A method for extracting sodium alginate from giant kelp by plasma discharge

Abstract

The present invention discloses a method for extracting sodium alginate from giant kelp by plasma discharge. Take dry giant kelp powder, add anhydrous ethanol and soak overnight, and obtain decolorized and degreased giant kelp powder after drying; pulverize and sieve to obtain pretreated giant kelp powder; soak the pretreated giant kelp powder in NaCl or CaCl2 solution and then ultrasonicate; place the mixed solution in a plasma discharge device for treatment, and after the reaction ends, centrifuge and take the supernatant; add CaCl2 to complex alginic acid, let it stand for 2 h, and then centrifuge to obtain calcium alginate as the precipitate; add sodium chloride solution to calcium alginate for ion exchange, and finally perform alcohol precipitation to obtain sodium alginate. The discharge plasma technology is used to replace the sodium carbonate digestion in the traditional sodium alginate extraction step. The high-voltage pulse wave instantaneously causes great damage to the plant cell wall and cell membrane, accelerating the dissolution of the substances to be extracted in the system. Compared with the traditional method, it has the advantages of less treatment time, less energy consumption, and less environmental pollution.

Description

Technical Field

[0001] The present invention relates to the technical field of polysaccharide extraction, and particularly relates to a method for extracting sodium alginate from giant kelp by applying plasma discharge technology. Background Art

[0002] Giant kelp, a plant of the family Lessoniaceae in the phylum Phaeophyta, has a brown thallus and is the largest among algae. Giant kelp is the main raw material for the preparation of high-quality alginic acid and is an irreplaceable raw material in the industrial preparation industry of alginic acid.

[0003] Sodium alginate is a natural polysaccharide composed of L-guluronic acid and D-mannuronic acid, which is extracted from brown algae plants and is widely used in the fields of food, medicine, cosmetics and industry. Because of its unique physical and chemical properties, such as thickening, gelling and stabilizing properties, it performs excellently in many applications. At present, the preparation process of alginic acid mainly includes - raw material pretreatment, digestion, pre-neutralization and dilution, and calcium coagulation. After calcium coagulation, 15% sodium chloride solution is used as the eluent for ion exchange. The generated sodium alginate is dried and pulverized to obtain the product. However, the traditional alkaline solution extraction method has some problems, such as environmental pollution caused by the discharge of alkaline solution, low extraction efficiency, high energy consumption, cumbersome operation steps, high labor cost, etc. Therefore, it is extremely necessary to find a more low-cost, environmentally friendly and efficient alternative extraction scheme, which has great application prospects for cost reduction, efficiency increase and green preparation. Summary of the Invention

[0004] Aiming at the above problems, the purpose of the present invention is to provide a method for extracting sodium alginate from giant kelp to solve the problems of large pollution and long time consumption in the prior art.

[0005] Aiming at the above purpose, the present invention provides a method for extracting sodium alginate from giant kelp by using plasma discharge, which includes the following steps:

[0006] (1) Add anhydrous ethanol to dry giant kelp powder for soaking, and after drying, obtain decolorized and defatted giant kelp powder;

[0007] (2) Grind and sieve the decolorized and defatted giant kelp powder obtained in step (1) to obtain pretreated giant kelp powder;

[0008] (3) Soak the pretreated giant kelp powder obtained in step (2) in NaCl or CaCl2 solution for 1 - 2 h and then perform ultrasonic treatment, place the mixture in a plasma discharge device for treatment, and after the reaction ends, centrifuge and take the supernatant;

[0009] (4) Add anhydrous CaCl2 for complexing to the supernatant obtained in step (3) while stirring, let it stand for 2 h, and then centrifuge and take the precipitate to obtain calcium alginate;

[0010] (5) Add an NaCl solution to the calcium alginate obtained in step (4) for ion exchange to remove calcium.

[0011] (6) Add absolute ethanol with a volume to the solution obtained in step (5) for alcohol precipitation, precipitate a white solid, dry and crush it to obtain the finished sodium alginate.

[0012] In one embodiment of the present invention, in step (1), the addition amount of absolute ethanol is 2 - 4 times the volume of the kelp powder.

[0013] In one embodiment of the present invention, in step (1), the soaking time in absolute ethanol is 6 - 12 h.

[0014] In one embodiment of the present invention, in step (1), the drying temperature is 40 - 60 °C.

