A preparation method of porous starch with controllable multi-level pore sizes and morphology

Abstract

The present invention discloses a method for preparing porous starch with controllable multi-level pore sizes and morphology, belonging to the technical field of starch processing. In the present invention, purified amylose and amylopectin are used as raw materials for compounding, and the chain length distribution after the starch chain gelatinization and unwinding in the ultrasonic physical field is finely regulated. According to the amylose-amylopectin ratio, ethanol polar molecules are quantitatively introduced to efficiently combine with the exposed hydroxyl sites at all levels, and agglomerate to form a V-type complex. Furthermore, during the concentration and drying process, the interfacial tension formed by ethanol evaporation and water migration is utilized to stretch the amylose and amylopectin with different chain lengths in the system to form a "perfect circle" pore shape. By regulating the amylose-amylopectin ratio of starch (quantitative compounding), maintaining the exposure of starch hydroxyl sites (concentration and cultivation), and constructing interfacial tension to stretch starch chains (convective drying) and other principles, the porous starch with controllable pore size and morphology designed and prepared is expected to become a good material for adsorbing and embedding bioactive substances with different molecular weights.

Description

Technical Field

[0001] The present invention relates to a preparation method of porous starch with multi-level pore sizes and controllable morphology, in particular to a processing method of porous starch that utilizes the interfacial tension formed by ethanol evaporation and water migration to stretch amylose and amylopectin with different chain length distributions in the system to form a "perfect round" pore shape and has multi-level controllable pore sizes, belonging to the technical field of starch processing. Background Art

[0002] Porous starch (PS), as a commonly used modified starch, has higher adsorption efficiency, solubility and swelling ability than natural starch, and is widely used as a carrier for bioactive substances, an adsorbent for pollutants and an encapsulant for dietary supplements. Traditional methods for preparing PS, such as the bioenzyme method, ethanol-alkali method, acid hydrolysis method and molecular insertion method, often cause large shrinkage and deformation of starch granules, and so far there is no technical means to show that the characteristics such as pore size, pore volume, porosity and morphology of pores can be regulated. Summary of the Invention

[0003] The present invention utilizes technical processes such as compounding and purifying amylose and amylopectin, ultrasonic depolymerization to expose hydroxyl groups, ethanol antisolvent method, and convective drying to form pores by stretching, innovatively connects processes such as regulating the amylose-amylopectin ratio, catalyzing the dispersed state of starch chains, forming a starch-ethanol V-type complex, and constructing interfacial tension stretching, and combines the negative correlation between the starch pore size and the amylose-amylopectin ratio to form a set of PS preparation methods with controllable pore sizes. Through the present invention, "round holes" with regular morphology are formed, which is expected to become a new method for guiding the production of efficient and environmentally friendly PS.

[0004] The present invention adopts the following technical solution: a preparation method of porous starch with controllable pore size, the method comprising: using an ultrasonic physical field to promote the pasting and depolymerization of a compound starch solution with different amylose-amylopectin ratios, promptly performing alcohol precipitation in the formed homogeneous starch chain dispersed state pasting solution, slowly drying with air after aggregating starch chains to form PS with a "perfect round" pore shape and multi-level pore sizes, that is, the PS with multi-level pore sizes and controllable morphology. Specifically, treating a compound starch suspension with ultrasonic waves, and then performing alcohol precipitation treatment on the ultrasonic solution; after concentrating and culturing the ultrasonic-alcohol precipitation solution, performing solid-liquid separation, taking the precipitate for drying to obtain the porous starch with multi-level pore sizes and controllable morphology; the compound starch includes at least amylose and amylopectin, and the mass ratio of amylose to amylopectin is less than (4:1); the smaller the proportion of amylose, the larger the pore size of the prepared porous starch.

