Transformation method of L-serine
By using hexadecyltrimethylammonium bromide to treat whole cells and combining graded decolorization with gradient crystallization, the problems of low L-serine yield and purity in the enzymatic conversion method were solved, and efficient L-serine preparation was achieved.
Patent Information
- Application Number
- CN202510853664.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-24
- Publication Date
- 2025-09-23
AI Technical Summary
The yield and purity of L-serine in the prior art are relatively low. In particular, in the enzymatic conversion method, there is a problem of many impurities in the conversion liquid, which affects the extraction yield and purity.
The whole cells were treated with hexadecyltrimethylammonium bromide to release the enzyme, which was then decolorized twice using activated carbon combinations with different ratios and combined with a gradient crystallization method to prepare L-serine.
The yield and purity of L-serine are significantly improved, the influence of impurities is reduced, and the quality of the product is improved.
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of L-serine preparation, and in particular to a method for converting L-serine. Background Art
[0002] As an important amino acid with multiple benefits, L-serine has demonstrated significant application value in numerous fields. In the pharmaceutical field, it is not only a core component of numerous anti-cancer drugs but also plays a key role in physiological processes such as immune regulation and neurotransmitter synthesis, making it crucial for maintaining normal human physiological functions. In the food industry, L-serine can be used as a food additive, adding unique flavor and nutritional value. Furthermore, in cosmetics, as a key moisturizing factor, it is widely used in various high-end skincare products, effectively improving skin condition and enhancing its ability to retain moisture and repair itself.
[0003] At present, the production methods of L-serine mainly include chemical synthesis, enzymatic conversion and microbial fermentation. The chemical synthesis method often uses petroleum derivatives as raw materials. Although the process is relatively simple, it has serious environmental pollution problems, and the products are mostly DL-serine racemates. Subsequent complex and costly splitting steps are required to obtain L-serine, which greatly limits the large-scale application of this method; the microbial fermentation method faces many challenges. For example, the synthesis pathway of L-serine is extremely complex and is subject to multiple feedback inhibitions. During the fermentation process, bacteria will preferentially use L-serine for their own metabolism rather than secreting it outside the cell, resulting in limited accumulation of intracellular L-serine; the enzymatic conversion method has the advantages of mild reaction conditions and high selectivity, but traditional serine conversion uses whole cells for conversion, and there are many impurities in the conversion liquid, which affects the extraction yield and purity.
[0004] Therefore, it is necessary to propose a conversion method for L-serine to improve the yield and purity of L-serine. Summary of the Invention
[0005] The present invention provides a method for converting L-serine, which solves the problem of low yield and purity of L-serine when L-serine is prepared by an enzymatic conversion method in the related art.
[0006] The technical solution of the present invention is as follows: The present invention provides a method for converting L-serine, characterized in that it comprises the following steps: S1, the fermentation broth is filtered, concentrated, washed, and filtered and concentrated again to obtain a heavy liquid, cetyltrimethylammonium bromide is added to the heavy liquid, and the mixture is stirred and centrifuged to obtain a clear liquid; S2, adding the clear solution to a glycine aqueous solution, stirring and dissolving, then aerating, adding tetrahydrofolic acid and controlling the pH to 6.5-7.5 to carry out a conversion reaction to obtain a conversion solution; S3, the conversion liquid is decolorized, filtered, and concentrated to obtain a concentrated solution, and the concentrated solution is crystallized, filtered, and dried to a constant weight to obtain L-serine; The decolorization is carried out twice. The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2-3. The activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1-2:8.
[0007] As a further technical solution, the mass of the activated carbon used for the first decolorization is 0.15% of the mass of the conversion liquid, the temperature is 60~70°C, and the time is 0.5~1h. The mass of the activated carbon used for the second decolorization is 0.15% of the mass of the conversion liquid, the temperature is 70~80°C, and the time is 1~1.5h.
[0008] As a further technical solution, in step S1, the materials used for the filtration and concentration and the re-filtration and concentration are both ceramic membranes, and the pore size of the ceramic membrane is 50 nm.
[0009] As a further technical solution, in step S1, the volume ratio of the filtered and concentrated heavy liquid to the fermentation liquid is 1:3.
[0010] As a further technical solution, in step S1, the volume ratio of the heavy liquid after the second filtration and concentration to the fermentation liquid is 1:3.
[0011] As a further technical solution, in step S1, the solution used for washing is physiological saline, and the volume ratio of the physiological saline to the heavy liquid is 1-2:1.
[0012] As a further technical solution, in step S1, the mass ratio of the hexadecyltrimethylammonium bromide to the heavy liquid is 0.03-0.08:100.
