Preparation method for co-production of fructo-oligosaccharide and isomaltulose

Abstract

The invention belongs to the technical field of functional sugar production, and discloses a preparation method for co-production of fructo-oligosaccharide and isomaltulose, which comprises the following steps: preparing sucrose into a 700-750g / L aqueous solution, heating, adding fructosyl transferase for conversion, then carrying out enzyme deactivation, cooling, adding sucrose isomerase for conversion to obtain a mixed solution containing fructo-oligosaccharide and isomaltulose, and carrying out freeze-drying to obtain the fructo-oligosaccharide and isomaltulose co-product. The mixed solution is subjected to activated carbon decoloration, ion exchange resin desalination and separation to obtain fructo-oligosaccharide and mother liquor, and the mother liquor is subjected to evaporation concentration, cooling, crystallization, centrifugation and drying to obtain isomaltulose. According to the method, the fructo-oligosaccharide and the isomaltulose can be simultaneously produced by using the cane sugar, the production process is simple, the cane sugar conversion rate is high, the product purity is high, and the production cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of functional sugar production, and particularly relates to a method for preparing oligofructose and isomaltulose. Background Art

[0002] Fructo-oligosaccharides, also known as fructooligosaccharides or sucrose trisaccharide oligosaccharides, are natural active substances found in natural plants and are usually found in fruits and vegetables. As a non-reducing sugar, the viscosity, water activity, moisture retention, thermal stability under acidic conditions, and processability of fructo-oligosaccharides are similar to those of sucrose. It has many superior physiological effects, such as regulating intestinal flora, proliferating bifidobacteria, promoting calcium absorption, regulating blood lipids, immune regulation, and anti-caries. It is known as the most promising new generation of additives after the era of antibiotics - probiotics.

[0003] Isomaltulose is a natural sugar found in products such as sugarcane and honey. Its sweetness is about 42% of that of sucrose. It has a pure sweetness, basically the same as sucrose, and is widely used in various foods and sweeteners.

[0004] Currently, both fructo-oligosaccharides and isomaltulose can be produced using sucrose as raw material. However, the conversion rate of sucrose in the production of fructo-oligosaccharides is ≤55%, and the raw material utilization rate is low. In addition, a large number of enzyme preparations are used in the production process of fructo-oligosaccharides, and sucrose and other miscellaneous sugars need to be decomposed by microbial fermentation and enzymatic methods, resulting in waste. In addition, the production process is complicated. Therefore, in order to improve the utilization rate of sucrose, it is necessary to seek new sucrose processing methods while ensuring the conversion rate of fructo-oligosaccharides and improving production efficiency. Therefore, sucrose is used as raw material to produce fructo-oligosaccharides by enzymatic conversion, and the unconverted sucrose in the conversion liquid is used to co-produce isomaltulose, thereby improving the utilization rate of sucrose and reducing production costs. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method of oligofructose and isomaltulose, which overcomes the defects of the prior art and can simultaneously produce oligofructose and isomaltulose using sucrose, and has a simple production process, a high sucrose conversion rate, a high product purity and a low production cost.

[0006] In order to solve the above technical problems, the technical solution of the present invention is:

[0007] A method for preparing oligofructose and isomaltulose, comprising the following steps:

[0008] a: Prepare sucrose into a 700-750 g / L aqueous solution, adjust the pH value to 6.5-7.0, heat to 45-50°C and keep warm for 30 min, then add fructosyltransferase in an amount of 0.4-0.5 L / T sucrose dry basis, convert at 45-50°C for 24 h, and then end the conversion;

[0009] b: inactivate the enzyme of the material converted in step a at 90-100° C. for 10-20 min, and then cool the conversion solution to 30-35° C.;

[0010] c: adding sucrose isomerase to the conversion liquid cooled in step b, with the added amount being 20% ​​of the sucrose dry basis content, converting at 30-35° C. for 4 hours, then heating at 90-100° C. to inactivate the enzyme for 10-20 minutes, to obtain a mixed solution containing oligofructose and isomaltulose;

[0011] d: decolorizing the mixed solution containing oligofructose and isomaltulose obtained in step c with activated carbon, the amount of activated carbon added is 0.5% to 3%, keeping warm at 60° C. for 2 hours, and then removing the activated carbon with a diatomaceous earth filter;

[0012] e: Desalting the decolorized material with ion exchange resin, and then separating it by simulated moving bed chromatography to obtain oligofructose and mother liquor;

[0013] f: The mother liquor obtained in step E is concentrated by evaporation, crystallized by gradient cooling, centrifuged and dried to obtain isomaltulose.

