Frozen rice starch-based food based on ultrasonic-assisted hydrothermal modification and preparation method thereof

Abstract

The invention discloses a frozen rice starch-based food based on ultrasonic-assisted hydrothermal modification and a preparation method thereof. The rice starch is subjected to ultrasonic and hydrothermal dual modification, modified starch with good aging resistance is screened and added into the rice starch, and the rice starch-based frozen food with high moisture content can be prepared. The prepared mixed starch system has better aging resistance, thermal stability and digestion resistance, and the disintegration value and the retrogradation value are gradually reduced along with the increase of the modified starch and are respectively reduced by 64.79% and 33.93% compared with those of a pure rice starch system. Meanwhile, the hardness, elasticity, adhesiveness and chewiness of the prepared high-moisture-content mixed starch gel are all improved compared with those of pure rice starch gel. The gel hardness of the frozen and stored mixed starch is gradually reduced along with the addition of the modified starch, and the elasticity is gradually enhanced, so that the modified starch can better reduce the damage of ice crystals and aging to a starch gel network in the freezing and storing process.

Description

Technical Field

[0001] The invention relates to a frozen rice starch-based food based on ultrasound-assisted hydrothermal modification and a preparation method thereof, belonging to the technical field of food processing. Background Art

[0002] With the prevalence of fast-paced lifestyles, the ready-to-eat food market is experiencing significant growth, with frozen starch products becoming a core growth category due to their convenience. Frozen foods can be categorized into four main categories: livestock frozen foods, agricultural frozen foods, aquatic frozen foods, and prepared frozen foods. Prepared frozen foods include frozen starch-based foods, which account for approximately 30% to 40% of the global frozen food market, with starch-based staple foods being the largest subcategory.

[0003] However, frozen starch-based foods are susceptible to microstructural damage from ice crystals during the freezing process, and their main component, starch, is prone to aging during the gelatinization and freezing process. This can cause rice, rice noodles, and rice bread to become hard, crack, and have a poor taste, resulting in significant losses to food resources and the economy. Research has shown that combining rapid freezing with low-temperature storage can minimize the damage to food quality caused by ice crystals. Ultra-low-temperature rapid freezing can better maintain the original quality of food than slow freezing. At a freezing temperature of -40°C or lower, the core temperature of the food is reduced to the required -18°C to lock in freshness within a few minutes to ten minutes, and then frozen for storage.

[0004] Although freezing technology can effectively maintain the original quality of food, its sensory attributes, such as texture and flavor, still differ significantly from those of fresh products. Starch retrogradation occurs when gelatinized starch is naturally cooled at low temperatures, molecular motion weakens, and the broken hydrogen bonds between starch molecules recombine. The starch molecules tend to arrange in parallel, transforming from a disordered state to an ordered state, and amylose and amylopectin reform into an ordered structure in this process. Starch retrogradation includes the short-term gelatinization of amylose and the long-term recrystallization of amylopectin. Researchers believe that there is a synergistic effect between the short-term retrogradation of amylose and the long-term retrogradation of amylopectin. The microcrystals formed during the short-term retrogradation of amylose can further provide crystal nuclei for the long-term retrogradation of amylopectin. Amylopectin recrystallization slowly grows around this center and gradually forms crystals. By adjusting the amylose / amylopectin ratio in starch and adding proteins, lipids, carbohydrates, etc., starch retrogradation can be delayed. Amylose, with its linear structure and long chain length, promotes molecular rearrangement and crystallization, thereby accelerating starch retrogradation. Conversely, the highly branched structure of amylopectin inhibits the formation of an ordered structure, slowing the retrogradation process. This inherent structural difference results in a prolonged retrogradation of amylopectin-rich starches. The rate and extent of starch retrogradation also depend significantly on moisture content. DSC measurements of the recrystallization enthalpy of amylopectin indicate that the effect of water content on starch retrogradation does not follow a single linear trend. Both high and low moisture contents are detrimental to starch retrogradation, with maximum retrogradation occurring at a starch gel moisture content of 40% to 45%. Ding Wenping's study of the retrogradation properties of rice starch at different moisture contents found that amylopectin recrystallized most rapidly at a moisture content of 60%, indicating the most significant long-term retrogradation effect in starch systems. Currently, the effects of moisture content on the physicochemical properties and structure of starch have primarily focused on moisture contents below 80%, while limited research has examined the frozen storage of starch products with moisture contents above 80%, such as rice cheese and rice milk. Summary of the Invention

[0005] To address the deficiencies of the prior art, the present invention uses ultrasonic and hydrothermal dual modification of rice starch, screens for modified starch with good anti-aging properties, and adds it to the rice starch to prepare a quick-frozen and frozen storage food made from rice starch with a high moisture content (above 80%). By manipulating the physicochemical properties of the raw material, the rice starch's resistance to ice crystal destruction is enhanced and its regeneration rate is slowed, providing a theoretical basis for improving the quality of frozen starch-based foods and for processing and improving the quality of traditional foods using rice starch as the primary raw material.

