Starch modification method adopting radio frequency dry-heat combined annealing and application of starch modification method

Abstract

The invention discloses a radio frequency dry-heat combined annealing starch modification method, which comprises the following steps: heating starch in an RF system, cooling to room temperature, uniformly mixing the starch and distilled water according to a certain proportion, and then carrying out ANN treatment in a water bath. And finally, centrifuging the mixture, collecting the precipitate, and drying by using a freeze dryer. The method can increase the particle size of the starch, reduce the swelling force of the starch, reduce the sensitivity of the starch to amylase, enhance the crystal structure of starch particles, improve the short-range ordered structure of the starch, improve the thermal stability of the starch and improve the thermal capacity of the starch.

Description

Technical Field

[0001] The invention relates to the field of starch modification, and in particular to a starch modification method of radio frequency dry heating combined with annealing and an application thereof. Background Art

[0002] As the public becomes increasingly aware of the threat of chronic diseases to human health, health-promoting foods with low glycemic index (GI) and low digestibility properties are gaining more and more attention. Rice (Oryza sativa L.) is one of the main sources of carbohydrates, which are the main energy source in most people's diets. However, compared with other starchy foods, rice and rice-based foods are easily digested by the human gastrointestinal tract and result in a high GI, which may increase the risk of diet-related diseases such as type 2 diabetes, obesity, and cardiovascular disease. Therefore, reducing the digestibility of rice foods is of great significance for improving health foods. Starch is the main component of rice, accounting for 60% to 80% of its mass. The academic community divides starch into three categories according to its digestibility: rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS). SDS and RS help avoid the risk of rapid blood sugar increases caused by the continuous release of glucose, thereby reducing the burden on the body's blood sugar regulation system. Therefore, changing the composition of starch by increasing the content of SDS or RS to reduce the digestibility of starch has attracted increasing attention.

[0003] Therefore, it is urgent to develop a treatment and modification method that can significantly reduce the swelling power of starch, but increase the particle size, solubility and amylose content of starch, as well as significantly reduce the gelatinization viscosity and increase the gelatinization transition temperature, that is, significantly improve the gelatinization and thermal stability. Summary of the invention

[0004] The object of the present invention is to provide a method for modifying starch by radio frequency dry heating combined with annealing in view of the shortcomings of the prior art, comprising:

[0005] The starch was heated in the RF system, cooled to room temperature, and then mixed with water, followed by ANN in a water bath. The mixture was then centrifuged, and the precipitate was collected and dried using a freeze dryer.

[0006] Furthermore, the RF system is 1-15 kW, 15-40 MHz, for example, 5 kW, 27.12 MHz.

[0007] Further, the starch was heated in a 5 kW, 27.12 MHz RF system for 1-3 h.

[0008] Furthermore, the ratio of starch to water is starch / water = 1-3:2-15, w / v. For example, starch / water = 1:6, w / v.

[0009] Further, the container for uniformly mixing starch and water and the water bath is an organic bag, such as a polyethylene bag.

[0010] Further, annealing treatment is performed in a water bath at 40-65° C. for 6-18 h, for example, annealing in a water bath at 55° C. and ANN treatment for 12 h.

[0011] Furthermore, the starch is rice starch, corn starch, wheat starch, or potato starch, preferably rice starch.

[0012] Technical effects: The digestibility, physicochemical properties and multi-scale structure of rice starch were controlled by radio frequency dry heat treatment and annealing (RF-DHT-ANN). After treatment, the swelling power of starch was significantly reduced, but the particle size, solubility and straight chain starch content of starch were increased. In addition, RF-DHT-ANN treatment reduced the content of rapidly digestible starch, but increased the content of slowly digestible starch and rice starch. Compared with other starches, the gelatinization viscosity of the modified starch treated with RF-DHT-ANN was significantly reduced, but the gelatinization transition temperature was increased, that is, the gelatinization and thermal stability were significantly improved.

[0013] Explanation of terms

[0014] Annealing (ANN) is a hydrothermal treatment that can improve the thermal stability and physicochemical properties of starch. Excessive annealing treatment between the glass transition temperature and gelatinization temperature of starch may destroy the most unstable structure of starch, resulting in recrystallization or rearrangement of starch molecular chains. ANN can also enhance the gelatinization properties of starch, reduce its swelling power and solubility, and enhance the microcrystalline structure of starch, making it difficult for enzymes to penetrate and hydrolyze starch inside, thereby reducing the starch hydrolysis rate. However, the effect of ANN on starch varies depending on the source. It has a significant effect on some starches, but not much on others, probably due to the interweaving of starch chains and amylopectin, which hinders the movement and rearrangement of starch, thereby limiting the effectiveness of annealing treatment. In addition, the rigid structure of native starch granules may limit the efficiency of annealing.