[0005] In the present invention, the suspension of the compounded full - scale linear - branched starch mixture is subjected to ultrasonic chain scission to promote the starch chains to maintain a uniform dispersed state in the solution, giving a good opportunity for ethanol polar molecules to enter the spiral cavities of the starch chains, that is, an anti - solvent method is constructed. After the quantitatively introduced ethanol polar molecules are efficiently combined with the hydroxyl sites at all levels of the starch chains, they agglomerate to form a V - type complex, and its position will determine the spatial distribution of the pores in the later stage. The present invention emphasizes drying the precipitate in an open - air drying oven at 35 - 75 °C with a convection environment, which can provide the interfacial tension formed by ethanol evaporation and water migration, and then stretch the linear and branched starches with different chain lengths in the system to form "round" pore shapes. When the content of linear starch increases from 0% to 80%, the starch pore diameter gradually decreases.

[0006] Further, in the step (1), the plant sources of the purified linear starch and the purified branched starch are any one of corn, potato, wheat, cassava, sweet potato, and rice.

[0007] Further, in the step (1), the concentration of the compounded starch suspension is 2.5 - 15 wt%.

[0008] Further, in the step (1), the ultrasonic treatment conditions are as follows: the ultrasonic mode is one of probe ultrasonic or water - bath ultrasonic, the ultrasonic power is 20 - 60 W / ml, the ultrasonic frequency is 20 - 60 kHz, the ultrasonic time is 10 - 30 min, and the ultrasonic solution temperature is 60 - 100 °C.

[0009] Further, in the step (1), the alcohol precipitation treatment is as follows: anhydrous ethanol is uniformly dripped into the ultrasonic solution at a rate of 5 - 25 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1 - 3:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C.

[0010] Further, in the step (1), the centrifugation conditions are: the centrifugal force is 3500 - 6500×g, and the centrifugation time is 10 min.

[0011] The beneficial technical effects of the invention are as follows:

[0012] 1. The present invention explores the negative correlation between the ratio of linear - branched starch and the pore size, and uses this rule to seek reasonable process means to prepare PS with multi - level pore sizes and controllable morphology. A set of established process flows can be used to accurately guide the production practice of PS.

[0013] 2. The present invention proposes a green, environmentally friendly, simple and efficient physical processing flow for preparing PS. The product is safe and edible, and is a good material for adsorbing and embedding bioactive substances with different molecular weights, expanding its application scope in the fields of food and medicine industries. Description of the Drawings

[0014] Figure 1 Process flow chart of PS with controllable multi - level pore size and morphology prepared according to the present invention;

[0015] Figure 2 Scanning electron microscope (SEM) micrograph of PS with controllable multi - level pore size and morphology prepared according to the present invention, used to directly prove the pore size of each level of the PS prepared by the present invention;

[0016] Figure 3 Pore size distribution diagram and corresponding mercury intrusion / extrusion curve of PS with controllable multi - level pore size and morphology prepared according to the present invention, used to directly prove the pore size content distribution of each level of the PS prepared by the present invention;

[0017] Figure 4 Pore size distribution diagrams at different amylose contents in Example 2;

[0018] Figure 5 Scanning electron microscope (SEM) micrograph of PS prepared in Comparative Example 1 of the present invention, used to directly prove the influence of the ultrasonic treatment process on the pore morphology formation in the preparation method of the present invention.

[0019] Figure 6 Scanning electron microscope (SEM) micrograph of PS prepared by vacuum freeze - drying treatment in Comparative Example 2 of the present invention, used to directly prove the influence of the blast drying process on the pore morphology formation in the preparation method of the present invention. Detailed implementation manners

[0020] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following, in combination with the accompanying drawings and preferred embodiments, details the specific implementation manners, structures, features and their effects of the present invention as follows.

[0021] The following further illustrates the present invention through examples. The following examples are for illustrative purposes and not for limiting the scope of the present invention.

[0022] Example 1

[0023] A preparation method of PS with controllable multi - level pore size and morphology is as follows:

[0024] (1) Prepare a compound starch suspension: Compound amylose and amylopectin from corn sources so that the proportion of amylose is 0, 20%, 50%, 80% respectively, and then prepare a 2.5 wt% starch suspension with deionized water;

[0025] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 50 W / ml, the frequency to 50 kHz, and the time to 10 min, and perform ultrasonic-assisted gelatinization while maintaining a water bath heating at 90 °C;