[0006] The present invention is achieved through the following technical solutions:

[0007] The first object of the present invention is to provide a method for preparing high-resistant rice starch based on ultrasound-assisted hydrothermal modification, comprising the following steps:

[0008] S1. Add rice starch to water to prepare a starch suspension, and treat the suspension by ultrasonic treatment for 10 to 30 minutes;

[0009] S2. The starch suspension after ultrasonic treatment is heated and stirred in a water bath at 60-75° C., the starch suspension after treatment is freeze-dried, and ground to obtain the starch.

[0010] In one embodiment of the present invention, in step S1, the amplitude of the ultrasonic treatment is 250-350W, and the cycle is 1-3s on and 1-3s off.

[0011] In one embodiment of the present invention, in step S1, the mass fraction of rice starch in the starch suspension is 5% to 20%.

[0012] In one embodiment of the present invention, in step S1, during the ultrasonic treatment, the treatment temperature is 0-4°C.

[0013] In one embodiment of the present invention, in step S2, the stirring speed is 250-350 rpm.

[0014] In one embodiment of the present invention, in step S2, the heating and stirring treatment time is 20 to 40 minutes.

[0015] The second object of the present invention is to provide high-resistant rice starch prepared by the method.

[0016] The third object of the present invention is to provide the use of the high-resistant rice starch in the preparation of a frozen rice starch-based food based on ultrasound-assisted hydrothermal modification.

[0017] In one embodiment of the present invention, the application is to add 5% to 45% of high-resistant rice starch to the raw rice starch for preparing frozen rice starch-based food.

[0018] A fourth object of the present invention is to provide a frozen rice starch-based food based on ultrasound-assisted hydrothermal modification.

[0019] Beneficial effects of the present invention:

[0020] The present invention is to carry out ultrasonic combined with hydrothermal dual modification of rice starch, and screen modified starch with better anti-aging performance and add it to the rice starch. The invention can be used to prepare a quick-frozen-frozen storage food of rice starch with a higher moisture content (more than 80%), such as products with higher moisture content such as rice cheese and rice milk. The prepared mixed starch system has better anti-aging, thermal stability and digestibility, and the disintegration value and retrogradation value gradually decrease with the increase of modified starch, respectively reducing by 64.79% and 33.93% compared with the pure rice starch system. At the same time, the hardness, elasticity, stickiness and chewiness of the high-moisture content mixed starch gel prepared by the present invention are all improved compared with the pure rice starch gel. The hardness of the mixed starch gel after frozen storage gradually decreases with the addition of modified starch, and the elasticity gradually increases, indicating that the modified starch can better reduce the destruction of ice crystals and aging to the starch gel network during frozen storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 The rheological properties of mixed starch gel before and after frozen storage;

[0023] Figure 2 The surface microstructure of mixed starch gel before and after frozen storage;

[0024] Figure 3 Cross-sectional microstructure of mixed starch gel before and after frozen storage. DETAILED DESCRIPTION

[0025] The present invention is further described below in conjunction with specific examples. These implementation cases are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, after reading the content taught by the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims appended hereto.

[0026] Source of raw materials

[0027] Rice starch, Shanghai Yuanye Biotechnology Co., Ltd.;

[0028] Porcine pancreatic α-amylase (≥5 units / mg), Sigma Reagent Co., Ltd.;

[0029] Glucosidase (100,000 units / ml) was purchased from Shanghai Aladdin Biological Reagent Co., Ltd.

[0030] Detection method:

[0031] (1) Determination of gelatinization properties

[0032] According to GB / T24852-2010 “Rapid Viscometer Method for Determination of Gelatinization Properties of Rice and Rice Flour”, the gelatinization properties of the mixed starch were determined by gelatinizing the mixed starch using a rapid visco analyzer (RVA).

[0033] (2) Determination of thermodynamic properties

[0034] The thermal properties of starch samples were measured using DSC. First, 3.0 mg of sample was weighed into an aluminum pan, 9.0 μL of deionized water was added, the pan was sealed tightly, and the mixture was allowed to stand at room temperature overnight. The DSC measurement procedure was as follows: the equilibrium temperature was 20°C, and the temperature was increased from 20°C to 100°C at a rate of 10°C / min. The gelatinization onset temperature (T0), gelatinization peak temperature (TP), gelatinization end temperature (TC), and gelatinization enthalpy (ΔH) were recorded from the DSC curve.

[0035] (3) Determination of digestion characteristics

[0036] The method for determining the in vitro digestibility of starch was based on the Englyst method with slight modifications. 200 mg of starch sample was dispersed in 15 mL of sodium acetate buffer (0.2 M, pH 5.2) and mixed evenly. 10 mL of freshly prepared enzyme solution (porcine pancreatic α-amylase, 290 U / g; glucosidase, 50 U / mL) was added and incubated in a 37°C water bath at 120 r / min with gentle magnetic stirring. 0.5 mL of enzymatic hydrolysate was taken out at 20 and 120 min, inactivated by adding 4.5 mL of anhydrous ethanol, and 2 mL of DNS reagent was added to a boiling water bath for 7 min. After cooling with running water, the volume was filled to 15 mL with distilled water, and the absorbance was measured at a wavelength of 540 nm. The digestible starch content was calculated as the glucose content × 0.9. The contents of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) were calculated as follows:

[0037]

[0038]

[0039] RS(%)=1-SDS-RDS

[0040] Where TS is the total starch mass, mg; FG is the free glucose content in starch, mg; G120 and G20 are the glucose contents produced within 120 min and 20 min of starch hydrolysis, respectively, mg.

