Preparation method of structural stable porous starch

Abstract

The application discloses a preparation method of structural stable porous starch, which comprises the following steps: adding a hydrophilic colloid into deionized water to prepare a solution, then adding starch, mixing uniformly to obtain a compound suspension, heating the compound suspension at a crystal starting melting temperature, then performing vacuum freeze drying, grinding and sieving to obtain melting coated starch, adding the melting coated starch into a citric acid-sodium phosphate buffer solution, mixing uniformly to obtain a starch-buffer solution suspension, adding an enzyme to perform oscillation reaction, adjusting the reaction solution to alkaline to terminate the reaction after the reaction is completed, then adjusting the reaction solution to neutral, finally performing centrifugation, washing, vacuum freeze drying, grinding and sieving to obtain the structural stable porous starch. The method has the advantages of simple process and low manufacturing cost, and the prepared porous starch has excellent adsorption, structural stability and thermal stability, and solves the problems of complex manufacturing process, long preparation time, poor structural stability of the prepared porous starch and limited pore forming effect in the prior art.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of modified starch processing, and particularly relates to a preparation method of structural stable porous starch. BACKGROUND

[0002] Porous starch is a modified starch with honeycomb hole structure and high specific surface area. Compared with natural starch, porous starch has better adsorption performance, and this characteristic makes it widely used in food, medicine, chemical industry, cosmetics and agriculture.

[0003] In existing academic papers, there are many studies on the preparation of porous starch using enzyme method, such as: Zhang Huanhuan et al. (Zhang Huanhuan, Qi Zhi. Preparation of chestnut microporous starch by double enzyme method [J]. Food industry, 2017, 38(02): 81-84.) prepared chestnut microporous starch by double enzyme method, which has good adsorption effect, but the time-consuming is long (20h); Benavent-Gil et al. (Benavent-Gil Y, Rosell M C. Comparison of porous starches obtained from different enzyme types and levels [J]. Carbohydrate Polymers, 2017, 157 533-540.) found that different enzymes have different effects on the formation of porous structure of starch, and the thermal properties of the starch treated by enzyme show lower melting onset temperature and melting peak temperature (usually lower melting temperature, poor thermal stability of particles). The porous starch prepared by physical method usually has good structural stability, such as Yu Liming (Yu Liming. Preparation and adsorption application of potato porous starch [D]. Gansu Agricultural University, 2020.) said that the porous starch prepared by physical method has stable structure, but the pore forming effect is limited. In order to improve the pore forming effect of porous starch, researchers also combine physical method with enzyme method to prepare porous starch, but there is still a problem of difficult balance between stability and pore forming effect, such as Zahra et al. (Zahra D, Hossein MA, Mohsen B. Porous corn starch obtained from combined cold plasma and enzymatic hydrolysis: Microstructure and physicochemical properties [J]. International journal of biological macromolecules, 2022, 223 (PA): 790-797.) combined cold plasma treatment and enzymatic hydrolysis to prepare corn porous starch, which significantly improved the adsorption effect of the starch, but compared with single enzyme hydrolysis, the crystallinity of the particles and the gelatinization enthalpy were reduced, indicating that the stability of the particle structure was reduced; Zhang Tiantian et al. (Zhang Tiantian, Hou Mengxing, Liu Peiling. Preparation of cross-linked porous starch by high pressure and enzyme method and its properties [J]. Chinese Journal of Food Science, 2022, 22(12): 153-164.) used high pressure, α-amylase and glucoamylase to prepare high pressure porous starch, and then modified the starch with sodium trimetaphosphate. The prepared porous starch has excellent pore forming effect, but the Fourier infrared results show that the short-range ordered structure is destroyed and the crystallinity is reduced, indicating that the prepared porous starch has poor particle stability, and the preparation method is complex.

[0004] A Chinese invention patent with application number 202311273085.6 discloses a porous starch with free radical scavenging function and its preparation method and application. The patent uses enzyme method to prepare a porous starch with free radical scavenging function and discusses its adsorption. The research finds that the prepared porous starch has good pore-forming effect, but the thermogravimetric result shows that the initial thermal decomposition temperature of the prepared porous starch is low, and the porous starch has poor heat resistance. A Chinese invention patent with application number 202310131810.X discloses a modified starch and its preparation method. A porous starch with good adsorption effect on harmful gases in flue gas is prepared by combining enzyme method and crosslinking method. Although it has good adsorption and stability, a crosslinking agent, epoxy chloropropane (toxic and corrosive), is needed to introduce chemical groups, amino groups, in the crosslinking process, and the preparation method is complex. A Chinese invention patent with application number 202210118863.3 discloses crosslinked corn porous starch loaded curcumin composite gel microspheres and a preparation method thereof. Corn porous starch is prepared by using pulse electric field assisted enzymatic hydrolysis, and further treated by crosslinking method. Carboxymethyl cellulose and other substances are used as adjuvants to prepare composite gel microspheres for loading and embedding curcumin. The prepared sample has good embedding effect, but the preparation method is complex and the cost is high.

