Preparation method of flaky-structure sweet potato III-type resistant starch

Abstract

The invention belongs to the technical field of food processing, and relates to a preparation method of flaky sweet potato type III resistant starch. The flaky-structure sweet potato III-type resistant starch (sweet potato RS3) is prepared by combining microwave gelatinization pretreatment with an ultrasonic-enzyme method. Meanwhile, the sweet potato RS3 with the porous structure is prepared by combining hydrothermal or pressure heat gelatinization pretreatment with an ultrasonic-enzyme method. Through comparison, compared with the sweet potato RS3 with the porous structure, the sweet potato RS3 with the scaly structure has the highest short-range order degree and crystallinity, the maximum amylose content, gelatinization enthalpy and resistant starch content, the lowest solubility, expansion potential and starch hydrolysis rate, and the strongest anti-digestion capability; and the special processing requirements of type 2 diabetes patients and obese people on food can be met.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of food processing and relates to a preparation method of sweet potato type-III resistant starch with a scale structure. BACKGROUND

[0002] Resistant starch (RS) is also called enzyme-resistant starch or indigestible starch. It cannot be enzymatically hydrolyzed in the small intestine of healthy human body, but can be fermented by intestinal microorganisms in the colon to produce short-chain fatty acids. Compared with ordinary dietary fiber, resistant starch has higher physiological activity. Type-III resistant starch, which is also called retrograded starch, is formed by recrystallization of gelatinized starch after cooling. Type-III resistant starch has high safety. Due to its good functions and safety characteristics, type-III resistant starch is widely used in the food industry and has become a research hotspot.

[0003] In recent years, the research on type-III resistant starch mainly focuses on the preparation process and physicochemical characterization. The influence of different gelatinization pretreatments on the structure and physicochemical properties of type-III resistant starch under the same gelatinization degree and how this influence acts on the anti-digestive ability of type-III resistant starch have not been fully explored. Different gelatinization pretreatments can reshape the arrangement and crystalline structure of starch molecules, and then form type-III resistant starch with different structures. The structural differences will lead to different interaction modes of type-III resistant starch with digestive enzymes, and finally affect the anti-digestive ability. However, this key field has been lack of in-depth research for a long time, which restricts the precise development and efficient application of functional foods. SUMMARY

[0004] In view of the above problems and defects, the application provides a preparation method of sweet potato type-III resistant starch with a scale structure. The preparation method of the application uses microwave gelatinization pretreatment combined with ultrasonic-enzyme method to prepare sweet potato type-III resistant starch with a scale structure. The type-III resistant starch with a scale structure has strong anti-digestive ability, which provides a theoretical basis for realizing the differential preparation of type-III resistant starch with specific structures to improve the anti-digestive ability of starch.

[0005] In a first aspect, the application provides a preparation method of sweet potato type-III resistant starch with a scale structure. The preparation method uses microwave gelatinization pretreated sweet potato starch. After gelatinization, the sweet potato starch is subjected to ultrasonic treatment to obtain starch milk. After enzymatic hydrolysis of the starch milk, cold storage, freeze-drying, crushing and sieving are performed to obtain sweet potato type-III resistant starch with a scale structure.

[0006] Further, in the preparation method of sweet potato type-III resistant starch with a scale structure, the gelatinization degree of the microwave gelatinization pretreated sweet potato starch is 80%.

[0007] Further, in the preparation method of sweet potato type-III resistant starch with a scale structure, the power of the ultrasonic treatment is 40 W, and the time of the ultrasonic treatment is 16 min.

[0008] Further, in the preparation method of the scale structure sweet potato type III resistant starch provided by the application, the enzymolysis comprises: keeping the starch milk in a water bath at constant temperature, adding pullulanase, enzymolysis, and boiling water bath for enzyme inactivation after the enzymolysis is completed.

