A starch-based capsule and a method for its preparation

Abstract

The application relates to the technical field of drug carrier materials, in particular to a starch-based capsule and a preparation method thereof, which comprises the following steps: S1, after dialdehyde starch is gelatinized, the dialdehyde starch is mixed with a tannic acid solution, an iron chloride solution is added, uniform mixing and defoaming are carried out, and a film-forming solution is obtained; S2, a mold is immersed in the film-forming solution, preliminary drying is carried out under a set temperature and humidity condition after pulling up, and a gel film layer is formed; S3, the gel film layer after preliminary drying is demolded, humidity balance and drying are sequentially carried out, and the starch-based capsule is obtained. By adding tannic acid and a suitable amount of iron chloride, an effective crosslinking network structure is formed among starch molecules, the mechanical strength of the capsule shell is enhanced, and the hygroscopicity is reduced.

Description

Technical Field

[0001] This invention relates to the field of drug carrier materials technology, and in particular to a starch-based capsule and its preparation method. Background Technology

[0002] Starch capsules are plant-derived empty capsules made primarily from natural starch or its modified products as the film-forming material. As a significant alternative to gelatin capsules, they have garnered widespread attention in the pharmaceutical and health product sectors. Starch capsules possess characteristics such as wide availability, renewability, and biodegradability. In practical applications, starch capsules are not only used to encapsulate conventional chemical drug powders or granules for convenient drug administration, but their excellent biocompatibility and modifiability also make them particularly suitable for emerging and niche markets. For example, they can better protect heat- and moisture-sensitive biological agents or natural active ingredients; through specific modifications or process control, colon-targeted delivery can be achieved for the treatment of intestinal diseases; in the health supplement field, they are also an ideal carrier for packaging pH-sensitive live bacteria preparations such as probiotics. Furthermore, in functional foods and special medical foods, starch capsules provide a safe and neutral option for encapsulating oily liquids or flavoring substances.

[0003] The invention patent with publication number CN108546344A discloses a method for preparing starch gum and starch-based plant capsules. The method prepares starch gum with high amylose content through synergistic treatment of amylase and debranching enzyme, and obtains starch-based plant capsules with low brittleness and good disintegration performance.

[0004] In existing technologies, compared with traditional gelatin capsules, pure starch or modified starch capsules generally have lower mechanical strength and higher brittleness, making them more prone to breakage or deformation during high-speed filling, packaging, and transportation. Furthermore, starch itself has strong hydrophilicity, making the capsules highly sensitive to environmental humidity. In high humidity environments, they easily absorb moisture, soften, and stick together, while in low humidity environments, they easily lose water and become brittle. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a starch-based capsule and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing starch-based capsules, characterized by comprising the following steps:

[0008] S1. After gelatinizing the dialdehyde starch, mix it with the tannic acid solution, then add the ferric chloride solution, mix evenly and degas to obtain the film-forming solution; the ferric chloride solution is an aqueous solution of ferric chloride hexahydrate.

[0009] Specifically, the preparation method of the film-forming solution is as follows: Dialdehyde starch is dispersed in deionized water and gelatinized at 70-90℃ for 20-40 minutes to obtain a starch paste; the starch paste is cooled to 30-50℃, and a tannic acid solution is added, and the mixture is stirred for 20-40 minutes; then, a ferric chloride solution is added dropwise to the mixture, and stirring is continued for 10-20 minutes to obtain the film-forming solution. The order of addition of the tannic acid solution and the ferric chloride solution can be interchanged, or they can be premixed and added together. Degassing is performed using at least one of the following methods: static degassing, vacuum degassing, or centrifugal degassing, until no visible bubbles are present in the film-forming solution.

[0010] In the film-forming solution, the concentration of dialdehyde starch is 4%-6% (w / v), for example, 4.5%, 5.0%, or 5.5% (w / v). In the tannic acid solution, the mass ratio of tannic acid to dialdehyde starch is (0.1-0.3):1, for example, 0.15:1, 0.2:1, or 0.25:1. In the ferric chloride solution, the mass ratio of ferric chloride hexahydrate to dialdehyde starch is (0.04-0.06):1, for example, 0.045:1, 0.05:1, or 0.055:1.