[0005] In summary, in the prior art, the enzyme method for preparing porous starch has the problem of unstable particle structure, and chemical treatment introduces chemical groups. Although physical treatment has the advantage of being clean, the current common physical methods such as ultrasonic and cold plasma combined with enzyme method still cannot well balance the inherent common problem of pore-forming-structure stability. Preparing gel microspheres by embedding active substances in porous starch is a common method to improve the retention rate of active substances and the stability of particles, but it still has limitations such as complex operation and long preparation period. SUMMARY

[0006] The purpose of the present application is to solve the above-mentioned problems of the prior art, and to provide a preparation method of structure stable porous starch. The method has simple process and low manufacturing cost, and the prepared porous starch has excellent adsorption, structure stability and thermal stability.

[0007] To achieve the purpose of the present application, the following technical solutions are adopted:

[0008] A preparation method of structure stable porous starch, comprising the following steps:

[0009] (1) Melt coating: add hydrophilic colloid into deionized water to prepare a solution, then add starch and mix uniformly to obtain a compound suspension. Heat treat it at the crystal starting melting temperature, then vacuum freeze-dry and sieve to obtain melt coated starch;

[0010] (2) Directional hydrolysis: melt-coated starch is added to a citric acid-sodium phosphate buffer solution, mixed evenly to obtain a starch-buffer solution suspension, an enzyme is added to perform a shaking hydrolysis reaction, after the reaction is completed, the reaction solution is adjusted to alkaline to terminate the reaction, then adjusted to neutral, finally centrifuged, washed, vacuum freeze-dried, ground and sieved to obtain a structure-stable porous starch.

[0011] Further, in step (1), the hydrophilic colloid is one or a mixture of two or more of plant-derived hydrophilic colloids (pectin, locust bean gum, carrageenan, gum arabic), animal-derived hydrophilic colloids (gelatin, chitosan), microorganism-derived hydrophilic colloids (such as dextran, xanthan gum), and non-starch polysaccharides (such as cellulose, hemicellulose) in any proportion.

[0012] Further, in step (1), the starch is one or a mixture of two or more of cereal starches (normal corn starch, waxy corn starch, rice starch, millet starch, wheat starch), potato starches (potato starch, tapioca starch, sweet potato starch), legume starches (green bean starch, pea starch), and other vegetable sources (water caltrop starch, lotus root starch) in any proportion.

[0013] Further, in step (1), the hydrophilic colloid is added in an amount of 0.1% to 10% based on the dry mass of the starch.

[0014] Further, in step (1), the heating treatment is performed at a temperature that fluctuates by ±5°C around the temperature at which the starch crystals completely melt as determined by a differential scanning calorimeter, and the heating treatment is performed for 15 min to 100 min.

[0015] Further, in step (2), the amount of melt-coated starch added is (25-45) g per 100 mL of the citric acid-sodium phosphate buffer solution, and the pH of the citric acid-sodium phosphate buffer solution is 5.6-6.4.

[0016] Further, in step (2), the enzyme is one or a mixture of two or more of α-amylase, β-amylase, γ-amylase, isoamylase, or glucoamylase in any proportion, and the enzyme is added in an amount of 40 U / g to 180 U / g based on the dry mass of the starch.

[0017] Further, in step (2), the shaking hydrolysis reaction is performed at a speed of 120-200 r / min, at a temperature of 25°C to 60°C, and for 4 h to 12 h.

[0018] Further, in step (2), the centrifugation is performed at a speed of 3000-4000 r / min for 5 min to 25 min, and the sieving is performed through a 200-mesh sieve.

[0019] The porous starch prepared by the preparation method.

[0020] Advantages of the present application:

[0021] 1) The present application provides a preparation method of structural stable porous starch, which innovatively uses a two-step method of "melt coating" and "directional hydrolysis" to prepare structural stable porous starch. After melt treatment, the starch granules swell, exposing the weak structural sites in the granules, and the hydrophilic colloid is spread and coated on the surface of the starch granules and associated with the soluble starch leached during the melt process to form a non-structured network structure on the surface of the granules, which is a melt coating for the granules, to ensure the steady-state hydrolysis of the starch granules. At the same time, the micro-concave sites formed by the accumulation of hydrophilic colloid on the surface of the starch granules provide reaction sites for enzymatic hydrolysis, so as to realize directional hydrolysis of the enzyme.

[0022] 2) The method of the present application has simple preparation process and low manufacturing cost, and the prepared porous starch has obvious pore structure, and the scanning electron microscope results and ordinary microscope show that the particle structure is complete. Compared with the original starch, the adsorption rates of the porous starch prepared by the present application to water, oil, vitamin C and methylene blue are increased by 103.24%, 138.33%, 55.90% and 73.70% respectively, and the structural stability and thermal stability are better (the crystallinity is increased by 3.75%, and the melt initiation temperature is increased by 7℃), which provides an effective way for embedding adsorption and slow release of bioactive ingredients.