[0015] Dry heating treatment (DHT) is another environmentally friendly starch modification method. It can change the physicochemical properties of starch without destroying the starch granule structure. Studies have found that dry heat treatment improves the relative crystallinity, solubility and paste properties of quinoa starch. The digestibility of potato starch and yam starch was found to decrease after DHT. Therefore, dry heat treated starch is expected to achieve better starch structure reorganization and rearrangement during the subsequent annealing process and significantly inhibit starch hydrolysis. Compared with traditional DHT, radio frequency (RF) heating has the advantages of volumetric heating, deep penetration, high efficiency and suitability for heating low-moisture materials. It has high heating efficiency and low energy consumption, which is more in line with the concept of energy conservation and emission reduction. Radio frequency electromagnetic waves can cause oscillation and migration of polar molecules in starch, which may change the formation of starch molecular chains and thus change the crystalline structure of starch. Recently, studies have reported that RF energy changes the physicochemical properties and multi-scale structure of rice starch, buckwheat starch and sweet potato starch.

[0016] The starch treated with dry heat and annealing was labeled as RS-RFD and RS-A, respectively, and the starch treated with RF-DHT-ANN was labeled as RS-RFD-A. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a diagram of the radio frequency dry heat combined annealing equipment;

[0018] Figure 2 The microscopic morphology of starch granules under different treatment conditions;

[0019] Figure 3 The infrared structure, XRD, DSC and gelatinization curves of starch treated differently;

[0020] Figure 4 This is a diagram of the particle size of starch in different treatment groups;

[0021] Figure 5 Comparison of starch solubility and tension in different treatment groups;

[0022] Figure 6 The change of total hydrolysis degree of samples with different treatments over time. DETAILED DESCRIPTION

[0023] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments, and the contents mentioned in the implementation modes are not intended to limit the present invention.

[0024] Example 1

[0025] A method for modifying starch by radio frequency dry heating combined with annealing, comprising:

[0026] The starch was heated in the RF system, cooled to room temperature, and then mixed with distilled water in a certain ratio and then treated with ANN in a water bath. Finally, the mixture was centrifuged, and the precipitate was collected and dried using a freeze dryer.

[0027] Furthermore, the starch was heated in the RF system for 2 h.

[0028] Further, after cooling to room temperature, starch and distilled water were uniformly mixed in a polyethylene bag.

[0029] Further, the annealing treatment was carried out in a water bath for 10 h.

[0030] Furthermore, the starch is rice starch.

[0031] Furthermore, the RF system is 5kW, 27.12MHz.

[0032] Furthermore, the ratio of starch and water uniformly mixed is starch / water=1:6, w / v.

[0033] Further, annealing treatment was performed in a water bath at 55°C.

[0034] Example 2

[0035] A method for modifying starch by radio frequency dry heating combined with annealing, comprising:

[0036] The starch was heated in the RF system, cooled to room temperature, and then mixed with distilled water in a certain ratio and then treated with ANN in a water bath. Finally, the mixture was centrifuged, and the precipitate was collected and dried using a freeze dryer.

[0037] Furthermore, the starch was heated in the RF system for 1 h.

[0038] Further, after cooling to room temperature, starch and distilled water were uniformly mixed in a polyethylene bag.

[0039] Furthermore, the annealing treatment was carried out in a water bath for 6 h.

[0040] Furthermore, the starch is rice starch.

[0041] Furthermore, the RF system is 1 kW, 40 MHz.

[0042] Furthermore, the ratio of starch and water uniformly mixed is starch / water=1:15, w / v.

[0043] Further, annealing treatment was performed in a water bath at 40°C.

[0044] Example 3

[0045] A method for modifying starch by radio frequency dry heating combined with annealing, comprising:

[0046] The starch was heated in the RF system, cooled to room temperature, and then mixed with distilled water in a certain ratio and then treated with ANN in a water bath. Finally, the mixture was centrifuged, and the precipitate was collected and dried using a freeze dryer.