[0009] Further, in the preparation method of the scale structure sweet potato type III resistant starch provided by the application, the constant temperature is 55 ℃. The enzyme activity of the pullulanase is 10 U / g. The enzymolysis temperature is 55 ℃, and the enzymolysis time is 4 h.

[0010] Further, in the preparation method of the scale structure sweet potato type III resistant starch provided by the application, the enzyme inactivation time is 10 min.

[0011] Further, in the preparation method of the scale structure sweet potato type III resistant starch provided by the application, the refrigeration temperature is 4 ℃, and the refrigeration time is 24 h.

[0012] Further, in the preparation method of the scale structure sweet potato type III resistant starch provided by the application, the mesh of the screening is 100 mesh.

[0013] In the second aspect, the application provides a scale structure sweet potato type III resistant starch, which is prepared by the preparation method of the scale structure sweet potato type III resistant starch.

[0014] Further, in the scale structure sweet potato type III resistant starch provided by the application, the gelatinization enthalpy of the scale structure sweet potato type III resistant starch is 23-26 J / g, the amylose content is 23%-24%, and the resistant starch content is 54%-55%.

[0015] Compared with the prior art, the technical scheme provided by the application has at least the following beneficial effects or advantages: (1) The scale structure sweet potato RS3 is prepared by the microwave pregelatinization combined with ultrasonic-enzyme method, and the hole structure sweet potato RS3 is prepared by the hydrothermal or autoclave gelatinization pretreatment combined with ultrasonic-enzyme method. Compared with the hole structure sweet potato RS3, the scale structure sweet potato RS3 (MRS) has the maximum amylose content and resistant starch content, which respectively reach 23.54% and 54.72%, the highest gelatinization enthalpy (ΔH) of up to 24.69 J / g, and the lowest starch hydrolysis rate, which is only 39.81% after 180 min of hydrolysis. ΔH

[0016] ​(2) The MRS of sweet potato showed the most compact granule structure, the highest double helix degree, thermal stability and amylose content, and the lowest solubility and swelling power. This resulted in the MRS of sweet potato having the relatively largest resistant starch content and the relatively lowest starch hydrolysis rate, which embodied the relatively strongest anti-digestive capacity, and was suitable for the processing needs of food for type 2 diabetes and obese people. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, a brief introduction will be given below to the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0018] Figure 1 The scanning electron microscope and polarizing microscope images of sweet potato starches with different structures. Among them, NS is the original sweet potato starch, HRS is the hydrothermal pre-gelatinized treatment of the hole structure of sweet potato type III resistant starch, MRS is the scale structure of sweet potato type III resistant starch, and ARS is the hole structure of sweet potato type III resistant starch treated by pressure pre-gelatinization. The images from left to right are 500x, 2000x and 5000x scanning electron microscope and 200x polarizing microscope.

[0019] Figure 2 The particle size distribution curve of sweet potato starch with different structures. Among them, NS is the original sweet potato starch, HRS is the hydrothermal pre-gelatinized treatment of the hole structure of sweet potato type III resistant starch, MRS is the scale structure of sweet potato type III resistant starch, and ARS is the hole structure of sweet potato type III resistant starch treated by pressure pre-gelatinization.

[0020] Figure 3 The X-ray diffraction pattern of sweet potato starch with different structures. Among them, NS is the original sweet potato starch, HRS is the hydrothermal pre-gelatinized treatment of the hole structure of sweet potato type III resistant starch, MRS is the scale structure of sweet potato type III resistant starch, and ARS is the hole structure of sweet potato type III resistant starch treated by pressure pre-gelatinization.

[0021] Figure 4 The Fourier transform infrared spectrum of sweet potato starch with different structures. Among them, NS is the original sweet potato starch, HRS is the hydrothermal pre-gelatinized treatment of the hole structure of sweet potato type III resistant starch, MRS is the scale structure of sweet potato type III resistant starch, and ARS is the hole structure of sweet potato type III resistant starch treated by pressure pre-gelatinization.

