A method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction

Abstract

The application relates to a method for preparing modified starch through synergistic pretreatment and phase transfer catalytic gradient reaction, comprising the following steps: S1, starch is prepared into starch milk, and is subjected to composite enzyme hydrolysis treatment and ultrahigh pressure-ultrasonic synergistic treatment in sequence to obtain pretreated starch milk; S2, a phase transfer catalyst is added, the pH is adjusted to 11.0-12.0, a chemical modifier is added, and a three-stage gradient temperature control program is adopted for reaction: the first stage is 20-25 DEG C for 30-40 min, the second stage is 25-30 DEG C for 3-5 h, and the third stage is 35-40 DEG C for 5-7 h; S3, the temperature is lowered to 20-25 DEG C, the pH is adjusted to 5.5-6.0, and post-treatment is carried out to obtain modified starch. The application exposes active hydroxyl groups through composite enzyme hydrolysis and physical field synergistic pretreatment, strengthens mass transfer through phase transfer catalysis, realizes step-by-step penetration and directional reaction of reagents through gradient temperature control, inhibits side reactions through temperature reduction and neutralization, synergistically improves reaction efficiency and substitution degree, and reduces cost and pollution.

Description

Technical Field

[0001] This application relates to the field of modified starch preparation technology, and in particular to a method for preparing modified starch through synergistic pretreatment and phase transfer catalytic gradient reaction. Background Technology

[0002] Modified starches (such as hydroxypropyl starch, acetate starch, and acetylated starch) are widely used in food, pharmaceuticals, textiles, and papermaking, primarily serving as thickeners, stabilizers, and emulsifiers. Currently, industrial production of modified starches mostly employs wet processes, where starch is reacted with etherifying or esterifying agents in an aqueous phase under alkaline conditions. However, this process suffers from significant technical bottlenecks: the reaction efficiency of chemical reagents is generally low; for example, the etherification efficiency of propylene oxide is typically only 30%–50%, and the esterification efficiency of acetic anhydride is only 20%–30%. Large amounts of unreacted reagents not only increase raw material costs but also generate high-concentration organic wastewater during post-processing, significantly increasing the difficulty of wastewater treatment and environmental costs. Furthermore, the dense internal crystalline regions of starch granules, poor accessibility of active hydroxyl groups, and the susceptibility of reagents to hydrolysis side reactions, uneven solid-liquid phase contact, and localized overheating leading to starch degradation have long troubled those skilled in the art.

[0003] To improve the reaction efficiency of modified starch, researchers have explored various improvement strategies. Mechanical activation solid-phase methods disrupt starch crystalline regions through ball milling, but suffer from uneven activation and reagent distribution, resulting in limited efficiency gains. Moist heat pretreatment utilizes temperature gradients to open starch crystalline regions, but relies solely on physical exposure of hydroxyl groups, failing to suppress hydrolytic side reactions. Enzymatic hydrolysis degrades starch branched structures using single or complex enzyme systems, initially exposing active sites, but without synergy with catalytic or temperature control measures in the reaction system, significant efficiency improvements are difficult to achieve. Microwave or semi-dry methods can shorten reaction time and reduce wastewater, but easily lead to localized overheating and degradation of starch, resulting in uneven product substitution. Furthermore, phase transfer catalysis has been attempted in starch reactions such as benzylation and hydroxyethylation to improve solid-liquid mass transfer efficiency; however, current reports mostly focus on the addition of single catalysts, without combining it with deep starch pretreatment and precise temperature control of the reaction process, still resulting in low catalytic efficiency and difficulty in optimizing reaction conditions.

[0004] In summary, existing modified starch preparation methods generally suffer from the following core problems: insufficient activation of the starch crystalline region leads to inadequate exposure of hydroxyl groups; chemical reagents are prone to hydrolysis side reactions in the aqueous phase; the solid-liquid contact efficiency between starch granules and reagents is low; and localized overheating during the reaction process causes starch degradation or substituent removal. Although various single or combined improvement methods have been developed, no preparation method has yet been found that can simultaneously solve the above four key problems. There is an urgent need in this field for a universal modified starch preparation process that can synergistically achieve efficient starch activation, sufficient reagent contact, effective suppression of side reactions, and a mild and controllable reaction process, in order to improve reagent reaction efficiency, reduce raw material consumption, and lower environmental costs. Summary of the Invention

[0005] To achieve efficient starch activation, sufficient reagent contact, effective suppression of side reactions, and a mild and controllable reaction process, this application provides a method for preparing modified starch through synergistic pretreatment and phase transfer catalytic gradient reaction.

[0006] A method for preparing modified starch through synergistic pretreatment and phase transfer catalytic gradient reaction includes the following preparation steps: S1. Mix starch with water to prepare starch milk, add compound enzyme for enzymatic hydrolysis, and then place the enzymatically hydrolyzed starch milk in an ultra-high pressure-ultrasound combined device for treatment under the conditions of pressure 300-400MPa, ultrasonic frequency 20-40kHz, and temperature 35-40℃ to obtain pretreated starch milk. S2. Add a phase transfer catalyst to the pretreated starch milk, adjust the pH of the pretreated starch milk to 11.0-12.0, add a chemical modifier to obtain a reaction solution, and carry out the reaction solution using a three-stage gradient temperature control program to obtain the reaction product. S3. Cool the reaction product to 20-25℃ and adjust the pH to 5.5-6.0, and obtain the modified starch through post-treatment; The three-stage gradient temperature control procedure in step S2 includes: the first stage: controlling the temperature of the reaction solution to 20-25℃ and reacting for 30-40 minutes; the second stage: continuing to control the temperature of the reaction solution to 25-30℃ and reacting for 3-5 hours; and the third stage: continuing to control the temperature of the reaction solution to 35-40℃ and reacting for 5-7 hours.

[0007] The inventors discovered that by employing a synergistic pretreatment of compound enzymatic hydrolysis and ultra-high pressure-ultrasonic physical field, combined with a phase transfer catalytic gradient reaction process, it is unexpectedly possible to simultaneously achieve a significant increase in reaction efficiency, an increase in product substitution degree, and effective suppression of side reactions under conditions of low chemical modifier dosage.

[0008] Specifically, the starch milk first undergoes a complex enzymatic hydrolysis treatment to initially expose surface hydroxyl groups and create enzymatic hydrolysis channels. Then, it undergoes a combined ultra-high pressure and ultrasonic treatment. The hydrostatic pressure of the ultra-high pressure disrupts the intermolecular hydrogen bonds in the crystalline regions inside the starch granules, while the cavitation effect of the ultrasound generates localized high-temperature, high-pressure microjets, further dissociating the crystalline regions and preventing particle aggregation. The two processes complement each other in their spatial interaction—enzymatic hydrolysis opens channels from the surface inwards, while the physical field disrupts the crystalline regions from the inside out, thus fully exposing the active hydroxyl groups of the starch from the surface to the interior, providing ample active sites for subsequent reactions.