[0013] As a further technical solution, in step S1, the stirring time is 60 minutes and the stirring temperature is 35°C.
[0014] As a further technical solution, in step S1, the centrifugal speed is 10000 rpm, the centrifugal temperature is 10°C, and the centrifugal instrument is a cup centrifuge.
[0015] As a further technical solution, in step S2, the temperature for stirring and dissolving is 30-50°C, the ventilation gas is nitrogen, and the ventilation time is 30 minutes.
[0016] As a further technical solution, in step S2, the mass volume ratio of glycine to water in the glycine aqueous solution is 400~700g:2000mL.
[0017] As a further technical solution, in step S2, the volume ratio of the clear liquid to the glycine aqueous solution is 2-6:25, and the mass volume ratio of the tetrahydrofolic acid to the clear liquid is 1-2 g:80 mL.
[0018] As a further technical solution, in step S2, the temperature of the conversion reaction is 30-40°C, the pH is 6.5-7.5, and the time is 18-24 hours.
[0019] As a further technical solution, in step S2, the solution used to control the pH is formaldehyde.
[0020] As a further technical solution, in step S3, the concentration is vacuum concentration, the instrument used for the vacuum concentration is a rotary evaporator, the concentration temperature is 70~80°C, the concentration vacuum degree is -0.9~-0.95MPa, and the concentration rotation speed is 40~50rpm.
[0021] As a further technical solution, in step S3, the volume ratio of the concentrated liquid to the converted liquid is 1:2-4.
[0022] As a further technical solution, in step S3, the crystallization is gradient crystallization, and the gradient crystallization is divided into a first gradient crystallization and a second gradient crystallization, and the first gradient crystallization and the second gradient crystallization have different cooling rates.
[0023] As a further technical solution, in step S3, the crystallization process includes the following steps: the concentrated solution is first subjected to a first gradient crystallization, cooled to 40°C, and then subjected to a second gradient crystallization, cooled to 20°C.
[0024] As a further technical solution, the cooling rate of the first gradient crystallization is 10°C / h, and the cooling rate of the second gradient crystallization is 5°C / h.
[0025] In the preparation process of L-serine of the present invention, gradient crystallization is adopted and different cooling rates are set. The first gradient crystallization is carried out at a faster cooling rate of 10°C / h, which can quickly form a large number of crystal nuclei in the solution, provide sufficient core sites for crystal growth, avoid solute supersaturation and precipitation due to insufficient number of crystal nuclei, reduce the generation of amorphous precipitates, thereby ensuring that the target product can be efficiently converted into a crystal form, laying the foundation for improving the yield. At the same time, the faster cooling rate can quickly reduce the solute solubility in the early stage, promote the preferential crystallization of L-serine, and reduce the chance of impurity co-crystallization; the second gradient crystallization is carried out at a faster cooling rate of 10°C / h, which can quickly form a large number of crystal nuclei in the solution, provide sufficient core sites for crystal growth, avoid solute supersaturation and precipitation due to insufficient number of crystal nuclei, reduce the generation of amorphous precipitates, thereby ensuring that the target product can be efficiently converted into a crystal form, and lay the foundation for improving the yield. At the same time, the faster cooling rate can quickly reduce the solubility of the solute in the early stage, promote the preferential crystallization of L-serine, and reduce the chance of impurity co-crystallization; The crystallization process slows down the cooling rate to 5℃ / h, providing more sufficient growth time for the crystals, allowing the formed crystal nuclei to evenly and orderly adsorb solute molecules and grow into crystals with full particles and dense structure. The slow cooling process can reduce the stress inside the crystals, reduce crystal breakage and impurity inclusion, and significantly improve the purity of the crystals. Moreover, slow cooling is conducive to the uniformity of crystal size, avoiding the dissolution of small crystals and the excessive growth of large crystals, making the final crystal product particles regular, facilitating subsequent filtration and drying operations, reducing crystal loss, and further improving the yield of L-serine.
[0026] As a further technical solution, in step S3, the crystallization time is 2 hours.
[0027] As a further technical solution, in step S3, the drying temperature is 50-70°C.
