Method for preparing structured starch coating liquid with oil-proof and moisture-absorbing dual functions

Abstract

The present application relates to the technical field of starch coating, in particular to a preparation method of structured starch coating liquid with oil-proof and moisture-absorbing dual functions. The method comprises the following steps: firstly, performing mild acidolysis or selective oxidation pretreatment on cassava starch; then, preparing starch milk by using the pretreated starch, and performing synergistic enzymolysis on the starch milk under mild conditions by using a composite enzyme composed of alpha-amylase and glucoamylase in a specific ratio; after the enzymolysis, performing enzyme inactivation treatment, and then adding a bio-based crosslinking agent for compounding. The prepared coating has a specific bimodal distribution structure of starch derivative molecular weight, which is a key to realize function balance. After the coating liquid is coated on a substrate such as paper and dried and solidified, the formed coating not only has excellent oil-proof performance, but also exhibits good water vapor absorption capacity. The raw materials of the present application are green and the process is environmentally friendly, and the prepared multifunctional coating can be effectively applied to the fields of food packaging, medical protection and other fields which need to resist oil and regulate humidity at the same time.

Description

Technical Field

[0001] This invention relates to the field of starch coating technology, and more specifically, to a method for preparing a structured starch coating liquid with dual functions of oil resistance and moisture absorption. Background Technology

[0002] Cellulose-based materials such as paper and cardboard are ideal alternatives to plastic packaging due to their biodegradable and renewable properties. However, their porous structure makes them susceptible to grease penetration and their limited moisture absorption capacity makes them unsuitable for complex applications that require both grease barrier and moisture absorption (such as packaging for fried foods, liners for fresh produce trays, and medical dressing packaging).

[0003] In existing technologies, fluorinated compounds or polyethylene (PE) coatings are mainly used to impart oil resistance to paper, which pose potential environmental and health risks, and the latter is difficult to degrade. To improve the hygroscopicity of paper, superabsorbent resins (such as sodium polyacrylate) are often coated, but their biodegradability is poor, and they usually cause the material to become completely hydrophilic and lose its oil resistance.

[0004] Starch, as a natural and fully biodegradable polymer, has been widely studied for the preparation of environmentally friendly coatings. However, natural starch films are brittle, have poor water resistance, and limited functionality. While single chemical modifications (such as acetylation and octenyl succinic anhydride modification) can improve hydrophobicity and oil resistance, they sacrifice hydrophilicity. Conversely, preparing porous starch or highly hydrolyzed starch can enhance hygroscopicity, but this severely compromises the continuity and barrier properties of the film. Although some studies have used enzymatic modifications to improve starch properties, these methods typically have a single objective (such as only improving film-forming properties or preparing dextrin) or employ complex enzyme systems and cumbersome processes.

[0005] More importantly, existing technologies lack a deep understanding and proactive design of the structure-property relationship between the microstructure of starch-based coatings and their macroscopic oil-repellent / moisture-absorbing dual functions. Oil repellency requires a dense, continuous coating with a high degree of network cross-linking; while rapid moisture absorption requires abundant hydrophilic channels or regions in the material. These two structural requirements are traditionally considered contradictory. Therefore, developing a simple starch modification method that can spontaneously construct this contradictory yet unified microstructure has become a pressing technical challenge in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a structured starch coating liquid with dual functions of oil resistance and moisture absorption, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, on the one hand, this invention provides a method for preparing a structured starch coating liquid with dual functions of oil resistance and moisture absorption. This method, through precise control of pretreatment and compound enzymatic hydrolysis, ensures that the molecular weight of the final enzymatic hydrolysis product exhibits a specific bimodal distribution, and based on this, forms a microphase separation structure in subsequent film formation. The method includes the following steps: S1. Structural Pre-regulation - Pretreatment: Cassava starch is subjected to mild acid hydrolysis or selective oxidation pretreatment to obtain pretreated starch; the core control index of the pretreatment is: This increases the solubility of pretreated starch to 15%-35%, while its particle morphology is partially eroded but not completely disintegrated.

[0008] The mild acid hydrolysis is carried out using 0.5%-1.5% dilute hydrochloric acid at 45-55°C for 1-2 hours. The selective oxidation is carried out using a low dose of oxidant (sodium hypochlorite with an effective chlorine content of 0.5%-2%), under alkaline conditions (pH 8-9.5) and at 35-45°C for 0.5-1.5 hours.