[0009] Meanwhile, under alkaline conditions, the phase transfer catalyst transfers reactive species from the aqueous phase to the organic phase or starch particle interface, significantly reducing the solid-liquid interfacial tension and allowing the chemical modifier to more effectively contact the exposed hydroxyl groups. The pretreated and activated starch slurry then enters a gradient temperature-controlled reaction stage. The three-stage gradient temperature control program is designed based on precise control of the main and side reactions at each stage: the first stage is a low-temperature reaction at 20-25℃, allowing the chemical modifier to be uniformly adsorbed and gradually penetrate the starch particle surface, while avoiding hydrolysis side reactions caused by high temperatures; the second stage is a medium-temperature reaction at 25-30℃, where the chemical modifier fully combines with the exposed active hydroxyl groups on the surface, completing the main substitution reaction; the third stage is a high-temperature reaction at 35-40℃, promoting the diffusion of the chemical modifier into the interior of the starch particles, allowing the internal hydroxyl groups to also be substituted, thereby improving the uniformity of the product's substitution degree. After the reaction is completed, the temperature is immediately lowered to 20-25℃ and the pH is adjusted to 5.5-6.0 to quickly terminate the reaction, avoiding the degradation and substituent removal of starch particles under high-temperature and high-alkaline conditions that occur in traditional processes.

[0010] This application achieves comprehensive exposure of hydroxyl groups in the spatial dimension through synergistic pretreatment of compound enzymatic hydrolysis and ultra-high pressure-ultrasonic physical field; enhances mass transfer in the interfacial dimension through phase transfer catalysis; achieves gradual penetration and directional reaction of chemical modifiers in the temporal dimension through three-stage gradient temperature control; and suppresses side reactions in the termination dimension through cooling and neutralization. The processes in each dimension are interconnected and interdependent, unexpectedly achieving simultaneous improvement in reaction efficiency, product quality and environmental friendliness.

[0011] In one specific implementation, the complex enzyme comprises pullulanase and α-amylase, wherein the mass ratio of pullulanase to α-amylase is 1:(0.8-1.2).

[0012] By employing the aforementioned composite enzymes, pullulanase and α-amylase exhibit a synergistic effect at the specified mass ratio. Pullulanase specifically hydrolyzes the α-1,6-glycosidic bonds in the branched structure of starch, cleaving the branches and exposing the linear regions previously obscured by the branched structure. α-amylase, on the other hand, randomly hydrolyzes the α-1,4-glycosidic bonds via endocleavage, breaking down long-chain starch molecules into appropriately short chain fragments. The two enzymes complement each other at their sites of action—pullulanase debranching exposes new accessible sites, while α-amylase endocleavage generates more reducing ends and segments with moderate polymerization, collectively significantly increasing the abundance of active hydroxyl groups on the surface and near-surface regions of starch granules. If the pullulanase ratio is too high, excessive debranching leads to the release of numerous short chain fragments, which weakens the structural integrity of the starch granules and is detrimental to the contact efficiency of subsequent solid-liquid reactions. Conversely, if the α-amylase ratio is too high, excessive endocleavage results in excessively short starch molecular chains, causing a sharp drop in viscosity and reducing the binding sites for chemical modifiers. Only within a mass ratio range of 1:(0.8-1.2) can the degree of debranching and endo-cleavage be optimally balanced, which preserves the basic morphology of starch granules to ensure the effectiveness of solid-liquid contact, while maximizing the exposure of active hydroxyl groups available for chemical modification, thus laying the structural foundation for subsequent phase transfer catalysis and gradient reactions.

[0013] More preferably, the pullulanase is a hydrophobically modified pullulanase, and the modification and preparation steps of the hydrophobically modified pullulanase are as follows: Pullulanase was dissolved in phosphate buffer at pH 7.0-8.0, 2-octenyl succinic anhydride was added, and the reaction was carried out at room temperature. The hydrophobically modified pullulanase was obtained after post-treatment.

[0014] The inventors discovered that pullulanase undergoes significant changes in its molecular structure, interfacial behavior, and catalytic performance after being hydrophobically modified with 2-octenyl succinic anhydride (2-OSA), resulting in unexpected technical effects in synergy with subsequent processes.

[0015] First, the cyclic anhydride group in the 2-OSA molecule can undergo a specific acylation reaction with the free amino groups of lysine residues on the surface of pullulanase molecules, covalently grafting the octenyl hydrophobic long chain onto the enzyme molecule surface. This chemical modification significantly enhances the surface hydrophobicity of the modified pullulanase while maintaining its catalytic activity in hydrolyzing α-1,6-glycosidic bonds. When the modified pullulanase is introduced into the starch milk system, the octenyl hydrophobic segments on its surface can generate stronger affinity adsorption with the hydrophobic microregions inside and on the surface of starch granules, guiding the enzyme molecule from the traditional random diffusion adsorption mode to a hydrophobic anchoring mode. This allows the modified pullulanase to more efficiently locate the active site region on the surface of starch granules, thereby improving the enzymatic hydrolysis efficiency.

[0016] Secondly, protein acylation converts the positively charged amino group of lysine residues into an uncharged amide bond, resulting in a decrease in the isoelectric point and an increase in the net negative charge on the molecular surface of the modified pullulanase. In the starch milk system (pH 4.5-5.0), this charge change enhances the electrostatic repulsion between modified pullulanase molecules, effectively inhibiting enzyme aggregation and precipitation, and improving the dispersion stability and accessibility of the enzyme in the liquid phase.

[0017] Furthermore, chemical modification can induce a rearrangement of the secondary structure of pullulanase molecules—altering the ratio of α-helices, β-sheets, and random coils, thus giving the enzyme molecule a certain degree of structural rigidity. This moderate enhancement of structural rigidity helps maintain the conformational stability of the enzyme's active site, enabling it to resist conformational changes and inactivation caused by temperature fluctuations or shear forces under enzymatic reaction conditions (40-45℃).

[0018] The multiple effects of the aforementioned hydrophobic modification (such as enhanced hydrophobic affinity, improved dispersion stability, and improved structural rigidity) enable the modified pullulanase to more efficiently recognize and hydrolyze the α-1,6-glycosidic bonds of amylopectin in the starch slurry system, significantly improving enzymatic hydrolysis efficiency. More importantly, the octenyl hydrophobic long chains grafted onto the surface of the modified pullulanase remain on the starch granule surface or within the enzymatic channels after hydrolysis, forming a localized hydrophobic microenvironment. This microenvironment forms an affinity match with the amphiphilic structure of the phase transfer catalyst (alkyl glycoside APG-12) in subsequent steps, lowering the adsorption energy barrier of the phase transfer catalyst at the starch-water interface. This allows the phase transfer catalyst to be positioned more quickly and stably at the reaction interface, thereby enhancing the mass transfer efficiency between the solid and liquid phases and enabling the chemical modifier to penetrate more efficiently into the starch granules and react with the active hydroxyl groups.