[0015] In one embodiment of the present invention, in step (1), the drying time is 4 - 8 h.

[0016] In one embodiment of the present invention, in step (2), the mesh number of sieving is 100 - 800 meshes.

[0017] In one embodiment of the present invention, in step (3), the mass concentration of the NaCl or CaCl₂ solution is 0.01 - 0.1%, and the soaking time is 1 - 2 h.

[0018] In one embodiment of the present invention, in step (3), the ultrasonic time is 20 - 60 min, the ultrasonic temperature is 30 - 60 °C, and the ultrasonic power is 200 - 400 w.

[0019] In one embodiment of the present invention, in step (3), the mass ratio of the pretreated kelp powder to the NaCl or CaCl₂ solution is 1:10 - 1:30.

[0020] In one embodiment of the present invention, in step (3), the discharge voltage during plasma discharge treatment is 20 - 150 kV, the discharge time is 1 - 60 min, and the electrode spacing is 12 - 32 mm.

[0021] In one embodiment of the present invention, in step (4), the addition amount of anhydrous CaCl₂ is 1 - 3% of the volume of the supernatant.

[0022] In one embodiment of the present invention, in step (5), the concentration of the NaCl solution is 15 - 20%, and the ion exchange time for removing calcium is 1 - 2 h.

[0023] In one embodiment of the present invention, in step (5), the volume of the NaCl solution is 10 - 30 times the volume of the calcium alginate.

[0024] In one embodiment of the present invention, in step (6), the addition amount of absolute ethanol is 2 - 4 times the volume of the solution.

[0025] In one embodiment of the present invention, in step (6), the precipitation time of absolute ethanol is 6 - 12 h.

[0026] The beneficial effects of the present invention are as follows:

[0027] (1) The plasma discharge technology is used to replace the sodium carbonate digestion process in the traditional sodium alginate extraction step. The plasma discharge technology uses high - voltage pulse waves to instantaneously cause great damage to the plant cell wall and cell membrane, accelerating the dissolution of the substances to be extracted in the system. Compared with the traditional method, it has the advantages of less processing time, less energy consumption, and less environmental pollution, providing a new and efficient method for sodium alginate extraction.

[0028] (2) Before the plasma discharge treatment, the giant kelp powder pretreated by soaking in a low - concentration NaCl or CaCl2 solution and ultrasonic treatment is used. This process does not introduce new salt ions, which can be removed together in the subsequent calcium precipitation step, thus not affecting the overall extraction process.

[0029] (3) Since the giant kelp has a thick rope - shaped stem - like stipe and is leathery, it is completely different from ordinary algae such as kelp. The alginic acid is tightly combined with the fiber, and in the traditional extraction process, the fiber must be damaged with alkali solution to release the alginic acid. The combination of low - concentration NaCl or CaCl2 solution and ultrasonic treatment can soften the plant fiber before the plasma discharge, strip the alginic acid from the fiber, and thus cooperate with the plasma discharge to completely release the alginic acid, which is more conducive to the dissolution of alginic acid in the system, thereby increasing the extraction rate of sodium alginate. If the step of combining low - concentration NaCl or CaCl2 solution with ultrasonic treatment is not added and only the plasma discharge treatment is used, the extraction rate will be very low.

[0030] (4) In the present invention, soaking and ultrasonic treatment with a low - concentration NaCl or CaCl2 solution are carried out before the plasma discharge treatment, increasing the contact area and promoting the free release of alginic acid, further increasing the extraction rate of sodium alginate. The extraction rate of sodium alginate extracted by the method of the present invention can be as high as 49.8%. Description of the Drawings

[0031] Figure 1 Shows the influence of plasma discharge - related parameters on the extraction rate of sodium alginate.

[0032] Figure 2 Shows the extraction rates of sodium alginate in each example. Detailed Embodiments

[0033] It should be noted that all professional and scientific terms used herein have the same meaning as those familiar to persons skilled in the art, unless otherwise defined. The reagents or raw materials used in the present invention can all be obtained by conventional means. Unless otherwise specified, the reagents or raw materials used in the present invention are used in the conventional manner in the art or in accordance with the product instructions. The present invention will be further described below in conjunction with specific embodiments.

[0034] Example 1

[0035] A method for extracting sodium alginate from giant kelp by plasma discharge, comprising the following steps:

[0036] (1) Add 3 times the volume of absolute ethanol to the dried giant kelp powder and soak for 8 h, then dry at 50 °C for 6 h to obtain decolorized and degreased giant kelp powder. Pass through a 200-mesh sieve to obtain pretreated giant kelp powder for standby.

