Modified corn starch and preparation method thereof

Abstract

The invention relates to the technical field of natural high polymer material modification, in particular to modified corn starch and a preparation method thereof. Comprising the following steps: selecting plump corn kernels, disinfecting, washing after disinfection, and soaking in water for later use; the soaked corn kernels are pretreated with a static magnetic field, the magnetic field intensity is 10-50 mT, the treatment time is 1-4 h, and the treatment temperature is 23-27 DEG C; putting the corn kernels treated by the static magnetic field into a germination plate, adding a small amount of distilled water, and germinating for 24-48 hours under the air humidity of 25-75%; the method comprises the following steps: performing water milling, sieving, washing and standing on germinated corn kernels, removing supernate, re-suspending precipitates with water, centrifuging, removing upper-layer yellow substances, centrifuging, drying separated starch at 40-50 DEG C for 14-24 hours, grinding, and sieving with a 100-mesh sieve. The method has the advantages that the static magnetic field is used for pretreatment, germination and starch extraction, and the method is safe, non-toxic, green and environment-friendly.

Description

Technical Field

[0001] The invention relates to the technical field of natural polymer material modification, in particular to modified corn starch and a preparation method thereof. Background Art

[0002] Corn is one of the important food crops in my country. It is rich in nutrients such as starch, protein, vitamins, dietary fiber, etc., and is loved by many consumers. Among them, starch is a macromolecule, accounting for about 60-70% of the weight of corn kernels, and is widely used in the food industry. In addition, starch is a polysaccharide composed of several connected glucose units, which can be divided into amylose and amylopectin. Amylose and amylopectin molecules provide functional diversity, making natural and modified corn starch a valuable industrial resource. Therefore, starch has been widely used in the food industry as a thickener, gelling agent, expander and water retaining agent. However, when starch is processed, the particles are broken and the internal crystal structure is decomposed, which further increases the digestibility of starch, resulting in a sharp increase in blood sugar levels after people eat starch-containing foods, and causing excessive insulin secretion, resulting in insulin resistance and the development of fatty liver. Chronic hyperglycemia can induce obesity, type II diabetes, cardiovascular disease and other metabolic diseases, causing serious harm to human health. Now, with the increasing awareness of food nutrition and quality, today's consumers are also demanding healthier foods. Therefore, low GI food is an important research trend at present.

[0003] In recent years, many scholars have paid extensive attention to improving the thermal stability of starch granules and strengthening the interaction between the crystal structure and the molecules in the non-crystalline region in order to improve the digestibility of starch. At present, physical, chemical and enzymatic methods can effectively reduce the digestibility of starch. However, chemically modified starch has problems such as solvent residue, and some countries prohibit or restrict the application of chemically modified starch in food; due to the high cost and complex operation of the enzyme modification method, it is easily affected by environmental factors. In recent years, there are many physical techniques that can modify starch, such as ultrasound, electromagnetic field radiation and high hydrostatic pressure. These physical techniques all affect the digestibility of starch by changing the structure of starch. As an emerging non-thermal physical technology, static magnetic field has attracted widespread attention from the academic community. In addition, static magnetic field is a non-toxic, green and controllable treatment method that can reduce the use of chemicals and improve food and environmental safety. Static magnetic field treatment can change the microstructure of starch, make the surface of starch granules rough, and improve its crystal structure and oil absorption characteristics, so as to achieve an ideal application in food or non-food industry. Germination is a necessary stage in plant growth, during which the components of grains, such as starch, protein and fat, can be changed, so germination can be used to improve the edible quality and nutritional value of corn. At present, there are no reports on the use of this technology in the modification of germinated corn starch. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a modified corn starch and a preparation method thereof.

[0005] The present invention provides a method for preparing modified corn starch, which specifically comprises the following steps: S1. Pretreatment of corn kernels: Select corn kernels with full grains and disinfect them. After disinfection, rinse and soak them in water for later use; S2. Static magnetic field pretreatment: The soaked corn kernels are pretreated with a static magnetic field with a magnetic field intensity of 10-50 mT, a treatment time of 1-4 h, and a treatment temperature of 23-27 °C; S3. Corn kernel germination: Place corn kernels treated with a static magnetic field into a germination tray, add a small amount of distilled water so that the embryo of the corn kernels is completely in contact with the distilled water, and germinate at an air humidity of 25-75% for 24-48 hours. S4. Extraction of germinated corn starch: Grind the germinated corn kernels with water, sieve, wash, and stand, remove the supernatant, resuspend the precipitate with water, and centrifuge to obtain germinated corn starch; S5. Remove the upper yellow substance, centrifuge, dry the separated starch at 40-50°C for 14-24 hours, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

[0006] Preferably, the magnetic field intensity in step S2 is 20-30 mT, and the processing temperature is 25°C.

[0007] Preferably, the magnetic field intensity in step S2 is 30 mT and the processing time is 2 h.

[0008] Preferably, the air humidity in step S3 is 75% and the germination time is 36 hours.

[0009] Preferably, in step S4, the germinated corn kernels are added with distilled water in a volume ratio of 1:2 and then water-milled.