[0009] S2. Bimodal Structure Construction - Complex Enzymatic Hydrolysis: The pretreated starch obtained in S1 is prepared into a starch milk of 8%-15%, and the pH is adjusted to 5.5-6.2. At 52-58℃, a complex enzyme composed of α-amylase and saccharifying enzyme is added for time-process controlled enzymatic hydrolysis, with a total reaction time of 1.5-2.2 hours. The ratio of enzyme activity units of α-amylase to saccharifying enzyme is 1:(1.0-1.3), and the total amount of complex enzyme added is 0.08%-0.18% based on the dry weight of the pretreated starch.

[0010] S3, Functional Curing - Post-treatment: After inactivating the enzyme in the enzymatic hydrolysate obtained in S2, immediately add a specific combination of functional additives, including plasticizers (glycerol, sorbitol, added at 8%-12% of the system solids) and bio-based crosslinking agents (citric acid, added at 4%-7% of the system solids). After stirring evenly, a uniform and stable structured starch coating liquid is obtained.

[0011] Secondly, the present invention provides a structured starch coating liquid with dual functions of oil resistance and moisture absorption prepared by the above preparation method. The starch derivative contained in the coating liquid has a bimodal molecular weight distribution, wherein the first peak is located in the range of 5000-20000 Da (accounting for 60%-75%) and the second peak is located in the range of <1000 Da (accounting for 25%-40%).

[0012] Thirdly, the present invention provides a structured starch coating with dual functions of oil resistance and moisture absorption, which is obtained by coating the above-mentioned coating liquid onto the surface of a substrate and then drying and curing it into a film.

[0013] Fourthly, the present invention provides the application of the above-mentioned structured starch coating with dual functions of oil resistance and moisture absorption in the preparation of oil-resistant food packaging paper, medical protective pads or fresh food preservation packaging linings with moisture-wicking function.

[0014] Furthermore, this invention achieves both oil-repellent and moisture-absorbing functions by employing a precisely oriented process to in-situ construct a coating system with a framework and micro-regional microphase separation structure from natural cassava starch. Its reaction and structural evolution are described below: Phase 1 (S1 Pretreatment): The starting point for structural pre-activation and homogenization. Natural starch granules have a dense structure and uneven enzyme attack sites. Mild acid hydrolysis or selective oxidation preferentially and gently erodes the granule surface and amorphous regions, partially breaking long chains and introducing polar groups (carboxyl groups), causing granule swelling and increased solubility. The core of this step is to create a large number of uniform weak points that are easily attacked by enzymes, rather than deep destruction, thereby ensuring that subsequent enzymatic hydrolysis can start from a relatively uniform starting point, avoiding local over-hydrolysis, and laying the foundation for obtaining narrowly distributed and predictable enzymatic hydrolysis products.

[0015] Phase 2 (S2 complex enzymatic hydrolysis): Precise construction of kinetics and bimodal structure. Within a specific pH, temperature and narrow enzyme ratio (α-amylase: saccharifying enzyme = 1:1.0-1.3) and time (1.5-2.2 h) window, the two enzymes do not act independently, but produce a synergistic effect of kinetics.

[0016] α-Amylase acts as the main disintegrant, randomly endocleaving starch chains and rapidly degrading macromolecules into a series of medium-length dextrins and oligosaccharides (Mw 5k-20k Da), which are the main source of the future coating continuous phase backbone.

[0017] As fine-tuners, saccharifying enzymes rapidly convert oligosaccharides into small sugar molecules (Mw < 1 kDa) by exocleaving from the non-reducing ends. This conversion is slightly higher than that of α-amylase, ensuring that sufficient small sugar molecules that can serve as hydrophilic units are generated simultaneously while α-amylase produces enough skeletal material.

[0018] The rates of the two enzymes are optimally matched within this window. If there is too little saccharifying enzyme, the yield of small molecule sugars will be insufficient, resulting in poor hydrophilicity of the coating. If there is too much, the framework material will be over-trimmed, making it impossible to form a continuous membrane. Terminating the reaction at the preferred time point can freeze this dynamic process and obtain a bimodal distribution mixture with a relatively fixed ratio of framework material to hydrophilic units. This bimodal distribution is the material prerequisite for the subsequent formation of microphase separation structure.