[0022] Figure 5Solubility and swelling power of different structure sweet potato starch. Wherein, a is solubility, b is swelling power; NS is sweet potato starch, HRS is hydrothermal pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch, MRS is flaky structure sweet potato type Ⅲ resistant starch, ARS is autoclave pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch.

[0023] Figure 6 Hydrolysis curve of different structure sweet potato starch. Wherein, NS is sweet potato starch, HRS is hydrothermal pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch, MRS is flaky structure sweet potato type Ⅲ resistant starch, ARS is autoclave pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch.

[0024] Figure 7 In vitro digestion characteristics of different structure sweet potato starch. Wherein, NS is sweet potato starch, HRS is hydrothermal pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch, MRS is flaky structure sweet potato type Ⅲ resistant starch, ARS is autoclave pre-gelatinization treatment of hole structure sweet potato type Ⅲ resistant starch. DETAILED DESCRIPTION

[0025] Hereinafter, the technical solutions of the present application will be described in conjunction with examples, but the present application is not limited to the following examples. The experimental methods and detection methods described in each example are all conventional methods unless otherwise specified; the reagents and materials described are all commercially available unless otherwise specified. In the following examples, % is the mass percentage unless otherwise specified. In the following examples, the ratio is the mass ratio unless otherwise specified.

[0026] Example 1 The present embodiment provides a preparation method of flaky structure sweet potato type Ⅲ resistant starch.

[0027] 5.0 g of sweet potato starch was weighed and adjusted to a concentration of 20% starch milk. After stirring uniformly in a beaker, it was sealed with plastic wrap to prevent water loss and placed in the center of the microwave oven. Under the condition of microwave power 800 W, the microwave gelatinization time was 40 s. In this pre-gelatinization treatment method, the gelatinization degree of sweet potato starch was 80%.

[0028] The microwave pregelatinized sweet potato starch was naturally cooled to room temperature, and was ultrasonically treated under the condition of ultrasonic power 40 W for 16 min. After ultrasonic treatment, the starch milk was placed in a 55 ℃ water bath to keep constant temperature, 10 U / g pullulanase was added into the starch milk, and the starch milk was enzymatically hydrolyzed in a 55 ℃ water bath for 4 h. After the enzymatic hydrolysis was completed, the enzyme was inactivated in a boiling water bath for 10 min. After the starch milk was cooled to room temperature, it was quickly placed in a 4 ℃ refrigerator for cold storage for 24 h. After the cold storage was completed, the starch milk was taken out for freeze-drying. The freeze-dried product was crushed and passed through a 100-mesh sieve, and was sealed and stored. The sweet potato RS3 prepared by the microwave pregelatinization combined with ultrasonic-enzyme method was marked as MRS.

[0029] As shown in Figure 1 , the surface of the native sweet potato starch (NS) granules was smooth and the structure was complete, and the granules were mainly spherical and a small amount was irregular in shape. Some granule surfaces had depressions. The NS granules had the complete Maltese cross of starch granules, and the center of the cross was located at the navel point of the granule, and was uniformly distributed. The surface of the microwave pretreated sweet potato resistant starch (MRS) appeared part of the depression and scale structure, which was due to the appropriate microwave radiation promoting the ordered migration and rearrangement of short linear molecules, and forming a dense microcrystal on the surface of the starch.

[0030] Comparative Example 1 This comparative example provides a preparation method of a hole structure sweet potato type Ⅲ resistant starch (hydrothermal pregelatinization combined with ultrasonic-enzyme method).

[0031] 5.0 g of sweet potato starch was weighed, and was adjusted to a concentration of 20% starch milk with distilled water. The starch milk was gelatinized in a 100 ℃ constant temperature water bath for 20 min. In this pregelatinization treatment method, the gelatinization degree of the sweet potato starch was 80%.