[0010] Preferably, the sieving in step S4 is performed by sieving through 100 mesh and 200 mesh sieves in sequence.

[0011] Preferably, the disinfection in step S1 uses 0.1% sodium hypochlorite solution; after the rinsing, the pH is adjusted to neutral.

[0012] Preferably, the centrifugal conditions in step S4 are: centrifugal speed 6000r / min, centrifugal time 20min.

[0013] Preferably, the number of centrifugation in step S5 is 3 times; the drying temperature is 45° C. and the drying time is 24 hours.

[0014] The second object of the present invention is to provide a modified corn starch prepared by the modified corn starch preparation method.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects: The preparation method of the modified corn starch provided by the present invention is to extract starch by pretreatment in a static magnetic field, then germinate it, and observe the structure, physicochemical and digestion characteristics of the starch; this process has the advantages of safety, non-toxicity, green environmental protection, simple process, etc. The prepared modified corn starch has an enhanced ordered structure, high crystallinity, strong thermal stability, low swelling power and solubility, and is not easy to digest. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the in vitro digestion test result of the modified corn starch after being treated with a static magnetic field according to an embodiment of the present invention.

[0017] Figure 2 The present invention provides a scanning electron microscope image of modified corn starch after static magnetic field treatment according to an embodiment of the present invention; (A) natural germinated corn starch (GC), germination 0h; (A1) modified corn starch (SMF) after static magnetic field treatment, germination 0h; (B) GC, germination 24h; (B1) SMF, germination 24h; (C) GC, germination 36h; (C1) SMF, germination 36h; (D) GC, germination 72h; (D1) SMF, germination 72h.

[0018] Figure 3 This is an infrared spectrum of modified corn starch after being treated with a static magnetic field according to an embodiment of the present invention.

[0019] Figure 4 It is an X-ray diffraction spectrum diagram of modified corn starch after static magnetic field treatment provided by an embodiment of the present invention.

[0020] Figure 5 This is a thermal stability curve of modified corn starch after being treated with a static magnetic field according to an embodiment of the present invention.

[0021] Figure 6 1 is the gelatinization curve of the modified corn starch after being treated with a static magnetic field according to an embodiment of the present invention.

[0022] Figure 7 1 is the rheological characterization result of the modified corn starch after the static magnetic field treatment provided by the embodiment of the present invention; A in the figure represents the storage modulus (G'), and B in the figure represents the loss modulus (G''). DETAILED DESCRIPTION

[0023] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0024] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0025] Example 1 This embodiment provides a method for preparing modified corn starch, which specifically comprises the following steps: S1. Pretreatment of corn kernels: Select corn kernels with full grains and soak them in 0.1% sodium hypochlorite (NaClO) solution for disinfection for 15 minutes; rinse them with distilled water for 3-4 times until the pH is adjusted to neutral, and then soak them in distilled water for 6 hours for later use; S2. Static magnetic field (SMF) pretreatment: The soaked corn kernels were pretreated with a static magnetic field with a magnetic field intensity of 30 mT, a treatment time of 2 h, and a treatment temperature of 25 °C (the optimal SMF condition was selected based on low digestibility); S3. Corn kernel germination: Place corn kernels treated with a static magnetic field (SMF) into a germination tray, add distilled water so that the embryo of the corn kernel is completely in contact with the distilled water; germinate in an incubator with an air humidity of 75% for 36 hours; the size of the germination tray is 34x25x4.5cm; S4. Extraction of germinated corn (GC) starch: Grind the germinated corn kernels with distilled water in a ratio of 1:2, and sieve through 100-mesh and 200-mesh sieves; wash with distilled water for several times, let stand at room temperature for 4 hours, remove the supernatant, resuspend the precipitate with distilled water, and centrifuge at 6000 r / min for 20 minutes to obtain germinated corn starch; S5. Remove the yellow substance on the upper layer and centrifuge for 3 more times. Dry the separated starch at 40°C (in a forced air drying oven) for 24 h, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

[0026] Example 2 This embodiment provides a method for preparing modified corn starch, which specifically comprises the following steps: S1. Pretreatment of corn kernels: Select corn kernels with full grains and soak them in 0.1% sodium hypochlorite (NaClO) solution for disinfection for 15 minutes; rinse them with distilled water for 3-4 times until the pH is adjusted to neutral, and then soak them in distilled water for 6 hours for later use; S2. Static magnetic field (SMF) pretreatment: The soaked corn kernels were pretreated with a static magnetic field with a magnetic field intensity of 30 mT, a treatment time of 1 h, and a treatment temperature of 25°C; S3. Corn kernel germination: Place corn kernels treated with a static magnetic field (SMF) into a germination tray, add distilled water so that the embryo of the corn kernel is completely in contact with the distilled water; germinate in an incubator with an air humidity of 75% for 36 hours; S4. Extraction of germinated corn (GC) starch: Grind the germinated corn kernels with distilled water in a ratio of 1:2, and sieve through 100-mesh and 200-mesh sieves; wash with distilled water for several times, let stand at room temperature for 4 hours, remove the supernatant, resuspend the precipitate with distilled water, and centrifuge at 6000 r / min for 20 minutes to obtain germinated corn starch; S5. Remove the upper yellow substance and centrifuge for 3 more times. Dry the separated starch at 45°C (in a forced air drying oven) for 20 h, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

