Difunctional coating based on enzymolysis modified cassava starch as well as preparation method and application of difunctional coating

Abstract

The invention belongs to the field of biomass-based functional materials, relates to a difunctional coating based on enzymolysis modified cassava starch as well as a preparation method and application of the difunctional coating, and innovatively solves the technical problem that a single coating gives consideration to oil prevention and water absorption through a synergistic technical path of pretreatment and composite enzymolysis. The core mechanism is as follows: the starch particle structure is destroyed through pretreatment, and the degradation degree of starch is accurately controlled through the synergistic effect of alpha-amylase and saccharifying enzyme, so that an unexpected technical effect is achieved, and an enzymolysis product with a film-forming skeleton (medium molecular weight dextrin) and a hydrophilic unit (small molecular saccharide) is formed; meanwhile, a compact and continuous network structure can be formed by a film-forming framework, and grease permeation is blocked; and the hydrophilic unit endows the coating with a rapid capillary water absorption effect.

Description

Technical Field

[0001] This invention belongs to the field of biomass-based functional materials, and relates to a bifunctional coating based on enzymatically modified cassava starch, its preparation method, and its application. Background Technology

[0002] Paper, cardboard, and other fibrous substrates are ideal alternatives to plastic products due to their recyclability and biodegradability. However, their porous structure makes them susceptible to grease penetration, and their inherent water absorption capacity is limited, restricting their application in oily or absorbent applications (such as food packaging boxes and medical pads). Currently, imparting oil resistance to substrates mainly involves using fluorinated compounds or polyethylene (PE) coatings. The former poses environmental and health risks, while the latter is difficult to degrade, leading to "white pollution." Improving water absorption capacity largely relies on synthetic polymer resins, which also face environmental challenges.

[0003] Starch is a widely available, fully biodegradable natural polymer, but its film-forming properties, flexibility, and functionality are insufficient. Coating paper with a starch layer is an effective and common method to improve its oil resistance; however, a simple starch coating only provides basic, short-term oil resistance. It cannot resist prolonged contact with or penetration by highly penetrating greases (such as certain mechanical lubricants). For applications requiring extremely high oil resistance (such as long-term storage of fried foods), a simple starch coating is insufficient. Furthermore, starch coatings dissolve or soften when exposed to water or steam, losing their barrier function; therefore, they are also unlikely to provide moisture or water resistance.

[0004] Existing technologies for modifying starch to prepare coatings often have the following limitations: (1) Single function: They either focus on improving hydrophobicity and oil resistance through esterification and etherification, or on preparing porous starch with high adsorption capacity, making it difficult to achieve a balance between oil resistance and water absorption in the same coating. (2) Complex process: They often use multi-step chemical modification or continuous treatment with multiple enzymes, which is lengthy, costly, and may introduce chemical residues. (3) Poor performance: Coatings obtained by direct enzymatic hydrolysis of natural starch are brittle and prone to cracking, and their overall performance is unstable.

[0005] Therefore, developing a starch-based coating that uses green raw materials, has a simple process, and can impart excellent oil-repellent and water vapor-absorbing functions to the substrate has significant practical importance and market value. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the present invention aims to provide a simple, green, and safe bifunctional coating based on enzymatically modified cassava starch and its preparation method. This coating simultaneously provides the substrate with excellent oil resistance and its own rapid water vapor absorption capacity.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention discloses a method for preparing a bifunctional coating liquid based on enzymatically modified cassava starch, comprising the following steps: (1) Pretreatment: Cassava starch is acid-hydrolyzed or oxidized to obtain pretreated cassava starch; Acid hydrolysis can moderately reduce the molecular weight of starch and increase the non-crystalline region, creating favorable conditions for subsequent enzymatic hydrolysis.

[0008] Oxidative modification introduces carboxyl and carbonyl groups, which enhances the hydrophilicity and dispersibility of starch.

[0009] (2) Compound enzymatic hydrolysis: The pretreated cassava starch obtained in step (1) is prepared into a starch milk with a mass concentration of 5%-20%, and the pH of the system is adjusted to 5.0-6.5 (this is the optimal pH range for the synergistic effect of α-amylase and saccharifying enzyme). At a temperature of 50-65℃, a compound enzyme composed of α-amylase and saccharifying enzyme is added for enzymatic hydrolysis for 2-2.5 hours; the ratio of enzyme activity units of α-amylase to saccharifying enzyme is 1:0.8-1.5. The core of complex enzymatic hydrolysis lies in the synergistic effect of α-amylase (endopeptidase) and glucoamylase (exopeptidase), rather than a simple addition.

[0010] The key to achieving optimal bifunctional balance is to strictly control the enzyme activity ratio of α-amylase to glucoamylase to 1:(0.8-1.5) as described in this invention.

[0011] If the proportion of saccharifying enzymes is too low, insufficient small molecule sugars will be generated, resulting in poor hydrophilicity and water absorption of the coating, which may manifest as substandard water absorption.