[0019] Phase Three (S3 Post-treatment and Film Formation): Function-oriented assembly and curing. The addition of citric acid plays a triple role in the subsequent drying and curing process: Crosslinking agent: Its carboxyl groups undergo esterification with the hydroxyl groups on the starch chain, crosslinking medium molecular weight backbone segments into a three-dimensional network, which greatly enhances the density, mechanical strength and oil resistance of the coating.

[0020] Hydrophilic agents: Their unreacted carboxyl groups are strong hydrophilic groups, which together with small sugar molecules constitute a strong hydrophilic component.

[0021] Structure directing agent: Under the action of water evaporation and heat, polar small molecule sugars and citrate carboxyl groups tend to migrate from the hydrophobic starch backbone network and self-aggregate due to thermodynamic incompatibility, forming nanoscale hydrophilic microdomains, which are uniformly dispersed in the continuous, cross-linked starch network backbone. Plasticizers (glycerol / sorbitol) play a role in softening the network, promoting the formation of microdomains and preventing film brittleness.

[0022] Ultimately, through the aforementioned three-step continuous process, a stable microphase separation structure was successfully constructed within a single coating. The continuous, cross-linked starch network endows the coating with excellent oil-repellent properties, while the uniformly dispersed nano-hydrophilic microdomains provide efficient channels for the rapid adsorption and diffusion of water vapor, achieving moisture absorption. These two structures coexist at the microscopic level and synergize at the macroscopic level, thus resolving the contradiction between oil repellency and moisture absorption.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: The preparation method of this structured starch coating liquid with dual functions of oil resistance and moisture absorption involves guiding the starch raw material to self-assemble into a coating system with an inherent microphase separation structure at the molecular and supramolecular levels through pretreatment directional weakening, compound enzymatic hydrolysis, and post-treatment in-situ curing processes. This structure resolves the contradiction between dense oil resistance and rapid moisture absorption in macroscopic requirements, achieving dual functionality in one unit. The coating simultaneously possesses excellent oil resistance and rapid water vapor absorption capabilities, meeting the needs of complex application scenarios for multifunctional material integration.

[0024] Secondly, the entire technical route uses natural and renewable cassava starch as the main raw material and adopts a biological enzymatic method. Food-grade or bio-based additives (such as citric acid and glycerin) are used in the post-processing. The resulting coating material is completely biodegradable, avoiding the environmental and health risks brought by traditional fluorinated compounds or plastic coatings, and meeting the requirements of sustainable development.

[0025] Meanwhile, the preparation method has clear steps and mild conditions (such as medium-temperature reaction), requiring no complex equipment or harsh synthesis environment. By precisely controlling several key process parameters (pretreatment degree, enzyme ratio, reaction time), products with consistent performance can be obtained stably and repeatedly, which has the potential for large-scale industrial production.

[0026] The prepared bifunctional coating can be directly applied to low-cost substrates such as paper and cardboard, giving them high-value-added functional properties. This material is particularly suitable for occasions that require simultaneous barrier against grease and absorption of moisture, water vapor or a small amount of condensate, such as high-end takeaway packaging (especially fried food), fresh food preservation liners, medical protective sheets, etc., with a clear market application prospect. Attached Figure Description

[0027] Figure 1 This is an overall flowchart of Embodiment 1 of the present invention. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] First, all raw materials are commercially available products, and the testing methods are as follows: Starch solubility: According to the standard method, take a dry sample, dissolve it in boiling water, centrifuge, take the supernatant, dry it, weigh it, and calculate the solubility.

[0030] Molecular weight distribution: Gel permeation chromatography (GPC) was performed using a differential refractive index detector, calibrated with amylopectin standards.

[0031] Oil resistance: Kit value was determined according to GB / T 22805.2-2008 (Determination of grease resistance of paper and paperboard).

[0032] Water vapor absorption rate: Referring to the principle of GB / T 1540-2002 (Determination of water absorption of paper and paperboard, Koebner method), the weight gain (g / m²) of coated paper per unit area per unit time was measured under the conditions of 23±1℃ and 50±5% RH. 2 ·h).

[0033] Coating preparation: Unless otherwise specified, the coating liquid is applied using a wire rod to a 40 g / m² coating. 2 On food-grade white kraft paper, the dry coating amount is controlled at 8±0.5 g / m². 2 Dry in hot air at 105℃ for 90 seconds.