[0037] (2) Soak 1 kg of pretreated giant kelp powder and 0.01% CaCl2 solution by mass in a liquid-solid mass ratio of 1:25 for 1 h, then ultrasonicate at 40 °C and 300 W for 40 min. Place the mixed solution in a plasma discharge device and process for 4 min. During the plasma discharge treatment, the electrode spacing is 22 mm and the voltage is 40 kV. After the reaction, centrifuge and take the supernatant.

[0038] (3) While stirring, add anhydrous CaCl2 accounting for 2% of the volume of the supernatant to the supernatant for complexation. After standing for 2 h, centrifuge and take the precipitate to obtain calcium alginate.

[0039] (4) Add 20 L of 15% NaCl solution to the calcium alginate for ion exchange demineralization for 1 h.

[0040] (5) Add 3 times the volume of absolute ethanol to the mixed solution in step (4) for alcohol precipitation for 10 h to precipitate a white precipitate. Dry and pulverize to obtain the finished product of sodium alginate.

[0041] The extraction rate of sodium alginate obtained by the above method is 44.3%, and the gel viscosity is 855.0 mPa·s. Compared with the currently adopted extraction technology, the recovery rate of sodium alginate reaches 86%. Moreover, this example only consumes a small amount of electric energy, omits the operation steps such as alkali removal and alkali discharge, and also reduces the labor cost. Therefore, it has the advantages of reducing labor costs, reducing pollution emissions, while achieving a similar sodium alginate extraction effect.

[0042] Example 2

[0043] A method for extracting sodium alginate from giant kelp by plasma discharge, comprising the following steps:

[0044] (1) Add anhydrous ethanol with a volume twice that of the dry kelp powder and soak for 12 h. After drying at 40 °C for 8 h, obtain the decolorized and defatted kelp powder. Pass through a 100-mesh sieve to obtain the pretreated kelp powder for standby.

[0045] (2) Soak 1 kg of the pretreated kelp powder and a CaCl2 solution with a mass concentration of 0.09% at a solid-liquid ratio of 1:20 (w / w) for 2 h. Then, perform ultrasonic treatment at 60 °C and 200 W for 20 min. Place the mixed solution in a plasma discharge device and treat it at an electrode spacing of 22 mm and 40 kV for 6 min. After the reaction ends, centrifuge and take the supernatant.

[0046] (3) While stirring, add anhydrous CaCl2 with a volume of 1% of the supernatant volume to the supernatant. After standing for 2 h, centrifuge and take the precipitate to obtain calcium alginate.

[0047] (4) Add 10 L of 15% NaCl solution to the calcium alginate and perform ion exchange to remove calcium for 2 h.

[0048] (5) Add anhydrous ethanol with a volume twice that of the mixed solution in step (4) to perform alcohol precipitation for 12 h. A white precipitate will precipitate. Dry and crush it to obtain the finished sodium alginate.

[0049] The extraction rate of sodium alginate obtained by the above method is 42.5%, and the gel viscosity is 745.0 mPa·s. Compared with the currently adopted extraction technology, the recovery rate of sodium alginate reaches 82%. Moreover, this implementation only consumes a small amount of electric energy, omits the operation steps such as alkali solution removal and alkali solution discharge, and also reduces the labor consumption. Therefore, it has the advantages of reducing labor costs, reducing pollution emissions, and achieving a similar alginic acid extraction effect at the same time.

[0050] Example 3

[0051] A method for extracting sodium alginate from kelp using plasma discharge, comprising the following steps:

[0052] (1) Add anhydrous ethanol with a volume three times that of the dry kelp powder and soak for 10 h. After drying at 40 °C for 8 h, obtain the decolorized and defatted kelp powder. Pass through a 200-mesh sieve to obtain the pretreated kelp powder for standby.

[0053] (2) Soak 1 kg of the pretreated kelp powder and a NaCl solution with a mass concentration of 0.1% at a solid-liquid ratio of 1:20 (w / w) for 1 h. Then, perform ultrasonic treatment at 40 °C and 300 W for 30 min. Place the mixed solution in a plasma discharge device and treat it at an electrode spacing of 22 mm and 60 kV for 4 min. After the reaction ends, centrifuge and take the supernatant.