[0027] Example 3 The conditions optimization experiment of static magnetic field treatment is as follows: 1. Treatment methods of modified starch S1. Pretreatment of corn kernels: Select corn kernels with full grains and soak them in 0.1% sodium hypochlorite (NaClO) solution for disinfection for 15 minutes; rinse them with distilled water for 3-4 times until the pH is adjusted to neutral, and then soak them in distilled water for 6 hours for later use; S2. Static magnetic field (SMF) pretreatment: The soaked corn kernels were pretreated with a static magnetic field. The static magnetic field conditions were: magnetic field intensity 0, 10, 20, 30, 40, 50 mT; treatment time 0, 1h, 2h, 3h, 4h; treatment temperature 25°C (the best SMF condition was selected with low digestibility as an indicator); seeds treated with the same conditions but not treated with SMF (0mT, 0h group) served as the control group; S3. Corn kernel germination: Place corn kernels treated with a static magnetic field (SMF) into a germination tray, add distilled water so that the embryo of the corn kernel is completely in contact with the distilled water; germinate in an incubator with an air humidity of 75% for 36 hours; the size of the germination tray is 34x25x4.5cm; S4. Extraction of germinated corn (GC) starch: Grind the germinated corn kernels with distilled water in a ratio of 1:2, and sieve through 100-mesh and 200-mesh sieves; wash with distilled water for several times, let stand at room temperature for 4 hours, remove the supernatant, resuspend the precipitate with distilled water, and centrifuge at 6000 r / min for 20 minutes to obtain germinated corn starch; S5. Remove the upper yellow substance and centrifuge for 3 more times. Dry the separated starch at 40°C (in a forced air drying oven) for 24 hours, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

[0028] 2. Determination of in vitro digestibility of starch Place 500 mg of sample in a 50 ml centrifuge tube, add 10 ml of sodium acetate buffer (0.1 M, pH 5.2) and 6-8 glass beads. Then add 5 ml of mixed enzyme solution and shake for 180 min at a constant temperature shaker (160 r / min) at 37 ° C. Finally, take 0.1 ml of digestion solution at 0, 20, and 120 min, respectively, and add 0.9 mL of anhydrous ethanol to stop enzymatic hydrolysis. After centrifugation at 10000 r / min for 5 min, the glucose content in the supernatant was determined using a glucose detection kit. Use the following formula to calculate the RDS (rapidly digestible starch), SDS (slowly digestible starch), and RS (resistant starch) components in the starch sample: RDS(%)=0.9(G20−G0) / TS×100%; SDS(%)=0.9(G120−G20) / TS×100%; RS(%)=(1−(RDS%+SDS%))×100%; Where G0, G20 and G120 are the glucose contents released at 0 min, 20 min and 120 min respectively, 0.9 is the glucose conversion coefficient, and TS represents the total starch content in the sample (mg).

[0029] In order to further determine the effect of SMF treatment on GC starch, the magnetic field intensity and magnetic field time were screened. The digestion characteristics of GC starch treated with different SMF intensities were different. The results are shown in Table 1 and Figure 1 , Table 1 and Figure 1 The GC0h~GC72h groups were all germinated corn starch that had not been treated with a static magnetic field.

[0030] Table 1 In vitro digestion of modified corn starch under different static magnetic field intensities and magnetic field exposure times

[0031] Note: Data are mean ± standard deviation; different superscript letters in the same column indicate significant differences (P<0.05).

[0032] Table 1 or Figure 1 It can be seen that under the conditions of 30mT and 2h, RDS was significantly lower than GC starch by 9.69%, while SDS and RS were increased by 27.71% and 9.43% respectively (P<0.05). Therefore, the optimal SMF conditions were screened out with low digestibility as the index: magnetic field intensity of 30mT and treatment time of 2h.

[0033] Example 4 The optimization experiment of germination time is as follows: 1. Treatment methods of modified starch S1. Pretreatment of corn kernels: Select corn kernels with full grains and soak them in 0.1% sodium hypochlorite (NaClO) solution for disinfection for 15 minutes; rinse them with distilled water for 3-4 times until the pH is adjusted to neutral, and then soak them in distilled water for 6 hours for later use; S2. Static magnetic field (SMF) pretreatment: The soaked corn kernels were pretreated with a static magnetic field. The static magnetic field conditions were: magnetic field intensity 30 mT, treatment time 2 h, treatment temperature 25 °C (the optimal SMF conditions were screened out with low digestibility as an indicator); seeds treated with the same conditions but not treated with SMF (0 mT, 0 h group) served as the control group; S3. Corn kernel germination: Place corn kernels treated with a static magnetic field (SMF) into a germination tray, add distilled water so that the embryo of the corn kernel is completely in contact with the distilled water; germinate in an incubator with an air humidity of 75%, with germination times of 0h, 12h, 24h, 36h, 48h, 60h, and 72h (measured every 12h); the germination tray size is 34x25x4.5cm; S4. Extraction of germinated corn (GC) starch: Grind the germinated corn kernels with distilled water in a ratio of 1:2, and sieve through 100-mesh and 200-mesh sieves; wash with distilled water for several times, let stand at room temperature for 4 hours, remove the supernatant, resuspend the precipitate with distilled water, and centrifuge at 6000 r / min for 20 minutes to obtain germinated corn starch; S5. Remove the upper yellow substance and centrifuge for 3 more times. Dry the separated starch at 40°C (in a forced air drying oven) for 24 hours, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