[0032] The gelatinized and pretreated sweet potato starch was naturally cooled to room temperature, and was ultrasonically treated under the condition of ultrasonic power 40 W for 16 min. After ultrasonic treatment, the starch milk was placed in a 55 ℃ water bath to keep constant temperature, 10 U / g pullulanase was added into the starch milk, and the starch milk was enzymatically hydrolyzed in a 55 ℃ water bath for 4 h. After the enzymatic hydrolysis was completed, the enzyme was inactivated in a boiling water bath for 10 min. After the starch milk was cooled to room temperature, it was quickly placed in a 4 ℃ refrigerator for cold storage for 24 h. After the cold storage was completed, the starch milk was taken out for freeze-drying. The freeze-dried product was crushed and passed through a 100-mesh sieve, and was sealed and stored. The sweet potato RS3 prepared by the hydrothermal pregelatinization combined with ultrasonic-enzyme method was marked as HRS.

[0033] As shown in Figure 1 , the surface of the hydrothermal pretreated sweet potato resistant starch (HRS) appeared a network-like hole structure.

[0034] Comparative Example 2 The comparative example provides a preparation method of a hole structure sweet potato type III resistant starch (autoclave pre-gelatinization combined with ultrasonic-enzyme method). In the pre-gelatinization treatment method, the gelatinization degree of the sweet potato starch is 80%.

[0035] 5.0 g of sweet potato starch was weighed, and a starch milk with a concentration of 20% was prepared and placed in an autoclave for treatment at 121 ℃ for 15 min.

[0036] After the gelatinization pretreatment of the sweet potato starch, the sweet potato starch was naturally cooled to room temperature, and ultrasonic treatment was performed at an ultrasonic power of 40 W for 16 min. After the ultrasonic treatment, the starch milk was placed in a 55 ℃ water bath for constant temperature, 10 U / g of pullulanase was added to the starch milk, and enzyme hydrolysis was performed at 55 ℃ for 4 h. After the enzyme hydrolysis was completed, the enzyme was inactivated in a boiling water bath for 10 min. After the starch milk was cooled to room temperature, it was quickly placed in a 4 ℃ refrigerator for cold storage for 24 h. After the cold storage was completed, it was taken out and freeze-dried, crushed and passed through a 100 mesh sieve, and sealed for storage. The sweet potato RS3 prepared by the autoclave pre-gelatinization combined with ultrasonic-enzyme method was marked as ARS.

[0037] As shown in Figure 1 , the autoclave pretreated sweet potato resistant starch (ARS) also has a network-like hole structure on the surface, and the hole structure of ARS is more widely distributed than HRS. This is because the autoclave pretreatment has a stronger destructive force on the starch molecules than the hydrothermal pretreatment, thereby promoting the rearrangement of the starch molecular chains to form a disordered structure. Under the action of ultrasonic waves, this relatively loose structure leads to the formation of a large number of holes on the surface of the starch particles.

[0038] Example 2 This example provides the particle size of sweet potato type III resistant starch with different structures.

[0039] The particle size distribution curves of sweet potato type III resistant starch with different structures are shown in Figure 2 As shown in , the particle size of sweet potato NS presents three peaks, and the particle size difference is large, with peak values at 1.15 μm, 15.50 μm and 45.23 μm. After being prepared into sweet potato RS3, compared with sweet potato NS, HRS, MRS and ARS all present a single peak curve, and the peak type is more sharp. In addition, the peak values of HRS, ARS and MRS are respectively located at 52.66 μm, 52.71 μm and 59.37 μm, and the particle size distribution curves are similar but the curves are shifted to the right, showing a trend of increasing particle size.

[0040] As shown in Table 1, compared to sweet potato NS, sweet potato RS3 showed a significantly larger volume average particle size (P<0.05), with HRS at 54.83 μm, ARS at 50.83 μm, and MRS at 48.66 μm. The relatively larger volume average particle sizes of HRS and ARS are attributed to the formation of a network of pores on the surface of the starch particles after processing. Compared to microwave pretreatment, hydrothermal and pressure heating pretreatments caused the starch particles to become more collapsed and loose. Under the cavitation effect of ultrasound, cavities formed by the expansion and rupture of the starch particles appeared on the surface, resulting in a porous structure and thus increasing the volume of the starch particles.