[0023] 3) The method of the present application is suitable for different crystal forms and different particle sizes of starch raw materials, and has simple preparation method, high production efficiency, and advantages of green and clean (safe), high raw material utilization rate (cost), good steady-state loading effect (application value) and the like, which is beneficial to later application and promotes industrial development. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Optical microscope and polarizing microscope images of the porous starch prepared for Example 1 and Comparative Examples 1-2;

[0025] Figure 2 Scanning electron microscope and laser confocal microscope images of the porous starch prepared for Example 1 and Comparative Examples 1-2;

[0026] Figure 3 Scanning electron microscope images of the porous starch prepared for Examples 2-4;

[0027] Figure 4 Transparency test results of the porous starch prepared for Example 1 and Comparative Examples 1-2;

[0028] Figure 5 Hydrolysis rate and yield test results of the porous starch prepared for Example 1 and Comparative Example 2;

[0029] Figure 6 Thermal profile of the porous starch prepared for Example 1 and Comparative Examples 1-2;

[0030] Figure 7 Infrared spectra and corresponding deconvoluted spectra of the porous starch prepared for Example 1 and Comparative Examples 1-2;

[0031] Figure 8 Vitamin C adsorption rate, methylene blue adsorption rate, water adsorption rate and oil adsorption rate test results of the porous starch prepared for Example 1 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0032] In order for those skilled in the art to better understand the technical solutions in the present application, the technical solutions of the present application will be described clearly and completely in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0033] Example 1

[0034] A preparation method of a structurally stable porous starch, comprising the following steps:

[0035] (1) Melt-coating: 0.12 g of xanthan gum was added to 120 mL of deionized water to prepare a solution, then 40 g (dry basis) of ordinary corn starch was added and mixed uniformly to obtain a compound suspension, then the compound suspension was subjected to a 30 min heating treatment at a temperature at which the starch crystals began to melt (the melting onset temperature of the starch was determined in advance using a differential scanning instrument, which was used as the temperature at which the crystals began to melt), and finally vacuum freeze-drying (-80℃, 72 h), grinding and passing through a 200 mesh sieve to obtain a melt-coated starch;

[0036] (2) Directional hydrolysis: melt-coated starch was added into a citric acid-disodium hydrogen phosphate buffer solution (pH = 5.6) (3 mL of the citric acid-disodium hydrogen phosphate buffer solution was used per 1 g of the melt-coated starch), mixed uniformly to obtain a starch-buffer solution suspension, after 10 min of incubation at 55 °C, α-amylase was added (the enzyme was added in an amount of 80 U / g based on the dry starch basis), and the hydrolysis reaction was carried out at 55 °C for 9 h with oscillation (190 r / min). After the reaction was completed, the reaction solution was adjusted to be alkaline (pH = 10.8) by using a 1 mol / L sodium hydroxide solution to terminate the reaction, and 15 min later, the pH of the solution was adjusted to be neutral by using a 1 mol / L hydrochloric acid solution. Finally, the starch was centrifuged (at a speed of 3500 r / min for 10 min), washed, vacuum freeze-dried (-80 °C, 72 h), ground and sieved through a 200-mesh sieve to obtain the structure-stable porous starch.

[0037] Comparative Example 1

[0038] Preparation of melt-coated starch: 0.12 g of xanthan gum was uniformly dispersed in 120 mL of deionized water to prepare a solution, 40 g of dry ordinary corn starch was added, mixed uniformly to obtain a compound suspension, and then the compound suspension was subjected to 30 min of heating treatment at a temperature at which the starch crystals began to melt (the melting initiation temperature of the starch was determined in advance by using a differential scanning instrument, and the temperature was used as the temperature at which the crystals began to melt). Finally, the melt-coated starch was vacuum freeze-dried (-80 °C, 72 h), ground and sieved through a 200-mesh sieve.

[0039] Comparative Example 2

[0040] Preparation of enzyme-hydrolyzed porous starch: ordinary corn starch was mixed with a citric acid-disodium hydrogen phosphate buffer solution (pH = 5.6) at a ratio of 1:3 (3 mL of the citric acid-disodium hydrogen phosphate buffer solution was used per 1 g of the ordinary corn starch) to obtain a starch suspension, after 10 min of incubation at 55 °C, α-amylase was added to the starch suspension (the enzyme was added in an amount of 120 U / g based on the dry starch basis), and the hydrolysis reaction was carried out at 55 °C for 15 h with constant temperature oscillation (190 r / min). After the reaction was completed, the reaction solution was adjusted to be alkaline (pH = 10.8) by using a 1 mol / L sodium hydroxide solution to terminate the reaction, and 15 min later, the pH of the solution was adjusted to be neutral by using a hydrochloric acid solution. Finally, the starch was centrifuged (at a speed of 3500 r / min for 10 min), washed, vacuum freeze-dried (-80 °C, 72 h), ground and sieved through a 200-mesh sieve to obtain the enzyme-hydrolyzed porous starch.