[0019] More preferably, the α-amylase is a hydrophilically modified α-amylase, and the modification steps of the hydrophilically modified α-amylase are as follows: α-Amylase was dissolved in phosphate buffer at pH 7.0-8.0 and reacted with polyethylene glycol under the mediation of carbodiimide. The hydrophilic modified α-amylase was obtained after post-treatment.

[0020] The inventors discovered that after α-amylase was hydrophilically modified with polyethylene glycol (PEG), its molecular characteristics, interfacial behavior, and catalytic performance all changed significantly, resulting in unexpected technical effects in synergy with subsequent processes.

[0021] First, PEG is a linear, flexible, and highly hydrophilic polymer. Under the mediation of carbodiimide (EDC), PEG molecules can be covalently grafted onto the lysine amino or carboxyl groups on the surface of α-amylase molecules via amide bonds. The grafted PEG long chains form a hydrophilic hydration layer on the enzyme molecule surface. The introduction of this hydration layer produces multiple effects: on the one hand, it significantly improves the dispersion stability and solubility of α-amylase in the starch emulsion aqueous phase, effectively inhibiting the aggregation and precipitation of enzyme molecules in the aqueous phase due to hydrophobic interactions, thus maintaining the enzyme molecules in a highly dispersed monomolecular state in the reaction system; on the other hand, the PEG hydration layer acts as a spatial barrier, buffering the direct impact of subsequent ultra-high pressure-ultrasonic physical field treatment on the enzyme molecules, protecting the native conformation of the enzyme molecules from damage caused by the violent physical field.

[0022] Secondly, PEG grafting can alter the hydrophilic-hydrophobic balance on the surface of α-amylase molecules. Fluorescence spectroscopy studies show that the binding of PEG to amylase enhances the intrinsic fluorescence of the enzyme molecule and increases the hydrophobicity of the microenvironment containing tyrosine residues. This indicates that PEG grafting induces a subtle rearrangement of the secondary and tertiary structures of α-amylase molecules—the enzyme molecule tends to form a more compact and rigid conformation. This moderate enhancement of structural rigidity helps maintain the conformational stability of the enzyme's active site, enabling it to resist conformational changes and inactivation caused by temperature fluctuations or physical shear forces under subsequent enzymatic hydrolysis conditions (40-45℃). Furthermore, PEG-grafted α-amylase exhibits a more uniform distribution in polyurethane films than the natural enzyme, indicating that PEGylation improves the compatibility between the enzyme molecule and different interfaces. This property also contributes to the uniform adsorption and efficient action of enzyme molecules on the starch granule surface in the starch slurry system of this application.

[0023] The inventors further discovered that when hydrophilically modified α-amylase and hydrophobically modified pullulanase are used together in a complex enzyme system, significant functional complementarity and spatial synergy occur between them. This synergistic effect far exceeds the simple summation of the effects of a single modified enzyme, specifically manifested in the following aspects: First, the two enzymes exhibit natural complementarity in their spatial localization on starch granules. Pullulanase, with its surface grafted with octenyl hydrophobic chains, possesses a strong affinity for hydrophobic microregions both inside and on the surface of starch granules. It tends to "anchor" itself to hydrophobic regions of the starch granules (such as areas with dense amylopectin and the edges of crystalline regions), primarily responsible for hydrolyzing α-1,6-glycosidic bonds to remove branching. Conversely, the hydrophilic α-amylase, due to the high hydrophilicity of its surface PEG chains, tends to disperse more readily in the aqueous phase of starch emulsion. It uses its hydrophilic shell to affinity-adsorb onto exposed hydrophilic regions of the starch granule surface (such as amylose regions and the inner walls of pores created by enzymatic hydrolysis), hydrolyzing α-1,4-glycosidic bonds via an endo-glucanogenic approach. These two enzymes perform their respective functions spatially without interference, respectively localizing in the hydrophobic and hydrophilic regions of the starch granules. This avoids excessive competition for substrate within the same region, achieving full coverage of the starch granules from hydrophobic to hydrophilic areas, thus significantly improving enzymatic hydrolysis efficiency.

[0024] Secondly, the two enzymes form a relay-like "debranching-endocleavage" interaction in their substrate action. The hydrophobically modified pullulanase first acts on the α-1,6-glycosidic bonds of amylopectin, cleaving the branches and exposing the previously obscured linear regions and new α-1,4-glycosidic bonds. These newly exposed sites become the dominant substrate for the hydrophilically modified α-amylase, which then endocleaves these newly exposed α-1,4-glycosidic bonds, producing more reduced ends and short chain fragments with moderate polymerization. Conversely, the pores and fissures created by the endocleavage of α-amylase provide channels for the hydrophobically modified pullulanase to penetrate deeper into the starch granules, enabling it to act on the α-1,6-glycosidic bonds in the deeper crystalline regions. This mutually dependent and alternating relay-like interaction in substrate action overcomes the efficiency bottleneck caused by steric hindrance and substrate competition between the two enzymes in natural complex enzymes.

[0025] Third, the structural rigidity acquired by the two modified enzymes creates a "stability superposition" effect. As mentioned earlier, hydrophobic modification increases the α-helix ratio and strengthens the structural rigidity of pullulanase; hydrophilic modification makes the conformation of α-amylase tend to be more compact and rigid. When the two coexist in the same reaction system, their respective conformational stability allows them to maintain a high enzyme activity retention rate under the combined enzymatic hydrolysis conditions (40-45℃, pH 4.5-5.0, accompanied by subsequent physical field shearing forces). More importantly, the hydrophilic hydration layer formed by the PEG chains on the surface of α-amylase not only protects the α-amylase itself, but also acts as a "buffer barrier" to reduce the indirect impact of physical field treatment on the other component in the system—the hydrophobically modified pullulanase—to a certain extent (e.g., by reducing the overall ionic strength of the system and buffering pH fluctuations), making the overall stability of the composite enzyme system better than that of either single modified enzyme used alone.

[0026] Fourth, the spatial positioning difference between the two enzymes after opposite polarity modification unexpectedly produced a "polar prefitting" effect. During enzymatic hydrolysis, the hydrophobically modified pullulanase "pre-positioned" the octenyl hydrophobic segment on the hydrophobic region of the starch granule surface, while the hydrophilically modified α-amylase "pre-positioned" the PEG hydrophilic segment on the hydrophilic region. This polar spatial pre-differentiation precisely matches the interfacial distribution behavior of the phase transfer catalyst, alkyl glycoside APG-12 (an amphiphilic molecule), in the subsequent steps—the hydrophobic alkyl chain of the APG-12 molecule preferentially binds to the hydrophobically modified region of the starch granule surface, while its hydrophilic glycoside head group is compatible with the aqueous phase and the hydrophilized region. This "polar prefitting" lowers the adsorption energy barrier of the phase transfer catalyst at the starch-water interface, allowing it to arrange itself more quickly and orderly on the reaction interface, thereby further enhancing the mass transfer efficiency between the solid and liquid phases.