[0014] Preferably, in step a, sucrose is prepared into a 725 g / L aqueous solution, the pH value is adjusted to 6.8, the temperature is raised to 47.5° C. and kept warm for 30 min, and then fructosyltransferase is added in an amount of 0.45 L / T sucrose dry basis, and the solution is converted at 47.5° C. for 24 h.

[0015] Preferably, in step b, the material is inactivated at 95°C for 15 minutes, and then the conversion solution is cooled to 32.5°C.

[0016] Preferably, in step c, after adding sucrose isomerase, the mixture is converted at 32.5° C. for 4 hours, and then the temperature is raised to 95° C. to inactivate the enzyme for 15 minutes.

[0017] Preferably, the amount of activated carbon added in step d is 1.5%.

[0018] Preferably, in step e, the ion exchange resin is D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin, which are used in sequence for desalination, and the flow rate is 3.0-4.0 BV; the simulated moving bed chromatographic separation is performed using a calcium-type resin, the simulated moving bed chromatographic separation temperature is 60-70° C., the flow rate is 5-6 ml / s, and the unit resin processing capacity is 0.046-0.050; the separated product is divided into liquid A and liquid B, liquid A is the separated oligofructose liquid, the purity is ≥95%, and liquid B is the mother liquor containing isomaltulose.

[0019] Furthermore, in the step e, the simulated moving bed chromatography separation temperature is 65° C., the flow rate is 5.5 ml / s, and the unit resin processing capacity is 0.048.

[0020] Preferably, in step f, the evaporation concentration is carried out at 60-65° C. to a dry matter content of 70-75%, and then a gradient cooling method is adopted for crystallization, the cooling rate is 1-1.5° C. / h, the stirring speed is 30-40 rpm, the gradient cooling is maintained at 20° C. for 10-12 h, and then centrifuged at 6000-8000 rpm for 0.5-2 h, and then the crystals are placed in a constant temperature drying oven at 45-50° C. and dried for 15-20 h.

[0021] Furthermore, in the step f, the evaporation concentration is carried out at 65°C to a dry matter content of 75%, and then a gradient cooling method is adopted for crystallization, the cooling rate is 1.5°C / h, the stirring speed is 35rpm, the gradient cooling is maintained at 20°C for 11h, and then centrifuged at 8000rpm for 1.5h, and then the crystals are placed in a constant temperature drying oven at 45°C and dried for 20h.

[0022] Due to the adoption of the above technical solution, the beneficial effects achieved by the present invention are:

[0023] The present invention uses sucrose to generate oligofructose under the catalysis of fructosyltransferase, and then adds sucrose isomerase to the conversion liquid to convert the sucrose to isomaltulose, thereby solving the problem that a large amount of sucrose remaining in the conversion liquid cannot be used when sucrose is used alone to produce oligofructose or isomaltulose in the prior art.

[0024] The process provided by the invention is simple and feasible, and overcomes the defects that the conversion rate of sucrose in the production of oligofructose is less than or equal to 55%, the utilization rate of raw materials is low, and more enzyme preparations are used in the production process of oligofructose, and sucrose and other miscellaneous sugars need to be decomposed by microbial fermentation and enzymatic method, resulting in waste and complicated production process; the simulated moving bed chromatography separation technology and the purification process of cooling crystallization are adopted, the refining process is simple and easy, and high-purity isomaltulose is produced on the basis of oligofructose; oligofructose is produced after separation. The purity of the product is ≥95%, which is much higher than the purity of 86.5% produced by existing membrane separation and biological fermentation processes. The separated isomaltulose enriched liquid adopts the production process of evaporation concentration and cooling crystallization, and the isomaltulose in the enriched liquid is purified by crystallization. The purity of the final product is ≥98% (the existing purity is 78% and the conversion rate is 90%), realizing the purification process of segmented separation of fructooligosaccharides and isomaltulose, and the total conversion rate of sucrose is 100%; therefore, the production costs of fructooligosaccharides and isomaltulose are greatly reduced, which is conducive to their industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the chromatogram of the oligofructose product of Example 1 of the present invention