[0041] (4) Determination of moisture distribution

[0042] A 5g thawed sample was wrapped in plastic wrap to prevent water loss and measured using a low-field nuclear magnetic resonance (NMR) spectrometer. The following parameters were used: magnet temperature 32°C, main frequency SF = 20 MHz, sampling frequency SW = 200 kHz, sampling interval TW = 2500 ms, number of echoes NECH = 8000, and number of summations NS = 16.

[0043] (5) Determination of texture characteristics

[0044] Rice starch gel was placed in a sample box (2 cm inner diameter, 3 cm height) and equilibrated at room temperature for 2 hours after frozen storage. Its texture properties were then analyzed using a physical property analyzer. Hardness was measured using a P36 cylindrical probe with the following test parameters: 30% strain, 0.5 mm / s front speed, 0.5 mm / s rear speed, and 5.0 g trigger force.

[0045] When measuring the gel strength, the puncture test parameters were as follows: TA / 0.5 probe, pre-test speed 2.0 mm / s, test speed 1.0 mm / s, post-test speed 2.0 mm / s, test distance 15 mm, trigger force 5 g.

[0046] (6) Determination of rheological properties

[0047] Place the thawed sample on the rheometer platform. A 40mm diameter parallel plate fixture was used for testing. The test temperature was 25°C, the gap was 1.0mm, and the shear rate was 0.01 to 100s⁻¹. The Herschel-Bulkley model was used to fit the static rheological curve of starch. The shear stress was calculated as follows:

[0048] σ=σ0+Kγn

[0049] Where: σ is shear stress, Pa; σ0 is yield stress, Pa; γ is shear rate, s-1; K is consistency coefficient, Pa·sn; n is flow characteristic index.

[0050] (7) Determination of microstructure

[0051] The freeze-dried starch sample was cut into a cross-section using a single-sided blade, evenly spread on a sample stage with conductive glue, and gold-sprayed for 120 s. The sample morphology was observed using a scanning electron microscope with an operating voltage of 3 kV and a magnification of ×200.

[0052] The technical solutions of the present invention are described in detail below with reference to specific embodiments. In the following embodiments, unless otherwise specified, the reagents, materials and equipment used can be purchased from commercial sources, or prepared by conventional methods, or are commonly used in the industry.

[0053] Example 1:

[0054] (1) Ultrasonic treatment

[0055] Rice starch was added to water at a 10% w / w ratio to prepare a starch suspension. The starch suspension was then treated with an ultrasonic cell disruptor at an amplitude of 300 W, a 2-second on / 1-second off cycle, and ultrasonicated for 15 min. During the ultrasonication, the temperature was controlled at 0° C. in an ice-water bath. The starch suspension was freeze-dried, ground, and passed through a 120-mesh sieve. The prepared starch was labeled UT.

[0056] (2) Hydrothermal treatment

[0057] A starch suspension (1:3, w / w) was prepared using rice starch. The suspension was heated and stirred in a magnetically stirred water bath at 300 rpm for 30 minutes. The heating temperatures were 60°C (T0), 67°C (TP), and 75°C (TC), respectively, corresponding to the gelatinization initial temperature (RS) of native rice starch. The treated samples were freeze-dried, ground, and passed through a 120-mesh sieve. The prepared starches were labeled PR-T0, PR-TP, and PR-TC.

[0058] (3) Ultrasonic-hydrothermal treatment

[0059] The starch prepared in step (1) was added to water at a mass ratio of 1:3 (w / w) to prepare a starch suspension, which was then heated and stirred at 300 rpm in a magnetically stirred water bath for 30 min. The heating temperatures were the gelatinization initial temperature (T0), gelatinization peak temperature (TP), and gelatinization termination temperature (TC) of native rice starch RS. The treated samples were freeze-dried, ground, and passed through a 120-mesh sieve. The prepared modified starches were labeled UT-T0, UT-TP, and UT-TC.