[0027] In one specific implementation, in step S1, the starch is selected from one or more of tapioca starch, corn starch, and potato starch.

[0028] Cassava starch, corn starch, and potato starch can all be used as suitable raw materials for this method. Cassava starch has a high amylopectin content (approximately 75%-83%), and its loose granular structure and high swelling power make it more sensitive to the action of complex enzymes during the enzymatic pretreatment stage, resulting in high exposure efficiency of active hydroxyl groups. It is a preferred raw material for etherified modified starches such as hydroxypropyl starch. Corn starch (ordinary type) has an amylose content of approximately 25%-28%, and its granule surface is relatively dense. However, after complex enzymatic hydrolysis and ultra-high pressure-ultrasound synergistic pretreatment, its crystalline regions can also be effectively destroyed, resulting in sufficient exposure of active hydroxyl groups. Potato starch has a larger granule size (5-100 μm) and an amylose content of approximately 20%-25%. Its unique B-type crystalline structure and high phosphorus content give it better swelling properties and reactivity in alkaline reaction systems (pH 11.0-12.0).

[0029] Although the three types of starch differ in granule morphology, crystalline structure, linear / branched chain ratio, and reactivity, the method described in this application, through a series of processes including "compound enzymatic hydrolysis—ultra-high pressure—ultrasonic physical field synergy—phase transfer catalytic gradient reaction," can achieve universal activation and modification targeting the structural characteristics of different starches without requiring adjustments to core equipment or significant changes to process parameters due to changes in raw materials. This raw material compatibility fully demonstrates the versatility and industrial adaptability of this method, which is beneficial for reducing equipment investment and process switching costs in multi-variety production lines.

[0030] In one specific implementation, in step S2, the phase transfer catalyst is alkyl glycoside APG-12.

[0031] Alkyl glycoside APG-12 plays multiple synergistic roles in this application, specifically: First, APG-12 is a green, biodegradable nonionic surfactant. Synthesized from natural, renewable resources—starch (or glucose) and fatty alcohols—it possesses advantages such as low surface tension, high surface activity, and rapid and complete biodegradation. In the alkaline reaction system (pH 11.0-12.0) of this application, APG-12 exhibits excellent alkali resistance and stability, maintaining its activity throughout the entire etherification reaction process, with no harmful residues in the final product, meeting the safety requirements for food-grade modified starch.

[0032] Secondly, APG-12, as a phase transfer catalyst, can significantly improve the interfacial contact between starch (solid phase) and chemical modifiers (liquid phase). Starch particles are dispersed in a solid phase in alkaline starch emulsion, while chemical modifiers (such as propylene oxide) have limited solubility in the aqueous phase. The mass transfer efficiency between the two is a key bottleneck determining the reaction efficiency. The APG-12 molecule has an amphiphilic structure (including a hydrophobic alkyl chain (C12) and a hydrophilic glycosidic head group), which allows it to be directionally adsorbed at the starch-water interface, effectively reducing interfacial tension, improving the wetting and penetration of chemical modifiers into starch particles, and thus enhancing the mass transfer efficiency between the solid and liquid phases.

[0033] More importantly, APG-12 forms a functional connection and synergy with the preceding pretreatment steps. In step S1, the combined enzymatic hydrolysis and ultra-high pressure-ultrasound synergistic treatment have fully destroyed the crystalline regions of the starch granules, exposing the active hydroxyl groups from the surface to the interior. However, if the exposed hydroxyl groups cannot effectively contact the chemical modifier, the substitution reaction cannot occur efficiently. The presence of APG-12 precisely solves this "last mile" problem—the adsorption layer it forms at the starch-water interface provides a "channel" for the chemical modifier to diffuse into the interior of the starch granules, allowing the active hydroxyl groups exposed in step S1 to be fully captured and reacted by the chemical modifier, thereby effectively translating the activation effect of the pretreatment stage into an improvement in actual reaction efficiency.

[0034] Furthermore, APG-12 works synergistically with the subsequent gradient temperature control program. In the first stage of the reaction (20-25℃), the adsorption of APG-12 causes the chemical modifier to be uniformly enriched on the surface of starch granules, providing sufficient local concentration for the initial reaction at low temperatures. In the second stage (25-30℃), the continuous interfacial activity of APG-12 promotes the full binding of the chemical modifier with the surface hydroxyl groups. In the third stage (35-40℃), APG-12 further reduces the diffusion resistance inside the granules, helping the chemical modifier to penetrate deeper into the starch granules. Throughout all three stages, APG-12 acts as an "interfacial bridge," ensuring that the design intent of each stage of the gradient temperature control is fully realized.

[0035] In summary, the addition of APG-12 enables the phase transfer catalytic gradient reaction process of this application to form an "interfacial bridge" mechanism—by reducing interfacial tension, improving wetting and penetration, and enhancing solid-liquid mass transfer, it efficiently connects the active hydroxyl groups exposed in the S1 pretreatment stage with the chemical modifier. At the same time, it forms a synergistic effect with the three-stage gradient temperature control program throughout the entire process, thereby significantly improving the reaction efficiency of the chemical modifier and the uniformity of product substitution.

[0036] More preferably, the phase transfer catalyst further includes tetrabutylammonium bromide, and the mass ratio of the alkyl glycoside APG-12 to tetrabutylammonium bromide is 1:(0.5-1.2).

[0037] The inventors discovered that when alkyl glycoside APG-12 and tetrabutylammonium bromide (TBAB) are used in combination at the above mass ratio, they exhibit a significant synergistic effect in phase transfer catalysis. This is specifically reflected in the following aspects: First, the two catalysts complement each other in terms of their catalytic mechanisms, focusing on interfacial regulation and ion migration. APG-12, as a nonionic surfactant, can reduce the interfacial tension between starch particles and the aqueous phase, improving the wetting and penetration of chemical modifiers into the starch particles and creating a favorable interfacial environment for mass transfer. TBAB, on the other hand, is a classic quaternary ammonium salt phase transfer catalyst. Its tetrabutylammonium cation is lipophilic and can associate with the alkoxide anion formed by deprotonation of starch glucose units to form an ion pair, migrating the active species from the aqueous phase to the reaction site. This allows the two catalysts to work together to complete the entire catalytic chain from mass transfer improvement to active species delivery.

[0038] Secondly, the molecular structures of both molecules create a spatial synergy at the interface adsorption: the hydrophobic alkyl chain (C12) and hydrophilic glycoside head group of APG-12 enable it to oriented at the starch-water interface. The addition of TBAB results in the simultaneous presence of nonionic APG-12 molecules and positively charged TBA⁺ ions in the interfacial adsorption layer. This synergistic adsorption makes the interfacial arrangement more ordered and compact, further reducing interfacial tension and providing a smoother mass transfer channel for the chemical modifier.