[0026] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution prepared in step (2) at a constant rate of 5 ml / min until the volume ratio of ethanol to the ultrasonic solution is 2.5:1. During the process, there is no stirring, and the solution temperature is maintained at at least 50 °C;

[0027] (4) Incubate at a constant temperature and then centrifuge: Seal the mixed solution prepared in step (3), and let it stand and incubate at 50 °C for 30 min. After centrifuging for 10 min at a centrifugal force of 3500×g, remove the supernatant to obtain the precipitate;

[0028] (5) Blow-dry: Place the precipitate obtained in step (4) in a hot air drying oven at 35 °C with a blowing function, and dry it with the opening for 48 h to obtain the PS with controllable multi-level pore sizes and morphologies;

[0029] (6) Observation of porous morphology: Evenly spread a small amount of starch granules passing through a 75-μm sieve on the conductive adhesive and coat them with gold. Observe the morphology through a scanning electron microscope (SEM) at an acceleration voltage of 3 kV and a magnification of ×10,000; As Figure 2 shown, it can be seen that in this example, porous starch is obtained at different amylose ratios, and the pores are circular.

[0030] (7) Detection of pore size distribution: Apply an external pressure using a mercury intrusion porosimeter (MIP) to invade non-wetting mercury into the PS sample, and detect the voids at the micron scale through the intrusion-extrusion cycle. Record the pressure and the amount of mercury intrusion, and infer the pore structure parameters (such as pore radius, pore volume, pore surface area, pore size distribution) from the measured mercury intrusion curve. Assuming that the pores are composed of cylindrical pores, the relationship between the pore radius and the pressure can be revealed by the Washburn equation, and the formula is as follows:

[0031]

[0032] In the formula, P is the mercury intrusion pressure (MPa), r is the pore radius (μm) when mercury enters under pressure P, θ is the contact angle (130°), and γ is the interfacial tension of mercury (0.485 J / m 2 ).

[0033] Figure 3For the mercury intrusion / extrusion curve, it can be seen that as the proportion of amylose increases, the pore size of the prepared porous starch becomes smaller. Amylopectin has a generally short chain length and is prone to agglomeration, and it is not easily changed during the co-treatment, so the resulting pore structure is larger. On the contrary, amylose has greater rigidity, and the starch chain is easily broken under the influence of the ultrasonic physical field, generating more pores. Therefore, the pore size distribution of porous starch can be regulated by compounding amylose / amylopectin.

[0034] Example 2 (0 ≤ X < 10%)

[0035] A preparation method of PS with controllable multi-level pore size and morphology is as follows:

[0036] (1) Prepare a compound starch suspension: Compound amylose and amylopectin from corn sources so that the proportion of amylose is 0%, 4%, 6%, and 9% respectively; then prepare a 5 wt% starch suspension with deionized water respectively;

[0037] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 40 W / ml, the frequency to 40 kHz, and the time to 15 min for ultrasonic paste gelatinization, while maintaining a water bath heating at 80 °C;

[0038] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution obtained in step (2) at a rate of 10 ml / min until the volume ratio of ethanol to the ultrasonic solution is 2:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0039] (4) Centrifuge after constant temperature culture: Seal the mixed solution obtained in step (3) and statically culture it at 50 °C for 40 min. Centrifuge at a centrifugal force of 4500 × g for 10 min and then remove the supernatant to obtain a precipitate;

[0040] (5) Blow-dry: Place the precipitate obtained in step (4) in a hot air drying oven at 45 °C with a blowing function, and dry it with the opening for 36 h to obtain the PS with controllable multi-level pore size and morphology;

[0041] (6) Observation of porous morphology: Evenly spread a small amount of starch particles passing through a 75 μm sieve on the conductive adhesive and coat with gold. Observe the morphology through a scanning electron microscope (SEM) at an acceleration voltage of 3 kV with a magnification of ×10,000, and it can be seen that the starch surface presents circular pores.