[0034] 2. Determination of in vitro digestibility of starch Place 500 mg of sample in a 50 ml centrifuge tube, add 10 ml of sodium acetate buffer (0.1 M, pH 5.2) and 6-8 glass beads. Then add 5 ml of mixed enzyme solution and shake for 180 min at a constant temperature shaker (160 r / min) at 37 ° C. Finally, take 0.1 ml of digestion solution at 0, 20, and 120 min, respectively, and add 0.9 mL of anhydrous ethanol to stop enzymatic hydrolysis. After centrifugation at 10000 r / min for 5 min, the glucose content in the supernatant was determined using a glucose detection kit. Use the following formula to calculate the RDS (rapidly digestible starch), SDS (slowly digestible starch), and RS (resistant starch) components in the starch sample: RDS(%)=0.9(G20−G0) / TS×100%; SDS(%)=0.9(G120−G20) / TS×100%; RS(%)=(1−(RDS%+SDS%))×100%; Where G0, G20 and G120 are the glucose contents released at 0 min, 20 min and 120 min respectively, 0.9 is the glucose conversion coefficient, and TS represents the total starch content in the sample (mg).

[0035] Table 2 Effect of germination time on in vitro digestibility of corn starch

[0036] Note: Data are mean ± standard deviation. Values ​​with different capital letters in the same column represent significant differences in different treatment conditions at the same germination time (P<0.05); values ​​with different lowercase letters in the same column represent significant differences in different germination times at the same treatment conditions (P<0.05).

[0037] Results: See Table 2; From Table 2, it can be seen that the RDS of corn at 36h of germination was the lowest, and the SDS and RS were the highest (P<0.05), indicating that the starch order of corn at 36h of germination was the highest and the crystallization was perfect. This shows that germination can change the structure of starch, which will affect the digestion rate of starch. Most importantly, GC after SMF treatment can further change the structure of starch, thereby changing SDS and RS. From Table 2, it can be seen that when corn was germinated for 0-36h, the RDS of corn treated with SMF was significantly reduced, and the SDS and RS were significantly increased (P<0.05). This is because SMF further activated the starch hydrolyzing enzyme to preferentially act on the non-crystalline amorphous starch molecules, making the ordered structure of starch more highly crystalline, and the starch that is more easily hydrolyzed and digested has been mostly consumed. In this process, the starch that exists is more difficult to digest with a more crystalline structure. When corn was germinated for 48-72h, the RDS of corn treated with SMF increased, and the SDS and RS decreased. This is because long-term germination itself will cause damage and pores on the surface of some starch granules. SMF accelerates the penetration of enzymes into starch granules, resulting in the degradation of starch chains by starch degradation enzymes, so the digestibility of starch is improved. In short, the increase of SDS and RS of starch can stabilize intestinal health, effectively relieve postprandial blood sugar, have a sense of fullness, reduce chronic diseases such as obesity, and have a positive impact on the development of low GI foods. From this point of view, modifying starch by SMF plus germination treatment is harmless and non-polluting. It can be used to produce health products that are more beneficial to human health.

[0038] 3. Microstructure of Starch The morphological characteristics of starch samples were observed using an ultra-high resolution field emission scanning electron microscope at a magnification of 1000×. The samples were fixed with a special conductive double-sided adhesive and vacuum-sprayed gold, and placed in the instrument for photography and observation. The results are shown in Figure 2 .

[0039] like Figure 2 As shown in the figure, the natural corn starch granules are polygonal, irregular, elliptical and smooth in shape under 1000 times magnification. The GC starch granules with different germination time and SMF treatment have different effects on their shape and surface. Figure 2It can be seen that the surface of starch granules has roughness and pores. In the process of seed germination from 0 to 36 hours, it can be seen from the figure that only slight changes have occurred on the surface of starch granules, indicating that the enzymes that decompose starch have not been fully activated in this process; when the seeds germinate to 72 hours, the hydrolytic enzymes such as amylase and debranching enzyme in the seeds are activated until they penetrate the interior of the starch granules and decompose part of the starch to provide nutrients for seed germination. The granular morphology of GC starch treated with SMF will be rougher, with more dents or pores. This may be because the corn kernels treated with SMF absorb the residual water during soaking, resulting in water molecules promoting the rearrangement of the amorphous regions of starch granules, degrading the internal structure of starch granules and destroying them; another possibility is that the corn seeds treated with SMF accelerate the activity of amylase, resulting in a rougher surface of starch granules and more pores. The rough surface and more pores of starch granules are equivalent to an increase in the surface area of ​​starch granules, which can improve the hydrophilicity of starch, which will cause the properties of starch such as water absorption capacity, swelling capacity and adhesive properties to change.