[0047] Furthermore, the starch was heated in the RF system for 3 h.

[0048] Further, after cooling to room temperature, starch and distilled water were uniformly mixed in a polyethylene bag.

[0049] Furthermore, the film was annealed in a water bath for 12 h.

[0050] Furthermore, the starch is wheat starch.

[0051] Furthermore, the RF system is 15kW, 15MHz.

[0052] Furthermore, the ratio of starch and water uniformly mixed is starch / water=3:2, w / v.

[0053] Further, the container for uniformly mixing starch and water and the water bath is an organic bag, such as a polyethylene bag.

[0054] Further, annealing treatment was performed in a water bath at 65°C.

[0055] Comparative Example

[0056] A single radio frequency dry heat treatment (RF-DHT) and a single annealing treatment (ANN) were used as control groups, and other steps and parameters were exactly the same as those in Example 1.

[0057] Amylose content

[0058] Rice starch modified with RF-DHT-ANN has a higher amylose content, and starch with a high amylose content has a higher digestibility resistance.

[0059] Particle size distribution

[0060] The particle size of rice starch is closely related to its functional properties such as gelatinization, stability, and gel-forming ability. During the RF-DHT-ANN modification process, the partial gelatinization of starch granules and the improvement of the interaction between amylose and amylopectin led to the aggregation of starch granules, thereby increasing the granule volume, indicating that the RF-DHT-ANN modification treatment has a better inhibitory effect on starch hydrolysis.

[0061] Swelling power and solubility

[0062] Accurately weigh 0.1 g of starch sample (m) in a 50 mL centrifuge tube, and then record the total mass of the sample and the centrifuge tube (W 1). The starch sample was then dispersed in 5 mL of distilled water and shaken in a water bath. The starch dispersion was heated at different temperatures of 50, 60, 70, 80, and 90 °C for 30 min. Finally, the mixture was cooled to room temperature and centrifuged at 3500 r / min for 10 min. The supernatant was poured into an aluminum pan and kept at 105 °C for 8 h. The total mass of the centrifuge tube and the precipitate (W) was recorded. 2 ). The residue obtained after drying the supernatant represents the amount of starch dissolved in water. The swelling power is calculated as follows:

[0063] F=(W 1 -W 2 ) / m (1)

[0064] Where F is the swelling force, W is 1 is the total mass of the sample and the centrifuge tube, W 2 is the total mass of the precipitate and the centrifuge tube, and m is the mass of the starch weighed.

[0065] The experimental results showed that starch treated with RS-RFD-ANN showed higher solubility compared with untreated starch.

[0066] Microscopic morphology of particles

[0067] Normal rice starch granules are irregular polyhedrons with sharp edges and angles, and some grooves and cracks on the surface. Unlike natural starch, the edges and corners of starch granules were eroded after annealing, and the starch structure was reorganized. In contrast, the surface structure of starch after RF-DHT-ANN (magnified 1500 and 6000 times) did not change significantly, while some holes and cracks existed on the starch surface.

[0068] In vitro digestibility

[0069] The starch solution (150 mL) was added to 10 mL of sodium acetate buffer (pH 5.2, concentration 0.1 mol / L) and heated in a boiling water bath for 15 min to completely gelatinize it. The gelled sample was incubated in a 37°C water bath with continuous shaking for 10 min. Then 10 mL of pepsin solution was added to the reaction mixture and incubated at 37°C for 30 min. After that, 5 mL of an enzyme mixture containing trypsin and amyloglucosidase was added to the reaction mixture and incubated at 27°C for 120 min. 1 mL of the hydrolyzed solution was collected at different time intervals (0, 20 and 120 min), mixed with 4 mL of anhydrous ethanol, and then centrifuged at 5000 r / min for 10 min. The glucose content released in the supernatant was determined using a glucose oxidase peroxidase kit. The contents of RDS, SDS and RS were calculated as follows:

[0070] RDS(%)=(G 20 -G0 )×0.9×100 / TS (2)

[0071] SDS(%)=(G 120 -G 20 ) ×100 / TS (3)

[0072] RS(%)=(1-RDS-SDS)×100 (4)

[0073] Among them G 0 Indicates the amount of free glucose, G 20 and G 120 They represent the amount of glucose released after 20 min and 120 min of hydrolysis, respectively, and TS is the total starch content.