[0042] (7) Aperture distribution detection: Using a mercury intrusion porosimeter (MIP), an external pressure is applied to intrude non-wetting mercury into the PS sample, and the voids at the micron scale are detected through an intrusion-extrusion cycle. The pressure and the amount of mercury intrusion are recorded, and the pore structure parameters (such as pore radius, pore volume, pore surface area, aperture distribution) are inferred from the measured mercury intrusion curve. Assuming that the pores are composed of cylindrical pores, the relationship between the pore radius and the pressure can be revealed by the Washburn equation, as follows:

[0043]

[0044] In the formula, P is the mercury intrusion pressure (MPa), r is the pore radius (μm) when mercury enters under pressure P, θ is the contact angle (130°), and γ is the interfacial tension of mercury (0.485 J / m 2 ).

[0045] According to the mercury intrusion test, when the proportion of amylose is 0%, the proportion of small pores with 15 ≤ Y < 30 μm reaches 98.58%. When the proportion of amylose is 4%, the proportion of small pores with 15 ≤ Y < 30 μm reaches 96.90%. When the proportion of amylose is 6%, the proportion of small pores with 15 ≤ Y < 30 μm reaches 95.38%. When the proportion of amylose is 9%, the proportion of small pores with 15 ≤ Y < 30 μm reaches 95.31%. As Figure 4 . It can be seen that within the range of amylose proportion of 0 ≤ X < 10%, the pore-making success rate of small pores with 15 ≤ X < 30 μm reaches more than 90%.

[0046] Example 3 (10 ≤ X < 30%)

[0047] A preparation method of PS with controllable multi-level pore size and morphology is as follows:

[0048] (1) Prepare a compound starch suspension: Compound the amylose and amylopectin from corn sources so that the proportion of amylose is 10%, 16%, 23%, 29% respectively; then prepare a 7.5 wt% starch suspension with deionized water respectively;

[0049] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 30 W / ml, the frequency to 30 kHz, and the time to 20 min for ultrasonic gelatinization promotion, while maintaining a water bath heating at 70 °C;

[0050] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution obtained in step (2) at a constant rate of 15 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1.5:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0051] (4)Centrifugation after incubation at a constant temperature: Seal the mixed solution obtained in step (3), and incubate it statically at 50 °C for 50 min. After centrifuging at a centrifugal force of 5500×g for 10 min, remove the supernatant to obtain a precipitate;

[0052] (5)Drying with hot air blowing: Place the precipitate obtained in step (4) in a hot air drying oven at 55 °C with a hot air blowing function, and dry it with the opening uncovered for 24 h to obtain the PS with controllable multi-level pore sizes and morphology;

[0053] (6)Observation of porous morphology: Evenly spread a small amount of starch granules passing through a 75-μm sieve on a conductive adhesive and coat it with gold. Observe the morphology with a scanning electron microscope (SEM) at an accelerating voltage of 3 kV and a magnification of ×10,000. It can be seen that circular holes are presented on the surface of the starch.

[0054] (7)Detection of pore size distribution: Apply an external pressure using a mercury intrusion porosimeter (MIP) to intrude non-wetting mercury into the PS sample, and detect the voids at the micron scale through the intrusion-extrusion cycle. Record the pressure and the amount of mercury intrusion, and infer the pore structure parameters (such as pore radius, pore volume, pore surface area, pore size distribution) from the measured mercury intrusion curve. Assuming that the pores are composed of cylindrical pores, the relationship between the pore radius and the pressure can be revealed by the Washburn equation, and the formula is as follows:

[0055]

[0056] In the formula, P is the mercury intrusion pressure (MPa), r is the pore radius (μm) when mercury enters under the pressure P, θ is the contact angle (130°), and γ is the interfacial tension of mercury (0.485 J / m 2 ).

[0057] According to the mercury intrusion test, when the proportion of amylose is 10%, the proportion of small holes with 10 ≤ Y < 15 μm reaches 95.33%. When the proportion of amylose is 16%, the proportion of small holes with 10 ≤ Y < 15 μm reaches 93.74%. When the proportion of amylose is 23%, the proportion of small holes with 10 ≤ Y < 15 μm reaches 93.58%. When the proportion of amylose is 29%, the proportion of small holes with 10 ≤ Y < 15 μm reaches 94.08%. Thus, in the range of amylose proportion where 10 ≤ X < 30%, the hole-making success rate of small holes with 10 ≤ Y < 15 μm reaches more than 90%.