[0028] The working principle and beneficial effects of the present invention are: During the preparation process of L-serine, the present invention uses hexadecyltrimethylammonium bromide to treat the whole cells, which is different from the prior art that uses whole cells for conversion. The present invention releases the enzyme outside the cells, and relatively few impurities are present in the conversion system during the conversion, which is conducive to improving the purity and yield of L-serine. In the present invention, decolorization is carried out twice and activated carbon combinations with different ratios are used. In the first decolorization, a larger amount of wood powder activated carbon is used. The wood activated carbon has a rich pore structure and a strong adsorption capacity for large molecular pigments and impurities, and can quickly remove most of the coloring substances in the fermentation liquid. The method can remove the residual small molecular pigments and trace impurities, creating a purer environment for the subsequent conversion reaction; the amount of coconut shell powder activated carbon used in the second decolorization is relatively large, and the high adsorption performance of coconut shell powder activated carbon can remove residual small molecular pigments and trace impurities, especially the polar substances that affect the purity of the product. A small amount of wood activated carbon can supplement the adsorption of residual impurities with larger pores. The graded decolorization treatment of the two times ensures the efficient removal of impurities and significantly improves the purity of the final product, while minimizing the loss of the target product, thereby effectively improving the yield of L-serine. DETAILED DESCRIPTION
[0029] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.
[0030] In the following examples and comparative examples: Example 1 A method for converting L-serine comprises the following steps: S1. Take 2100 mL of fermentation broth, filter and concentrate it using a ceramic membrane with a pore size of 50 nm, concentrate it to 700 mL, add 700 mL of normal saline, filter and concentrate it again using a ceramic membrane with a pore size of 50 nm, and concentrate it to a volume of 700 ml to obtain a heavy liquid. Add 0.21 g of hexadecyltrimethylammonium bromide to the heavy liquid, stir it at 35 ° C for 60 min, and centrifuge it at a speed of 10000 rpm and a centrifugation temperature of 10 ° C to obtain a clear liquid; S2. Weigh 400 g of glycine, add 2000 mL of water to dissolve, add to a 5000 mL reactor, add 200 mL of the clear solution, add water to make the volume 3000 mL, stir and heat to 30 ° C to dissolve, pass nitrogen for 30 min, add 5 g of tetrahydrofolic acid, control the temperature to 30 ° C, add formaldehyde to adjust the pH to 6.5 for conversion reaction, react for 18 h to obtain a conversion solution; S3, to the conversion liquid, activated carbon with a mass of 0.15% of the conversion liquid was added for the first decolorization, the temperature for the first decolorization was 60 ° C, and the time was 0.5h, and after suction filtration, activated carbon with a mass of 0.15% of the conversion liquid was added for the second decolorization, the temperature for the second decolorization was 70 ° C, and the time was 1.5h, and after suction filtration, a rotary evaporator was used for vacuum concentration, the concentrated temperature was 70 ° C, the vacuum degree was-0.9MPa, and the speed was 40rpm, and the concentrated solution was obtained after concentration. The volume ratio of the concentrated solution to the conversion liquid was 1:2, and the concentrated solution was cooled to 20 ° C at a cooling rate of 10 ° C / h and crystallized. After crystallization for 2h, the crystals were suction filtered to obtain wet crystals, and the wet crystals were dried to constant weight at 50 ° C to obtain L-serine; The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:8; The yield of this embodiment is 67.1%, and the purity is 99.5%.
[0031] Example 2 A method for converting L-serine comprises the following steps: S1. Take 2100 mL of fermentation broth, filter and concentrate it using a ceramic membrane with a pore size of 50 nm, concentrate it to 700 mL, add 700 mL of normal saline, filter and concentrate it again using a ceramic membrane with a pore size of 50 nm, and concentrate it to a volume of 700 ml to obtain a heavy liquid. Add 0.35 g of hexadecyltrimethylammonium bromide to the heavy liquid, stir it at 35 ° C for 60 min, and centrifuge it at a speed of 10000 rpm and a centrifugation temperature of 10 ° C to obtain a clear liquid; S2. Weigh 550 g of glycine, add 2000 mL of water to dissolve, add to a 5000 mL reactor, add 400 mL of the clear solution, add water to make the volume 3000 mL, stir and heat to 40 ° C to dissolve, pass nitrogen for 30 min, add 7.5 g of tetrahydrofolic acid, control the temperature to 35 ° C, add formaldehyde to adjust the pH to 7 for conversion reaction, react for 21 h to obtain a conversion solution; S3, to the conversion liquid, 0.15% of the activated carbon of the conversion liquid mass was added for the first decolorization, the temperature for the first decolorization was 60 ° C, and the time was 0.5h, and after suction filtration, 0.15% of the activated carbon of the conversion liquid mass was added for the second decolorization, the temperature for the second decolorization was 70 ° C, and the time was 1.5h, and after suction filtration, a rotary evaporator was used for vacuum concentration, the concentrated temperature was 70 ° C, the vacuum degree was-0.92MPa, and the speed was 40rpm, and the concentrated solution was obtained after concentration. The volume ratio of the concentrated solution to the conversion liquid was 1:3, and the concentrated solution was cooled to 20 ° C at a cooling rate of 10 ° C / h and crystallized. After crystallization for 2h, the crystals were suction filtered to obtain wet crystals. After the wet crystals were dried to constant weight at 60 ° C, L-serine was obtained; The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:8; The yield of this embodiment is 68.4%, and the purity is 99.6%.