[0041] Table 1. Particle size distribution characteristics (%) of native sweet potato starch and sweet potato type III resistant starch with different gelatinization pretreatments.

[0042] Note: D [4,3] D is the volume average particle size. [3,2] D is the average particle size based on surface area. 10 D 50 D 90 The values ​​represent particle sizes with volume fractions of 10%, 50%, and 90%, respectively; different lowercase letters in the same column indicate significant differences (P < 0.05).

[0043] Example 3 This embodiment provides the differences in physicochemical properties of sweet potato type III resistant starch with different structures.

[0044] (1) Crystallographic structure analysis of sweet potato type III resistant starch with different structures Starch granules consist of amorphous and semi-crystalline regions. The crystalline configuration of starch can be determined using X-ray diffraction patterns. Figure 3 As shown, NS exhibits distinct diffraction peaks at 15.2°, 17.2°, 18.0°, and 23.1°, indicating a type A crystalline structure. HRS, MRS, and ARS, however, only show a strong diffraction peak at 17.0°, with a new weak peak appearing at 22.2°, suggesting that HRS, MRS, and ARS sweet potato type III resistant starches all possess a type B crystalline structure. Therefore, the three gelatinization pretreatment methods combined with ultrasound-enzyme methods, while altering the sweet potato starch structure, promote the transformation of sweet potato starch from type A crystals to the more compact type B crystals. Different starch crystal forms exhibit different enzymatic hydrolytic properties; compared to type A crystals, type B crystals have stronger digestibility, which also explains why sweet potato RS3 has stronger digestibility.

[0045] The crystallinities of sweet potato NS, HRS, MRS, and ARS are shown in Table 2, being 19.52%, 10.80%, 11.58%, and 9.34% respectively. The crystallinities of sweet potato RS3 with different structures are all lower than that of sweet potato NS. This is because the thermal energy during the gelatinization process enhances the interaction between water molecules and starch molecules, resulting in the destruction of the originally ordered crystal structure of sweet potato starch. The amylopectin molecules in the crystalline region unwind, which also enables the starch molecular chains to rearrange through intermolecular hydrogen bonds during the retrogradation process to form B-type crystals. The crystallinities of sweet potato RS3 with different structures are ranked as MRS > HRS > ARS in terms of size. Among them, the crystallinity of MRS is relatively large, indicating that compared with the porous-structured sweet potato RS3, the degree of damage to the crystalline region of starch granules in the flaky-structured sweet potato RS3 is relatively small. During the retrogradation process, the starch molecular chains are arranged orderly and recrystallized, increasing the degree of crystallization of the starch relatively.

[0046] Table 2 Crystallinities of type III resistant starch of sweet potato with different structures (%)

[0047] Note: Different lowercase letters in the same column indicate significant differences (P < 0.05).

[0048] (2) Analysis of the short-range ordered structure of type III resistant starch of sweet potato with different structures As Figure 4 shown, the infrared spectra of type III resistant starch of sweet potato with different structures are similar, and the characteristic peaks are respectively at 3402 cm -1 , 2930 cm -1 and 1647 cm -1 respectively, which are attributed to the stretching vibrations of O-H, C-H, and C=O. In the infrared spectrum of sweet potato RS3, there is no generation of new peaks or absence of peaks, indicating that the functional group compositions of sweet potato RS3 with different structures hardly change.