[0026] Figure 2 It is a chromatogram of the isomaltulose product of Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with embodiments:

[0028] Embodiment 1:

[0029] a: Prepare 75 kg of sucrose into a 725 g / L aqueous solution with distilled water, adjust the pH to 6.75, heat to 47.5°C and keep warm for 30 min, then add fructosyltransferase at an amount of 0.45 L / T sucrose dry basis, and convert at 47.5°C for 24 h to complete the conversion;

[0030] b: inactivate the enzyme in the material converted in step a at 95°C for 15 min, and then cool the conversion solution to 32.5°C;

[0031] c: Add sucrose isomerase to the cooled conversion liquid in step b, with the added amount being 20% ​​of the sucrose dry content, convert at 32.5°C for 4 hours, then heat and inactivate the enzyme at 95°C for 15 minutes to obtain 103.5 liters of a mixed solution containing oligofructose and isomaltulose;

[0032] d: decolorizing the mixed solution containing oligofructose and isomaltulose obtained in step c with activated carbon, the amount of activated carbon added is 1.5%, keeping warm at 60° C. for 2 h, and then removing the activated carbon with a diatomaceous earth filter;

[0033] e: The decolorized material was desalted using D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin in sequence at a flow rate of 3.5 BV / h, and then separated by simulated moving bed chromatography at a temperature of 65°C, a flow rate of 5.5 ml / s, and a unit resin processing capacity of 0.048 to obtain oligofructose with a purity of 95.6%, and 85 liters of oligofructose syrup with a solid content of 25%. After concentration and drying, 29.7 kg of oligofructose solids (75*0.55 conversion rate*0.8 chromatographic separation yield*0.9 loss) were obtained; and 125 liters of mother liquor with a solid content of 24% were obtained;

[0034] f: The mother liquor obtained in step E was evaporated and concentrated at 62.5°C to a dry matter content of 72.5%, and then crystallized by gradient cooling at a cooling rate of 1.25°C / h and a stirring speed of 35rpm. The temperature was gradually reduced to 20°C and maintained for 12h, and then centrifuged at 7000rpm for 1.5h. The crystals were then placed in a constant temperature drying oven and dried at 47.5°C for 20h to obtain 28.75 kg of isomaltulose with a purity of 100% (75*0.45 residual sucrose*0.85 conversion rate*0.67 crystallization yield).

[0035] Embodiment 2:

[0036] a: Prepare 75 kg of sucrose into a 750 g / L aqueous solution with distilled water, adjust the pH to 7.0, heat to 50°C and keep warm for 30 min, then add fructosyltransferase at an amount of 0.5 L / T sucrose dry basis, and convert at 50°C for 24 h to complete the conversion;

[0037] b: inactivate the enzyme in the material converted in step a at 100°C for 10 minutes, and then cool the conversion solution to 35°C;

[0038] c: adding sucrose isomerase to the conversion liquid after cooling in step b, with the addition amount being 20% ​​of the sucrose dry basis content, converting at 35° C. for 4 hours, and then heating at 100° C. to inactivate the enzyme for 10 minutes, to obtain 107 liters of a mixed solution containing oligofructose and isomaltulose;

[0039] d: decolorizing the mixed solution containing oligofructose and isomaltulose obtained in step c with activated carbon, the amount of activated carbon added is 3%, keeping the mixture at 60° C. for 2 h, and then removing the activated carbon with a diatomaceous earth filter;