[0041] Example 2

[0042] A method for preparing a structure-stable porous starch, comprising the following steps:

[0043] (1) melt-coating: xanthan gum 0.04 g was added to 120 mL of deionized water to prepare a solution, then 40 g (dry basis) of ordinary corn starch was added and mixed uniformly to obtain a compound suspension, then the compound suspension was subjected to a 30 min heat treatment at a temperature at which the starch crystals began to melt (the melting onset temperature of the starch was determined in advance using a differential scanning calorimeter, which was used as the temperature at which the crystals began to melt), and finally vacuum freeze-drying (-80°C, 72 h), grinding and passing through a 200 mesh sieve to obtain melt-coated starch;

[0044] (2) directional hydrolysis: the melt-coated starch was added to a citric acid-disodium hydrogen phosphate buffer solution (pH = 6.4) (3 mL of citric acid-disodium hydrogen phosphate buffer solution was used per 1 g of melt-coated starch), mixed uniformly to obtain a starch-buffer solution suspension, after incubation at 55°C for 10 min, α-amylase was added (the enzyme was added at a dosage of 80 U / g based on the dry starch basis), and the hydrolysis reaction was carried out at 55°C with shaking (190 r / min) for 12 h, after the reaction was completed, the reaction solution was adjusted to alkaline (pH = 10.8) using a 1 mol / L sodium hydroxide solution to terminate the reaction, and after 15 min, a 1 mol / L hydrochloric acid solution was used to adjust to neutral, and finally centrifugation (at a speed of 3500 r / min for 10 min), washing, vacuum freeze-drying (-80°C, 72 h), grinding and passing through a 200 mesh sieve to obtain structure-stable porous starch.

[0045] Example 3

[0046] A method for preparing structure-stable porous starch, comprising the following steps:

[0047] (1) melt-coating: xanthan gum 0.40 g was added to 120 mL of deionized water to prepare a solution, then 40 g (dry basis) of ordinary corn starch was added and mixed uniformly to obtain a compound suspension, then the compound suspension was subjected to a 30 min heat treatment at a temperature at which the starch crystals began to melt (the melting onset temperature of the starch was determined in advance using a differential scanning calorimeter, which was used as the temperature at which the crystals began to melt), and finally vacuum freeze-drying (-80°C, 72 h), grinding and passing through a 200 mesh sieve to obtain melt-coated starch;

[0048] (2) Directional hydrolysis: melt-coated starch was added into a citric acid-disodium hydrogen phosphate buffer solution (pH = 5.6) (3 mL of the buffer solution was used per 1 g of the melt-coated starch), mixed uniformly to obtain a starch-buffer solution suspension, and then, after incubation at 55°C for 10 min, α-amylase was added (60 U / g of the starch on a dry basis), and the hydrolysis reaction was carried out at 55°C for 12 h with oscillation (190 r / min). After the reaction, the reaction solution was adjusted to alkaline (pH = 10.8) with a 1 mol / L sodium hydroxide solution to terminate the reaction, and then, after 15 min, it was adjusted to neutral with a 1 mol / L hydrochloric acid solution. Finally, the starch was centrifuged (at 3500 r / min for 10 min), washed, vacuum freeze-dried (-80°C, 72 h), ground, and sieved through a 200-mesh sieve to obtain the structure-stable porous starch.

[0049] Example 4

[0050] A method for preparing a structure-stable porous starch, comprising the following steps:

[0051] (1) Melt coating: xanthan gum 0.20 g was added into 120 mL of deionized water to prepare a solution, and then, 40 g (dry basis) of ordinary corn starch was added and mixed uniformly to obtain a compound suspension. Then, the compound suspension was subjected to heating treatment at a temperature at which the starch crystals began to melt (the melting initiation temperature of the starch was determined in advance using a differential scanning calorimeter, and the temperature was used as the temperature at which the crystals began to melt). Finally, the compound was vacuum freeze-dried (-80°C, 72 h), ground, and sieved through a 200-mesh sieve to obtain the melt-coated starch.

[0052] (2) Directional hydrolysis: melt-coated starch was added into a citric acid-disodium hydrogen phosphate buffer solution (pH = 6) (3 mL of the buffer solution was used per 1 g of the melt-coated starch), mixed uniformly to obtain a starch-buffer solution suspension, and then, after incubation at 55°C for 10 min, α-amylase was added (100 U / g of the starch on a dry basis), and the hydrolysis reaction was carried out at 55°C for 6 h with oscillation (190 r / min). After the reaction, the reaction solution was adjusted to alkaline (pH = 10.8) with a 1 mol / L sodium hydroxide solution to terminate the reaction, and then, after 15 min, it was adjusted to neutral with a 1 mol / L hydrochloric acid solution. Finally, the starch was centrifuged (at 3500 r / min for 10 min), washed, vacuum freeze-dried (-80°C, 72 h), ground, and sieved through a 200-mesh sieve to obtain the structure-stable porous starch.