[0011] S2. Immerse the mold in the film-forming liquid, pull it up, and perform preliminary drying under set temperature and humidity conditions to form a gel film layer. This step is called dip-coating molding, and the specific conditions are: preheat the mold to 30-40℃, the immersion time in the film-forming liquid is 5-20 seconds, and the pulling speed is 3-8 mm / s. The mold is a needle-shaped mold for capsule molding. The pulling speed can be uniform or segmented, for example, first pull at a slow speed of 2-4 mm / s to allow the liquid film to initially adhere, and then pull at a fast speed of 6-10 mm / s to complete the molding. The preliminary drying conditions are: under ventilated conditions, control the wind speed at 0.1-0.5 m / s, the temperature at 20-30℃, the relative humidity at 40%-50%, and the drying time at 3-6 hours.

[0012] S3. Demold the pre-dried gel film layer to obtain a capsule blank, and then perform humidity equilibration and drying sequentially to obtain the starch-based capsule. Humidity equilibration involves placing the demolded capsule blank in an environment with a temperature of 20-30℃ and a relative humidity of 70%-80% for 12-36 hours. Drying is carried out at 30-50℃ until the moisture content of the capsule is below 10% (determined by the loss on drying method). Drying can be performed using a gradient temperature increase method, for example, first drying at 30-35℃ for 2-4 hours, then increasing to 40-45℃ until the moisture content meets the standard.

[0013] Secondly, the present invention provides a starch-based capsule prepared by any of the above-described preparation methods, characterized in that the capsule wall comprises dialdehyde starch, tannic acid, and Fe. 3+ A starch-based gel network is formed through cross-linking. The cross-linking includes Schiff base reactions and / or acetalization reactions between the aldehyde groups of dialdehyde starch and the phenolic hydroxyl groups of tannic acid, as well as Fe... 3+ Coordination complexation reaction between the ortho- and ortho-phenolic hydroxyl groups of tannic acid.

[0014] The aldehyde content of the dialdehyde starch is 0.5-0.8 mmol / g, and the dialdehyde starch can be obtained by selective oxidation of starch with sodium periodate.

[0015] Based on the mass of the dialdehyde starch, the amount of tannic acid is 10%-30%, and the Fe... 3+ The dosage of the ion, calculated as ferric chloride hexahydrate, is 4%-6%.

[0016] The capsule has a wall thickness of 0.05-0.15 mm and a disintegration time of less than 15 minutes.

[0017] The beneficial effects of this invention are:

[0018] 1. The microstructure of the capsule wall consists of a stable three-dimensional network framework formed by covalent cross-linking of the aldehyde groups on the dialdehyde starch molecular chains and the phenolic hydroxyl groups on the tannic acid molecules. Simultaneously, Fe... 3+ The metal-phenol coordination bond formed between the ion and the ortho-phenolic hydroxyl group of tannic acid acts as a dynamic and reversible non-covalent crosslinking point, which is uniformly interspersed and bonded in the network. When the capsule wall material is subjected to stress, the covalent bond network provides high strength and modulus, while the dynamic coordination bond can effectively dissipate energy through reversible fracture and recombination, preventing crack propagation, thereby endowing the capsule with excellent toughness and damage resistance.

[0019] 2. Tannic acid itself has natural antioxidant and antibacterial properties. Its introduction not only acts as a cross-linking agent but also brings potential antioxidant functions to the capsules, helping to protect oxygen-sensitive contents. 3+ The introduction of [a substance] while forming coordination bonds usually gives the capsule an elegant appearance ranging from light yellow to dark brown. This inherent color can act as a light shield, which helps protect photosensitive contents and reduces or eliminates the need for additional synthetic light shielding agents. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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.

[0021] Example 1: A method for preparing starch-based capsules, comprising the following steps:

[0022] S1. Disperse dialdehyde starch (aldehyde content 0.5 mmol / g) in deionized water to prepare a 4% (w / v) dispersion. Gelatinize the dispersion at 70°C for 40 minutes to obtain starch paste. Cool the starch paste to 30°C and add tannic acid solution (tannic acid to dialdehyde starch mass ratio of 0.1:1). Stir and mix for 40 minutes. Then add ferric chloride solution (aqueous solution of ferric chloride hexahydrate, ferric chloride hexahydrate to dialdehyde starch mass ratio of 0.04:1) dropwise to the mixture and continue stirring for 20 minutes. Use static degassing method to treat until there are no obvious visible bubbles in the film-forming solution to obtain the film-forming solution.