[0034] Example 1: This example provides a method for preparing a structured starch coating with dual functions of oil resistance and moisture absorption. S1 involves acid hydrolysis pretreatment (see...). Figure 1 ), including the following steps: S1. Pretreatment: Weigh 100g of cassava starch and disperse it in 200mL of 1.0% (w / w) hydrochloric acid solution. Stir the mixture in a 50℃ constant temperature water bath for 1.5 hours. After the reaction, neutralize the solution with 5% NaOH solution to pH 7.0. Filter the solution, wash it three times with deionized water, and dry it in a 55℃ oven to constant weight. The solubility of the pretreated starch was measured to be 26%.

[0035] S2. Compound Enzymatic Hydrolysis: Weigh 20g of the pretreated starch and add 180g of deionized water to prepare a 10% starch slurry. Adjust the pH to 5.8 with 0.1M NaOH solution, transfer to a 55℃ constant temperature water bath, and add the compound enzyme solution under mechanical stirring. The compound enzyme consists of α-amylase (enzyme activity 5000U / g) and saccharifying enzyme (enzyme activity 100000U / g), with a ratio of 1:1.1 based on enzyme activity units. The total amount of compound enzyme added is 0.12% of the dry weight of the pretreated starch. Continue the reaction for 2.0 hours.

[0036] S3. Post-treatment: Quickly place the reaction solution in a 92°C water bath and keep it for 20 minutes to completely inactivate the enzyme. Cool it to below 60°C, add glycerol (10% of the total solids in the system) and food-grade citric acid monohydrate (5.5% of the total solids in the system), and continue stirring for 40 minutes until completely homogeneous to obtain a transparent, slightly yellow coating solution, which is labeled as sample A.

[0037] test: GPC test: Take an appropriate amount of sample A, dilute and filter it, and then inject it. The results show that its molecular weight has a clear bimodal distribution: peak 1 (weight average molecular weight Mw≈16200Da, accounting for about 70%); peak 2 (Mw≈750Da, accounting for about 30%).

[0038] Coating and performance testing: Coated according to standard methods. The kit's oil repellency rating was measured to be 12, and its water vapor absorption rate was 21.5 g / m³. 2 ·h.

[0039] Example 2: This example provides a method for preparing a structured starch coating with dual functions of oil resistance and moisture absorption. S1 involves an oxidation pretreatment, including the following steps: S1. Pretreatment: Weigh 100g of cassava starch and disperse it in 200mL of water. Adjust the pH to 9.0 with 5% NaOH solution. Slowly add 10mL of sodium hypochlorite solution with an effective chlorine content of 1.5%. Stir and react at 40℃ for 1.0 hour. Add an appropriate amount of 10% NaHSO3 solution to quench the residual oxidant. Adjust the pH to neutral with dilute hydrochloric acid. The subsequent washing and drying are the same as in Example 1. The solubility of the pretreated starch was measured to be 31%.

[0040] S2. Compound enzymatic hydrolysis: Weigh 20g of the above oxidized starch and prepare 12% starch milk. Adjust the pH to 6.0 with 0.1M HCl. Add a compound enzyme with an enzyme activity ratio of 1:1.2 (α-amylase: saccharifying enzyme) at 56℃. The total amount added is 0.10% of the dry starch. React for 1.8 hours.

[0041] S3. Post-treatment: After enzyme inactivation, add sorbitol (9%) and citric acid (6%), stir evenly, and obtain coating solution sample B.

[0042] Testing: GPC showed a bimodal distribution (Mw 17500Da and 650Da). Coating performance: Kit value 10, water vapor absorption 19.0 g / m³. 2 ·h.

[0043] Example 3: This example is basically the same as Example 1, except that the ratio of the complex enzyme is adjusted to demonstrate the adjustability of the bimodal distribution ratio.

[0044] In step S2, the enzyme activity ratio of α-amylase to saccharifying enzyme is adjusted to 1:1.25, the total enzyme activity remains unchanged, the reaction time is still 2.0 hours, and the remaining steps are exactly the same as in Example 1.

[0045] Test results: GPC test: The molecular weight of the coating liquid shows a bimodal distribution: peak 1 (Mw≈14000Da, accounting for about 65%); peak 2 (Mw≈800Da, accounting for about 35%).

[0046] Coating performance: Kit has an oil resistance rating of 11 and a water vapor absorption rate of 23.0 g / m³. 2 ·h.

[0047] Comparative Example 1: No pretreatment control The purpose is to examine the necessity of the pretreatment step for forming a bimodal structure and final performance.