[0040] e: The decolorized material was desalted using D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin in sequence at a flow rate of 4.0 BV / h, and then separated by simulated moving bed chromatography at a temperature of 70°C, a flow rate of 6 ml / s, and a unit resin processing capacity of 0.050 to obtain oligofructose with a purity of 95.5%, and 95 liters of oligofructose syrup with a solid content of 21.5%. After concentration and drying, 28.9 kg of oligofructose solids were obtained; and 125 liters of mother liquor with a solid content of 19.7% isomaltulose was obtained;

[0041] f: The mother liquor obtained in step E was evaporated and concentrated at 65°C to a dry matter content of 75%, and then crystallized by gradient cooling at a cooling rate of 1.5°C / h and a stirring speed of 40rpm. The temperature was gradually cooled to 20°C and maintained for 12h, and then centrifuged at 8000rpm for 0.5h. The crystals were then placed in a constant temperature drying oven and dried at 50°C for 15h to obtain 27.48 kg of isomaltulose with a purity of 99%.

[0042] Embodiment 3:

[0043] a: Prepare 75 kg of sucrose into a 750 g / L aqueous solution with distilled water, adjust the pH to 6.5, heat to 45°C and keep warm for 30 min, then add fructosyltransferase at an amount of 0.4 L / T sucrose dry basis, convert at 45°C for 24 h, and then terminate the conversion;

[0044] b: inactivate the enzyme in the material converted in step a at 90°C for 20 min, and then cool the conversion solution to 30°C;

[0045] c: adding sucrose isomerase to the cooled conversion liquid in step b, with the added amount being 20% ​​of the sucrose dry content, converting at 30° C. for 4 hours, then heating and inactivating the enzyme at 90° C. for 20 minutes to obtain 100 liters of a mixed solution containing oligofructose and isomaltulose;

[0046] d: decolorizing the mixed solution containing oligofructose and isomaltulose obtained in step c with activated carbon, the amount of activated carbon added is 0.5%, keeping warm at 60° C. for 2 h, and then removing the activated carbon with a diatomaceous earth filter;

[0047] e: The decolorized material was desalted using D001-FD macroporous strong acid styrene cation exchange resin and D354-FD macroporous styrene weak base anion exchange resin in sequence at a flow rate of 3.0 BV / h, and then separated by simulated moving bed chromatography at a temperature of 60°C, a flow rate of 5 ml / s, and a unit resin processing capacity of 0.046 to obtain oligofructose with a purity of 85.7%, and 87 liters of oligofructose syrup with a solid content of 27.5%. After evaporation and drying, 38.67 kg of oligofructose solids were obtained; and 112 liters of mother liquor with a solid content of 22.5% were obtained;

[0048] f: The mother liquor obtained in step E was evaporated and concentrated at 60°C to a dry matter content of 70%, and then crystallized by gradient cooling at a cooling rate of 1.0°C / h and a stirring speed of 30 rpm. The temperature was gradually cooled to 20°C and maintained for 12 hours, and then centrifuged at 6000 rpm for 2 hours. The crystals were then placed in a constant temperature drying oven and dried at 45°C for 25 hours to obtain 28.35 kg of isomaltulose with a purity of 95.7%.

[0049] Comparative Example 1: Production process 1 for preparing oligofructose according to the prior art, the specific steps are as follows:

[0050] a: Prepare 75 kg of sucrose into a 750 g / L aqueous solution with distilled water, adjust the pH to 6.5, heat to 45°C and keep warm for 30 min, then add fructosyltransferase at an amount of 0.4 L / T sucrose dry basis, convert at 45°C for 24 h, and then terminate the conversion;

[0051] b: inactivate the enzyme in the material converted in step a at 90°C for 15 minutes, and then cool the conversion solution to room temperature;

[0052] c: decolorizing the conversion liquid obtained in step b with activated carbon, with an activated carbon addition amount of 1.5%, keeping the temperature at 60°C for 2h, and then removing the activated carbon with a diatomaceous earth filter;

[0053] d: The decolorized material was desalted using D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin in sequence at a flow rate of 3.0 BV / h.