[0054] (3) While stirring, add anhydrous CaCl2 with a volume of 2% of the supernatant volume to the supernatant. After standing for 2 h, centrifuge and take the precipitate to obtain calcium alginate.

[0055] (4) Add 20 L of 20% NaCl solution to calcium alginate, and perform ion exchange to remove calcium for 1 h;

[0056] (5) Add 2 times the volume of absolute ethanol to the mixed solution in step (4) for alcohol precipitation for 10 h to precipitate a white precipitate, dry and crush it to obtain the finished product of sodium alginate.

[0057] The extraction rate of sodium alginate obtained by the above method is 42.7%, and the gel viscosity is 1055.0 mPa·s. Compared with the currently adopted extraction technology, the recovery rate of sodium alginate reaches 83%, and this implementation only consumes a small amount of electric energy, omits the operation steps such as alkali solution removal and alkali solution discharge, and also reduces the labor consumption. Therefore, it has the advantages of reducing labor costs, reducing pollution emissions, and achieving a similar alginic acid extraction effect at the same time.

[0058] Example 4

[0059] A method for extracting sodium alginate from giant kelp by using plasma discharge, comprising the following steps:

[0060] (1) Add 4 times the volume of absolute ethanol to the dried giant kelp powder and soak for 6 h, dry at 60 °C for 4 h to obtain decolorized and defatted giant kelp powder, pass through an 800-mesh sieve to obtain pretreated giant kelp powder, and set aside;

[0061] (2) After soaking 1 kg of pretreated giant kelp powder and 0.1% NaCl solution with a solid-liquid ratio of 1:20 (w / w) for 1 h, perform ultrasonic treatment at 30 °C and 400 w for 60 min, place the mixed solution in a plasma discharge device for treatment at an electrode spacing of 27 mm and 40 kV for 4 min, and centrifuge to take the supernatant after the reaction;

[0062] (3) While stirring, add 3% of the volume of absolute CaCl2 to the supernatant for complexation, let it stand for 2 h, and centrifuge to obtain the precipitate, which is calcium alginate;

[0063] (4) Add 30 L of 15% NaCl solution to calcium alginate, and perform ion exchange to remove calcium for 1 h;

[0064] (5) Add 4 times the volume of absolute ethanol to the mixed solution in step (4) for alcohol precipitation for 6 h to precipitate a white precipitate, dry and crush it to obtain the finished product of sodium alginate.

[0065] The extraction rate of sodium alginate obtained by the above method is 39.4%, and the gel viscosity is 772.5 mPa·s. Compared with the currently adopted extraction technology, the recovery rate of sodium alginate reaches 80%, and this implementation only consumes a small amount of electric energy, omits the operation steps such as alkali solution removal and alkali solution discharge, and also reduces the labor consumption. Therefore, it has the advantages of reducing labor costs, reducing pollution emissions, and achieving a similar alginic acid extraction effect at the same time.

[0066] Example 5

[0067] A method for extracting sodium alginate from giant kelp by plasma discharge, comprising the following steps:

[0068] (1) Add anhydrous ethanol with a volume 2 times that of the dried giant kelp powder and soak for 12 h, then dry at 60 °C for 6 h to obtain decolorized and degreased giant kelp powder. Pass through a 100-mesh sieve to obtain pretreated giant kelp powder for standby;

[0069] (2) Soak 1 kg of pretreated giant kelp powder and 0.1% NaCl solution by mass concentration at a solid-liquid ratio of 1:20 (w / w) for 1 h, then ultrasonicate at 40 °C and 300 w for 30 min. Place the mixed solution in a plasma discharge device and treat it at an electrode spacing of 22 mm and 40 kV for 4 min. After the reaction ends, centrifuge and take the supernatant;

[0070] (3) While stirring, add anhydrous CaCl2 with a volume 2% of the supernatant volume to the supernatant for complexation. After standing for 2 h, centrifuge and take the precipitate to obtain calcium alginate;

[0071] (4) Add 20 L of 15% NaCl solution to the calcium alginate for ion exchange and decalcification for 1 h;

[0072] (5) Add anhydrous ethanol with a volume 3 times that of the mixed solution in step (4) for alcohol precipitation for 10 h. A white precipitate will precipitate out. Dry and crush it to obtain the finished product of sodium alginate.