[0053] Figure 1 Optical microscope images and polarizing microscope images of the porous starches prepared in Example 1 and Comparative Examples 1-2 are shown in Figure 1It can be seen that the common corn starch and the starch sample of Comparative Example 1 have larger starch granules and complete polarized cross structure, indicating that the melt-coating treated starch retains its original morphological and structural characteristics. In contrast, the radial shadow is observed in the starch prepared in Comparative Example 2 and Example 1, which is speculated to be the pore structure of the starch granule. Among them, the structure of Comparative Example 2 is fragile, and under the observation of an optical microscope, many broken starch granules can be clearly seen, and the polarized intensity is weakened, indicating that Comparative Example 2 (only enzymatic treatment) can form larger pores, but its structure is unstable. However, the porous starch prepared in Example 1 not only has obvious pore channels in the granule, but also most of the starch prepared in Example 1 still has a complete polarized cross structure, and the broken granules are few, indicating that the porous starch prepared in Example 1 has good structural stability.

[0054] Figure 2 The scanning electron microscope images and laser confocal images of the porous starch prepared in Example 1 and Comparative Examples 1-2 are shown in the following table. It can be seen from the observation of the scanning electron microscope that the edge of the common corn starch granule is clear, the surface is smooth, there are a small amount of micropores, the internal structure is complete, and there are no obvious channels and cracks. It can be seen from the laser confocal results that the edge of Comparative Example 1 granule is blurred after the "melt-coating" treatment. It can be seen from the electron microscope images that there is a mixture of soluble starch and hydrophilic colloid accumulated on the surface of Comparative Example 1. This indicates that the melt-coating method used in the present application can effectively perform "coating" on the non-grid interface distribution of the granule, thereby providing a site for "directional hydrolysis". Comparative Example 2 (only enzymatic treatment) and Example 1 both have obvious pore structure. Among them, the porous starch prepared in Comparative Example 2 has a significant pore-forming effect, but its structure is severely damaged, the inside is collapsed, and the original structural stability is lost. In contrast, Example 1 not only has a good pore-forming effect, but also the granule is more complete, and shows significant structural stability. This is consistent with the results of the optical microscope and the polarized microscope. In addition, the mesh membrane layer formed by the melt-coating can be clearly observed on the surface of Example 1. These membrane layers not only play a role in maintaining the structure of the granule, but also the recesses formed by the mesh membrane layer can also adsorb active substances on the surface of the granule, thereby improving the embedding rate.

[0055] Figure 3 The scanning electron microscope images of the porous starch prepared in Examples 2-4 are shown in the following table. It can be seen that the porous starch prepared in Examples 2-4 all have good pore-forming effect and structural stability, indicating that the method of preparing the structural stable porous starch by the two-step method of "melt-coating" and "directional hydrolysis" is feasible.

[0056] The transparency, hydrolysis rate and yield, water / oil absorption rate, vitamin adsorption rate and methylene blue adsorption rate of the porous starch prepared in Example 1 and Comparative Examples 1-2 were tested, and the related test methods are as follows:

[0057] 1. Transparency determination method: 0.40 g (dry basis) of ordinary corn starch, comparative examples 1-2 and porous starch prepared in example 1 were accurately weighed and mixed with 40 mL of deionized water to prepare a starch suspension, which was then heated in a boiling water bath and stirred uniformly for 20 min. After cooling for a period of time, an appropriate amount of starch paste was poured into a cuvette, and the transmittance of the starch paste was determined at 620 nm using a UV spectrophotometer. The transmittance of the starch paste was measured after 0 h, 24 h, 48 h and 72 h, respectively, i.e. the transparency.

[0058] 2. Hydrolysis rate and yield determination method: DNS method was used to determine the hydrolysis rate. (1) Preparation of glucose standard curve: 1 g of glucose was placed in an oven at 80°C and dried to constant weight. After being taken out, 0.0500 g of glucose was accurately weighed, dissolved in distilled water, and diluted to 50 mL to prepare a 1 mg / mL glucose standard solution. Distilled water was used to dilute the glucose solution to 0.1 mg / mL, 0.2 mg / mL, 0.3 mg / mL, 0.4 mg / mL and 0.5 mg / mL, respectively. 0.5 mL of glucose solution of different concentrations was placed in a light-proof test tube containing 1.5 mL of DNS solution. After boiling water bath reaction for 5 min, it was quickly cooled to room temperature. 4 mL of distilled water was added to each tube and left for 20 min. The corresponding absorbance was measured at 540 nm wavelength using a spectrophotometer to obtain the standard curve of glucose concentration (y) and absorbance (x), i.e. y = 2.2905x + 0.0213 (R 2 = 0.998).(2) Determination of sample hydrolysis rate: a certain amount of starch was weighed for enzymatic reaction. After the reaction was terminated, the starch milk was centrifuged. 0.5 mL of diluted supernatant was measured for absorbance according to the method of (1). The starch hydrolysis rate can be calculated by the following formula.(3) Determination of yield: the starch milk after enzymatic reaction was centrifuged. The precipitate was washed and dried. The yield was calculated by the following formula.