[0023] S2. Preheat the needle mold for capsule forming to 30°C, immerse it in the above film-forming solution for 20 seconds, lift it at a uniform speed of 3 mm / s, and then perform preliminary drying under ventilation conditions, controlling the wind speed at 0.1 m / s, temperature at 20°C, and relative humidity at 40%, and dry for 6 hours to form a gel film layer.

[0024] S3. Demold the pre-dried gel film layer to obtain capsule blanks; place the capsule blanks in an environment with a temperature of 20℃ and a relative humidity of 70% for 36 hours to complete humidity equilibrium; then dry using a gradient temperature method, first drying at 30℃ for 4 hours, and then increasing to 40℃ until the moisture content of the capsules is less than 10% (determined by the loss on drying method), to obtain starch-based capsules.

[0025] Example 2, a method for preparing starch-based capsules, comprising the following steps:

[0026] S1. Disperse dialdehyde starch (aldehyde content 0.65 mmol / g) in deionized water to prepare a 5% (w / v) dispersion. Gelatinize at 80℃ for 30 minutes to obtain starch paste. Cool the starch paste to 40℃ and add tannic acid solution (tannic acid to dialdehyde starch mass ratio of 0.2:1). Stir and mix for 30 minutes. Then add ferric chloride solution (aqueous solution of ferric chloride hexahydrate, ferric chloride hexahydrate to dialdehyde starch mass ratio of 0.05:1) dropwise to the mixture and continue stirring for 15 minutes. Use vacuum degassing method to treat until there are no obvious visible bubbles in the film-forming solution to obtain the film-forming solution.

[0027] S2. Preheat the needle mold for capsule forming to 35°C, immerse it in the above film-forming solution for 12 seconds, and use segmented variable speed lifting (first lift slowly at 3 mm / s to allow the liquid film to initially adhere, and then lift quickly at 8 mm / s to complete the forming). Then, perform preliminary drying under ventilated conditions, controlling the wind speed at 0.3 m / s, the temperature at 25°C, and the relative humidity at 45%, and dry for 4.5 hours to form a gel film layer.

[0028] S3. Demold the pre-dried gel film layer to obtain capsule blanks; place the capsule blanks in an environment with a temperature of 25℃ and a relative humidity of 75% for 24 hours to complete humidity equilibrium; then dry using a gradient temperature method, first drying at 32℃ for 3 hours, and then increasing to 42℃ until the moisture content of the capsules is less than 10% (determined by the loss on drying method), to obtain starch-based capsules.

[0029] Example 3, a method for preparing starch-based capsules, comprising the following steps:

[0030] S1. Disperse dialdehyde starch (aldehyde content 0.8 mmol / g) in deionized water to prepare a 6% (w / v) dispersion. Gelatinize the dispersion at 90℃ for 20 minutes to obtain starch paste. Cool the starch paste to 50℃ and add tannic acid solution (tannic acid to dialdehyde starch mass ratio of 0.3:1). Stir and mix for 20 minutes. Then add ferric chloride solution (aqueous solution of ferric chloride hexahydrate, ferric chloride hexahydrate to dialdehyde starch mass ratio of 0.06:1) dropwise to the mixture and continue stirring for 10 minutes. Centrifuge to remove bubbles until there are no visible bubbles in the film-forming solution to obtain the film-forming solution.

[0031] S2. Preheat the needle mold for capsule forming to 40°C, immerse it in the above film-forming solution for 5 seconds, lift it at a uniform speed of 8 mm / s, and then perform preliminary drying under ventilation conditions, controlling the wind speed at 0.5 m / s, temperature at 30°C, and relative humidity at 50%, and dry for 3 hours to form a gel film layer.

[0032] S3. Demold the pre-dried gel film layer to obtain capsule blanks; place the capsule blanks in an environment with a temperature of 30℃ and a relative humidity of 80% for 12 hours to complete humidity equilibrium; then dry using a gradient temperature method, first drying at 35℃ for 2 hours, and then increasing to 45℃ until the moisture content of the capsules is less than 10% (determined by the loss on drying method), to obtain starch-based capsules.