[0073] The extraction rate of sodium alginate obtained by the above method is 49.8%, and the gel viscosity is 707.5 mPa·s. Compared with the currently adopted extraction technology, the recovery rate of sodium alginate reaches 92%, and this implementation only consumes a small amount of electric energy, eliminating operation steps such as alkali removal and alkali discharge, and also reducing labor consumption. Therefore, it has the advantages of reducing labor costs, reducing pollution emissions, while achieving a similar sodium alginate extraction effect.

[0074] Comparative Example 1

[0075] A method for extracting sodium alginate, comprising the following steps:

[0076] (1) Add anhydrous ethanol with a volume 2 times that of the dried giant kelp powder and soak for 12 h, then dry at 60 °C for 6 h to obtain decolorized and degreased giant kelp powder. Pass through a 100-mesh sieve to obtain pretreated giant kelp powder for standby;

[0077] (2) Mix 1 kg of pretreated giant kelp powder and 2% Na2CO3 solution by mass concentration at a solid-liquid ratio of 1:30 (w / w), and digest at 50 °C for 3 h. After the reaction ends, centrifuge and take the supernatant;

[0078] (3) Adjust the pH of the supernatant to 7 - 8 with 1M HCl. While stirring the supernatant, add anhydrous CaCl₂ at 2% of the supernatant volume. After standing for 2 h, centrifuge to obtain the precipitate, which is calcium alginate.

[0079] (4) Add 20 L of 15% NaCl solution to the calcium alginate and perform ion exchange for 1 h to remove calcium.

[0080] (5) Add three times the volume of anhydrous ethanol to the mixed solution in step (4) and perform alcohol precipitation for 10 h. A white precipitate will precipitate out. Dry and crush it to obtain the finished product of sodium alginate.

[0081] The extraction rate of sodium alginate obtained by the traditional sodium carbonate digestion method in Comparative Example 1 is 39.7%, and the gel viscosity is 747.5 mPa·s. This method requires operation steps such as removal of the lye and discharge of the lye, and at the same time increases the labor cost.

[0082] Comparative Example 2

[0083] A method for extracting sodium alginate, comprising the following steps:

[0084] (1) Add anhydrous ethanol at twice the volume to the dried giant kelp powder and soak for 12 h. After drying at 40 °C for 8 h, obtain the decolorized and defatted giant kelp powder. Pass it through a 100 - mesh sieve to obtain the pretreated giant kelp powder for standby.

[0085] (2) Mix 1 kg of the pretreated giant kelp powder with deionized water at a solid - liquid ratio of 1:20 (w / w) and place it in a plasma discharge device for treatment at an electrode spacing of 22 mm and 40 kV for 6 min. After the reaction, centrifuge to obtain the supernatant.

[0086] (3) While stirring the supernatant, add anhydrous CaCl₂ at 1% of the supernatant volume for complexation. After standing for 2 h, centrifuge to obtain the precipitate, which is calcium alginate.

[0087] (4) Add 10 L of 15% NaCl solution to the calcium alginate and perform ion exchange for 2 h to remove calcium.

[0088] (5) Add twice the volume of anhydrous ethanol to the mixed solution in step (4) and perform alcohol precipitation for 12 h. A white precipitate will precipitate out. Dry and crush it to obtain the finished product of sodium alginate.

[0089] The extraction rate of sodium alginate obtained by the method of Comparative Example 2 is 30.7%, and the gel viscosity is 764.5 mPa·s.

[0090] Through the data of Examples 1 - 7, it is found that applying the plasma discharge technology can improve the problems of large environmental pollution and long time consumption caused by the traditional sodium carbonate digestion for extracting sodium alginate without reducing the extraction rate and viscosity.

[0091] Based on Example 1, by adjusting various parameters during the plasma discharge process, the influence of plasma discharge parameters on the extraction rate of sodium alginate was studied, and the results are as Figure 1 shown.

[0092] Figure 1 (A) is a graph showing the change in the extraction rate of sodium alginate obtained by adjusting the material-liquid ratio of kelp powder to low-concentration CaCl2 solution on the basis of Example 1. From Figure 1 (A), it can be seen that when the material-liquid ratio of kelp powder to 0.01% low-concentration CaCl2 solution is between 1:10 and 1:20 and ultrasonic treatment is carried out, the extraction rate of sodium alginate increases significantly with the increase of the material-liquid ratio, from 21.8% to 49.8%, reaching the maximum when the material-liquid ratio is 1:20. Continuing to increase the material-liquid ratio, the extraction rate of sodium alginate shows a downward trend.