[0032] Example 3 A method for converting L-serine comprises the following steps: S1. Take 2100 mL of fermentation broth, filter and concentrate it using a ceramic membrane with a pore size of 50 nm, concentrate it to 700 mL, add 700 mL of normal saline, filter and concentrate it again using a ceramic membrane with a pore size of 50 nm, and concentrate it to a volume of 700 ml to obtain a heavy liquid. Add 0.56 g of hexadecyltrimethylammonium bromide to the heavy liquid, stir it at 35 ° C for 60 min, and centrifuge it at a speed of 10000 rpm and a centrifugation temperature of 10 ° C to obtain a clear liquid; S2. Weigh 700 g of glycine, add 2000 mL of water to dissolve, add to a 5000 mL reactor, add 600 mL of the clear solution, add water to make the volume 3000 mL, stir and heat to 50 ° C to dissolve, pass nitrogen for 30 min, add 10 g of tetrahydrofolic acid, control the temperature to 40 ° C, add formaldehyde to adjust the pH to 7.5 for conversion reaction, react for 24 h to obtain a conversion solution; S3, to conversion liquid, activated carbon with a mass of 0.15% of conversion liquid was added for the first decolorization, the temperature for the first decolorization was 60 ° C, and the time was 0.5h, and after suction filtration, activated carbon with a mass of 0.15% of conversion liquid was added for a second decolorization, the temperature for the second decolorization was 70 ° C, and the time was 1.5h, and after suction filtration, rotary evaporator was used for vacuum concentration, the concentrated temperature was 70 ° C, the vacuum degree was-0.95MPa, and the rotating speed was 40rpm, and concentrated solution was obtained after concentration, the volume ratio of concentrated solution to conversion liquid was 1:4, and concentrated solution was cooled to 20 ° C at a cooling rate of 10 ° C / h and crystallized. After crystallization for 2h, the crystals were suction filtered to obtain wet crystals, and after the wet crystals were dried to constant weight at 70 ° C, L-serine was obtained; The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:8; The yield of this embodiment is 69.2%, and the purity is 99.7%.
[0033] Example 4 Compared with Example 2, the difference of Example 4 is that the activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:3; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 2:8; The yield of this embodiment is 69.3%, and the purity is 99.7%.
[0034] Example 5 Compared with Example 4, the difference of Example 5 is that the concentrated solution is cooled to 20°C at a cooling rate of 5°C / h for crystallization; The yield of this embodiment is 65.7%, and the purity is 99.3%.
[0035] Example 6 Compared with Example 4, the difference of Example 6 is that the concentrated liquid is first cooled to 40°C at a cooling rate of 10°C / h, and then cooled to 20°C at a cooling rate of 5°C / h; The yield of this embodiment is 71.5%, and the purity is 99.8%.
[0036] Example 7 Compared with Example 4, the difference of Example 7 is that the concentrated liquid is first cooled to 40°C at a cooling rate of 5°C / h, and then cooled to 20°C at a cooling rate of 10°C / h; The yield of this embodiment is 66.4%, and the purity is 99.4%.
[0037] Comparative Example 1 Compared with Example 2, the difference of Comparative Example 1 is that the activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 2:7; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:8; The yield of this comparative example is 64.3%, and the purity is 99.2%.
[0038] Comparative Example 2 Compared with Example 2, the difference of Comparative Example 2 is that the activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 8:1; The yield of this comparative example is 64.0%, and the purity is 99.1%.
[0039] Comparative Example 3 Compared with Example 2, the difference of Comparative Example 3 is that the activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:1; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1:1; The yield of this comparative example is 64.9%, and the purity is 99.4%.
[0040] Comparative Example 4 Compared with Example 2, the difference of Comparative Example 4 is that the activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 2:7; the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 8:1; The yield of this comparative example is 63.8%, and the purity is 99.0%.
[0041] Comparative Example 5 Compared with Example 2, the difference in Comparative Example 5 is that the activated carbon used in the first decolorization and the second decolorization is wood powder activated carbon; The yield of this comparative example is 60.6%, and the purity is 97.2%.