[0059]

[0060] In the formula, C is the concentration, g / mL, measured by DNS method after dilution and obtained from the regression straight line equation; N is the dilution multiple; V is the volume of supernatant, mL; M1 is the mass of starch, g; M2 is the mass of porous starch obtained after treatment, g.

[0061] 3. Water / oil absorption rate test method: 0.40 g (dry basis) of starch was accurately weighed in a high core tube. 5 mL of distilled water (soybean oil) was added to the tube and stirred uniformly. After 30 min at room temperature, the high core was removed for 10 min to remove the upper layer of distilled water (soybean oil). The total mass of the remaining precipitate and the centrifuge tube was accurately weighed. The water absorption rate (oil absorption rate) of the starch was calculated by the following formula:

[0062]

[0063] In the formula, the mass of the centrifuge tube and powder is taken and recorded as M1, g; the mass of the precipitate and centrifuge tube after adsorption is recorded as M2, g.

[0064] 4. Determination method of vitamin adsorption rate: (1) Determination of standard curve: prepare vitamin C solutions with a gradient of 5 mg / L and a concentration of 5-40 mg / L, and measure the absorbance A at 265 nm using an ultraviolet spectrophotometer to obtain the standard curve of the relationship between the concentration (y) of vitamin C and the absorbance (x), i.e. y = 29.555-1.274 (R 2 = 0.9978). (2) Adsorption of vitamin C: accurately take 1 g (dry basis) of the sample, then add 25 mL of a vitamin C solution with a concentration of 20 mg / mL, and adsorb for a period of time on a constant-temperature shaker (150 r / min, 25°C), and then centrifuge (3000 r / min, 25°C, 5 min) to obtain the supernatant. Measure the absorbance of the supernatant and convert it into the concentration by referring to the standard curve. The adsorption rate of the porous starch on the vitamin C solution can be calculated by the following formula:

[0065]

[0066] In the formula, V: volume of the vitamin C solution, mL; Co: initial concentration of the solution, mg / L; Ce: equilibrium concentration of the solution, mg / mL.

[0067] 5. Determination method of methylene blue adsorption rate: (1) Determination of standard curve: prepare methylene blue solutions with a gradient of 1 mg / L and a concentration of 1-10 mg / L, and measure the absorbance at 665 nm using an ultraviolet spectrophotometer to obtain the standard curve of the relationship between the concentration (y) of methylene blue and the absorbance (x), i.e. y = 5.9387x-0.235 (R 2 = 0.9981). (2) Adsorption of methylene blue: accurately take 100 mg (dry basis) of the sample, then add 40 mL of a methylene blue solution with a concentration of 50 mg / mL, and adsorb for a period of time on a constant-temperature shaker (150 r / min, 25°C), and then centrifuge (3000 r / min, 25°C) for 5 min to obtain the supernatant. Measure the absorbance of the supernatant and convert it into the concentration by referring to the standard curve. The adsorption rate of the porous starch on the methylene blue solution can be calculated by the following formula:

[0068]

[0069] In the formula, V: volume of the methylene blue solution added, mL; Co: initial concentration of the solution, mg / L; Ce: equilibrium concentration of the solution, mg / mL.

[0070] Figure 4 The results of the transparency test of the porous starch prepared in Example 1 and Comparative Examples 1-2 are shown in the figure. As can be seen from the figure, the transparency of the starch particles of Comparative Example 2 and Example 1 is significantly increased compared with the untreated normal corn starch and the starch prepared in Comparative Example 1, which is probably because the α-amylase acts on the α-1, 4 glycosidic bond, and after the enzymatic hydrolysis, the starch glycosidic bond is broken and the chain length is shortened, which is beneficial to the better dissolution of the starch particles in water, thus increasing the transparency. The transparency of the starch prepared in Comparative Example 1 is lower than that of the untreated normal corn starch, which indicates that the sample after the melt coating treatment is not easy to be gelatinized, which is beneficial to improve the stability. Compared with Comparative Example 2, the transparency of the porous starch prepared in Example 1 is significantly decreased, and the trend does not change with time (0h-72h), which indicates that the two-step method of "melt coating" and "directed hydrolysis" can significantly improve the thermal stability of the particles and the prepared starch has good storage performance.