[0033] Comparative Example 1

[0034] The difference between this comparative example and Example 1 is that no tannic acid solution was added in the preparation of the film-forming solution, while the other process parameters and operating steps are completely consistent with Example 1.

[0035] Comparative Example 2

[0036] The difference between this comparative example and Example 2 is that the mass ratio of ferric chloride hexahydrate to dialdehyde starch is 0.03:1, while the remaining process parameters and operating steps are completely consistent with Example 2.

[0037] Prepare capsule samples as described in Examples 1-3 and Comparative Examples 1-2. Before testing, all samples were placed in a constant temperature and humidity chamber at 25±1℃ and 50±5% for at least 48 hours to equilibrate. At least 5 parallel samples were prepared for each group of samples for each test.

[0038] 1. Mechanical strength test: Randomly select at least 10 intact capsule shells (i.e., unfilled capsule caps or bodies) from each group of capsule samples and equilibrate them for at least 48 hours in a constant temperature and humidity environment of 25±1℃ and 50±5% relative humidity. Before the test, use a sharp blade to cut the capsule shell horizontally along the middle, take the cap part, and ensure that the test area (center of the cap top) is flat and free of visible defects.

[0039] The tests were conducted using a texture analyzer. A cylindrical flat-bottomed probe (P / 2 model, 2 mm in diameter) was selected. Before testing, the instrument was calibrated for force and displacement.

[0040] The prepared capsule shell sample (with the outer surface of the cap facing upwards) is stably fixed on the instrument's flat sample stage or a specially designed grooved holder, ensuring the test point is centered on the top of the cap. The test procedure is set as follows: the probe descent speed before testing is 1.0 mm / s; the testing speed is 0.5 mm / s; the trigger force is 5 g (i.e., when the probe contacts the sample surface and reaches a force of 5 g, the displacement and force changes are recorded); the penetration depth is set to 2.0 mm (to ensure complete penetration of the capsule wall). The test is started, and the probe moves vertically downwards until the set penetration depth is reached. The peak force reached during the puncture of the capsule shell is read; this is the puncture strength of the sample, expressed in Newtons (N). The average value and standard deviation of each sample group are calculated. The results are shown in Table 1.

[0041] 2. Hygroscopicity test: Randomly select at least 5 whole capsules from each group of capsule samples and accurately weigh their initial weight (W0). To ensure representativeness, the capsules may be appropriately broken or whole capsules may be used, but the condition of the samples within the group must be kept consistent.

[0042] A programmable temperature and humidity chamber was used, with the following conditions set: temperature 40±1℃ and relative humidity 75±5%. A saturated sodium chloride solution was placed inside the chamber or a humidification system was used to maintain the specified humidity. The chamber conditions were verified using a calibrated thermometer and hygrometer.

[0043] Place the weighed sample in an open weighing bottle or petri dish, and quickly place it in a temperature and humidity incubator that has reached the set conditions. Start timing and leave it for 72 hours. During the incubation period, ensure that the samples do not pile up and that air can circulate freely around the samples.

[0044] After the specified time has elapsed, quickly remove the sample and gently wipe away any visible water droplets that may have condensed on the surface with a lint-free soft cloth or filter paper. Immediately weigh the sample on an analytical balance after it has absorbed moisture (W). t The entire weighing process should be completed within 2 minutes to avoid the sample absorbing moisture from the laboratory environment during the weighing process. The moisture absorption rate of each sample is calculated using the following formula: Moisture absorption rate (%) = [(W...] t -W0) / W0]×100%. The results are shown in Table 2:

[0045] 3. Disintegration Time Test: An intelligent disintegration apparatus conforming to the requirements of the Chinese Pharmacopoeia was used. The test media were: (a) Artificial gastric fluid: 7.65 mL of concentrated hydrochloric acid was dissolved in approximately 400 mL of water and 2.0 g of pepsin (1:3000). After shaking well, the solution was diluted to 1000 mL with water and the pH was adjusted to 1.20±0.05; (b) Artificial intestinal fluid: 3.40 g of potassium dihydrogen phosphate was dissolved in approximately 400 mL of water. The pH was adjusted to 6.80±0.05 with 0.1 mol / L sodium hydroxide solution, and 5.0 g of pancreatin was added. After mixing well, the solution was diluted to 1000 mL with water. Before the test, the medium temperature needed to be preheated and maintained at 37.0±0.5℃.