[0060] The thermodynamic properties of the original starch and the starch after treatment in steps (1), (2) and (3) are shown in Table 1. Ultrasonic treatment had no significant effect on the gelatinization temperature of rice starch (P < 0.05). Ultrasonic treatment significantly reduced ΔH from 10.32 J / g to 8.60 J / g (P < 0.05), indicating that the ultrasonic cavitation effect destroyed the amorphous and crystalline regions. Hydrothermal treatment (PR-T0, PR-T p ) makes T0, T p 、T C The results showed that the thermal stability of gelatinization was enhanced. ΔH decreased with the increase of hydrothermal temperature (T0→T CΔH decreased from 10.32 J / g to undetectable levels, indicating that hydrothermal treatment disrupted hydrogen bonds, forming a stable amorphous structure and gradually disintegrating the crystalline structure. Ultrasonic pretreatment significantly increased the gelatinization temperature of the hydrothermal treatment compared to the single hydrothermal treatment, likely due to ultrasound loosening the molecular structure and promoting amylose extraction. Furthermore, ΔH was lower than that of the single hydrothermal treatment, but the difference was not significant (P>0.05), indicating that hydrothermal treatment primarily disrupted crystal structure, while ultrasound assisted in the enhancement.

[0061] Table 1 Effects of modification treatments on the thermodynamic properties of rice starch

[0062]

[0063]

[0064] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0065] The gelatinization parameters of native rice starch and starch after treatment in steps (1), (2), and (3) are shown in Table 3. Ultrasonic treatment significantly increased the peak viscosity, final viscosity, and disintegration value of native rice starch and reduced its recovery value (P < 0.05). Peak viscosity is used to characterize the swelling power of starch. The decrease in peak viscosity indicates that the internal network structure of starch granules is destroyed during the gelatinization process. The increase in the viscosity of rice starch under ultrasonic treatment may be due to the destruction of the starch molecular structure by ultrasound, which reduces the starch particle size and increases the contact area between starch molecules and water during gelatinization. Therefore, it is easier to absorb water and swell, and the viscosity increases. The increase in disintegration value indicates that ultrasound reduces the shear stability of starch granules during the heating gelatinization process.

[0066] The peak viscosity of hydrothermally treated rice starches (RS-T0, RS-TP, and RS-TC) was significantly lower than that of native starch (P < 0.05). This is attributed to the disruption of the starch's multi-level structure by hydrothermal treatment: granule expansion leads to surface brittleness, decreased shear resistance, and thus reduced peak viscosity. As the treatment temperature increased, the viscosity initially decreased and then increased, with RS-TP having the lowest viscosity. This may be due to the incomplete gelatinization of the starch during T0 hydrothermal treatment, resulting in high water retention in the granules. During this treatment, the overflowed amylose formed a surface gelatinized layer at the local high temperature, which impeded water penetration and inhibited granule expansion, resulting in a decrease in viscosity. When the temperature reached the critical value for complete gelatinization, the amylose was fully gelatinized and reorganized, and the viscosity rebounded.

[0067] The disintegration values of both single hydrothermal modification and ultrasound-combined hydrothermal modification of starch showed a decreasing trend with increasing temperature, indicating that the gel stability of hydrothermally modified starch was enhanced. This may be because hydrothermal treatment induces starch chain rearrangement, strengthens the amorphous region structure, and thus improves the granule's resistance to decomposition. It is worth noting that the starch disintegration values at T0 and TP hydrothermal temperatures were higher than those of native starch, while the opposite was true for the fully gelatinized TC treatment group. This phenomenon reveals the dual effect of temperature on starch stability: before complete gelatinization (T0, TP), the crystalline region of the granules undergoes only partial changes, and the molecular chain interactions have not yet been significantly enhanced; while at the TC complete gelatinization temperature, high temperature promotes structural transformation of the crystalline region, increases the chain density of the amorphous region, and strengthens the intermolecular forces.

[0068] The retrogradation values of modified rice starches were lower than those of native rice starch. This may be because the hydrothermal treatment strengthens the interactions between amylopectin chains, and the migration and mobility of amorphous α-glucans promote the rearrangement of the double helical structure, forming a more compact and rigid structure. The retrogradation value of starch treated with ultrasound combined with hydrothermal treatment was significantly lower than that of starch treated with hydrothermal treatment alone (P < 0.05). In particular, the retrogradation value of starch treated with ultrasound-TC treatment was 46.50% lower than that of native rice starch. This may be because the ultrasonic pretreatment disrupts the starch granule structure, enhances water permeability, and promotes the formation of amorphous regions, making it less likely for starch to recrystallize during gelatinization and cooling.

[0069] Table 3 Effects of modification treatment on the gelatinization properties of rice starch

[0070]

[0071] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0072] In summary, compared with single hydrothermal treatment, ultrasound-combined hydrothermal treatment further increased gelatinization temperature and further decreased enthalpy, peak viscosity, final viscosity, disintegration value, retrogradation value, and amylose content, with these values gradually decreasing with increasing hydrothermal temperature. Ultrasound-TC treatment exhibited the lowest enthalpy, retrogradation value, order, relative crystallinity, and amylose content. Under the experimental conditions, ultrasound-TC hydrothermal treatment exhibited better anti-retrogradation properties and gel rheological properties than treatments at other temperatures. Therefore, modified starches obtained using ultrasound-combined TC hydrothermal treatment were selected for subsequent experiments.