[0049] The characteristic absorption peaks of the infrared spectrum of starch in the range of 1150 - 900 cm -1 are related to the short-range order degree of starch. 1047 / 1022 cm -1 and 995 / 1022 cm -1The infrared ratio of the deformed parameter (DO value) and the double helix parameter (DD value) of starch molecules were calculated, respectively. As shown in Table 3, the DO value and DD value of sweet potato NS were 0.930 and 0.942, respectively. Compared with sweet potato NS, the DO values of sweet potato HRS, MRS and ARS were significantly decreased, while the DD values were significantly increased (P<0.05), indicating that the order degree of different structure sweet potato RS3 was low, and the double helix degree was improved. This was because the ordered crystal structure of amylopectin in different structure sweet potato RS3 was destroyed, so that the amylose was intertwined in the form of double helix during starch retrogradation, which improved the double helix degree of starch molecules. Among the three different structures of sweet potato RS3, MRS had a relatively high DD value of 0.963, indicating that the scale structure sweet potato RS3 formed a higher degree of double helix structure during the retrogradation process, which increased its resistance to digestive enzymes.

[0050] Table 3 DO and DD values of different structure sweet potato RS3

[0051] Note: The same column and different lowercase letters represent significant difference (P<0.05).

[0052] (3) Analysis of amylose content and amylopectin content of different structure sweet potato RS3 The amylose content and amylopectin content of sweet potato starch and sweet potato RS3 were shown in Table 4. Compared with sweet potato NS, the amylose content of sweet potato RS3 was significantly increased (P<0.05), from 15.54% to 19.19%~23.54%; while the amylopectin content was significantly decreased (P<0.05), from 64.22% to 53.69%~57.50%. The ratio of amylopectin / amylose could indicate the anti-digestion of starch, and the smaller the ratio, the stronger the anti-digestion of starch. MRS showed the smallest amylopectin / amylose ratio, which was 2.28. This indicated that compared with the hole structure RS3, the scale structure RS3 had the strongest anti-digestion ability.

[0053] Table 4 Amylose and amylopectin content of different structure sweet potato RS3

[0054] Note: The same column and different lowercase letters represent significant difference (P<0.05).

[0055] (4) Analysis of solubility and swelling power of different structure sweet potato RS3 The solubility and swelling power of sweet potato NS and sweet potato RS3 at different temperatures were shown in Table 5. Figure 5The solubility and swelling power of sweet potato NS and sweet potato RS3 generally showed an upward trend with the increase of temperature. In the range of 40 ℃ to 60 ℃, the solubility and swelling power of sweet potato RS3 were higher than those of NS. When the temperature was greater than 60 ℃, the solubility and swelling power of NS increased sharply, eventually reaching 15.70% and 18.77, respectively, which were greater than those of sweet potato RS3 (p<0.05). This may be due to the fact that in the lower temperature range, the gel structure of sweet potato RS3 is in an amorphous state, which is easy to absorb water and dissolve at low temperature. In addition, the crystal of sweet potato RS3 is composed of a large number of amylose molecules winding and associating, which has strong water-locking ability. Therefore, the solubility and swelling power of sweet potato RS3 are larger at low temperature. Since NS has not been processed, the particle integrity is high, so the swelling and water absorption capacity is limited. When the temperature is greater than 60 ℃, with the further increase of temperature, the starch granules of NS are destroyed after reaching the gelatinization temperature, and the microcrystalline bundle structure is loose, which can absorb a large amount of water, resulting in a rapid increase in solubility and swelling power. However, the temperature does not reach the melting temperature of the crystal of sweet potato RS3, which cannot destroy its compact structure. Therefore, the solubility and swelling power of sweet potato RS3 are less affected by temperature at high temperature.

[0056] The solubility and swelling power of the three structural sweet potato RS3 at 90 ℃ were ranked as ARS>HRS>MRS, in which MRS had the relatively lowest solubility and swelling power (p<0.05), which were 7.15% and 9.10, respectively. This is related to the surface structure of starch granules and gelatinization enthalpy. Compared with ARS and HRS, the relatively dense surface structure and relatively high gelatinization enthalpy of MRS increase its thermal stability, so its solubility and swelling power are less affected by temperature changes. The hole structure on the surface of ARS and HRS makes water molecules more easily penetrate into the interior of starch granules, thereby destroying the hydrogen bonds between starch granules, resulting in an increase in solubility and swelling power. Therefore, compared with the hole structure sweet potato RS3, the scale structure sweet potato RS3 is more difficult to be dissolved by heat in water, which reduces the dissolution of starch and makes it difficult to be digested and decomposed.

