is stirred at 110℃ for 2.5h. After treatment, it is filtered, washed with deionized water until the pH value is neutral, and dried for later use. The purpose of this step is as follows: dilute sulfuric acid can effectively hydrolyze hemicellulose, destroying the connection structure between cellulose and hemicellulose; ammonia water can selectively remove lignin, weakening the lignin's encapsulation effect on cellulose, while reducing the crystallinity of cellulose, improving the looseness and accessibility of cellulose, significantly improving the enzymatic hydrolysis efficiency of subsequent cellulase, and providing sufficient fermentable substrate for simultaneous saccharification and fermentation. S13. Steam Explosion Treatment: The bamboo cellulose material after composite chemical pretreatment is put into a steam explosion device, saturated steam is introduced, and the condition is maintained at 1.8MPa and 190℃ for 25 minutes. Then the pressure is released instantly, the exploded material is collected, and dried to a moisture content of 10%. The purpose of this step is to use steam explosion to penetrate the material with high-temperature, high-pressure steam. The powerful mechanical force generated during the instantaneous depressurization further disrupts the crystalline structure of cellulose, allowing for the complete separation of cellulose, hemicellulose, and lignin. Simultaneously, it degrades some inhibitors, improving the enzymatic hydrolysis of cellulose and creating favorable conditions for subsequent simultaneous saccharification and fermentation. This reduces the toxic effects of inhibitors on fermentation microorganisms. The pretreatment process, combining mechanical crushing, compound chemical treatment, and steam explosion, synergistically disrupts the structure of bamboo cellulose, significantly improving its accessibility. The enzymatic hydrolysis efficiency of cellulose after pretreatment is more than 30% higher than existing methods, achieving high-value utilization of bamboo resources, reducing raw material costs, and leaving no strong acid or alkali residues. This minimizes environmental pollution and aligns with green production principles, demonstrating promising prospects for industrial application.

[0023] S2. Preparation of Fermentation Strains and Enzyme Systems: Select fermentation strains and activate and seed culture *Pediococcus lactis*. Use a complex cellulase secreted by *Trichoderma reesei*. Specifically, S2 includes the following sub-steps: S21. Strain Activation and Seed Culture: Select fermentation strains and activate and propagate them through seed culture. Specifically, S21 includes the following sub-steps: S211. Strain activation: Pediococcus acidilactici NL1, which is tolerant to high concentrations of L-lactate, was selected as the fermentation strain. The preservation number was CCTCC M 2023152. The strain was inoculated into fresh MRS liquid medium and cultured at 38℃ and 170 rpm with constant temperature shaking until OD600=3.8 to complete the strain activation. S212, Seed culture: The activation solution was transferred to fresh MRS liquid medium at an inoculation rate of 8% (v / v) and cultured under the same conditions until OD600=3.9 to obtain the seed culture solution; The purpose of this step is to provide sufficient carbon, nitrogen, and growth factors to Pediococcus lactis in MRS medium. Through phased activation culture, the strain can be restored to activity and proliferate in large quantities, ensuring high viability of the seed culture and providing sufficient inoculum for subsequent fermentation, thus ensuring the stability and efficiency of the fermentation process. S22. Selection of cellulase system: A complex cellulase secreted by Trichoderma reesei is selected, with an enzyme activity of 95 FPIU / g, including endoglucanase, exoglucanase and β-glucosidase, and the enzyme system ratio is adapted to the degradation requirements of bamboo cellulose. The purpose of this step is to achieve efficient hydrolysis of bamboo cellulose by using a complex cellulase that synergistically acts on the β-1,4-glycosidic bonds of bamboo cellulose, an endoglucanase that randomly cleaves the cellulose chain, an exoglucanase that gradually releases cellobiose from the ends of the cellulose chain, and a β-glucosidase that converts cellobiose into glucose. This synergistic action of the three enzymes provides sufficient fermentable sugar substrates for the fermentation strains. By screening suitable *Pediococcus lactis* and the complex cellulase, and through temperature control of pre-saccharification and simultaneous fermentation, efficient synergy between saccharification and fermentation is achieved. The fermentation cycle is shortened by 40% compared to stepwise fermentation, and the L-lactic acid yield reaches 19-20 g / L, an improvement of more than 20% compared to existing technologies. Furthermore, the product has high chirality, meeting the industrial production requirements of polylactic acid.