[0039] More importantly, the combination of these two components produces a stable complementary effect in alkaline reaction systems (pH 11.0-12.0). APG-12 exhibits good alkali resistance under alkaline conditions, but it lacks ion exchange capacity. While TBAB possesses strong catalytic activity, its stability is relatively weak in highly alkaline environments. By combining them, the alkali resistance of APG-12 provides a relatively mild interfacial microenvironment for TBAB, buffering the impact of strong alkali on TBAB to a certain extent and extending its effective catalytic lifetime. The complementary stability of the two components ensures that the combined system maintains consistently high catalytic activity throughout the alkaline reaction process.

[0040] Furthermore, a deep synergy was formed between the compound system and the preceding pretreatment steps. In step S1, the crystalline regions of the starch granules were sufficiently disrupted, and the active hydroxyl groups were fully exposed from the surface to the interior. By improving interfacial wetting with APG-12, TBAB continuously migrated the alkoxide anions activated under alkaline conditions to the reaction sites. The compound system of APG-12 and TBAB significantly improved reaction efficiency and product substitution degree by efficiently capturing the active hydroxyl groups exposed in step S1 with chemical modifiers.

[0041] In one specific implementation scheme, in step S2, while adding the phase transfer catalyst to the pretreated starch milk, a hydroxyl-functionalized imidazole ionic liquid is also added, wherein the amount of the hydroxyl-functionalized imidazole ionic liquid added is 0.5-5 wt% of the starch mass; more preferably, the hydroxyl-functionalized imidazole ionic liquid is selected from one or more of 1-(2,3-dihydroxypropyl)-3-methylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, and 1-(3-hydroxypropyl)-3-methylimidazolium dodecyl sulfate.

[0042] The inventors discovered that by introducing hydroxyl-functionalized imidazole ionic liquids into the alkaline reaction system (pH 11.0-12.0) of this application, the liquids significantly promote starch activation, mass transfer enhancement, and reaction efficiency improvement through multiple hydrogen bonding interactions, destruction of starch crystallization regions, optimization of the reaction medium, and synergistic cooperation with the phase transfer catalytic system.

[0043] First, hydroxyl-functionalized imidazole ionic liquids can effectively disrupt the strong hydrogen bond network between starch molecules, achieving deep activation of starch granules. Natural starch, due to the presence of numerous strong hydrogen bonds both between and within molecules, forms a dense crystalline structure, resulting in extremely low accessibility to active hydroxyl groups. Studies have shown that the cations and anions of hydroxyl-functionalized ionic liquids (such as 1-(2-hydroxyethyl)-3-methylimidazolium chloride) can simultaneously interact with starch to form new hydrogen bonds, thereby disrupting the original strong hydrogen bond network of starch.

[0044] More importantly, the hydroxyl-functionalized ionic liquid and the APG-12 / TBAB complex phase transfer catalytic system in this application form a synergistic "activation-mass transfer" relay. The hydroxyl groups on the imidazole cations in the ionic liquid can form strong hydrogen bonds with the starch hydroxyl groups, weakening the original strong hydrogen bond network between starch molecules, causing the starch particles to further swell and loosen their structure in the reaction system. This effect complements and reinforces the combined enzymatic hydrolysis and ultra-high pressure-ultrasound synergistic pretreatment in step S1: the S1 pretreatment has already achieved full exposure of hydroxyl groups through enzymatic hydrolysis and physical field; while the addition of the ionic liquid further weakens the residual hydrogen bonding at the molecular level, making the microenvironment of the exposed hydroxyl groups more relaxed, reducing the steric hindrance and energy barrier of the subsequent substitution reaction. At the same time, the APG-12 / TBAB complex system is responsible for migrating reactive species in the aqueous phase to the starch-water interface. The three components work together in a relay-like process: "ionic liquid loosens hydrogen bonds → APG-12 opens interfacial channels → TBAB migrates active species." This allows the ionic liquid to lower the reaction energy barrier at the molecular level, APG-12 to reduce mass transfer resistance at the interfacial level, and TBAB to supply active species at the ionic level, enabling the chemical modifier to reach and act on the active hydroxyl sites more efficiently.

[0045] Furthermore, ionic liquids can act as green solvents and co-catalysts in reactions, further enhancing reaction efficiency. Studies have shown that ionic liquids can serve as excellent media for starch modification, playing a solvent and catalytic role in reactions such as etherification and esterification. Among different imidazole carboxylate ionic liquids, the degree of substitution and reaction efficiency of acetylated corn starch increase with decreasing alkyl chain length of the cation or anion. In the alkaline system of this application, the amount of hydroxyl-functionalized imidazole ionic liquid added is controlled within the range of 0.5-5 wt% of starch mass, which fully utilizes its hydrogen bond breaking and activation effects on starch while avoiding the problem of excessively high system viscosity due to excessive ionic liquid, thus affecting mass transfer efficiency.

[0046] In summary, hydroxyl-functionalized imidazole ionic liquids, through a mechanism of "hydrogen bond disruption—dissociation of crystalline regions—molecular-level activation," form a cross-scale relay with the combined enzymatic hydrolysis and ultra-high pressure-ultrasound synergistic pretreatment in this application. Macroscopic pretreatment opens structural channels, and ionic liquids loosen the hydrogen bond network at the molecular level, jointly enabling the active hydroxyl groups of starch to be deeply and fully exposed from the particle surface to the interior of the crystalline region. Furthermore, with the APG-12 / TBAB complex phase transfer catalytic system, a synergistic system of "activation-mass transfer-reaction" is formed, further significantly enhancing the reaction efficiency of the chemical modifier on the basis of existing improvements.

[0047] In one specific implementation scheme, in step S2, the chemical modifier is an etherifying agent, an esterifying agent, or a crosslinking agent; the etherifying agent is propylene oxide, the esterifying agent is acetic anhydride, and the crosslinking agent is sodium trimetaphosphate or phosphorus oxychloride.

[0048] By employing the aforementioned chemical modifiers, the method of this application can be applied to the preparation of various types of modified starches, achieving the goal of "one set of processes, multiple products" for universal production, as detailed below: Firstly, when propylene oxide is selected as the etherifying agent, this method is used to prepare hydroxypropyl starch. Propylene oxide undergoes a nucleophilic addition reaction with starch hydroxyl groups under alkaline conditions (pH 11.0-12.0) to generate hydroxypropyl starch ether. The key to this reaction lies in the accessibility of the active hydroxyl groups in the starch granules and the diffusion efficiency of propylene oxide. After pretreatment by combined enzymatic hydrolysis and ultra-high pressure-ultrasound in step S1, the starch crystalline region is fully destroyed, and the active hydroxyl groups are fully exposed from the particle surface to the interior. The addition of a phase transfer catalyst in step S2 significantly improves the dispersion and mass transfer efficiency of propylene oxide (oil-soluble) in starch emulsion (aqueous phase). The combination of these two factors significantly improves the etherification reaction efficiency of propylene oxide, while also resulting in a more uniform degree of substitution in the product. After the reaction, a cooling and neutralization step in step S3 effectively prevents the loss of hydroxypropyl groups under high temperature and high alkalinity conditions, ensuring the stability of product quality.