[0071] Figure 5 The results of the hydrolysis rate and yield of the porous starch prepared in Example 1 and Comparative Example 2. Generally, the yield is negatively correlated with the degree of hydrolysis, and the hydrolysis rate is positively correlated with the degree of hydrolysis, and the figure also shows such a trend. The degree of hydrolysis of Comparative Example 2 is higher, but combined with the electron microscope result analysis, the particle structure of Comparative Example 2 collapses to form a large cavity, and the particle structure stability is poor, which indicates that single enzymatic hydrolysis makes the particle over-hydrolyzed, resulting in poor particle structure stability. In contrast, the porous starch prepared by the two-step method of "melt coating" and "directed hydrolysis" can form significant pore structure, while having good structure stability. In addition, the higher yield makes the porous starch prepared in Example 1 also has certain advantages in economic benefits.

[0072] Figure 6 The thermal property diagram of the porous starch prepared in Example 1 and Comparative Examples 1-2. As can be seen from the figure, the melting onset temperature of the porous starch prepared in Comparative Example 2 and Example 1 is significantly increased compared with the untreated normal corn starch and Comparative Example 1, and the melting onset temperature of Example 1 is higher than that of Comparative Example 2. Generally, the initial melting temperature is positively correlated with the thermal stability, and the initial melting temperature of Example 1 is 73.84℃, which is significantly higher than that of the normal corn starch (66.16℃) and Comparative Example 2 (70.21℃), which indicates that the two-step method of "melt coating" and "directed hydrolysis" can significantly improve the thermal stability of the starch particles.

[0073] Figure 7 The infrared spectrum and the corresponding deconvolution spectrum of the porous starch prepared in Example 1 and Comparative Examples 1-2 are shown in the figure, wherein, Figure 7 A is the infrared spectrum, Figure 7 B is the corresponding deconvolution spectrum. As can be seen from the figure, the position of the characteristic absorption peak of the starch does not change before and after each treatment, which indicates that the treatment does not cause the change of the related groups of the starch. The starch particles have a characteristic absorption peak at 1047cm-1 and 1022cm -1 The absorption peaks at these locations are related to the crystalline and amorphous structures, respectively. Therefore, the ratio R(1047 / 1022 cm⁻¹) is often used. -1 The short-range order of starch structure is characterized by the R value, with a larger R value indicating a higher degree of structural order. Compared to ordinary corn starch, Comparative Example 2 showed a decreased short-range order, and particle breakage caused by excessive random hydrolysis is likely the main reason for this reduced order. Meanwhile, Example 1 exhibited a significantly higher short-range order than Comparative Example 2, increasing from 0.74 cm⁻¹. -1 (Comparative Example 2) Increased to 0.93cm -1 (Example 1). This is mainly because "melt coating" provides a rough surface for the particles, which is beneficial for enzyme adhesion. Furthermore, the unstructured coating on the particle surface allows the enzyme to target the exposed areas of the coating, providing hydrolysis sites for subsequent "directional hydrolysis." Therefore, "melt coating" can effectively promote the directional hydrolysis of amorphous regions of starch particles by enzyme molecules, thereby increasing the crystallinity and structural stability of the porous starch particles prepared by the two-step method of "melt coating" and "directional hydrolysis."

[0074] Table 1

[0075]

[0076] Table 1 shows the long-range crystal structure parameters of the starch granules prepared in Example 1 and Comparative Examples 1-2. The relative crystallinity RC% is the proportion of the crystalline region area in the total area (the sum of the crystalline and amorphous regions) in the diffraction pattern, and is usually used to characterize the long-range crystal structure of granules. Enzymatic hydrolysis usually preferentially hydrolyzes amorphous regions, so the starch granules after enzymatic hydrolysis have a higher degree of crystallinity.

[0077] As shown in Table 1, the characteristic peaks of Comparative Examples 1-2 and Example 1 remained unchanged, indicating that the crystal type of the particles remained unchanged, still being type A crystal structure. The relative crystallinity of Example 1 increased from 20.35% (ordinary corn starch) to 24.10%, demonstrating that the two-step method of "melt coating" and "directional hydrolysis" significantly improved particle stability. The relative crystallinity of Comparative Example 2 was lower than that of ordinary corn starch, possibly because the particles underwent excessive random hydrolysis, leading to structural damage and a decrease in relative crystallinity. The relative crystallinity of Example 1 was much higher than that of the Comparative Examples and ordinary corn starch, indicating that the two-step method of "melt coating" and "directional hydrolysis" had a good promoting effect on particle structural stability.