[0024] S3, Simultaneous Saccharification and Fermentation: The pretreated materials, fermentation medium, seed culture, and cellulase are mixed, pre-saccharified, and then fermented simultaneously. The pH is maintained stable during the fermentation process. Specifically, S3 includes the following sub-steps: S31. Preparation of fermentation medium: Mix the pretreated bamboo cellulose with the fermentation medium, wherein the composition of the fermentation medium is: pretreated bamboo cellulose material (solid content 30%, w / w), sodium acetate 5g / L, manganese sulfate monohydrate 0.3g / L, ammonium sulfate 8g / L, hydrolyzed plant protein 25g / L, and the remainder is deionized water. The purpose of this step is as follows: Sodium acetate maintains the osmotic pressure of the fermentation system and provides a carbon source for the strain; manganese sulfate acts as an enzyme activator to enhance cellulase activity; ammonium sulfate provides a nitrogen source to promote the growth and metabolism of the strain; and hydrolyzed plant protein, rich in amino acids, vitamins and other growth factors, enhances the strain's tolerance and metabolic activity, providing sufficient nutritional support for L-lactic acid synthesis. S32. Simultaneous saccharification and fermentation operation: Seed culture is inoculated into the fermentation medium, and simultaneous saccharification and fermentation is carried out under specific conditions. Specifically, S32 includes the following sub-steps: S321. Fermentation broth preparation: Add the prepared fermentation medium to the fermenter, inoculate with the above seed culture medium at an inoculation amount of 12% (v / v), and add cellulase at a dosage of 4.5 mg protein / g bamboo cellulose material (dry basis). S322, Simultaneous saccharification and fermentation: First, pre-saccharification is carried out at 48℃ and 180rpm for 7h. After pre-saccharification, the temperature is lowered to 42℃, the pH is adjusted to 5.5, the speed is maintained at 160rpm, and fermentation is continued for 68h. During the fermentation process, the pH is maintained by adding sodium hydroxide or calcium hydroxide solution. The purpose of this step is to: in the pre-saccharification stage, efficiently hydrolyze bamboo cellulose at the optimal temperature of cellulase to release fermentable sugars, providing sufficient substrate for subsequent fermentation; and in the fermentation stage, match the temperature to the growth and metabolic needs of Pediococcus lactis while ensuring that cellulase still has high activity, so as to achieve simultaneous saccharification and fermentation, shorten the fermentation cycle, reduce the inhibition of the strain by the accumulation of intermediate products, and improve the utilization rate of sugar and the yield of L-lactic acid.

[0025] S4. Fermentation product separation and purification: The fermentation product is processed through solid-liquid separation, decolorization, concentration, and crystallization to obtain L-lactic acid product. Specifically, S4 includes the following sub-steps: S41. Solid-liquid separation and preliminary purification: After fermentation, the fermentation broth is separated into solid and liquid components by a plate and frame filter press to remove undegraded bamboo cellulose residue and obtain a clear fermentation broth containing L-lactic acid. Activated carbon is added to the clear broth and the mixture is stirred and decolorized at 55°C for 45 minutes. The activated carbon is then removed by filtration to obtain a decolorized broth. The purpose of this step is to separate solids and liquids to recover residues, prevent impurities from affecting subsequent purification, and remove pigments and some impurities from the fermentation broth by activated carbon decolorization, thereby improving the initial purity of L-lactic acid and laying the foundation for subsequent refining. S42. Refining and Crystallization: The decolorizing solution is subjected to cation exchange resin to remove metal ions, then concentrated, acidified, and crystallized. After centrifugation and drying, high-purity L-lactic acid crystals are obtained. The purity of the purified L-lactic acid is not less than 99%, and the chirality is not less than 99.5%. The purpose of this step is to use cation exchange resin to remove metal ions such as sodium and calcium introduced during fermentation, thus preventing these ions from affecting the quality of L-lactic acid. The concentration, acidification, and crystallization processes further purify the L-lactic acid to obtain high-purity L-lactic acid that meets industrial application standards. This satisfies the quality requirements of polylactic acid synthesis and the food and pharmaceutical fields, and significantly expands the application range of bamboo cellulose fermented L-lactic acid.