[0046] Six intact capsules were randomly selected from each sample group. To simulate a filled state, the capsule caps were locked. Filling tests were performed using an inert filler that did not contain the analyte, or empty capsules were used directly.

[0047] Artificial gastric fluid test: Add 900 mL of the above-mentioned artificial gastric fluid to each glass tube of the disintegrator basket. Place 6 capsules into each of the 6 glass tubes of the basket, ensuring that the capsules are completely submerged in the fluid. Start the disintegrator, set the basket reciprocating frequency to 30 times / minute, and the amplitude to 55 mm. Observe and record the time (in minutes) required for each capsule to completely rupture and for all capsule wall fragments to pass through the sieve. If the capsule is not completely disintegrated after 60 minutes, stop the test and record it as ">60 minutes".

[0048] Artificial intestinal fluid test: After completing the gastric fluid test (or reaching 60 minutes), remove the basket along with the incompletely disintegrated capsules and gently rinse with purified water. Replace the medium with 900 mL of preheated artificial intestinal fluid in each glass tube. Place the basket back into the disintegration apparatus and start the instrument. Observe and record the complete disintegration time of each capsule in the intestinal fluid.

[0049] The disintegration time of each capsule was recorded in both media, with 6 capsules tested in each group. The results are shown in Table 3:

[0050]

[0051] As shown in Table 1, the puncture strengths of Examples 1-3 were 8.52±0.41 N, 10.37±0.38 N, and 9.68±0.45 N, respectively, all significantly higher than those of the comparative example. This indicates that the preparation process of the examples can effectively improve the mechanical strength of the capsule shell. From the preparation process, tannic acid solution was added in the examples. Tannic acid and dialdehyde starch may undergo a cross-linking reaction, forming a denser network structure, enhancing intermolecular forces, and making the capsule shell structure more compact, thereby improving the puncture strength. Simultaneously, the appropriate amount of ferric chloride added (the mass ratio of ferric chloride hexahydrate to dialdehyde starch in the examples was 0.04:1, 0.05:1, and 0.06:1, respectively) may act as a cross-linking agent, further promoting the cross-linking between starch molecules and enhancing the mechanical properties of the capsule shell.

[0052] Comparative Example 1, without the addition of tannic acid solution, had a puncture strength of only 2.15 ± 0.63 N, far lower than the example. This is because the lack of tannic acid reduces the degree of cross-linking between starch molecules, resulting in a less dense network structure and a decrease in the mechanical strength of the capsule shell. In Comparative Example 2, the mass ratio of ferric chloride hexahydrate to dialdehyde starch was 0.03:1, lower than the example, and the puncture strength was 6.24 ± 0.58 N, also lower than the example. This indicates that insufficient ferric chloride content leads to inadequate cross-linking and cannot effectively enhance the mechanical strength of the capsule shell.

[0053] As shown in Table 2, the 72-hour moisture absorption rates of Examples 1-3 were 8.7%, 6.3%, and 7.5%, respectively, all lower than the comparative example. This is attributed to the relatively dense capsule shell structure formed by the preparation process of the examples. The addition of tannic acid caused the starch molecules to form a cross-linked network, reducing the gaps between molecules and decreasing the chance of moisture entering the capsule interior, thereby reducing the moisture absorption rate. The appropriate amount of ferric chloride added also promoted the cross-linking reaction, further enhancing the density of the capsule shell and improving its moisture resistance.

[0054] Comparative Example 1, without the addition of tannic acid, had a moisture absorption rate as high as 22.4%. Due to the lack of cross-linking structure, the gaps between starch molecules were relatively large, allowing moisture to easily enter the capsule interior, resulting in an increased moisture absorption rate. Comparative Example 2, with insufficient ferric chloride, had a moisture absorption rate of 15.8%, also higher than the example. This indicates that the amount of ferric chloride added affects the degree of cross-linking; insufficient addition cannot effectively reduce the gaps between molecules, thus failing to enhance the hygroscopicity of the capsule shell.

[0055] As shown in Table 3, Examples 1-3 did not disintegrate in simulated gastric fluid (>60 minutes), while the disintegration times in simulated intestinal fluid were 12.5 minutes, 8.5 minutes, and 10.8 minutes, respectively. This may be because the capsule shells prepared in these examples have specific structures and components, making them relatively stable and less prone to disintegration in the acidic environment of the stomach (pH 1.20±0.05). However, in simulated intestinal fluid (pH 6.80±0.05), the change in environment may have disrupted the cross-linked structure of the capsule shell, causing it to disintegrate rapidly. This characteristic is beneficial for drug release in the intestine and improves drug bioavailability.