[0093] Figure 1 (B) is a graph showing the change in the extraction rate of sodium alginate obtained by adjusting the discharge time on the basis of Example 2. The results show that when the discharge time is between 3 and 5 minutes, the extraction rate of sodium alginate increases with the increase of the discharge time, from 38.8% to 47.2%. When the discharge time increases to 5 minutes, the extraction rate of sodium alginate is the largest. When it is greater than 5 minutes, the extraction rate of sodium alginate shows an obvious downward trend. It may be because sodium alginate is degraded, resulting in a decrease in the extraction rate.

[0094] Figure 1 (C) is a graph showing the change in the extraction rate of sodium alginate obtained by adjusting the discharge voltage on the basis of Example 3. The results show that when the discharge voltage is between 30 and 50 kV, the extraction rate of sodium alginate increases with the increase of the discharge voltage, from 19.7% to 49.1%. When the discharge voltage increases to 50 kV, the extraction rate of sodium alginate is the largest. When it is greater than 50 kV, the extraction rate of sodium alginate shows an obvious downward trend.

[0095] Figure 1 (D) is a graph showing the change in the extraction rate of sodium alginate obtained by adjusting the electrode spacing on the basis of Example 4. The results show that when the electrode spacing is between 12 and 22 mm, the extraction rate of sodium alginate increases with the increase of the discharge time, from 14.6% to 45.3%.

[0096] The embodiments provided above are not intended to limit the scope covered by the present invention, nor are the described steps intended to limit their execution order. Obvious improvements made by those skilled in the art to the present invention in combination with the existing well-known common sense also fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for extracting sodium alginate from giant kelp using plasma discharge, characterized in that, It includes the following steps: (1) Add absolute ethanol to dry kelp powder for soaking. After drying, decolorized and defatted kelp powder is obtained; (2) Grind and sieve the decolorized and defatted kelp powder obtained in step (1) to obtain pretreated kelp powder; (3) Soak the pretreated kelp powder obtained in step (2) in NaCl or CaCl₂ solution for 1 - 2 h and then perform ultrasonic treatment. Place the mixture in a plasma discharge device for treatment. After the reaction ends, centrifuge and take the supernatant. The mass concentration of the NaCl or CaCl₂ solution is 0.01 - 0.1%, the soaking time is 1 - 2 h, and the mass ratio of the pretreated kelp powder to the NaCl or CaCl₂ solution is 1:20 - 1:25; the ultrasonic time is 20 - 60 min, the ultrasonic temperature is 30 - 60 °C, and the ultrasonic power is 200 - 400 W; when performing plasma discharge treatment, the discharge voltage is 40 - 60 kV, the discharge time is 4 - 5 min, and the electrode spacing is 22 mm; (4) While stirring, add anhydrous CaCl₂ to the supernatant obtained in step (3) for complexing. After standing for 2 h, centrifuge and take the precipitate to obtain calcium alginate; (5) Add NaCl solution to the calcium alginate obtained in step (4) for ion exchange to remove calcium; (6) Add absolute ethanol with a volume 2 - 4 times that of the solution to the solution obtained in step (5) for alcohol precipitation. A white precipitate is precipitated, dried and pulverized to obtain the finished product of sodium alginate.

2. The method according to claim 1, characterized in that, In step (1), the addition amount of absolute ethanol is 2 - 4 times the volume of the kelp powder, and the soaking time of the absolute ethanol is 6 - 12 h.

3. The method according to claim 1, wherein In step (1), the drying temperature is 40 - 60 °C, and the drying time is 4 - 8 h.

4. The method according to claim 1, characterized in that, In step (2), the mesh number of sieving is 100 - 800 meshes.

5. The method according to claim 1, wherein In step (4), the addition amount of anhydrous CaCl₂ is 1 - 3% of the volume of the supernatant.

6. The method according to claim 1, wherein In step (5), the concentration of the NaCl solution is 15 - 20%, the ion exchange time for removing calcium is 1 - 2 h, and the volume of the NaCl solution is 10 - 30 times the volume of the calcium alginate.

7. The method according to claim 1, wherein In step (6), the addition amount of absolute ethanol is 2 - 4 times the volume of the solution, and the alcohol precipitation time of the absolute ethanol is 6 - 12 h.

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