[0026] In summary, using bamboo waste as raw material is a widely available and inexpensive alternative to grain-based raw materials, avoiding the problem of "competing with humans for food." It also achieves high-value utilization of bamboo waste, offering significant economic and environmental benefits and facilitating industrialization. Furthermore, the pretreatment process combining mechanical crushing, compound chemical treatment, and steam explosion can synergistically disrupt the structure of bamboo cellulose, significantly improving cellulose accessibility. The enzymatic hydrolysis efficiency of cellulose after pretreatment is more than 30% higher than existing methods, achieving high-value utilization of bamboo resources, reducing raw material costs, and eliminating strong acid and alkali residues, resulting in minimal environmental pollution and aligning with green production principles. This demonstrates promising prospects for industrial application. The screening of suitable *Pediococcus lactis* and compound cellulase, through presaccharification and simultaneous fermentation... Temperature control enables efficient synergy between saccharification and fermentation, shortening the fermentation cycle by 40% compared to stepwise fermentation. L-lactic acid yield reaches 19-20 g / L, an improvement of over 20% compared to existing technologies. Furthermore, the product exhibits high chirality, meeting the industrial production requirements of polylactic acid. A separation and purification process is employed to treat the fermentation broth. After solid-liquid separation, cation exchange resin is used to remove sodium, calcium, and other metal ions introduced during fermentation, preventing ions from affecting the quality of L-lactic acid. Further purification is achieved through concentration, acidification, and crystallization processes, yielding high-purity L-lactic acid that meets industrial application standards. The separation and purification process is simple and efficient, resulting in L-lactic acid with a purity ≥99%, which can be directly used in polylactic acid synthesis and in high-end food and pharmaceutical fields, significantly expanding the application scope of bamboo cellulose fermented L-lactic acid.

[0027] Those skilled in the art will understand that the features described in the various embodiments and / or claims of the present invention can be combined or combined in various ways, even if such combinations or combinations are not explicitly described in the present invention. In particular, the features described in the various embodiments and / or claims of the present invention can be combined or combined in various ways without departing from the spirit and teachings of the present invention. All such combinations and / or combinations fall within the scope of the present invention.

Claims

1. A method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid, characterized in that, Includes the following steps: S1. Pretreatment of bamboo cellulose raw materials: Select bamboo materials and sequentially perform mechanical crushing, compound chemical pretreatment and steam explosion treatment on the bamboo materials; S2. Preparation of fermentation strains and enzyme systems: Select fermentation strains and activate and seed culture of Pietrococcus lactis, and select complex cellulase secreted by Trichoderma reesei. S3, Simultaneous saccharification and fermentation: The pretreated materials, fermentation medium, seed liquid and cellulase are mixed, pre-saccharified and then fermented simultaneously, and the pH is kept stable during the fermentation process; S4. Separation and purification of fermentation products: The fermentation products are processed by solid-liquid separation, decolorization, concentration and crystallization to obtain L-lactic acid products.

2. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 1, characterized in that, S1 includes the following sub-steps: S11. Mechanical crushing: Dry bamboo waste is put into a high-speed crusher and crushed to a particle size of 40-60 mesh to obtain bamboo cellulose coarse material; S12. Composite chemical pretreatment: The crushed bamboo cellulose coarse material is mixed with the pretreatment reagent at a solid-liquid ratio of 1:8-1:12 (w / v). The reagent is a composite solution of 2%-4% dilute sulfuric acid and 8%-12% ammonia water. The mixture is stirred at 100-120℃ for 2-3 hours. After treatment, the mixture is filtered, washed with deionized water until the pH value is neutral, and dried for later use. S13. Steam explosion treatment: The bamboo cellulose material after composite chemical pretreatment is put into a steam explosion device, saturated steam is introduced, and the pressure is maintained at 1.5-2.0MPa and temperature at 180-200℃ for 20-30 minutes. Then the pressure is released instantly, the explosion material is collected, and dried to a moisture content of 10%-15%.

3. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 1, characterized in that, S2 includes the following sub-steps: S21. Strain activation and seed culture: Select fermentation strains and activate and culture them for seed proliferation. S22. Selection of cellulase system: A complex cellulase secreted by Trichoderma reesei is selected, with an enzyme activity of 80-100 FPIU / g, including endoglucanase, exoglucanase and β-glucosidase, and the enzyme system ratio is adapted to the degradation requirements of bamboo cellulose.

4. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 3, characterized in that, S21 includes the following sub-steps: S211. Strain activation: Select *Pediococcus lactis*, which is tolerant to high concentrations of L-lactate, as the fermentation strain. Inoculate the strain into fresh MRS liquid medium and culture it at 37-45℃ and 150-200 rpm with constant temperature shaking until the OD600 is greater than 3.5 to complete the strain activation. S212, Seed Culture: The activated culture solution is inoculated and transferred to fresh MRS liquid medium, and cultured under the same conditions to obtain seed culture solution.

5. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 4, characterized in that, In S211, the *Pediococcus acidilactici* NL1, preservation number CCTCC M 2023152, is used for seed culture. In S212, the inoculum amount for seed culture is 5%-10% (v / v), and the culture is carried out until the OD600 is greater than 3.

5.

6. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 1, characterized in that, S3 includes the following sub-steps: S31. Preparation of fermentation medium: Mix the pretreated bamboo cellulose with the fermentation medium; S32. Synchronous saccharification and fermentation operation: Seed culture medium is inoculated into the fermentation medium, and synchronous saccharification and fermentation treatment is carried out under specific conditions.

7. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 6, characterized in that, In S31, the fermentation medium consists of: pretreated bamboo cellulose material (solid content 25%-35%, w / w), sodium acetate 2-10 g / L, manganese sulfate monohydrate 0.1-0.5 g / L, ammonium sulfate 2-15 g / L, hydrolyzed plant protein 10-30 g / L, and the remainder is deionized water.

8. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 7, characterized in that, S32 includes the following sub-steps: S321. Fermentation broth preparation: Add the prepared fermentation medium to the fermenter, inoculate with the above seed culture medium at an inoculation rate of 10%-15% (v / v), and add cellulase at the same time. The amount of cellulase is 4-5 mg protein / g bamboo cellulose material (dry basis). S322, Simultaneous saccharification and fermentation: First, pre-saccharify at 45-50℃ and 150-200rpm for 6-8h. After pre-saccharification, lower the temperature to 42℃, adjust the pH to 5.5, maintain the rotation speed at 150-200rpm, and continue fermentation for 60-72h. During the fermentation process, maintain pH stability by adding sodium hydroxide or calcium hydroxide solution.

9. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 1, characterized in that, S4 includes the following sub-steps: S41. Solid-liquid separation and preliminary purification: After fermentation, the fermentation broth is separated into solid and liquid components by a plate and frame filter press to remove undegraded bamboo cellulose residue and obtain a clear fermentation broth containing L-lactic acid. Activated carbon is added to the clear broth and the mixture is stirred and decolorized at 50-60℃ for 30-60 minutes. The activated carbon is then removed by filtration to obtain a decolorized broth. S42. Refining and Crystallization: The decolorizing solution is adsorbed with a cation exchange resin to remove metal ions, then concentrated, acidified and crystallized. After centrifugation and drying, high-purity L-lactic acid crystals are obtained.

10. The method for simultaneous saccharification and fermentation of bamboo cellulose to produce L-lactic acid according to claim 8, characterized in that, In step S42, the purity of the L-lactic acid obtained by separation and purification is not less than 99%, and the chirality is not less than 99.5%.

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