[0049] Secondly, when acetic anhydride is selected as the esterifying agent, this method is used to prepare starch acetate. Acetic anhydride reacts with the hydroxyl groups of starch under alkaline conditions to form starch acetate. Acetic anhydride is prone to hydrolysis in the aqueous phase, producing acetic acid, which is the main reason for the low reaction efficiency of acetic anhydride in traditional processes (usually only 20%-30%). In this application, the pretreatment in step S1 fully activates the starch particles and improves the reactivity; the addition of the APG-12 / TBAB complex phase transfer catalytic system in step S2 allows acetic anhydride molecules to be more efficiently enriched at the starch-water interface and contact the active hydroxyl groups; the low-temperature initial stage (20-25℃) in the three-stage gradient temperature control program effectively suppresses the hydrolysis side reaction of acetic anhydride. The synergistic effect of these three measures significantly improves the esterification reaction efficiency of acetic anhydride. The cooling and neutralization in step S3 further avoids the hydrolysis and detachment of esterified groups under high-temperature alkaline conditions, ensuring the degree of substitution and stability of the product.

[0050] Furthermore, when sodium trimetaphosphate or phosphorus oxychloride is selected as the crosslinking agent, this method is used to prepare crosslinked starch (such as the pre-crosslinking step of hydroxypropyl distarch phosphate, or crosslinked starch alone). Sodium trimetaphosphate (STMP) is a food-grade crosslinking agent that undergoes a nucleophilic substitution reaction with starch hydroxyl groups under alkaline conditions to form phosphate diester crosslinking bonds. Phosphorus oxychloride (POCl3) is a highly efficient crosslinking agent with much higher reactivity than sodium trimetaphosphate, achieving a high degree of crosslinking at lower dosages. Both share the characteristic that the crosslinking reaction mainly occurs on the surface and near-surface region of starch granules, with relatively low requirements for the diffusion depth of the crosslinking agent into the granule interior. In the method of this application, the hydroxyl groups on the surface of starch granules, fully exposed by the S1 pretreatment, provide sufficient accessible sites for the crosslinking reaction; the phase transfer catalytic system efficiently transports the crosslinking agent molecules to the reaction sites; and the gradient temperature control program ensures a mild and controllable crosslinking reaction, avoiding product performance defects caused by over- or uneven crosslinking.

[0051] More importantly, although the three chemical modifiers (propylene oxide, acetic anhydride, and sodium trimetaphosphate / phosphorus oxychloride) belong to etherifying agents, esterifying agents, and crosslinking agents, respectively, and their reaction mechanisms with starch hydroxyl groups are different, they are all suitable for the core process architecture of this application. The combined enzymatic hydrolysis and ultra-high pressure-ultrasonic synergistic pretreatment in S1 are responsible for exposing active hydroxyl groups; the phase transfer catalytic system in S2 is responsible for enhancing solid-liquid mass transfer, and gradient temperature control is responsible for directionally regulating the reaction process; and the cooling neutralization in S3 is responsible for terminating side reactions. This makes the process of this application universal—it can achieve the efficient preparation of various modified starches such as hydroxypropyl starch, acetate starch, crosslinked starch, and composite modified starches (such as hydroxypropyl distarch phosphate) without adjusting the core process parameters or replacing key equipment due to changing chemical modifiers, fully demonstrating the wide applicability of this method in industrial production.

[0052] In summary, this application includes at least one of the following beneficial technical effects: 1. This application achieves gradual penetration and directional reaction of chemical modifiers from the surface to the interior by adopting a three-stage gradient temperature control program, which significantly improves reaction efficiency and the uniformity of product substitution.

[0053] 2. This application achieves spatial synergistic localization and substrate relay coordination of the two enzymes on starch granules by using a combination of hydrophobically modified pullulanase and hydrophilically modified α-amylase, which greatly increases the exposure of active hydroxyl groups.

[0054] 3. This application uses a combination of alkyl glycoside APG-12 and tetrabutylammonium bromide as a phase transfer catalyst, which achieves a functional relay of interface regulation and ion transport at a specific ratio, significantly enhancing the solid-liquid mass transfer efficiency.

[0055] 4. This application further enhances the reaction efficiency by adding hydroxyl-functionalized imidazole ionic liquids to deeply disrupt the residual hydrogen bond network of starch at the molecular level, forming a cross-scale synergy with the pretreatment and phase transfer catalytic system. Detailed Implementation

[0056] The present application will be further described in detail below with reference to embodiments and comparative examples: Some of the raw materials used in the examples and comparative examples: 2-Octenylsuccinic anhydride (CAS: 42482-06-4), alkyl glycoside APG-12, i.e., dodecyl glucoside (CAS: 110615-47-9), and 1-(2-hydroxyethyl)-3-methylimidazolium chloride, i.e., 1-(2-hydroxyethyl)-3-methylimidazolium chloride (CAS: 61755-34-8), are all from Sigma-Aldrich.

[0057] Polyethylene glycol, brand name PEG-3000, was purchased from Lotte in South Korea; pullulanase, product number hy-027, and α-amylase, product number hy-011, were both purchased from Nanjing Hongyou Biotechnology Co., Ltd.

[0058] This application takes the preparation of hydroxypropyl starch as an example. Unless otherwise specified, all raw materials used in the examples and comparative examples are commercially available products.

[0059] Preparation Example 1 The modification and preparation steps of hydrophobically modified pullulanase are as follows: 100 g of pullulanase was dissolved in 5000 mL of 0.05 mol / L pH 7.5 phosphate buffer, and 1.25 g of 2-octenylsuccinic anhydride was added. The mixture was stirred at 25 °C for 3 h, and the pH of the reaction system was maintained at 7.5 using 0.1 mol / L NaOH solution. After the reaction, the reaction solution was transferred to a dialysis bag with a molecular weight cutoff of 10 kDa and dialyzed with deionized water at 4 °C for 36 h, with the dialysate changed every 8 h. After dialysis, the liquid in the dialysis bag was freeze-dried for 60 h (-50 °C, vacuum degree of 15 Pa) to obtain hydrophobically modified pullulanase.