[0073] Example 2:

[0074] The modified starch was partially substituted for rice starch at a substitution level of 5% to obtain a mixed starch. Water was added to prepare the mixed starch into a starch suspension with a starch mass fraction of 12%. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then frozen in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0075] Example 3:

[0076] The modified starch was partially substituted for rice starch at a substitution level of 15% to obtain a mixed starch. Water was added to prepare the mixed starch into a starch suspension with a starch mass fraction of 12%. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then placed in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0077] Example 4:

[0078] The modified starch was partially substituted for rice starch at a substitution level of 25% to obtain a mixed starch. Water was added to prepare the mixed starch into a starch suspension with a starch mass fraction of 12%. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then frozen in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0079] Example 5:

[0080] The modified starch was partially substituted for rice starch at a substitution level of 35% to obtain a mixed starch. Water was added to prepare the mixed starch into a starch suspension with a starch mass fraction of 12%. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then frozen in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0081] Example 6:

[0082] The modified starch was partially substituted for rice starch at a substitution level of 45% to obtain a mixed starch. Water was added to prepare the mixed starch into a starch suspension with a starch mass fraction of 12%. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then frozen in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0083] Comparative Example 1:

[0084] Water was added to prepare a starch suspension with a starch mass fraction of 12% by weight of rice starch. The starch suspension was gelatinized to obtain a rice starch gel sample. The sample was naturally cooled to 25°C at room temperature and then quickly frozen in a -40°C quick-freezer until the center temperature reached about -18°C. The sample was then frozen in a -18°C refrigerator for 5 days to obtain a frozen rice starch-based gel sample.

[0085] Test example:

[0086] Part of the starch gel samples of Examples 2 to 6 and Comparative Example 1 was freeze-dried and ground through a sieve with a diameter of 0.125 mm; part of the starch gel samples was thawed and equilibrated at room temperature for 2 h before testing.

[0087] (1) As shown in Table 3, the disintegration value and retrogradation value of the mixed starch are significantly lower than those of pure rice starch, and the peak viscosity and final viscosity also show similar trends. When the addition amount of modified starch is between 5% and 35%, the disintegration value and retrogradation value gradually decrease with the increase of modified starch, the disintegration value decreases from 363.50cp to 128.00cp, and the retrogradation value decreases from 975.50cp to 644.50cp, respectively. The disintegration value and retrogradation value are reduced by 64.79% and 33.93% compared with the pure rice starch system. It is worth noting that when the addition amount of modified starch is further increased to 45%, the disintegration value and retrogradation value show an upward trend.

[0088] Table 3 Effect of modified starch addition on the gelatinization properties of rice starch

[0089]

[0090]

[0091] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0092] (2) The thermodynamic properties of starch can well reflect the thermal stability of the ordered structure formed by hydrogen bonds within and between starch molecules. Its properties are related to the crystal structure, particle size and amylose content of starch granules. The effect of the addition of modified starch on the thermodynamic properties of rice starch is shown in Table 4. The gelatinization temperature range of pure rice starch is 60.65℃~73.33℃, and the enthalpy value △H is 10.82J / g. Compared with pure rice starch, the gelatinization starting temperature T0 and peak temperature TP of the mixed starch system are lower, but there is no significant difference (P<0.05). The end temperature gradually increases with the addition of modified starch, and the enthalpy value gradually decreases. When the modified starch addition amount is 35%, the enthalpy value is 5.29J / g. This shows that the modified starch forms a pre-gelatinized network in the mixed system, providing nucleation sites for native starch and promoting starch gelatinization. However, the energy demand in the initial stage is still dominated by the crystalline region of native starch, which is manifested by the fact that T0 and TP do not shift significantly. The increase in TC may be related to the gradual dissolution of short-chain amylopectin produced by ultrasonic shearing in the modified starch during the later stages of gelatinization, requiring higher temperatures to support the continued extension of its molecular chains. Hydrothermal treatment leaves the modified starch chains in an extended state, requiring no additional energy to break down the crystalline structure during reheating. A greater proportion of modified starch in the mixture results in a more significant decrease in overall thermal enthalpy.

[0093] Table 4 Effect of modified starch addition on the thermodynamic properties of rice starch

[0094]

[0095] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0096] (3) Table 5 shows the effect of modified starch addition on the in vitro digestion parameters of rice starch, including RDS, SDS and RS contents. The addition of modified starch significantly reduced the RDS content, and significantly increased the SDS and RS contents (P < 0.05). During the digestion process, compared with the pure rice starch system, when the modified starch addition amount was 45%, the RDS content decreased from 48.26% to 35.48%, the SDS content increased from 22.90% to 28.05%, and the RS content increased from 28.84% to 36.47%. For the mixed system, as the proportion of modified starch increased, it showed that the inhibitory effect of modified starch on rice starch digestion had a strong dependence, that is, modified rice starch can reduce the digestibility of rice starch, which may help blood sugar control. Combined with Figure 2The SEM results showed that the gel network structure of the mixed starch was denser and the internal pores were reduced. This structure may help to limit the diffusion of enzyme molecules and slow down the hydrolysis rate, especially when the addition amount of modified rice starch was 25% to 35%. When the modified starch was further increased, as shown by SEM observation, when the addition amount was 45%, the interlayer spacing expanded, the pores increased, the enzyme was easier to penetrate, the RDS content rebounded, and the inhibitory effect on digestion weakened.