[0048] Procedure: Except for omitting the pretreatment step and directly using natural cassava starch for enzymatic hydrolysis, all other conditions (enzyme ratio, dosage, time, and post-treatment) were exactly the same as in Example 1. The resulting coating solution was labeled as Sample C. Test Results: GPC analysis: The molecular weight distribution shows a broad and asymmetrical single peak, ranging from several thousand to hundreds of thousands of Daltons, without obvious bimodal separation.

[0049] Coating performance and characteristics: Leveling during application was slightly poor; after drying, the film showed minor cracks; oil resistance was only grade 3; water vapor absorption was low and uneven, averaging 7.8 g / m³. 2 ·h.

[0050] Conclusion: Without pretreatment, starch granules have intact structures, and enzyme attack is random and inefficient, making it impossible to start from a uniform starting point for synergistic degradation. This results in a wide molecular weight distribution and uncontrollable structure of the products, making it impossible to form effective continuous and dispersed phases, and both bifunctional phases are very poor.

[0051] Comparative Example 2: Single α-amylase control The aim was to investigate the key role of saccharifying enzymes in synergistic effects and the generation of hydrophilic units.

[0052] Procedure: Pretreatment was the same as in Example 1. In the enzymatic hydrolysis stage, only α-amylase with the same total enzyme activity as in Example 1 was added (i.e., the amount of saccharifying enzyme added was 0). The reaction time was 2.0 hours, and the remaining steps remained unchanged, yielding coating solution sample D. Test Results: GPC analysis: It presents a single main peak with a molecular weight of approximately 38,000 Da, and there is almost no signal in the low molecular weight region (<1,000 Da).

[0053] Coating performance: The coating is continuous and glossy, with an oil resistance rating of 9, but its water vapor absorption rate is extremely low, only 3.5 g / m³. 2 ·h.

[0054] Conclusion: The lack of saccharifying enzymes prevents the effective generation of sufficient small-molecule hydrophilic sugars. The products are almost entirely medium- to high-molecular-weight fragments, which can form a dense oil-resistant film, but the severe lack of hydrophilic components renders the moisture absorption function almost ineffective.

[0055] Comparative Example 3: Enzyme Imbalance Control (Insufficient Glycoamylase) The purpose is to investigate the effect when the enzyme ratio deviates from the scope of this invention.

[0056] Procedure: Pretreatment was the same as in Example 1. During enzymatic hydrolysis, the enzyme activity ratio of α-amylase to saccharifying enzyme was adjusted to 1:0.5 (i.e., the proportion of saccharifying enzyme was lower), while the total enzyme activity remained unchanged. The reaction was carried out for 2.0 hours to obtain coating solution sample E. Test Results: GPC analysis: The high molecular weight fraction accounts for a large proportion, with a strong peak in the Mw~30000 Da range; the low molecular weight fraction has only one weak peak (estimated to account for <10%).

[0057] Coating performance: Oil resistance grade 10, water vapor absorption rate 8.2 g / m³ 2 ·h.

[0058] Conclusion: The proportion of saccharifying enzymes was insufficient, the yield of small molecule sugars was inadequate, and the product was more similar to the effect of a single α-amylase in Experiment 4. Although it had some oil-resistant properties, the improvement in hygroscopic performance was limited due to the insufficient number of hydrophilic microregions.

[0059] Comparative Example 4: Imbalanced Enzyme Ratio Control (Excess Saccharifying Enzyme) The aim is to further investigate the effect of deviations in enzyme ratios.

[0060] Procedure: Pretreatment was the same as in Example 1. During enzymatic hydrolysis, the enzyme activity ratio of α-amylase to saccharifying enzyme was adjusted to 1:2.0 (i.e., saccharifying enzyme in excess), while the total enzyme activity remained unchanged. The reaction was carried out for 2.0 hours to obtain coating solution sample F. Test results: GPC analysis: High molecular weight peaks almost disappeared, and the spectrum mainly showed a series of low molecular weight oligosaccharide and sugar peaks, with the main Mw below 5000 Da.

[0061] Coating performance: The coating cannot form a complete, continuous film, exhibiting a porous and brittle state. Oil resistance is extremely poor, only grade 2; although the water vapor absorption rate is as high as 24 g / m³... 2 ·h, but it has completely lost its basic function as a barrier coating.