[0060] Preparation Example 2 The modification steps for hydrophilic modification of α-amylase are as follows: 100 g of α-amylase was dissolved in 5000 mL of 0.05 mol / L pH 7.5 phosphate buffer, and 1.25 g of carbodiimide and 1 g of N-hydroxysuccinimide were added. The mixture was activated at 4 °C for 45 min, and then 3 g of polyethylene glycol was added. The reaction mixture was stirred at 25 °C for 4 h, and the pH of the reaction system was maintained at 7.5 using 0.1 mol / L NaOH solution. After the reaction, the reaction solution was transferred to a dialysis bag with a molecular weight cutoff of 10 kDa and dialyzed with deionized water at 4 °C for 36 h, with the dialysate changed every 8 h. After dialysis, the liquid in the dialysis bag was freeze-dried for 60 h (-50 °C, vacuum degree 15 Pa) to obtain hydrophilically modified α-amylase. Example Example

[0061] The preparation method of hydroxypropyl starch is as follows: S1. Prepare a starch slurry with a solid content of 40% by adding water to 500g of cassava starch. Adjust the pH of the starch slurry to 4.8 using 0.1mol / L sodium hydroxide solution. Add 0.3g of compound enzyme (the compound enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) and enzymatically hydrolyze the starch slurry at 45℃ for 25min, maintaining a stirring speed of 125r / min during the process to keep the pH of the starch slurry stable at 4.8. Then, place the enzymatically hydrolyzed starch slurry in an ultra-high pressure-ultrasound combined device and treat it at a pressure of 350MPa, an ultrasonic frequency of 30kHz, and a temperature of 38℃ to obtain pretreated starch slurry. S2. After cooling the pretreated starch milk to 25℃, add 3.75g of alkyl glycoside APG-12 and sonicate to dissolve for 15min. Adjust the pH of the reaction system to 11.5 with 4% sodium hydroxide solution. After sealing the reactor, replace the air inside with nitrogen. Measure 28g of propylene oxide into the reactor with stirring, then stir at 350r / min for 15min to obtain the reaction solution. Perform an etherification reaction using a three-stage gradient temperature control program (first stage: react at 25℃ for 35min; second stage: continue reacting at 30℃ for 4h; third stage: continue reacting at 40℃ for 6h) to obtain the reaction product. S3. After the reaction is complete, turn on the external circulating cooling water to cool the reaction product to 25°C. Adjust the pH to 6.0 with 10% dilute sulfuric acid solution and stir for 25 minutes to neutralize. After neutralization, wash the product slurry three times with twice the volume of water. Centrifuge the washed product to remove water, and vacuum dry it at 45°C for 12 hours. The dried product is then pulverized and passed through a 100-mesh sieve to obtain the hydroxypropyl starch product. Example

[0062] The difference between Example 2 and Example 1 is that in step S1 of Example 2, 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) is replaced with 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g of the hydrophobically modified pullulanase prepared in Preparation Example 1 and 0.15g α-amylase). Example

[0063] The difference between Example 3 and Example 1 is that in step S1 of Example 3, 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) is replaced with 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g of the hydrophilic modified α-amylase prepared in Preparation Example 2). Example

[0064] The difference between Example 4 and Example 1 is only that in step S1 of Example 4, 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) is replaced with 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g of the hydrophobic modified pullulanase prepared in Preparation Example 1 and 0.15g of the hydrophilic modified α-amylase prepared in Preparation Example 2). Example

[0065] The difference between Example 5 and Example 4 is that in step S2 of Example 5, 3.75g of alkyl glycoside APG-12 is replaced with 2g of alkyl glycoside APG-12 and 1.75g ​​of tetrabutylammonium bromide. Example

[0066] The only difference between Example 6 and Example 4 is that in step S2 of Example 6, 3.75g of alkyl glycoside APG-12 is replaced with 1.5g of alkyl glycoside APG-12 and 2.25g of tetrabutylammonium bromide. Example

[0067] The only difference between Example 7 and Example 4 is that in step S2 of Example 7, 3.75g of alkyl glycoside APG-12 is replaced with 2.75g of alkyl glycoside APG-12 and 1g of tetrabutylammonium bromide. Example

[0068] The difference between Example 8 and Example 5 is that in step S2 of Example 8, while adding 2g of alkyl glycoside APG-12 and 1.75g ​​of tetrabutylammonium bromide to the pretreated starch milk, 20g of 1-(2-hydroxyethyl)-3-methylimidazolium chloride (added in the form of an aqueous solution, with a mass ratio of ionic liquid to water of 1:9) is also added.

[0069] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that in step S1 of Comparative Example 1, 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) is replaced with 0.3g pullulanase.

[0070] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that in step S1 of Comparative Example 2, 0.3g of the complex enzyme (the complex enzyme is a mixture of 0.15g pullulanase and 0.15g α-amylase) is replaced with 0.3g of α-amylase.

[0071] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that in step S2 of Comparative Example 3, the reaction solution was subjected to a three-stage gradient temperature control program for etherification (first stage: the reaction solution was heated to 25°C and reacted for 35 min; second stage: the reaction solution was heated to 30°C and reacted for 4 h; third stage: the reaction solution was heated to 40°C and reacted for 6 h), instead of being heated to a constant temperature of 40°C and reacted for 10 h and 35 min.

[0072] The hydroxypropyl starch prepared in each example and comparative example was tested according to the following standards: (1) Determination of degree of hydroxypropyl substitution (MS): Spectrophotometry was used. The specific determination conditions were as follows: detection wavelength 590 nm, heating in a boiling water bath for 3 min, cooling to 15 °C in an ice bath, adding ninhydrin, and standing in a 25 °C water bath for 100 min. The absorbance was measured with a reagent blank as a reference. The hydroxypropyl content was calculated according to the standard curve. The molar substitution degree was then calculated using the formula MS=(WH×2.84) / (100-WH), where MS is the molar substitution degree of hydroxypropyl starch; WH is the content of hydroxypropyl (%); and 2.84 is the conversion factor for mass fraction to MS.

[0073] (2) Calculation of hydroxypropyl reaction efficiency: The theoretical maximum degree of substitution is calculated based on the actual amount of propylene oxide fed, and the hydroxypropyl reaction efficiency is calculated according to the formula "reaction efficiency (%) = (measured MS / theoretical MS) × 100%".

[0074] (3) Determination of residual propylene oxide and by-product content: Gas chromatography was used, with a headspace sampler and an SH-I-624SilMS column for separation, and flame ionization detector (FID) was used for detection. The content of residual propylene oxide and hydrolysis by-product propylene glycol in the reaction products and waste liquid was quantitatively determined by external standard method.

[0075] (4) Gelatinization temperature determination: Using a rapid viscosity analyzer, accurately weigh 3.0 g of sample (based on 14% wet basis) into a measuring cup, add 25 mL of deionized water, and run the following program: keep at 50℃ for 1 min, increase the temperature to 95℃ at 6℃ / min, keep at 95℃ for 5 min, decrease the temperature to 50℃ at 6℃ / min, keep at 50℃ for 2 min, and record the gelatinization initiation temperature.