[0097] Table 5 Effect of modified starch addition on the digestibility of rice starch

[0098]

[0099] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0100] (4) The water distribution of pure rice starch and mixed starch gel before and after frozen storage was measured by low-field nuclear magnetic resonance technology, as shown in Table 6. T21, T22 and T23 represent the transverse relaxation time of strongly bound water, weakly bound water and free water, respectively. A21, A22 and A23 represent the percentage of the peak area of the relaxation spectrum corresponding to T21, T22 and T23, respectively, to characterize the content ratio of water molecules in the corresponding state. Before frozen storage, the strongly bound water content in the mixed starch gel was significantly lower than that in the pure rice starch gel system, while the weakly bound water and free water contents were on the contrary. This shows that the addition of modified starch promotes the conversion of bound water in the starch gel to free water, reducing the gel's ability to retain water molecules. When the modified starch addition amount is 25%, the strongly bound water content of the mixed starch gel system is the lowest, only 30.29% of that of the pure rice starch gel. Water molecules bind to the hydroxyl groups of strong water sites in the crystalline regions of starch molecules through hydrogen bonds. Ultrasound causes cavitation damage to starch granules, resulting in a loss of the ordered structure that facilitates water transport and gelatinization. This significantly reduces the number of strong water sites, such as hydroxyl groups in the crystalline regions, that were originally bound by ordered hydrogen bonds. During further gelatinization, the starch molecular chains swell and rearrange due to heat, further disrupting the residual crystalline structure and forming more amorphous regions. These regions have weaker water binding capacity, relying primarily on loose hydrogen bonds or van der Waals forces. As a result, the amount of strongly bound water decreases, while the ratio of weakly bound water to free water increases. After frozen storage, the strongly bound water content of mixed starch gels is significantly lower than that of unfrozen gels, while the free water content increases significantly. This is related to the aging of starch gels during frozen storage. Aging reduces the viscoelasticity of the starch gel network, weakening its ability to bind water molecules. Furthermore, the mechanical pressure generated by ice crystal growth during storage deteriorates the starch gel network structure, making it difficult for water molecules released during thawing to be absorbed by the gel network. It is worth noting that the strongly bound water content in the mixed starch gel system after frozen storage increases with the addition of modified starch, while the free water content gradually decreases. The reason may be that during the freezing process, the pregelatinized starch molecular chains of the modified starch are fully stretched by hydrothermal reaction and form an interpenetrating network with the native starch. Figure 3 Scanning electron microscopy revealed that gels containing 35% and 45% modified starch exhibited a denser cross-sectional layered structure after freezing, restricting free water migration. Furthermore, the modified starch's short-chain molecules rapidly restructured their network through hydrogen bonding during thawing. Subsequent rheological experiments revealed that the G′ and G″ values of some mixed starch gel samples were greater than those of pure rice starch gels, thus reducing water loss caused by repeated freezing and thawing.

[0101] Table 6 Moisture distribution of mixed starch gel before and after frozen storage

[0102]

[0103] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0104] (5) Table 7 summarizes the changes in the hardness, elasticity, adhesiveness and chewiness of starch gel before and after frozen storage. As can be seen from the table, the addition of modified starch significantly increased the hardness, elasticity, adhesiveness and chewiness of rice starch gel (P < 0.05). When the addition amount of modified starch was 5% to 25%, the hardness and adhesiveness of the gel gradually increased. The cavitation effect of ultrasound creates micropores on the surface of starch particles, increases the specific surface area and reactive sites, promotes the penetration of water molecules, and at the same time, the short-chain molecules produced easily interact with each other to enhance the gel network structure. After hydrothermal modification, the starch particles are fully gelatinized, the molecular chains unfold and rearrange, and stronger interactions are formed with the native starch molecules through hydrogen bonds, hydrophobic interactions, etc., further strengthening the gel network. With the increase of the proportion of modified starch, the number of modified starch molecular chains in the gel system increases, and the synergistic effect between modified starch and native starch becomes more significant with the increase of addition amount. The intermolecular force and cross-linking degree are enhanced, further improving the hardness and adhesiveness of the gel. Compared with unfrozen mixed starch gel samples, frozen storage increased the hardness, adhesiveness, and chewiness of the mixed starch gel, while decreasing its elasticity. The presence of amylose has a greater impact on gel hardness than amylopectin. During frozen storage, the amylose molecules in the mixed starch gel are more likely to undergo retrogradation, forming a more ordered crystalline structure, thereby increasing the gel's hardness and chewiness. Some amylopectin molecules rearrange during freezing, forming a tighter network structure but reducing the gel's elasticity. Furthermore, during freezing, free water in the gel forms ice crystals. The growth of ice crystals squeezes the starch molecular chains, disrupting the original gel network structure. At the same time, water migrates out of the gel network and concentrates in the ice crystal region, enhancing the interaction between the starch molecular chains and making the gel network tighter. These changes increase the gel's hardness and adhesiveness, but also disrupt its uniformity and reduce its elasticity. It is worth noting that the hardness of the mixed starch gel after frozen storage decreases and the elasticity increases with the addition of modified starch. When the modified starch addition amount is 45%, the hardness decreases by 22.95% and the elasticity increases by 16.19%, indicating that during the frozen storage process of the gel, the cross-linking effect between the modified rice starch molecular chains and the native rice starch molecular chains can delay the starch regeneration.