[0074] RF-DHT-ANN significantly reduced the RDS content and increased the RS content, but the SDS level remained unchanged, indicating that RF-DHT-ANN reduced the sensitivity of starch to amylase. The higher SR and lower RDS may be attributed to the rearrangement of starch structure and the improvement of interactions in starch chains during the RF-DHT-ANN process.

[0075] Analysis of the short-range ordered structure of starch

[0076] Fourier transform infrared spectroscopy analysis of the close-range ordered structure was performed using the potassium bromide tablet method. The starch sample was mixed with potassium bromide (1 / 100, w / w), and the mixture was thoroughly ground and then pressed into a transparent tablet, and the Fourier transform infrared pattern was collected in a dark room. The acquisition resolution was 4 cm -1 , scanning range 400-4000cm -1 The sample was scanned 32 times. The spectrum was analyzed using Peakfit4.12 software. The changes in the short-range ordered structure of starch were analyzed by the absorbance ratio 1047 / 1022 (R 1047 / 1022 ) is used to calculate the changes in the short-range ordered structure of starch. The changes in the short-range ordered structure of starch are usually measured by infrared spectroscopy in the range of 1200-800 cm -1 In addition, 1047cm -1 and 1022cm -1 The absorbance of represents the crystalline structure and amorphous region of starch respectively, and the ratio can reflect the degree of short-range structure in starch.

[0077] RS-RFD-A R 1050 / 1020 is 1.53. During the ANN treatment, the synergistic effect of moisture and heat energy destroyed the ordered double helical structure of starch molecules, which can be attributed to R 1050 / 1020However, contradictory effects of ANN treatment on the short-range ordered structure of starch were observed. In addition, during the drying and heating process, high temperature may destroy some weak hydrogen bonds in starch molecules, which may cause the dissociation of the ordered double helical structure in the crystalline region, resulting in the decrease of 1050-1020cm -1 The short-range ordered structure is significantly improved.

[0078] X-ray diffraction analysis of crystal structure

[0079] The diffraction intensity was collected from scanning angles of 5° to 40° (2θ). The areas of the crystalline peaks and amorphous regions were integrated by Organic software. The combined treatment did not change the crystal type of rice starch, but the diffraction intensity changed after the modification treatment. The characteristic peak of RF-DHT-ANN modified starch at 20.2° (2θ) was significantly weakened in the figure (P<0.05).

[0080] Differential Scanning Calorimetry (DSC) Analysis

[0081] Starch (3 mg) and distilled water (9 mg) were added to an aluminum crucible. The crucible was sealed and equilibrated (4 °C, 12 h) to allow starch and water to be fully mixed. The scanning temperature range and heating rate were 25-120 °C and 10 °C / min, respectively. The starting temperature (T 0 ), peak temperature (T p ), end temperature (T c ) and gelatinization enthalpy (ΔH).

[0082] The exothermic peak of the RF-DHT-ANN modified starch sample shifted to the right relative to the native starch, indicating that the modification changed the thermal properties of rice starch. The ΔH of the native starch and RS-RFD-A were 1.80 and 1.47, respectively. Compared with the native starch, the ΔH after modification was significantly decreased (P<0.05).

[0083] Sticky properties

[0084] The starch sample (3 g, dry basis) and distilled water (25 g) were mixed in an aluminum can to form a dispersion. The gelatinization temperature (GT), peak viscosity (PV), final viscosity (FV), through viscosity (TV), breakdown, setback (SB) and peak time (PT) were recorded.

[0085] Native and modified rice starches showed significantly different gelatinization properties, which indicated that the modification treatments altered the gelatinization properties of rice starch. RS-RFD-A showed an improvement in GT compared to both normal rice starch and RS-RFD, indicating an increase in the temperature required for starch gelatinization after the combined treatments. RS-RFD showed a higher PV compared to normal rice starch (1105.00 cP and 1286.02 cP). After ANN treatment and RS-RFD-ANN treatment, the PV (609.00 and 562.00 cP) and FV (866.34 and 797.67 cP) of starch decreased significantly compared to native starch. Through-flow viscosity (TV) is the lowest viscosity at the terminal heating stage. The results showed that the TV of RS-RFD increased compared to native, but ANN and dual treatments significantly reduced TV (P<0.05). RS-RFD showed higher BD and SB than its native counterpart, and there were no significant changes in peak time (PT) and gelatinization temperature (GT) (P>0.05).