[0040] 4. Short-range ordered structure of starch The starch samples were analyzed by infrared spectroscopy using an FTIR spectrometer. The starch samples were mixed with potassium bromide at a mass ratio of 1:100 (w / w), ground evenly, and pressed into thin sheets. The scanning range was 4000-400 cm -1 The spectrum was baseline corrected using Omnic 8.2, and the 1200-800 cm -1 Deconvolution is performed on the range.

[0041] In this study, the infrared spectra were deconvoluted using the OMNIC program to determine the absorption ratios of different peaks. Specifically, at 1047 cm -1 The absorption peak at 1022cm is related to the crystallinity of starch molecules, which corresponds to the ordered structure of starch molecules; -1 The absorption peak at 995cm is related to the non-crystalline region of starch, which corresponds to the amorphous structure of starch; -1 The absorption peak at 1047 / 1022cm corresponds to the number of hydrogen bonds in starch. -1 (R1) and 1022 / 995cm -1 The ratio of the two absorption peaks (R2) was used to compare the short-range order of starch and the hydrated starch structure of the amorphous structure. The results are shown in Tables 3 and Figure 3 .

[0042] Table 3 Short-range ordered structure, relative crystallinity and amylose content of modified corn starch after static magnetic field treatment

[0043] Table 3 represents 1047 / 1022cm -1 and 1022 / 995cm -1 From the values ​​of GC starch, it can be seen that the R1 value of GC starch shows a trend of first increasing and then decreasing, among which the value at 36h is the highest (P<0.05), and the R2 value shows a trend of first decreasing and then increasing, among which the value at 36h is the lowest (P<0.05); when the corn is germinating for 0h-36h, the R1 value after SMF treatment is further increased, and the R2 value is the opposite. This shows that SMF treatment can destroy the amorphous region and branched molecular chains of starch, and the generated short-chain molecular chains form a new ordered structure through hydrogen bonds; when the corn is germinating for 48h-60h, the R1 value after SMF treatment is lower than that of the control group, and the R2 value is the opposite. This is because the long germination time of corn may cause the double helix structure and the straight chain molecular chain to break. At this time, SMF treatment will further destroy the crystallinity and amorphous part of the starch granules, forming an irregular helical structure, so the R1 value is reduced. The results showed that germination treatment of corn for 36 hours could stabilize the short-range ordered structure of starch. However, the R1 value of GC starch treated with SMF increased by 19.67% (P<0.05) compared with the control group, indicating that the short-range ordered structure of GC starch treated with SMF was more stable.

[0044] Figure 3 A in the figure represents the original spectrum. Figure 3 B in the figure shows the deconvoluted spectrum. It can be seen from the figure that the position of the absorption peak is almost unchanged for both GC starch and GC starch treated with SMF, indicating that these two treatments do not lead to the generation of new molecular groups and chemical bonds in starch. -1 -3100cm -1 There are broad absorption peaks in the range of 1200 cm -1 -950cm -1 The bands in the range are sensitive to the starch molecular structure. Although the position of the absorption peak of starch did not change after germination and SMF treatment, the intensity of the absorption peak changed, which may be due to the change in the orderly arrangement of the starch crystal structure.

[0045] 5. Long-range ordered structure of starch The crystal structure of starch samples was analyzed by X-ray diffractometer. The X-ray source used was Cu-Ka. The X-ray diffraction analysis conditions were: scanning rate 5° / min, power 40kV, current 30mA, step width 0.01°. The diffraction angle (2θ) varied from 5° to 40°.

[0046] The relative crystallinity (RC) of each sample was analyzed according to the following formula: RC(%)=Ac / (Ac+Aa)×100% Ac: crystalline integrated area; Aa: amorphous integrated area; In order to further evaluate the effect of SMF treatment on the crystalline structure of GC starch, the relative crystallinity (RC) value was calculated. The RC value is the ratio of the crystalline area to the non-crystalline area of ​​starch. The results are shown in Table 3 and Figure 4 ;from Figure 4 It can be seen that the main X-ray diffraction peaks at 2θ are 15°, 17°, 18° and 23°, among which single-peak diffraction peaks appear at 15° and 23°, and the strongest double-peak diffraction peaks appear at 17° and 18°, which is the typical a-type crystal morphology of corn starch. SMF treatment has no significant effect on the position of the diffraction peaks, which is related to the tightly arranged double helix structure in the a-type crystalline starch structure. However, the diffraction peaks of GC starch treated with SMF are more obvious than those of the control, indicating that GC starch treated with SMF forms more starch crystal structures.