[0105] Table 7 Texture characteristics of mixed starch gel before and after frozen storage

[0106]

[0107] Note: Data are expressed as mean ± standard deviation. Different letters in the same column indicate significant differences (P<0.05).

[0108] (6) Rheological properties of pure rice starch gel and mixed starch gel before and after frozen storage Figure 1As shown. As can be seen from the figure, before and after frozen storage, the pure rice starch gel and the mixed starch gel system G′>G″, tanδ<1, that is, the elastic properties of the starch gel under the experimental conditions are dominant, showing stronger solid behavior. Before frozen storage, when the modified starch addition amount is 5%, the mixed starch gel G′ is higher than that of the pure rice starch gel, and with the further addition of modified starch, the mixed starch gel G′ is lower than the pure starch gel system. Within the modified starch addition range, the mixed starch gel system G′ with 45% modified starch addition is relatively low; the G″ curves of the pure rice starch gel and the mixed starch gel system show a trend of basic overlap. The changes in rheological properties can be attributed to the dual regulatory effects of ultrasonic-hydrothermal modified starch on the gel network structure. When 5% modified starch was added, the exposure of its surface hydrophilic groups and the penetration effect of small molecular segments may enhance the penetration of water molecules into starch molecules during gelatinization through hydrogen bonds, promoting the dissolution of amylose, which is manifested as an increase in G′; when the modified starch addition amount was 45%, the excessive introduction of modified starch may lead to competitive hydration and phase separation between molecules, weakening the original continuous cross-linked structure of amylopectin. At the same time, the plasticizing effect of amylose and the increase in gel volume during heating reduced the rigidity of the system, resulting in a decrease in G′. The convergence of the G″ curves indicates that the viscosity is mainly controlled by the viscoelastic behavior of unmodified starch, and the free amylose in the modified starch does not form significantly different energy dissipation in dynamic shear. After frozen storage, the G′ and G″ of pure starch gel and mixed starch gel are significantly improved. The reason may be that the mechanical extrusion and regeneration of ice crystals synergistically strengthen the network: during the freezing process, ice crystals squeeze the gel network, forcing the molecular chains to arrange closely, while promoting the regeneration of amylose and enhancing the cross-linking density, resulting in a significant increase in G′ and G″. It is worth noting that within the range of modified starch addition, the viscoelasticity of the mixed starch gel with a 35% addition is significantly improved. The mixed starch gel with 45% addition was relatively good, while the worst at 35% addition, indicating that the compatibility of modified starch and native starch was better, the ice crystals were evenly distributed, guiding the directional rearrangement of the molecular chains to form a more uniform strengthening structure, while at 45%, the molecular chains at the phase interface were disordered, and interface defects were formed between the modified starch-rich area and the pure starch area, weakening the freeze-strengthening effect. The results corresponded to the scanning electron microscopy results: the mixed starch gel with 35% modified starch addition had a relatively good structure before and after frozen storage. The loss tangent value tanδ is the ratio between G″ and G′. The larger the tanδ, the more dominant the viscosity characteristics in the system. Before and after frozen storage, the tanδ value of the mixed starch gel with a higher modified starch addition was higher than that of the pure rice starch gel, indicating that increasing the proportion of modified starch can make the viscosity of rice starch more significantly increase.

[0109] (7) Figure 2The surface microstructures of pure rice starch gel and mixed starch gel at 200× magnification are shown. Electron microscopy reveals that the surface of the freeze-dried pure rice starch gel is characterized by densely packed, circular pores of varying sizes and a generally uneven surface. As the modified starch content reaches 35%, the surface pores of the freeze-dried sample gradually decrease, with a small number of ductile cracks forming instead. During the freeze-drying process, free water in the pure rice starch gel forms ice crystals, which sublime and leave pores that match the ice crystal shape. Because the pure starch gel network is relatively loose, ice crystal growth is unrestricted, resulting in the formation of randomly distributed, circular pores of varying sizes. The modified starch, while having shorter and looser molecular chains, forms crosslinks with the pure starch molecular chains through hydrogen bonding and hydrophobic interactions, creating a denser gel network. This dense network restricts the size and growth direction of ice crystals, reducing the number of large pores after freeze-drying. Furthermore, the increased crosslinking of the mixed starch gel enhances its elasticity, better resisting shrinkage stress during freeze-drying, reducing brittle fracture and instead forming a small number of ductile cracks. The improved hardness and elasticity of the blended starch gels are directly related to their dense network structure. SEM results further confirm this structural advantage, demonstrating reduced pores and cracks and enhanced mechanical strength. The reappearance of a porous structure at 45% modified starch addition is likely due to excessive crosslinking density resulting from the excessive amount of modified starch, which increases the network rigidity. This prevents stress release through elastic deformation during freeze-drying, leading to brittle fracture and the formation of pores. After frozen storage, pure rice starch gels and blended starch gels exhibited a coexistence of pores and ductile cracks, with an increased density of pores and cracks. During frozen storage, water within the gel migrates from amorphous regions to ice crystals, resulting in localized water accumulation. During freeze-drying, ice crystals in these regions sublime, forming dense pores. These pores then become interconnected by cracks due to network disruption. Furthermore, frozen storage causes rearrangement of the amylose chains, forming more crystalline regions. This increases the differential shrinkage between the crystalline and amorphous regions, generating internal stresses during freeze-drying and leading to network disruption.