[0058] Example 4 (30 ≤ X < 50%)

[0059] A preparation method of PS with controllable multi-level pore sizes and morphology is as follows:

[0060] (1) Prepare a compound starch suspension: Compound amylose and amylopectin from corn sources so that the proportion of amylose is 30%, 35%, 44%, and 49% respectively, and then prepare 10 wt% starch suspensions with deionized water respectively;

[0061] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 20 W / ml, the frequency to 20 kHz, and the time to 25 min for ultrasonic promoting gelatinization, while maintaining a water bath heating at 60 °C;

[0062] (3) Alcohol precipitation treatment: Slowly add absolute ethanol to the ultrasonic solution obtained in step (2) at a constant rate of 20 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0063] (4) Incubate at a constant temperature and then centrifuge: Seal the mixed solution obtained in step (3), and let it stand and incubate at 50 °C for 60 min. Centrifuge at a centrifugal force of 6500×g for 10 min and then remove the supernatant to obtain a precipitate;

[0064] (5) Blow-dry: Place the precipitate obtained in step (4) in a hot air drying oven at 65 °C with a blowing function, and dry it with an open mouth for 12 h to obtain the PS with controllable multi-level pore sizes and morphologies;

[0065] (6) Observation of porous morphology: Evenly spread a small amount of starch particles passing through a 75 μm sieve on the conductive adhesive and coat them with gold. Observe the morphology through a scanning electron microscope (SEM) at an acceleration voltage of 3 KV with a magnification of ×10,000, and it can be seen that the surface of the starch presents circular holes.

[0066] (7) Detection of pore size distribution: Apply an external pressure using a mercury intrusion porosimeter (MIP) to intrude non-wetting mercury into the PS sample, and detect the voids at the micron scale through the intrusion-extrusion cycle. Record the pressure and the amount of mercury intrusion, and infer the pore structure parameters (such as pore radius, pore volume, pore surface area, pore size distribution) from the measured mercury intrusion curve. Assuming that the pores are composed of cylindrical pores, the relationship between the pore radius and the pressure can be revealed by the Washburn equation, and the formula is as follows:

[0067]

[0068] In the formula, P is the mercury intrusion pressure (MPa), r is the pore radius (μm) when mercury enters under pressure P, θ is the contact angle (130°), and γ is the interfacial tension of mercury (0.485 J / m 2 ).

[0069] According to mercury intrusion tests, when the proportion of amylose is 30%, the proportion of small pores with 1 ≤ Y < 10 μm reaches 98.58%. When the proportion of amylose is 35%, the proportion of small pores with 1 ≤ Y < 10 μm reaches 96.90%. When the proportion of amylose is 44%, the proportion of small pores with 1 ≤ Y < 10 μm reaches 95.38%. When the proportion of amylose is 49%, the proportion of small pores with 1 ≤ Y < 10 μm reaches 95.03%. Thus, in the range of amylose proportion of 30 ≤ X < 50%, the hole-making success rate of small pores with 1 ≤ Y < 10 μm reaches more than 90%.

[0070] Example 5 (50 ≤ X < 80%)

[0071] A preparation method of PS with controllable multi-level pore sizes and morphologies is as follows:

[0072] (1) Prepare a compound starch suspension: Compound the amylose and amylopectin from corn sources so that the proportion of amylose is 50%, 60%, 70%, and 79% respectively, and then prepare 10 wt% starch suspensions with deionized water respectively;

[0073] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 20 W / ml, the frequency to 20 kHz, and the time to 25 min for ultrasonic paste gelatinization, while maintaining a water bath heating at 60 °C;

[0074] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution obtained in step (2) at a rate of 20 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0075] (4) Centrifuge after constant temperature cultivation: Seal the mixed solution obtained in step (3), and statically cultivate it at 50 °C for 60 min. After centrifuging at a centrifugal force of 6500 × g for 10 min, remove the supernatant to obtain a precipitate;

[0076] (5) Blow-dry: Place the precipitate obtained in step (4) in a hot air drying oven at 65 °C with a blowing function, and dry it open for 12 h to obtain the PS with controllable multi-level pore sizes and morphologies;

[0077] (6) Observation of porous morphology: Evenly spread a small amount of starch particles passing through a 75 μm sieve on a conductive adhesive and coat them with gold. Observe the morphology through a scanning electron microscope (SEM) at an acceleration voltage of 3 KV with a magnification of ×10,000, and it can be seen that the starch surface presents relatively dense circular holes.