[0047] As can be seen from Table 3, the crystallinity of starch was the highest at 36h of corn germination, and the RC value of GC starch treated with SMF was 16.11% higher than that of the control group (P<0.05), indicating that GC starch treated with SMF promoted the rearrangement of starch molecular chains and formed more crystalline structures of starch. It also showed that the double helix content of branched starch increased and the content of amylose decreased, proving that the starch treated with SMF preferentially acted on the non-crystalline region, making its area smaller, thereby increasing the ratio of the crystalline region area to the non-crystalline region area. However, when corn germinated from 48 to 72h, the RC value of starch was further reduced after SMF treatment. During the germination process, starch was hydrolyzed to provide energy, and the amorphous region of starch granules was hydrolyzed first, and then the crystalline region was hydrolyzed. SMF treatment further hydrolyzed the crystalline region of starch granules, resulting in partial destruction of the crystal structure and reduced interaction between molecular chains, resulting in reduced crystallinity of starch. This result is consistent with the results of infrared and digestibility, indicating that starch granules with longer long-range ordered structures are less digestible.

[0048] 6. Thermal stability of starch The thermodynamic properties of starch samples were analyzed by differential scanning calorimetry. 3 mg of starch sample was placed in a DSC crucible aluminum box, 9 μl of deionized water was added, the box was sealed and equilibrated at 4 °C for 24 h, and an empty crucible aluminum box was used as a blank control. The test procedure was as follows: the heating rate was 10 °C / min, the temperature increased from 30 °C to 120 °C, and the nitrogen flow rate was 50 mL / min. The starting temperature (To), peak temperature (Tp), end temperature (Tc) and enthalpy change (ΔH) values ​​were recorded. The results are shown in Table 4 and Figure 5 shown.

[0049] Table 4 Thermodynamic parameters of modified corn starch after static magnetic field treatment

[0050] from Figure 5 It can be seen that corn starch has an obvious absorption peak between 60℃ and 80℃, which is the characteristic peak of starch endothermicity and the energy required to open the double helix structure of starch. T0 is the initial temperature of starch gelatinization, which is related to the stability of the amorphous region of starch granules; T P is the peak temperature of starch gelatinization, which is related to the stability of starch crystals; T C It is the termination temperature of starch gelatinization, indicating the melting temperature of perfect crystallization. From Table 4, it can be seen that as corn germinates to 72h, T0, T P , T C and ΔH have slightly changed, showing a trend of first increasing and then decreasing. P , T C The increase indicates that the temperature required for starch gelatinization increases and the starch is more stable. When corn germinates from 0h to 36h, SMF treatment increases the T0 and T P , T C and ΔH increased, especially at 36h, reaching the highest state, which increased by 1.85%, 3.11%, 1.43% and 16.16% compared with the control group, respectively (P<0.05). This shows that at this moment, GC starch treated with SMF requires higher temperature and heat to stick, the energy required to open the double helix structure of amylopectin is the highest, and the ordered structure of starch is the strongest. When corn germinated for 48-72h, SMF treatment increased the T0 and T P , T C The further decrease of ΔH may be due to the degradation and consumption of starch by SMF in the process of promoting corn germination, which leads to the reduction of the number of double helical structures of starch or the destruction of the double helical structure of starch granules. This result is consistent with the XRD and infrared results. The DSC results show that the ordered structure of starch is enhanced after magnetic field treatment, the crystallinity of starch is large, and the thermal stability is strong, which is conducive to the conversion of SDS and RS from RDS, thereby reducing its starch digestibility.

[0051] VII. Determination and results of oil (water) absorption, swelling power, solubility, transmittance, gelatinization and rheological properties of starch 1. Put 1g starch into a dried centrifuge tube and mix it with 10ml oil (water) at room temperature (add oil first and then starch sample to avoid incomplete contact between oil and starch sample), vortex for 20 minutes, let stand for 30 minutes, centrifuge at 4000 r / min for 30 minutes, pour off the oil (water), and accurately weigh the centrifuge tube and precipitate.

[0052] Oil (water) absorption = (W2-W1) / W0 x100% In the formula: W0: dry weight of sample / g; W1: weight of centrifuge tube plus dry sample / g; W2: weight of centrifuge tube plus sediment / g.

[0053] 2. Mix 0.4 g starch with 40 ml distilled water to prepare a 1% suspension. Then heat the suspension in boiling water, stir continuously for 30 min, and cool to room temperature. Centrifuge at 7000 r / min for 20 min, and dry the supernatant at 105°C to constant weight.

[0054] The expansion power (SP) and solubility (S) are calculated as follows: SP = m p / (m0-m s )x100%;S=m s / m0x100% (where: m0: sample dry weight / g; m s : Weight after drying / g; m p : Precipitation weight / g. ) 3. Mix 0.1 g starch with 10 ml distilled water to prepare a 1% suspension. Then heat the suspension in boiling water for 30 min and cool to room temperature. Then measure the transmittance of the starch paste at 650 nm with distilled water as a blank.