[0042] Comparative Example 6 Compared with Example 2, the difference in Comparative Example 6 is that the activated carbon used in the first decolorization and the second decolorization is coconut shell powder activated carbon; The yield of this comparative example is 61.5%, and the purity is 97.8%.
[0043] Comparative Example 7 Compared with Example 2, the difference in Comparative Example 7 is that the activated carbon used in the first decolorization is wood powder activated carbon, and the activated carbon used in the second decolorization is coconut shell powder activated carbon; The yield of this comparative example is 62.7%, and the purity is 98.3%.
[0044] Comparative Example 8 Compared with Example 2, the difference in Comparative Example 8 is that the activated carbon used in the first decolorization is coconut shell powder activated carbon, and the activated carbon used in the second decolorization is wood powder activated carbon; The yield of this comparative example is 63.1%, and the purity is 98.9%.
[0045] Comparative Example 9 Compared with Example 2, the difference of Comparative Example 9 is that hexadecyltrimethylammonium bromide is not added; The yield of this comparative example is 56.3%, and the purity is 94.7%.
[0046] By comparing Examples 1 to 4 with Comparative Examples 1 to 9, it can be seen that in the preparation process of L-serine, hexadecyltrimethylammonium bromide is first added for treatment, and the decolorization is carried out twice. The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2 to 3, and the activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1 to 2:8. When the purity and yield of L-serine can be improved.
[0047] From the comparison of Examples 4 to 7, it can be seen that when the crystallization in the preparation process of L-serine is gradient crystallization, and the cooling rate of the first gradient crystallization is 10°C / h and the cooling rate of the second gradient crystallization is 5°C / h, the purity and yield of L-serine can be further improved.
[0048] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.
Claims
1. A method for converting L-serine, characterized in that: The following steps are involved: S1, the fermentation broth is filtered, concentrated, washed, and filtered and concentrated again to obtain a heavy liquid, cetyltrimethylammonium bromide is added to the heavy liquid, and the mixture is stirred and centrifuged to obtain a clear liquid; S2, adding the clear solution to a glycine aqueous solution, stirring and dissolving, then aerating, adding tetrahydrofolic acid and controlling the pH to 6.5-7.5 to carry out a conversion reaction to obtain a conversion solution; S3, the conversion liquid is decolorized, filtered, and concentrated to obtain a concentrated solution, and the concentrated solution is crystallized, filtered, and dried to a constant weight to obtain L-serine; The decolorization is carried out twice. The activated carbon used in the first decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 7:2-3. The activated carbon used in the second decolorization is composed of wood powder activated carbon and coconut shell powder activated carbon in a mass ratio of 1-2:
8.
2. A method for converting L-serine according to claim 1, characterized in that, The mass of the activated carbon used for the first decolorization is 0.15% of the mass of the conversion liquid, the temperature is 60~70℃, and the time is 0.5~1h. The mass of the activated carbon used for the second decolorization is 0.15% of the mass of the conversion liquid, the temperature is 70~80℃, and the time is 1~1.5h.
3. A method for converting L-serine according to claim 1, characterized in that, In step S1, the solution used for washing is physiological saline, and the volume ratio of the physiological saline to the heavy liquid is 1-2:
1.
4. The method for converting L-serine according to claim 1, wherein In step S1, the mass ratio of the hexadecyltrimethylammonium bromide to the heavy liquid is 0.03-0.08:
100.
5. A method for converting L-serine according to claim 1, characterized in that, In step S2, the mass volume ratio of glycine to water in the glycine aqueous solution is 400-700 g:2000 mL.
6. A method for converting L-serine according to claim 1, characterized in that, In step S2, the volume ratio of the clear liquid to the glycine aqueous solution is 2-6:25, and the mass volume ratio of the tetrahydrofolic acid to the clear liquid is 1-2 g:80 mL.
7. A method for converting L-serine according to claim 1, characterized in that, In step S2, the temperature of the conversion reaction is 30-40°C.
8. The method for converting L-serine according to claim 1, wherein In step S3, the concentration is vacuum concentration, the concentration temperature is 70-80° C., the concentration vacuum degree is -0.9-0.95 MPa, and the concentration rotation speed is 40-50 rpm.
9. The method for converting L-serine according to claim 1, wherein In step S3, the crystallization is gradient crystallization, and the gradient crystallization is divided into a first gradient crystallization and a second gradient crystallization, and the first gradient crystallization and the second gradient crystallization have different cooling rates.
10. The method for converting L-serine according to claim 9, wherein: The cooling rate of the first gradient crystallization is 10° C. / h, and the cooling rate of the second gradient crystallization is 5° C. / h.