[0078] (7) Aperture distribution detection: Using a mercury intrusion porosimeter (MIP), an external pressure is applied to intrude non-wetting mercury into the PS sample, and the voids at the micron scale are detected through the intrusion-extrusion cycle. Record the pressure and the amount of mercury intrusion, and infer the pore structure parameters (such as pore radius, pore volume, pore surface area, aperture distribution) from the measured mercury intrusion curve. Assuming that the pores are composed of cylindrical pores, the relationship between the pore radius and the pressure can be revealed by the Washburn equation, as follows:

[0079]

[0080] In the formula, P is the mercury intrusion pressure (MPa), r is the pore radius (μm) when mercury enters under pressure P, θ is the contact angle (130°), and γ is the interfacial tension of mercury (0.485 J / m 2 ).

[0081] According to the mercury intrusion test, when the proportion of amylose is 50%, the proportion of small pores with 1 ≤ Y < 1000 nm reaches 93.96%. When the proportion of amylose is 60%, the proportion of small pores with 1 ≤ Y < 1000 nm reaches 92.33%. When the proportion of amylose is 70%, the proportion of small pores with 1 ≤ Y < 1000 nm reaches 91.63%. When the proportion of amylose is 79%, the proportion of small pores with 1 ≤ Y < 1000 nm reaches 92.51%. It can be seen that within the range of amylose proportion of 50 ≤ X < 80%, the pore formation success rate of small pores with 1 ≤ Y < 1000 nm reaches more than 90%.

[0082] Comparative Example 1

[0083] A preparation method of PS with controllable multi-level pore size and morphology is as follows:

[0084] (1) Prepare a compound starch suspension: Compound the amylose and amylopectin from corn sources in four ratios, namely 0:1, 1:4, 1:1, and 4:1, that is, the amylose contents are 0%, 20%, 50%, and 80% respectively, and then prepare a 5 wt% starch suspension with deionized water;

[0085] (2) To prove the influence of ultrasonic treatment on the formation law of pore size in the present invention, this comparative example does not set the ultrasonic treatment step, that is, directly use the starch suspension prepared in step (1) for alcohol precipitation treatment;

[0086] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution prepared in step (1) at a constant rate of 20 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1.5:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0087] (4) Centrifugation after incubation at a constant temperature: Seal the four kinds of mixed solutions prepared in step (3), and incubate them statically at 50 °C for 30 min. Centrifuge at a centrifugal force of 3500 - 6500×g for 10 min respectively, and then remove the supernatant to obtain four kinds of precipitates;

[0088] (5) Drying by blowing air: Place the precipitates obtained in step (4) in a hot air dryer at 50 °C with a blowing function, and dry them in the open air for 24 h to obtain four kinds of PS prepared without the ultrasonic step;

[0089] (6) Observation of porous morphology: Evenly spread a small amount of starch granules passing through a 75 - μm sieve on the conductive adhesive and coat them with gold. Observe the morphology by scanning electron microscope (SEM) at an acceleration voltage of 3 kV and a magnification of ×10,000.

[0090] For the PS prepared in this comparative example, the formed pore sizes are uniform, and there is no correlation law with the ratio of amylose - amylopectin, that is, PS with controllable pore sizes fails to be successfully prepared.