[0062] Conclusion: Excessive saccharifying enzymes and synergistic imbalance with α-amylase lead to excessive degradation of the starch backbone. The product lacks sufficient medium molecular weight fragments to construct a continuous phase, thus failing to form a film for oil prevention. Despite its strong hydrophilicity, it has no practical value.

[0063] Comparative Example 5: Control with insufficient enzymatic hydrolysis time The aim is to examine the impact of reaction time on the collaborative process.

[0064] Procedure: Pretreatment was the same as in Example 1, and enzymatic hydrolysis conditions were also the same as in Example 1, but the reaction time was shortened to 1.0 hour to obtain coating solution sample G. Test Results: GPC analysis: The molecular weight distribution is still dominated by high molecular weight (more components with Mw>50000Da). Although there are signs of initiation in the low molecular weight region, two separate peaks have not yet formed.

[0065] Coating performance: The coating structure is relatively loose. Oil resistance is grade 4, and water vapor absorption is 10.1 g / m³. 2 ·h.

[0066] Conclusion: Insufficient reaction time, incomplete synergistic degradation process, inadequate degradation of the framework material, and insufficient generation of hydrophilic units resulted in unsatisfactory performance in both aspects.

[0067] Comparative Example 6: Control with excessively long enzymatic hydrolysis time The aim is to further investigate the effect of reaction time.

[0068] Procedure: Pretreatment was the same as in Example 1, and enzymatic hydrolysis conditions were also the same as in Example 1, but the reaction time was extended to 3.0 hours to obtain coating solution sample H. Test Results: GPC analysis: Similar to Experiment 6 (excess saccharifying enzyme), the high molecular weight component was greatly reduced, and the product was mainly low molecular weight.

[0069] Coating performance: The coating is brittle and easily peels off; oil resistance is grade 3; water vapor absorption rate is 17.5 g / m³. 2 ·h.

[0070] Conclusion: When the reaction time is too long, the medium molecular weight fragments that originally served as the backbone are further hydrolyzed into smaller molecules under the continuous action of the complex enzyme, which destroys the bimodal structure and leads to an imbalance in performance.

[0071] The key process parameters, structural features, and final performance of the above embodiments and comparative examples are summarized in Table 1 below to clearly demonstrate their correlation.

[0072] Table 1: Comprehensive Comparison of Processes, Structures, and Performance of Each Test Example sample Preprocessing Enzyme ratio (α: sugar) Enzymatic hydrolysis time (h) Molecular weight distribution characteristics (GPC) Kit oil resistance rating <![CDATA[Water vapor absorption rate (g / m 2 ·h)]]> Dual-function synergy evaluation Example 1 Mild acid hydrolysis 1:1.1 2.0 Typical bimodal (16.2k & 0.75kDa) 12 21.5 Excellent (high in both academic and professional fields) Example 2 Selective oxidation 1:1.2 1.8 Typical bimodal (17.5k & 0.65kDa) 10 19.0 Excellent (high in both academic and professional fields) Example 3 Mild acid hydrolysis 1:1.25 2.0 Typical bimodal (14.0k & 0.80kDa) 11 23.0 Excellent (high in both academic and professional fields) Comparative Example 1 none 1:1.1 2.0 Broad single peak, no separated double peak 3 7.8 Poor (double bottom) Comparative Example 2 Mild acid hydrolysis 1:0 (α enzyme only) 2.0 High molecular weight single peak (~38kDa) 9 3.5 Imbalance (high oil resistance, low moisture absorption) Comparative Example 3 Mild acid hydrolysis 1:0.5 2.0 The peaks are mainly high molecular weight, with weaker peaks for low molecular weight. 10 8.2 Imbalanced (decent oil resistance, insufficient moisture absorption) Comparative Example 4 Mild acid hydrolysis 1:2.0 2.0 Mainly low molecular weight 2 24.0 Imbalance (low oil resistance, high moisture absorption) Comparative Example 5 Mild acid hydrolysis 1:1.1 1.0 Insufficient degradation, no bimodal peaks 4 10.1 Poor (both low) Comparative Example 6 Mild acid hydrolysis 1:1.1 3.0 Excessive hydrolysis, similar to low molecular weight single peaks 3 17.5 Poor (poor oil resistance, average moisture absorption) As shown in Table 1, the molecular weight distribution characteristics of the coating solution are directly and reproducibly correlated with the coating performance formed on the substrate. Only when the enzymatic hydrolysis products exhibit a typical bimodal molecular weight distribution, i.e., when there are significant medium molecular weight peaks (5000-20000 Da) and low molecular weight peaks (<1000 Da) simultaneously, can the prepared coating achieve high oil resistance (Kit ≥ 9 grade) and high moisture absorption (≥ 15 g / m³). 2 The simultaneous enhancement of h) (as in Examples 1-3). Conversely, any product that deviates from this specific structural characteristic (such as a broad single peak, a single high molecular weight peak, or a single low molecular weight peak) cannot enable the coating to possess both excellent properties simultaneously.