[0057] (4) Analysis of the thermodynamic properties of sweet potato type III resistant starch with different structures As shown in Table 5, the To, Tp, and Tc of sweet potato NS were 62.58 ℃, 74.26 ℃, and 87.41 ℃, respectively. Compared with sweet potato NS, the gelatinization temperatures (To, Tp, and Tc) of the three structural sweet potato RS3 were significantly increased (P<0.05), in which the To ranged from 89.41 to 98.65 ℃, the Tp ranged from 97.38 to 105.55 ℃, and the Tc ranged from 102.76 to 109.90 ℃. The ΔH20.76 J / g, 24.69 J / g and 13.14 J / g, which were significantly higher than that of sweet potato NS (6.28 J / g) (P<0.05). The pasting enthalpy of the three different structures of sweet potato RS3 was in the order of MRS>HRS>ARS. The difference in pasting enthalpy was not only related to the double helix structure in starch, but also related to the structural characteristics of the surface of starch granules. The pores and cracks on the surface of starch granules could more easily lead to the infiltration of water, which in turn penetrated into the crystalline region of starch to cause ΔH According to the results of scanning electron microscopy Figure 1 It can be seen that the surfaces of HRS and ARS granules have pore structures, and the pores on the surface of ARS are more widely distributed, thus more easily leading to the diffusion of water.

[0058] Table 5 Thermodynamic properties of different structure sweet potato type III resistant starch

[0059] Note: The same column and different lowercase letters represent significant differences (P<0.05).

[0060] Example 4 This example provides the in vitro digestion performance of different structure sweet potato type III resistant starch.

[0061] After the appropriate amount of sweet potato starch was baked to dryness, 0.1 g of sweet potato starch was taken in a 50 mL centrifuge tube, 15 mL of 0.1 mol / L phosphate buffer was added to form a mixed solution, and then it was placed in a boiling water bath for gelatinization for 30 min. After the starch paste was cooled, 5 mL of 0.01 mol / L NaOH solution was added, followed by the addition of 0.085 g of α-amylase, 0.01 g of glucoamylase and 0.072 g of trypsin (porcine pancreas). Then the enzyme solution was shaken in a constant temperature water bath at 37 ℃ for 3 h, and 1 mL of the enzyme solution was taken at 0, 20, 60, 90, 120, 150, 180 min after the enzyme was added, respectively, into a 25 mL colorimetric tube, and 4 mL of anhydrous ethanol was added to terminate the reaction. The amount of reducing sugar G t (t is the enzyme hydrolysis time when sampling) was determined by the DNS method, and the contents of rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) in the starch sample and the starch hydrolysis rate were calculated according to the following formula:

[0062]

[0063]

[0064]

[0065] wherein: G 0、 G 20 , G 120 are the reducing sugar content in supernatant at 0, 20, 120 min of hydrolysis, respectively / mg; G 淀 is the total starch content in sweet potato starch / mg; G t is the reducing sugar content in supernatant at t time / mg; 0.9 is the starch glucose conversion coefficient; m is the starch sample mass / mg.

[0066] The digestion rate curves of sweet potato NS and sweet potato RS3 are shown in Figure 6 In the range of 0~20 min, the hydrolysis rates of sweet potato NS and sweet potato RS3 both rapidly rise, which belongs to the rapid digestion starch RDS. In the range of 20~180 min, the rising rates of the hydrolysis rates of sweet potato NS and sweet potato RS3 become slow and tend to be stable after 120 min. Overall, the hydrolysis rate of sweet potato RS3 is much lower than that of sweet potato NS. The starch hydrolysis rates of sweet potato NS, ARS, HRS and MRS at 180 min are 68.54%, 47.75%, 43.46% and 39.81% respectively, indicating that compared with the hole structure sweet potato RS3, the scale structure sweet potato RS3 has a lower hydrolysis rate and is not easy to be hydrolyzed by enzymes.