[0078] Figure 8 The results of vitamin C adsorption rate, methylene blue adsorption rate, water absorption rate, and oil absorption rate of the porous starches prepared in Example 1 and Comparative Examples 1-2 are as follows: Figure 8 A represents the test results for vitamin C adsorption rate and methylene blue adsorption rate.Figure 8 B is the water absorption and oil absorption test results, as can be seen from the figure, compared with the smooth surface of the original starch, the starch particles after "melting coating" have more significant adsorption effect. This may be due to the presence of the mixed accumulation layer of hydrophilic colloid and soluble starch on the surface of the "melting coating" particles, which not only increases the adsorption sites on the surface of the particles, but also can adsorb more substances, and because of the unique hydrophilicity of the hydrophilic colloid, the water absorption effect of Comparative Example 1 is more significant. The adsorption effect of the particles after enzymolysis is significantly higher than that of Comparative Example 1 and ordinary corn starch, which shows that Comparative Example 2 and Example 1 both have good pore structure, which is consistent with the SEM results in the foregoing. However, under the observation of the microscope and the scanning electron microscope, the particle structure of Comparative Example 2 collapses, so the adsorbed substances are easy to fall off. The adsorption effect of Example 1 is obviously better than that of Comparative Example 2, whether it is the adsorption rate of water, oil, vitamin C or methylene blue, all show the same trend. This may be because the "melting coating" and "directional hydrolysis" two-step method not only forms pores, but also has little damage to the particle structure, and the particles still have a relatively complete structure, which is more conducive to the adsorption and slow release of bioactive factors by porous starch. Compared with the original starch, the adsorption rate of water molecules, oil molecules, vitamin C and methylene blue of the porous starch prepared by the "melting coating" and "directional hydrolysis" two-step method is increased by 103.24%, 138.33%, 55.90% and 73.70%, respectively, which fully shows that the "melting coating" and "directional hydrolysis" two-step method has significant advantages in improving the adsorption effect of starch particles.

[0079] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and cannot limit the protection scope of the present application. Any change, modification, replacement, combination and simplification made without departing from the spirit and principles of the present application shall be covered within the protection scope of the present application.

Claims

1. A method for preparing a structurally stable porous starch, characterized by, The method comprises the following steps: (1) melt coating: hydrophilic colloid is added to deionized water to prepare a solution, then starch is added and mixed uniformly to obtain a compound suspension, which is heated at the starting melting temperature of starch crystals, then vacuum freeze-dried and sieved to obtain melt-coated starch; (2) directional hydrolysis: melt-coated starch is added to a citric acid-disodium hydrogen phosphate buffer solution, mixed uniformly to obtain a starch-buffer solution suspension, an enzyme is added for oscillation hydrolysis reaction, after the reaction is completed, the reaction solution is adjusted to alkaline to terminate the reaction, then adjusted to neutral, finally centrifuged, washed, vacuum freeze-dried and ground and sieved to obtain structure-stable porous starch.

2. The method for preparing the structurally stable porous starch according to claim 1, wherein, In step (1), the hydrophilic colloid is one or a mixture of two or more of pectin, locust bean gum, carrageenan, gum arabic, gelatin, chitosan, dextran, xanthan gum, cellulose or hemicellulose in any proportion.

3. The method for preparing structurally stable porous starch as described in claim 1, characterized in that, In step (1), the starch is one or a mixture of two or more of ordinary corn starch, waxy corn starch, rice starch, millet starch, wheat starch, potato starch, cassava starch, sweet potato starch, mung bean starch, pea starch, water caltrop starch or lotus root starch in any proportion.

4. The method for preparing the structurally stable porous starch according to claim 1, wherein In step (1), the hydrophilic colloid is added in an amount of 0.1% to 10% based on the dry mass of the starch.

5. The method for preparing structurally stable porous starch as described in claim 1, characterized in that, In step (1), the heating temperature is the temperature at which the starch crystals completely melt, with a fluctuation of ±5℃, and the heating time is 15 min to 100 min.

6. The method for preparing structurally stable porous starch as described in claim 1, characterized in that, In step (2), the amount of melt-coated starch added is (25-45) g per 100 mL of citric acid-disodium hydrogen phosphate buffer solution, and the pH value of the citric acid-disodium hydrogen phosphate buffer solution is 5.6-6.

4.

7. The method for preparing structurally stable porous starch as described in claim 1, characterized in that, In step (2), the enzyme is one or a mixture of two or more of alpha-amylase, beta-amylase, gamma-amylase, isoamylase or glucoamylase in any proportion, and the amount of enzyme added is 40 U / g to 180 U / g based on the dry mass of the starch.

8. The method for preparing the structurally stable porous starch according to claim 1, wherein In step (2), the oscillation hydrolysis reaction is carried out at 120-200 r / min, 25℃-60℃ and for 4 h-12 h.

9. The method for producing the structurally stable porous starch according to any one of claims 1 to 8, characterized by, In step (2), the centrifugation is carried out at 3000-4000 r / min for 5 min-25 min, and the sieving is carried out through a 200-mesh sieve.

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