[0055] 4. The gelatinization properties of starch samples were measured using a fast Visco analyzer. Pour 3 g of corn starch into a jar and add 25 mL of distilled water to mix thoroughly. The mixture was stirred at 960 r / min for 10 s, and then changed to 160 r / min during the rest of the analysis. Test procedure: keep at 50°C for 1 min, then increase to 95°C at a rate of 6°C / min, keep the temperature at 95°C for 2.5 min, then cool to 50°C at a rate of 6°C / min and keep for 2 min. The total time for the whole process was 13 min. The peak viscosity (PV), valley viscosity (TV), disintegration value (BD), final viscosity (FV) and regeneration value (SB) were recorded.

[0056] 5. The rheological properties of starch samples were tested using an MCR302 rheometer. The starch paste obtained from RVA was pressed with a flat plate (P40) for 5 min to balance the stress. The angular frequency range was 0.1~100 rad / s and the strain was 1%. The storage modulus (G') and loss modulus (G'') were measured as a function of angular frequency.

[0057] Table 5 Oil absorption, water absorption, swelling power, solubility and transmittance of modified corn starch after static magnetic field treatment

[0058] Results: From Table 5, it can be seen that the oil absorption rate of GC starch showed a trend of first decreasing and then increasing, while the water absorption rate showed a trend of increasing all the time. The oil absorption rate of corn starch treated with SMF for 36 hours was 35.00% lower than that of natural corn starch and 10.49% lower than that of the control group (P<0.05). This is because when the corn germinated from 0 to 36 hours, the starch granules were damaged after SMF treatment, and the surface became rougher, thereby reducing the oil absorption rate; at 48 to 72 hours, many pores appeared in the starch granules after SMF treatment, allowing oil molecules to enter the pores; in addition, through the action of SMF, the capillary attraction of oil molecules was also increased, so that the oil molecules were trapped in the channels, pores and gaps between the starch granules through capillary action, resulting in an increase in oil absorption rate. SEM also proved this. It shows that starch with low oil absorption rate has strong resistance to digestion. During the whole process of germination, the water absorption rate has been rising because the surface area of ​​starch granules continues to increase, increasing the contact area between water molecules and starch. GC starch treated with SMF produced a larger effective surface area in its starch granules, which facilitated more interactions between water molecules and starch chains, resulting in a higher water absorption capacity. Compared with native starch, the water absorption rate increased by 26.86% (P<0.05), indicating that SMF treatment reduced the external structural density of starch granules and formed cavities on the surface of starch granules, thereby accelerating water penetration.

[0059] Swelling power and solubility are important indicators for evaluating starch properties. Swelling power reflects the degree of starch absorption of water dissolution; while solubility reflects the degree of starch solubility, which refers to the straight-chain starch leached during the heating process, which is affected by starch composition and particle size. The integrity of starch granules and the interaction between starch chains in the amorphous and crystalline states are two important factors affecting starch swelling capacity and solubility. It can be seen from Table 5 that the solubility and swelling power of GC starch show a trend of first decreasing and then increasing with the increase of germination time. However, GC starch treated with SMF has a further effect on swelling power and solubility. When corn germinated from 0h to 36h, after SMF treatment, the swelling power and hydrolysis degree of starch were further reduced, which may be affected by the degree of starch crystallization. Starch with high crystallinity may inhibit the swelling power and solubility of starch granules. XRD proved this result. On the other hand, it may be that after SMF treatment, the interaction between amylose and branched starch in starch is enhanced, and the crystal and double helix are rearranged, which leads to the reduction of starch swelling power. When corn germinated from 48h to 72h, after SMF treatment, the swelling power and solubility of starch increased instead. This trend is consistent with the results of digestion, DSC and XRD. The reason for the further increase may be that the germination time is too long and the further treatment of SMF destroys the structure of starch, causing branched starch to break in the process to form shorter amylose chains, so that amylose achieves better leaching effect, so that starch absorbs more water. It may also be due to the increase of starch pores and the interference of amorphous regions, so that more water molecules penetrate into the pores of starch granules, so that both increase, which is proved by SEM. This indicates that starch with low swelling power and solubility is more difficult to digest.

[0060] Table 5 shows that the transmittance of GC starch increases first and then decreases. The content of amylose, the size of starch granules and the ratio of amylose to amylopectin will affect the transmittance of starch paste. When corn germinates for 0h-36h, the transmittance of starch is further reduced after SMF treatment, indicating that the GC starch treated with SMF rearranges the starch molecules into crystals, thereby reducing the transmittance; when germination is 48h-72h, the transmittance of starch increases after SMF treatment. This result is consistent with the results of some tests such as digestion, infrared, XRD and DSC, proving that the transmittance of starch is positively correlated with amylose and negatively correlated with ΔH. This shows that starch with low transmittance is not easy to digest.