[0054] e: Nanofiltration membrane separation was used for separation and purification. Nanofiltration membrane with a molecular weight cutoff of 150-300Da was selected. During the nanofiltration separation process, the operating pressure was generally 2.3MPa, the temperature was controlled at 25°C, the feed flow rate was controlled at 2m / s, and the feed concentration was controlled at 20%. The results showed that the final purity of oligofructose using nanofiltration membrane was increased to about 84.3%. 34.25kg of oligofructose was obtained.

[0055] Comparative Example 2: Production process 2 for preparing oligofructose using the prior art, the specific steps are as follows:

[0056] a: Prepare 75 kg of sucrose into a 750 g / L aqueous solution with distilled water, adjust the pH to 6.5, heat to 45°C and keep warm for 30 min, then add fructosyltransferase at an amount of 0.4 L / T sucrose dry basis, convert at 45°C for 24 h, and then terminate the conversion;

[0057] b: inactivate the enzyme in the material converted in step a at 90°C for 15 minutes, and then cool the conversion solution to room temperature;

[0058] c: decolorizing the conversion liquid obtained in step b with activated carbon, with an activated carbon addition amount of 1.5%, keeping the temperature at 60°C for 2h, and then removing the activated carbon with a diatomaceous earth filter;

[0059] d: The decolorized material was desalted using D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin in sequence at a flow rate of 3.0 BV / h.

[0060] e: The concentration of the above liquid was adjusted to 30%, and the glucose in the ordinary oligofructose was eliminated by the synergistic action of glucose oxidase and catalase, wherein the addition amount of glucose oxidase was 50 mg / kg and the addition amount of catalase was 10 mg / kg. The reaction was carried out at pH = 5.5 and 40°C for 30 hours, which could completely eliminate the glucose in the oligofructose product and obtain 38.4 kg of oligofructose with 86.5%.

[0061] Comparative Example 3: Production process 3 for preparing oligofructose using the prior art, the specific steps are as follows:

[0062] The steps are the same as a, b, c, and d in Comparative Example 2; the concentration of the reaction solution is adjusted to 25%, 10% of the highly selective, high-sugar-tolerant yeast SK2.003 is added thereto, and the reaction is carried out at 25°C and a stirring speed of 200 rpm for 24 hours to obtain 32.34 kg of oligofructose with a purity of 80.5%.

[0063] Comparative Example 4: Production process 1 for preparing isomaltulose according to the prior art, the specific steps are as follows:

[0064] a: Prepare 75 kg of sucrose into a 750 g / L aqueous solution with distilled water, adjust the pH value to 6.0, add sucrose isomerase in an amount of 20% of the dry content of sucrose, convert at 35°C for 5 hours, then heat and inactivate the enzyme at 90°C for 15 minutes.

[0065] b: The obtained conversion liquid was decolorized with activated carbon, the amount of activated carbon added was 1.5%, and the temperature was kept at 60°C for 2 hours, and then the activated carbon was removed by a diatomaceous earth filter;

[0066] c: The decolorized material is sequentially desalted using D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin at a flow rate of 2.0 BV / h;

[0067] d: The isomaltulose content of the reaction liquid after centrifugation was 84%. It was evaporated and concentrated to 70% dry matter content at 65°C, then naturally cooled and crystallized at room temperature, then centrifuged at 8000 rpm, and dried in a constant temperature drying oven at 40°C for 20 hours. The isomaltulose product was 41.86 kg with a purity of 94.2%.

[0068] The results of Examples 1-3 and Comparative Examples 1-4 were tabulated and analyzed, and the results are shown in Table 1:

[0069] Table 1 Results of Examples 1-3 and Comparative Examples 1-4

[0070]

[0071] Conclusion analysis:

[0072] By comparing the results of Examples 1-3 of the present invention and Comparative Examples 1-4, it can be seen that the production process of oligofructose and isomaltulose of the present invention uses sucrose as a raw material, adopts an enzymatic conversion method to produce oligofructose, and simultaneously utilizes the unconverted sucrose in the conversion liquid to co-produce isomaltulose, so that the sucrose conversion rate reaches 100% (refers to the complete consumption of sucrose, which does not mean that the sucrose is completely converted into oligofructose and isomaltulose, including glucose, fructose and other impurity sugars present in the mother liquor), and the sucrose conversion rate is increased by more than 45% compared with Comparative Examples 1-3. Through the chromatographic separation process and gradient cooling crystallization, the purity of the product is significantly improved, and the utilization rate of sucrose is improved, thereby reducing the production cost.