[0073] As can be seen from the performance data comparison, the performance of the coating obtained by using the complete process of this invention (such as Examples 1-3) is not something that the control groups (which only have a single functional tendency or poor performance in all aspects) can achieve. Specifically, while fully maintaining or even enhancing the extremely high oil resistance level, it significantly introduces excellent moisture absorption capacity, allowing two performance indicators that are usually mutually restrictive to coexist synergistically at a high level.

[0074] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a structured starch coating liquid with dual functions of oil resistance and moisture absorption, characterized in that, Through pretreatment, complex enzymatic hydrolysis, and post-treatment steps, the molecular weight of starch derivatives exhibits a bimodal distribution, ultimately forming a microphase separation structure in the coating. This process includes the following steps: S1. Pretreatment: Tapioca starch is subjected to mild acid hydrolysis or selective oxidation pretreatment to obtain pretreated starch with increased solubility of 15%-35%. S2. Compound Enzymatic Hydrolysis: The pretreated starch obtained in S1 is prepared into a starch milk with a content of 8%-15%, and the pH is adjusted to 5.5-6.

2. Enzymatic hydrolysis is then carried out at 52-58℃ by adding a compound enzyme consisting of α-amylase and saccharifying enzyme. The total reaction time is 1.5-2.2 hours. The ratio of enzyme activity units of α-amylase to saccharifying enzyme is 1:1.0-1.3, and the total amount of the compound enzyme added is 0.08%-0.18% based on the dry weight of the pretreated starch. S3. Post-treatment: After inactivating the enzyme in the enzymatic hydrolysate obtained in S2, add plasticizer and bio-based crosslinking agent, stir evenly to obtain a structured starch coating liquid; the plasticizer is glycerol or sorbitol, and its addition amount accounts for 8%-12% of the total solids in the system; the bio-based crosslinking agent is citric acid, and its addition amount accounts for 4%-7% of the total solids in the system.

2. The method for preparing the structured starch coating liquid with dual functions of oil resistance and moisture absorption according to claim 1, characterized in that, In step S1, the mild acidolysis is performed using dilute hydrochloric acid with a concentration of 0.5%-1.5% w / w at 45-55°C for 1-2 hours.

3. The method for preparing the structured starch coating liquid with dual functions of oil resistance and moisture absorption according to claim 1, characterized in that, In step S1, the selective oxidation uses a sodium hypochlorite solution with an effective chlorine content of 0.5%-2% as the oxidant, and reacts at pH 8-9.5 and 35-45°C for 0.5-1.5 hours.

4. A structured starch coating liquid with dual functions of oil resistance and moisture absorption, prepared by the method of any one of claims 1-3, characterized in that... The starch derivative contained in the coating liquid has a bimodal molecular weight distribution, with the first peak located in the weight-average molecular weight range of 5000-20000 Da.

5. The structured starch coating liquid with dual functions of oil resistance and moisture absorption according to claim 4, characterized in that, In the bimodal distribution, the first peak accounts for 60%-75%, and the second peak accounts for 25%-40%.

6. A structured starch coating with dual functions of oil resistance and moisture absorption, characterized in that, It is obtained by coating the substrate surface with the structured starch coating liquid with dual functions of oil resistance and moisture absorption as described in claim 4 or 5, and then drying and curing it into a film.

7. The structured starch coating with dual functions of oil resistance and moisture absorption according to claim 6, characterized in that, The substrate is paper or paperboard.

8. The application of the structured starch coating with dual functions of oil resistance and moisture absorption as described in claim 6 in the preparation of oil-resistant food packaging paper.

9. The application of the structured starch coating with dual functions of oil resistance and moisture absorption as described in claim 6 in the preparation of medical protective pads.

10. The application of the structured starch coating with dual functions of oil resistance and moisture absorption as described in claim 6 in the preparation of fresh food preservation packaging lining materials.

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