[0061] Table 6 Gelatinization parameters of modified corn starch after static magnetic field treatment

[0062] The RVA curve reflects the process of starch suspension viscosity changing with temperature. Pasting is a thermal process. The increase of shear force and water molecule activity can promote the water swelling capacity of starch and the leaching of amylose, thereby further accelerating the water swelling capacity to form pasting. Figure 6 Table 6 shows that the viscosity of corn in the whole germination process shows a trend of first increasing and then decreasing. When corn germinates for 0-36h, after SMF treatment, the peak viscosity, valley viscosity and final viscosity of starch are further increased. This stage shows that during the process of heating and stirring the starch in the water medium, SMF makes the starch more resistant to swelling, thereby improving the gelatinization degree of starch. In addition, it may rearrange the starch molecular chain, enhance the long-range and short-range ordered molecular structure, crystallinity and stability, thereby increasing the viscosity of starch. At 48-72h of germination, the peak viscosity, trough viscosity, final viscosity and retrogradation value of GC starch treated with SMF were further reduced. The reduction in peak viscosity was due to the change in the covalent bonds between amylose and amylopectin caused by SMF treatment, which led to the expansion of starch granules and reduced the thermal shear breakage rate. The retrogradation value was the difference between the final viscosity and the trough viscosity, reflecting the anti-aging properties of starch. SMF may have weakened the interaction between starch molecules, reduced the steric hindrance between starches, and led to a decrease in the retrogradation value. This proves that starch treated with SMF at this germination stage (including 24h and 36h) is not easy to age, improves the cold paste stability of starch, and has better viscosity stability.

[0063] Usually, the linkage and interaction forces between starch molecules are characterized by dynamic rheological measurements, and the corresponding parameters such as G' and G'' are used to reflect the toughness of the gel network and the organization after pasting. Figure 7It can be seen that whether it is GC starch or GC starch treated with SMF, their G' and G'' increase with the increase of amplitude frequency, indicating that these starches can easily form a gel network; the G' of all samples is greater than G'', indicating that these starches are typical weak gels. During the germination of corn, G' and G'' both tend to rise first and then fall. From 0 to 36 hours of corn germination, after SMF treatment, G' and G'' both increased further, which may indicate that SMF promotes the entanglement and interaction of starch molecular chains, increases the helical structure and crystal structure of starch, forms a strong gel network structure, and leads to a decrease in the content of amylose in starch and an increase in the ability to form gel; it is also possible that SMF treatment destroys the structure of the amorphous region of starch, allowing more water molecules to enter the starch granules, promoting gelatinization, increasing the interaction between starch molecules, making the entanglement more stable, and forming a network structure with stronger viscoelasticity. This indicates that starch with higher G' and G'' is not easy to digest, which is consistent with the DSC results. When corn germinated from 48 to 72 hours, G' and G'' were further reduced after SMF treatment. This may be because SMF treatment restricted the rearrangement of amylose molecules, reducing the elastic properties of starch. This shows that SMF may interfere with the formation of hydrogen bonds between starch chains at this germination stage, thereby reducing the stability of its gel network structure.

[0064] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0065] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing modified corn starch, characterized in that: The specific steps include: S1. Pretreatment of corn kernels: Select corn kernels with full grains and disinfect them. After disinfection, rinse and soak them in water for later use; S2. Static magnetic field pretreatment: The soaked corn kernels are pretreated with a static magnetic field with a magnetic field intensity of 10-50 mT, a treatment time of 1-4 h, and a treatment temperature of 23-27 °C; S3. Corn kernel germination: Place corn kernels treated with a static magnetic field into a germination tray, add a small amount of distilled water so that the embryo of the corn kernels is completely in contact with the distilled water, and germinate at an air humidity of 25-75% for 24-48 hours. S4. Extraction of germinated corn starch: Grind the germinated corn kernels with water, sieve, wash, and stand, remove the supernatant, resuspend the precipitate with water, and centrifuge to obtain germinated corn starch; S5. Remove the upper yellow substance, centrifuge, dry the separated starch at 40-50°C for 14-24 hours, grind it, and pass it through a 100-mesh sieve to obtain modified corn starch.

2. The method for preparing modified corn starch according to claim 1, characterized in that: The magnetic field intensity in step S2 is 20-30 mT, and the processing temperature is 25°C.

3. The method for preparing modified corn starch according to claim 2, characterized in that: The magnetic field intensity in step S2 is 30 mT, and the processing time is 2 h.

4. The method for preparing modified corn starch according to claim 3, characterized in that: The air humidity in step S3 is 75% and the germination time is 36 hours.

5. The method for preparing modified corn starch according to claim 4, characterized in that: In step S4, the germinated corn kernels and distilled water are added in a volume ratio of 1:2 and then water-milled.

6. The method for preparing modified corn starch according to claim 5, characterized in that: The sieving in step S4 is performed by sieving through 100 mesh and 200 mesh sieves in sequence.

7. The method for preparing modified corn starch according to claim 6, characterized in that: The disinfection in step S1 uses 0.1% sodium hypochlorite solution; after the rinsing, the pH is adjusted to neutral.

8. The method for preparing modified corn starch according to claim 7, characterized in that: The centrifugal conditions in step S4 are: centrifugal speed 6000 r / min, centrifugal time 20 min.

9. The method for preparing modified corn starch according to claim 8, characterized in that: The number of centrifugation in step S5 is 3 times; the drying temperature is 45° C. and the drying time is 24 hours.

10. A modified corn starch, characterized in that: The modified corn starch is prepared by the preparation method of any one of claims 1 to 9.

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