[0091] Comparative Example 2

[0092] (1) Preparation of a compound starch suspension: Compound the amylose and amylopectin from corn sources in four ratios, namely 0:1, 1:4, 1:1, and 4:1, that is, the amylose contents are 0%, 20%, 50%, and 80% respectively, and then prepare a 5 wt% starch suspension with deionized water;

[0093] (2) Ultrasonic treatment: Adopt the probe ultrasonic working mode, and insert the probe into the liquid surface to the center of the upper 1 / 3 of the liquid. Set the ultrasonic power to 20 W / ml, the frequency to 20 kHz, and the time to 20 min for ultrasonic gelatinization promotion, while maintaining a water bath at 70 °C;

[0094] (3) Alcohol precipitation treatment: Slowly add anhydrous ethanol to the ultrasonic solution prepared in step (1) at a rate of 20 ml / min until the volume ratio of ethanol to the ultrasonic solution is 1.5:1. There is no stirring during the process, and the solution temperature is maintained at at least 50 °C;

[0095] (4) Centrifugation after incubation at a constant temperature: Seal the four kinds of mixed solutions prepared in step (3), and incubate them statically at 50 °C for 30 min. Centrifuge at a centrifugal force of 3500 - 6500×g for 10 min respectively, and then remove the supernatant to obtain four kinds of precipitates;

[0096] (5) To prove the influence of the blowing - air drying process on the formation of pore morphology in the present invention, this comparative example uses vacuum freeze - drying instead of the blowing - air drying step. Place the precipitates obtained in step (4) in a vacuum freeze - dryer at 50 °C for 48 h to obtain four kinds of freeze - dried PS;

[0097] (6) Observation of porous morphology: A small amount of starch granules passing through a 75-μm sieve was evenly spread on a conductive adhesive and coated with gold. The morphology was observed by scanning electron microscopy (SEM) at an acceleration voltage of 3 kV and a magnification of ×10,000.

[0098] For the PS prepared in this comparative example, the formed pore size had a weak correlation with the amylose-amylopectin ratio, but the pore morphology presented an elliptical state due to the gravity of the ethanol drop addition, that is, a PS with controllable morphology was not successfully prepared.

[0099] The above are only the preferred embodiments of the present invention and do not impose any formal limitations on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or equivalent changes by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A preparation method of porous starch with controllable pore size, characterized in that: The compound starch suspension is treated by ultrasonic wave, and then the ultrasonic solution is subjected to alcohol precipitation treatment; after the ultrasonic-alcohol precipitation solution is concentrated and cultivated, centrifuged, and the precipitate is dried at a temperature of 35°C to 75°C to obtain the porous starch with controllable multi-level pore size and morphology; the compound starch at least includes amylose and amylopectin, and the mass ratio of amylose to amylopectin is below (4:1); the smaller the proportion of amylose, the larger the pore size of the prepared porous starch; The ultrasonic treatment conditions: the ultrasonic power is 20~60W / ml, the ultrasonic frequency is 20~60kHz, the ultrasonic time is 10~30min, and the ultrasonic solution temperature is 60~100°C; The alcohol precipitation treatment is: anhydrous ethanol is uniformly dripped into the ultrasonic solution at a rate of 5~25ml / min until the volume ratio of ethanol to the ultrasonic solution is 1~3:1, without stirring during the process, and the solution temperature is kept above 50°C; The conditions for concentration and cultivation: sealed and static, the cultivation temperature is 50°C~70°C, and the cultivation time is 30~70min; When the proportion of amylose is: 0 ≤ x < 10%, porous starch with a pore size y of 15 ≤ y < 30 μm is obtained; or when the proportion of amylose is: 10 ≤ X < 30%, porous starch with a pore size y of 10 ≤ y < 15μm is obtained; or when the proportion of amylose is: 30 ≤ X < 50%, porous starch with a pore size y of 1 ≤ y < 10 μm is obtained; or when the proportion of amylose is: 50 ≤ X < 80%, porous starch with a pore size y of 1 ≤ y < 1000 nm is obtained; the proportion of amylose mentioned refers to: amylose mass / total mass of amylose and amylopectin; The drying method is: drying for 12~48h, and the drying process needs to be carried out in hot air with a convection environment and dried in an open manner.

2. The preparation method according to claim 1, characterized in that, The plant sources of the amylose and amylopectin are any one of corn, potato, wheat, cassava, sweet potato, and rice.

3. The preparation method according to claim 1, characterized in that, The concentration of the compound starch suspension is 2.5~15wt%.

4. The preparation method according to claim 1, characterized in that, The centrifugation conditions: the centrifugal force is 3500~6500×g, and the centrifugation time is 10min.

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