[0067] The in vitro digestion results are shown in Figure 7 Compared with sweet potato NS, the RDS content of sweet potato RS3 significantly decreases, while the RS content and SDS content significantly increase (P<0.05). Among them, the RS content of MRS is the highest, which is 54.72%, significantly higher than the RS contents of sweet potato NS (21.69%), HRS (48.89%) and ARS (46.25%), which is related to the amylose content, granular structure, solubility and swelling potential and gelatinization enthalpy of MRS. The increase of amylose content in starch is the key factor leading to the decrease of enzymatic efficiency. This is because amylose can rearrange the hydrogen bond to form a dense crystal structure that resists enzymatic hydrolysis, so the higher amylose content of MRS increases its resistance to amylase, which shows an increase in RS content. In addition, the tight granular structure, relatively low solubility and swelling potential and relatively high gelatinization enthalpy of MRS all lead to its difficulty in gelatinization, and the starch is not easy to dissolve, so the digestion rate decreases. Therefore, compared with the hole structure sweet potato RS3, the scale structure sweet potato RS3 has the relatively strongest anti-digestion ability, which helps to reduce the increase of postprandial blood glucose level, and provides a choice for special dietary needs of people such as type 2 diabetes and obese patients.

[0068] The above description merely illustrates the principles of the application, the preferred embodiments and its advantages. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the application and are included within its spirit and scope.

Claims

1. A method for preparing a type III resistant starch from sweet potato with a flake-like structure, characterized in that, Sweet potato starch was pregelatinized using microwave; the gelatinized sweet potato starch was then ultrasonically treated to obtain starch milk; after enzymatic hydrolysis of the starch milk, it was refrigerated, freeze-dried, pulverized, and sieved to obtain scaly sweet potato type III resistant starch.

2. The method for preparing scaly sweet potato type III resistant starch according to claim 1, characterized in that, The degree of gelatinization of the microwave pregelatinized sweet potato starch is 80%.

3. The method for preparing type III resistant sweet potato starch with a scaly structure according to claim 1, characterized in that, The ultrasonic treatment power was 40 W, and the ultrasonic treatment time was 16 min.

4. The method for preparing type III resistant sweet potato starch with a scaly structure according to claim 1, characterized in that, The enzymatic hydrolysis includes: keeping the starch milk at a constant temperature in a water bath, adding pullulanase, performing enzymatic hydrolysis, and then inactivating the enzyme by boiling water after the enzymatic hydrolysis is completed.

5. The method for preparing type III resistant sweet potato starch with a scaly structure according to claim 4, characterized in that, The constant temperature is 55 ℃; The pullulanase has an enzyme activity of 10 U / g; The enzymatic hydrolysis temperature was 55 °C, and the enzymatic hydrolysis time was 4 h.

6. The method for preparing scaly sweet potato type III resistant starch according to claim 4, characterized in that, The enzyme inactivation time is 10 min.

7. The method for preparing type III resistant sweet potato starch with a flake-like structure according to claim 1, characterized in that, The refrigeration temperature is 4 ℃ and the refrigeration time is 24 h.

8. The method for preparing type III resistant sweet potato starch with a flake-like structure according to claim 1, characterized in that, The sieve used for sieving is 100 mesh.

9. A type III resistant starch from sweet potato with a scaly structure, characterized in that, It is prepared by the method for preparing scaly sweet potato type III resistant starch according to any one of claims 1 to 8.

10. The scaly structured sweet potato type III resistant starch according to claim 9, characterized in that, The scaly structure sweet potato type III resistant starch has a gelatinization enthalpy of 23-26 J / g, an amylose content of 23%-24%, and a resistant starch content of 54-55%.

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