[0086] The lower paste viscosity and higher gelatinization temperature of the starch samples treated with RF-DHT-ANN indicated that the starch granule structure was more enhanced and the intramolecular bonding force was more strongly strengthened, which led to the improvement of the gelatinization stability of the dual-treated starch samples.

[0087] Experimental results and conclusions:

[0088] (1) Amylose content

[0089] The amylose contents of native starch, ANN modified starch, RF-DHT modified starch and RF-DHT-ANN modified starch were 181.26 mg / g, 168.60 mg / g, 202.29 mg / g and 187.47 mg / g, respectively.

[0090] (2) Particle size distribution

[0091] The particle size of ordinary rice starch ranged from 1039-2211 nm, while the particle sizes of RS-A, RS-RFD, and RS-RFD-A were 1209-2571, 1209-2571, and 1635-3477 nm, respectively.

[0092] (3) Swelling power and solubility

[0093] The solubility and swelling power of all starch samples seem to increase with increasing temperature, obtaining the highest values ​​at 90 °C. High temperatures weaken the structure of starch granules, accelerate the leaching of amylose, and promote the swelling of granules. The solubility of modified starches is higher than that of native starch at all tested temperatures. In contrast, the swelling power of modified starches is lower than that of regular rice starch.

[0094] RS-RFD showed higher solubility compared to untreated starch, which may be related to the increased extraction of amylose during RF-DHT. RF-DHT significantly increased the solubility of starch compared to ANN and RF-DHT (P < 0.05), indicating that RF-DHT and ANN treatments had a synergistic effect.

[0095] In contrast, modified starches showed lower swelling power than native starch. RF-DHT showed the lowest swelling power (12.15 g / g) at 90 °C compared to single modification treatments. The possible reason is that high temperature degraded α-1,4 and α-1,6 glucose bonds and increased the molecular structure during the RF-DHT process. Subsequently, these modified structures and small components were easily reorganized during the ANN treatment, thereby regulating the swelling power of starch.

[0096] (4) Particle micromorphology

[0097] Unlike native starch, the corners of starch granules were eroded and the starch structure was reorganized after annealing. These phenomena indicate that ANN treatment destroyed the micrographs of starch granules, which may be attributed to the gelatinization of some starch granules during annealing treatment, leading to the collapse of starch structure. On the other hand, a possible reason is that sufficient moisture accelerates the rearrangement of starch chains. The surface structure of starch after RF-DHT (1500 and 6000 magnifications) did not change significantly, while some holes and cracks existed on the starch surface. The appearance of these holes and cracks may be due to the fact that high temperature accelerates the reorganization of amylose and amylopectin, leading to the collapse of starch granules. RF-DHT showed a surface structure similar to that of single annealing treatment. The results show that the effect of ANN treatment on the micromorphology of rice starch granules is more obvious.

[0098] (5) In vitro digestibility

[0099] The total hydrolysis rate of all starch samples increased with the extension of digestion time. The hydrolysis rate of all starch samples was the highest in the first 20 min. Because RDS hydrolyzes rapidly in the initial hydrolysis, this results in a higher hydrolysis rate for starch. The total hydrolysis rate of native starch in the first 20 min reached 70.08%, which was significantly higher than that of modified starch (P<0.05), indicating that the modification treatment reduced the sensitivity of the starch molecules to enzymatic hydrolysis.

[0100] Compared with native starch after ANN treatment, RS-A had lower RDS but higher SDS and RS. RF-DHT significantly reduced the RDS content and increased the RS content, but the SDS level remained unchanged, indicating that RF-DHT reduced the sensitivity of starch to amylase. Based on these findings, it was concluded that RF dry heat combined with annealing treatment significantly reduced the in vitro digestibility of rice starch and decreased the RDS level, but increased the SDS and RS content. The synergistic treatment had a better effect in reducing starch digestibility than the single modification treatment.

[0101] (6) Crystal structure

[0102] Natural starch and modified starch have a typical α-type crystalline structure with main diffraction peaks at approximately 15°, 17°, 18°, 20°, and 23° (2θ). Neither ANN nor RF-DHT changed the crystal type of rice starch. However, the diffraction intensity changed after modification treatment. The RCs of natural, ANN, RF dry heat, and dual treatments were 28.36%, 27.96%, 25.10%, and 28.38%, respectively. ANN and RF dry heat treatments reduced the RC of starch, but there was no difference between the dual treatment and the natural treatment (P>0.05).