[0110] Figure 3The cross-sectional microstructures of pure rice starch gel and mixed starch gel after freeze-drying at 200× magnification are shown. The cross-sections of pure rice starch gel and mixed starch gel exhibit a multilayered, lamellar, sponge structure that is loose and ordered. The formation of the multilayered sponge structure is primarily due to syneresis. When the modified starch addition ratio is between 5% and 15%, the mixed starch gel and pure rice starch gel exhibit similar lamellar, porous structures, indicating that the addition of modified starch is insufficient to significantly alter the original gel network. The gel network is likely dominated by pure starch, with the modified starch acting as a filler or slightly disrupting the gel structure, resulting in no significant structural changes. However, at a high enough ratio of 25% to 35%, the modified starch concentration begins to significantly affect the gel network, resulting in fewer pores and a denser stacking of the lamellar structure. After ultrasonic hydrothermal modification, the starch molecular chains are smaller and more easily cross-linked, increasing the density of the gel network and reducing the gaps between ice crystals, resulting in fewer pores. At this level, the modified starch may promote tighter molecular arrangement, resulting in a denser structure after freeze-drying, especially when the modified starch addition ratio is 35%. As the amount of modified starch increases, the interlamellar spacing increases, possibly due to phase separation or excessive cross-linking. At high concentrations, modified starch molecules form clusters due to hydrophobic interactions or hydrogen bonding, disrupting network continuity. This widens the interlamellar spacing after freeze-drying. Compared to samples stored unfrozen, pure rice starch gels and mixed starch gels exhibit similar lamellar structures after frozen storage, but with significantly fewer or no pores, and the lamellar wall structure thickens significantly, especially when the modified starch addition level ranges from 15% to 35%. During frozen storage, the amylose chains rearrange to form a double helix structure, increasing the proportion of crystalline regions and reducing the differential shrinkage between crystalline and amorphous regions. Freeze-drying results in a more uniform stress distribution, fewer lamellar pores, and a more continuous lamellar structure. This thickening of the lamellar wall structure may be due to the rearrangement of amylopectin chains to form crystallites, which strengthens the bonding between the lamellar layers, or to phase separation between starch and water.

[0111] The embodiments provided above are not intended to limit the scope of the present invention, nor are the steps described to limit their execution order. Any obvious improvements to the present invention made by those skilled in the art in combination with existing common knowledge shall fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing high-resistant rice starch based on ultrasound-assisted hydrothermal modification, characterized in that: The steps include: S1. Add rice starch to water to prepare a starch suspension, and treat the suspension by ultrasonic treatment for 10 to 30 minutes; S2. The starch suspension after ultrasonic treatment is heated and stirred in a water bath at 60-75° C., the starch suspension after treatment is freeze-dried, and ground to obtain the starch.

2. The preparation method according to claim 1, characterized in that In step S1, the amplitude of the ultrasonic treatment is 250-350W, and the cycle is 1-3s on and 1-3s off.

3. The preparation method according to claim 1, characterized in that In step S1, the mass fraction of rice starch in the starch suspension is 5% to 20%.

4. The preparation method according to claim 1, characterized in that In step S1, during the ultrasonic treatment, the treatment temperature is 0-4°C.

5. The preparation method according to claim 1, characterized in that In step S2, the stirring speed is 250 to 350 rpm.

6. The preparation method according to claim 1, characterized in that In step S2, the heating and stirring treatment time is 20 to 40 minutes.

7. High-resistant rice starch prepared by the method according to any one of claims 1 to 6.

8. Use of the high-resistant rice starch according to claim 7 in the preparation of a frozen rice starch-based food based on ultrasound-assisted hydrothermal modification.

9. The use according to claim 8, characterized in that The application is to add 5% to 45% of the high-resistant rice starch into the raw material rice starch for preparing frozen rice starch-based food.

10. A frozen rice starch-based food based on ultrasound-assisted hydrothermal modification, wherein the frozen rice starch-based food is prepared using rice starch to which 5% to 45% of the high-resistant rice starch according to any one of claims 1 to 6 is added as a raw material.

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