[0073] By comparing Example 1 of the present invention with Comparative Examples 1-3, it can be seen that the oligofructose conversion liquid adopts the process of calcium-type resin simulated moving bed chromatography separation, which greatly improves the product purity to more than 95% compared with the traditional process nanofiltration, microbial fermentation and enzymatic purification scheme. By comparing Example 1 with Comparative Example 4, it can be seen that the purity of isomaltulose is increased by more than 5% by using gradient cooling crystallization compared with natural cooling crystallization.

[0074] It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope limited by the appended claims of the application.

Claims

1. A method for preparing oligofructose and isomaltulose, characterized in that: The following steps are involved: a: Prepare sucrose into a 700-750 g / L aqueous solution, adjust the pH value to 6.5-7.0, heat to 45-50°C and keep warm for 30 min, then add fructosyltransferase in an amount of 0.4-0.5 L / T sucrose dry basis, convert at 45-50°C for 24 h, and then end the conversion; b: inactivate the enzyme of the material converted in step a at 90-100° C. for 10-20 min, and then cool the conversion solution to 30-35° C.; c: adding sucrose isomerase to the conversion liquid cooled in step b, with the added amount being 20% ​​of the sucrose dry basis content, converting at 30-35° C. for 4 hours, then heating at 90-100° C. to inactivate the enzyme for 10-20 minutes, to obtain a mixed solution containing oligofructose and isomaltulose; d: decolorizing the mixed solution containing oligofructose and isomaltulose obtained in step c with activated carbon, the amount of activated carbon added is 0.5% to 3%, keeping warm at 60° C. for 2 hours, and then removing the activated carbon with a diatomaceous earth filter; e: Desalting the decolorized material with ion exchange resin, and then separating it by simulated moving bed chromatography to obtain oligofructose and mother liquor; f: The mother liquor obtained in step E is concentrated by evaporation, crystallized by gradient cooling, centrifuged and dried to obtain isomaltulose.

2. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: In the step a, sucrose is prepared into a 725 g / L aqueous solution, the pH value is adjusted to 6.8, the temperature is raised to 47.5° C. and kept warm for 30 minutes, and then fructosyltransferase is added in an amount of 0.45 L / T sucrose dry basis, and the solution is converted at 47.5° C. for 24 hours.

3. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: In step b, the material is inactivated at 95°C for 15 minutes, and then the conversion solution is cooled to 32.5°C.

4. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: After adding sucrose isomerase in step c, the mixture is converted at 32.5° C. for 4 hours, and then the temperature is raised to 95° C. to inactivate the enzyme for 15 minutes.

5. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: The amount of activated carbon added in step d is 1.5%.

6. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: In the step e, the ion exchange resin is D001-FD macroporous strongly acidic styrene-based cation exchange resin and D354-FD macroporous styrene-based weakly basic anion exchange resin, which are used in sequence for desalination, and the flow rate is 3.0-4.0 BV; the simulated moving bed chromatographic separation is performed using a calcium type resin, the simulated moving bed chromatographic separation temperature is 60-70° C., the flow rate is 5-6 ml / s, and the unit resin processing capacity is 0.046-0.050; the separated product is divided into liquid A and liquid B, liquid A is the separated oligofructose liquid, and liquid B is the mother liquor containing isomaltulose.

7. The method for preparing oligofructose and isomaltulose according to claim 1, characterized in that: In the step f, the evaporation concentration is carried out at 60-65° C. to a dry matter content of 70-75%, and then a gradient cooling method is adopted for crystallization, the cooling rate is 1-1.5° C. / h, the stirring speed is 30-40 rpm, the gradient cooling is maintained at 20° C. for 12 hours, and then centrifuged at 6000-8000 rpm for 0.5-2 hours, and then the crystals are placed in a constant temperature drying oven at 45-50° C. and dried for 15-25 hours.

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