[0103] It is noteworthy that the RC of RS-RFD-A showed higher RC than the starches treated with single modification. It can be speculated that the small fraction of starch chains produced by high temperature destruction was reorganized and rearranged during ANN treatment, thereby increasing the RC of starch. This result indicates that ANN and RF-DHT have a synergistic effect and enhance the crystalline structure of starch granules compared with single modification treatment.

[0104] (7) Infrared spectroscopy analysis

[0105] No new characteristic peaks appeared or disappeared in all modified starch samples, and none of the three treatments significantly changed the pattern of FT-IR spectra (P < 0.05), indicating that no new functional groups or covalent bonds were formed in the modified rice starch. -1 / 1022cm -1 The proportion has changed.

[0106] R of natural, RS-A, RS-RFD and RS-RFD-A 1050 / 1020 Compared with native starch and RS-RFD-A, the R 1050 / 1020The R1050 / 1020 of RS-RFD-A was higher than that of RS-A and RS-RFD, which indicated that the synergistic treatment of RF dry heating and annealing significantly improved the short-range ordered structure compared with the single modification treatment.

[0107] (8) Thermal performance analysis

[0108] The modified starch showed a higher T than the native starch. 0 、T p and T c values, indicating that the modification treatment increased the transition temperature of starch. This result may be attributed to the enhanced interaction between amylose and amylopectin during the modification process, which limits the mobility of the amylopectin chain and leads to the transition temperature. Compared with the single treatment, the double treatment significantly increased the transition temperature of rice starch, indicating that RF dry heating and annealing treatment have a synergistic effect. Gelatinization enthalpy (ΔH) represents the ratio of crystalline region to non-crystalline region in the double helix structure of starch. The ΔH of native starch, RS-A, RS-RFD and RS-RFD-A were 1.80, 1.29, 1.41 and 1.47, respectively. Compared with native starch, the ΔH after modification treatment decreased significantly (P<0.05).

[0109] However, the combined treatment showed a higher ΔH compared with the ANN treatment and RF-DHT treatment. The possible reason is that the double helical structure destroyed by high temperature was reassembled in the subsequent ANN treatment. These observations suggest that the combined treatment has a better effect in improving the thermal stability of starch than the single treatment.

[0110] (9) Sticky properties

[0111] RS-RFD showed higher PV (1199.67 cP) and FV (1329.33 cP) compared to normal rice starch (1105.00 cP and 1286.02 cP). RS-RFD-A had the lowest PV, which indicated that the combination of RF-DHT-ANN had a synergistic effect on the improvement of starch thermal capacity.

[0112] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0113] It should be understood that the detailed description of the technical solutions of the present invention by means of the preferred embodiments is illustrative rather than restrictive. A person skilled in the art may modify the technical solutions described in the embodiments, or replace some of the technical features by equivalents, based on reading the specification of the present invention; and these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for modifying starch by radio frequency dry heating combined with annealing, characterized in that: include: The starch was heated in the RF system, cooled to room temperature, and then mixed with distilled water in a certain ratio and then treated with ANN in a water bath. Finally, the mixture was centrifuged, and the precipitate was collected and dried using a freeze dryer.

2. The method according to claim 1, characterized in that The starch was heated in the RF system for 1-3 h.

3. The method according to claim 1, characterized in that After cooling to room temperature, starch was evenly mixed with distilled water in a polyethylene bag.

4. The method according to claim 1, characterized in that: Anneal in a water bath for 6-12 h.

5. The method according to claim 1, characterized in that The starch is rice starch, corn starch, wheat starch, or potato starch, preferably rice starch.

6. The method according to claim 1, characterized in that RF systems are 1-15kW, 15-40 MHz.

7. The method according to claim 1, characterized in that Starch / water=1-3:2-15, w / v.

8. The method according to claim 1, characterized in that Annealing in a water bath at 40-65°C.

9. A modified starch prepared by the method according to any one of claims 1 to 8.

10. A method for modifying starch by radio frequency dry heating combined with annealing, for increasing the content of amylose, and / or increasing the particle size of starch, and / or reducing the swelling power of starch, and / or reducing the sensitivity of starch to amylase, and / or enhancing the crystalline structure of starch granules, and / or improving the short-range ordered structure of starch, and / or increasing the thermal stability of starch, and / or increasing the thermal capacity of starch, the method being the method of any one of claims 1-8.

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