[0056] The disintegration times of Comparative Example 1 in simulated gastric and intestinal fluids were 2.8 minutes and 1.5 minutes, respectively, while those of Comparative Example 2 were 45.2 minutes and 20.7 minutes, respectively, both shorter than those of the Example. Comparative Example 1 did not contain tannic acid, resulting in a less dense capsule shell structure that was easily damaged and disintegrated in gastric and intestinal fluids. In Comparative Example 2, the insufficient addition of ferric chloride and inadequate cross-linking also led to faster capsule shell disintegration in the medium.

[0057] In summary, the examples, by adding tannic acid and an appropriate amount of ferric chloride, enabled the formation of an effective cross-linked network structure between starch molecules, enhancing the mechanical strength of the capsule shell, reducing hygroscopicity, and achieving stability in simulated gastric fluid and rapid disintegration in simulated intestinal fluid. In contrast, the comparative examples, due to a lack of tannic acid or an inappropriate amount of ferric chloride, resulted in a less dense capsule shell structure, lower mechanical strength, higher hygroscopicity, and excessively rapid disintegration in the medium, failing to meet the requirements for sustained drug release and release at specific sites.

[0058] In the description of this specification, the reference to terms such as "embodiment," "various embodiments," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or preparation example is included in at least one embodiment of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0059] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing starch-based capsules, characterized in that, Includes the following steps: S1. After gelatinizing the dialdehyde starch, mix it with the tannic acid solution, then add the FeCl3 solution, mix evenly and degas to obtain the film-forming solution; S2. Perform adhesive molding by immersing the mold in the film-forming liquid, lifting it up and then performing preliminary drying under set temperature and humidity conditions to form a gel film layer. S3. Demold the pre-dried gel film layer to obtain a capsule blank, and perform humidity balancing and drying in sequence to obtain the starch-based capsule; In step S1, the preparation of the film-forming solution is specifically as follows: dispersing dialdehyde starch in deionized water and gelatinizing it at 70-90℃ for 20-40 minutes to obtain starch paste; cooling the starch paste to 30-50℃, adding tannic acid solution, and stirring for 20-40 minutes; then adding FeCl3 solution dropwise to the mixture and continuing to stir for 10-20 minutes to obtain the film-forming solution; In the film-forming solution, the concentration of dialdehyde starch is 4%-6% w / v; in the tannic acid solution, the mass ratio of tannic acid to dialdehyde starch is (0.1-0.3):1; in the FeCl3 solution, the mass ratio of FeCl3·6H2O to dialdehyde starch is (0.04-0.06):

1.

2. The preparation method according to claim 1, characterized in that, In step S2, the specific conditions for the adhesive-coated molding are as follows: the mold is preheated to 30-40°C, the dwell time in the film-forming liquid is 5-20 seconds, and the lifting speed is 3-8 mm / s.

3. The preparation method according to claim 1, characterized in that, In step S2, the preliminary drying conditions are: temperature of 20-30℃, relative humidity of 40%-50%, and drying time of 3-6 hours.

4. The preparation method according to claim 1, characterized in that, In step S3, the humidity balancing involves placing the demolded capsule blank in an environment with a relative humidity of 70%-80% for 12-36 hours to equilibrate.

5. The preparation method according to claim 1, characterized in that, In step S3, the drying is carried out at 30-50°C until the moisture content of the capsule is less than 10%.

6. A starch-based capsule prepared by the method according to any one of claims 1-5, characterized in that, The capsule wall comprises starch oxyacetate, tannic acid, and Fe. 3+ Starch-based gel networks formed through cross-linking.

7. A starch-based capsule according to claim 6, characterized in that, The aldehyde content of the dialdehyde starch is 0.5-0.8 mmol / g.

8. A starch-based capsule according to claim 6, characterized in that, Based on the mass of the dialdehyde starch, the amount of tannic acid is 10%-30%, and the Fe... 3+ The amount of ions used, calculated as FeCl3·6H2O, is 4%-6%.

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