[0076] The data from the above tests are summarized in Table 1 below. Table 1. Test results of hydroxypropyl starch prepared in each example and comparative example. Example 1 0.085 58.6 125 186 61.2 Example 2 0.096 66.2 118 172 60.5 Example 3 0.098 67.6 115 168 60.3 Example 4 0.118 81.4 98 142 59.1 Example 5 0.136 93.8 72 105 58.2 Example 6 0.105 72.4 108 158 60.8 Example 7 0.108 74.5 105 152 60.5 Example 8 0.152 94.6 68 98 57.6 Comparative Example 1 0.042 29.0 218 312 64.8 Comparative Example 2 0.045 31.0 212 305 64.5 Comparative Example 3 0.052 35.9 245 356 63.8 Based on Examples 1-8 and Table 1, it can be seen that Examples 1-8 all achieved high reaction efficiencies, with Examples 5 and 8 showing particularly significant improvements. The reasons for this may be as follows: After APG-12 and TBAB are combined, APG-12 reduces surface tension at the interface, improves reagent wetting and penetration, while TBAB transfers reactive species from the aqueous phase to the solid-phase reaction sites. This synergistic effect effectively enhances the mass transfer efficiency between the solid and liquid phases. The addition of the ionic liquid further exerts its effect at the molecular level; its hydroxyl groups form hydrogen bonds with starch hydroxyl groups, weakening the residual hydrogen bond network in the starch granules and making the microenvironment of the exposed active hydroxyl groups more relaxed, thereby reducing the steric hindrance and energy barrier of the substitution reaction. In contrast, although Examples 6 and 7 also used a combination of APG-12 and TBAB, the ratios were too high and too low, respectively, resulting in less effective results than Example 5. This may be because the imbalance in the ratio disrupts the synergistic balance between the two at the interface: when TBAB is in excess, too many cations may interfere with the orderly arrangement of APG-12 at the interface; when TBAB is insufficient, the transport efficiency of active species is limited, both of which lead to a decrease in phase transfer catalysis.

[0077] Combining Examples 1 and Comparative Examples 1-3, and referring to Table 1, it can be seen that the reaction efficiencies of Comparative Example 1 (single pullulanase) and Comparative Example 2 (single α-amylase) are significantly lower than those of Example 1. This may be because a single enzyme system can only hydrolyze α-1,6-glycosidic bonds or only endo-cleave α-1,4-glycosidic bonds, failing to achieve the synergistic effect of "debranching + endo-cleavage," resulting in insufficient destruction of the crystallization region and limited exposure of active hydroxyl groups. The reaction efficiency of Comparative Example 3 (reaction at isothermal 40°C) is also significantly lower than that of Example 1, and it produces more byproducts. This may be because propylene oxide undergoes extensive hydrolysis to generate propylene glycol under long-term reaction conditions at 40°C, leading to ineffective reagent consumption and a significant reduction in the effective concentration of the system participating in the etherification reaction. Furthermore, the isothermal high-temperature reaction may also cause degradation of starch granules and the removal of hydroxypropyl groups in a high-temperature, high-alkali environment, further affecting the increase in the degree of substitution.

[0078] This application utilizes a synergistic pretreatment process combining enzymatic hydrolysis and ultra-high pressure-ultrasonic physical field. This allows the complex enzyme to open starch granule channels from the surface inwards, ultra-high pressure to break intermolecular hydrogen bonds in the crystalline region from the inside out, and ultrasonic cavitation to further dissociate the crystalline region and prevent particle aggregation. These three processes complement each other, ensuring full exposure of the active hydroxyl groups in the starch. A phase transfer catalyst (preferably a combination of APG-12 and TBAB) is used to reduce interfacial tension and enhance solid-liquid mass transfer. A three-stage gradient heating process enables the gradual penetration and directional reaction of the chemical modifier, followed by rapid cooling and neutralization after the reaction to prevent starch granule degradation and substituent removal. These steps work synergistically to significantly improve the reaction efficiency of the chemical modifier, greatly increase the product's degree of substitution, effectively suppress side reactions, and reduce production costs and environmental impact.

[0079] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing modified starch through synergistic pretreatment and phase transfer catalytic gradient reaction, characterized in that, The preparation steps include the following: S1. Mix starch with water to prepare starch milk, add compound enzyme for enzymatic hydrolysis, and then place the enzymatically hydrolyzed starch milk in an ultra-high pressure-ultrasound combined device for treatment under the conditions of pressure 300-400MPa, ultrasonic frequency 20-40kHz, and temperature 35-40℃ to obtain pretreated starch milk. S2. Add a phase transfer catalyst to the pretreated starch milk, adjust the pH of the pretreated starch milk to 11.0-12.0, add a chemical modifier to obtain a reaction solution, and carry out the reaction solution using a three-stage gradient temperature control program to obtain the reaction product. S3. Cool the reaction product to 20-25℃ and adjust the pH to 5.5-6.0, and obtain the modified starch through post-treatment; The three-stage gradient temperature control procedure in step S2 includes: the first stage: controlling the temperature of the reaction solution to 20-25℃ and reacting for 30-40 minutes; the second stage: continuing to control the temperature of the reaction solution to 25-30℃ and reacting for 3-5 hours; and the third stage: continuing to control the temperature of the reaction solution to 35-40℃ and reacting for 5-7 hours.

2. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 1, characterized in that, The complex enzyme comprises pullulanase and α-amylase, wherein the mass ratio of pullulanase to α-amylase is 1:(0.8-1.2).

3. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 2, characterized in that, The pullulanase is a hydrophobically modified pullulanase, and the modification and preparation steps of the hydrophobically modified pullulanase are as follows: Pullulanase was dissolved in phosphate buffer at pH 7.0-8.0, 2-octenyl succinic anhydride was added, and the reaction was carried out at room temperature. The hydrophobically modified pullulanase was obtained after post-treatment.

4. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 2, characterized in that, The α-amylase is a hydrophilically modified α-amylase, and the modification steps of the hydrophilically modified α-amylase are as follows: α-Amylase was dissolved in phosphate buffer at pH 7.0-8.0 and reacted with polyethylene glycol under the mediation of carbodiimide. The hydrophilic modified α-amylase was obtained after post-treatment.

5. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 1, characterized in that, In step S1, the starch is selected from one or more of tapioca starch, corn starch, and potato starch.

6. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 1, characterized in that, In step S2, the phase transfer catalyst is alkyl glycoside APG-12.

7. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 6, characterized in that, The phase transfer catalyst further includes tetrabutylammonium bromide, and the mass ratio of the alkyl glycoside APG-12 to tetrabutylammonium bromide is 1:(0.5-1.2).

8. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 1, characterized in that, In step S2, while adding the phase transfer catalyst to the pretreated starch milk, a hydroxyl-functionalized imidazole ionic liquid is also added. The amount of the hydroxyl-functionalized imidazole ionic liquid added is 0.5-5 wt% of the starch mass.

9. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 8, characterized in that, The hydroxyl-functionalized imidazole ionic liquid is selected from one or more of 1-(2,3-dihydroxypropyl)-3-methylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, and 1-(3-hydroxypropyl)-3-methylimidazolium dodecyl sulfate.

10. The method for preparing modified starch by synergistic pretreatment and phase transfer catalytic gradient reaction according to claim 1, characterized in that, In step S2, the chemical modifier is an etherifying agent, an esterifying agent, or a crosslinking agent; the etherifying agent is propylene oxide, the esterifying agent is acetic anhydride, and the crosslinking agent is sodium trimetaphosphate or phosphorus oxychloride.