[0012] If the proportion of saccharifying enzymes is too high, it will over-cleave the starch skeleton, producing too many small molecules, which will destroy the compactness of the continuous phase and lead to a significant decrease in oil resistance.

[0013] By adjusting this ratio and the enzymatic hydrolysis time (2-2.5 hours), this invention can precisely control the ratio of the "oil-resistant skeleton" to the "hydrophilic unit," thereby achieving "on-demand customization" of the final coating's oil resistance level and water absorption rate to meet the specific needs of different application scenarios.

[0014] The innovation of this invention lies not only in the use of composite enzymatic hydrolysis, but also in guiding the starch degradation products to spontaneously form a stable composite structure of "dense framework for oil inhibition and hydrophilic units for water absorption" through a specific combination of enzymes (α-amylase + saccharifying enzyme) and precise process control (enzyme ratio 1:(0.8-1.5)). This is something that existing single-enzymatic hydrolysis or simple mixed-enzymatic hydrolysis techniques cannot achieve, and it is the fundamental reason why this invention can successfully endow the coating with dual functions. This step is the key to achieving dual functions, obtaining an ideal molecular weight distribution through the synergistic degradation of the two enzymes.

[0015] (3) Post-processing: The enzyme hydrolysate obtained in step (2) is subjected to enzyme inactivation treatment, and then plasticizer and cross-linking agent are added and stirred evenly to obtain the bifunctional coating liquid.

[0016] The specific post-treatment steps are as follows: After enzymatic hydrolysis, the system is heated to 85-95℃ and maintained for 30 minutes to completely inactivate the enzyme, resulting in an enzymatically modified starch solution. Subsequently, a plasticizer and a crosslinking agent are added to this solution. The plasticizer improves the flexibility of the coating and prevents cracking. The addition of the crosslinking agent (preferably citric acid) allows esterification crosslinking to occur during subsequent drying, enhancing the coating's water resistance and mechanical strength. After thorough stirring, a transparent or translucent bifunctional coating solution is obtained.

[0017] Preferably, in step (1), the acid hydrolysis modification is performed by dispersing cassava starch in a 0.5%-2% hydrochloric acid or sulfuric acid solution, reacting it at 40-55°C for 1-4 hours, neutralizing it, and then washing and drying it.

[0018] Preferably, in step (1), the oxidative modification is as follows: cassava starch is dispersed in water, the pH is adjusted to 8-10, sodium hypochlorite or hydrogen peroxide with an effective chlorine content of 1%-3% is added, the reaction is carried out at 30-50℃ for 0.5-2 hours, and the mixture is quenched, washed and dried.

[0019] Preferably, in step (2), the total amount of the compound enzyme added is 0.05%-0.3% of the dry weight of the pretreated cassava starch.

[0020] Preferably, in step (2), the total amount of the compound enzyme added is 0.15% of the dry weight of the pretreated cassava starch.

[0021] Preferably, in step (3), the plasticizer is at least one of glycerol, sorbitol, or polyethylene glycol, and the amount of plasticizer added is 8%-15% of the solid content of the enzymatic hydrolysate; The cross-linking agent is citric acid, and the amount of cross-linking agent added is 3%-8% of the solid mass of the enzymatic hydrolysate.

[0022] The second aspect of this invention discloses a bifunctional coating liquid based on enzymatically modified cassava starch, which is prepared by the above-described preparation method.

[0023] The third aspect of the present invention discloses a bifunctional coating material, which is formed by coating the surface of a substrate with the above-mentioned bifunctional coating liquid and then drying it.

[0024] The substrate can be paper or cardboard. Substrates also include natural fiber fabrics such as cotton and linen, non-woven fabrics, biodegradable plastic films such as polylactic acid (PLA) film and polyhydroxyalkanoate (PHA) film, wood, ceramics, etc.

[0025] The fourth aspect of this invention discloses the application of the above-mentioned bifunctional coating material in food packaging or medical products.

[0026] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention innovatively solves the technical challenge of a single coating simultaneously achieving oil resistance and water absorption through a synergistic technical approach of "pretreatment + compound enzymatic hydrolysis". Its core mechanism lies in: disrupting the starch granule structure through pretreatment, and then precisely controlling the degree of starch degradation through the synergistic action of α-amylase and saccharifying enzyme, achieving unexpected technical effects. This results in an enzymatic hydrolysis product that combines a film-forming framework (medium molecular weight dextrin) and hydrophilic units (small molecule sugars). Simultaneously, the film-forming framework forms a dense and continuous network structure, blocking oil penetration; while the hydrophilic units endow the coating with rapid capillary water absorption.

[0027] 2. By precisely controlling the composite enzymatic hydrolysis process, the cassava starch-based coating possesses both excellent oil resistance and rapid water vapor absorption, solving the problem of single function in traditional technologies.

[0028] 3. The entire process uses renewable starch as raw material, mainly adopts biological enzymatic modification, and optional chemical pretreatment is mild and controllable. The final product is safe, non-toxic and completely biodegradable.

[0029] 4. The coating obtained by this invention has a high oil resistance rating (Kit≥8) and a fast water absorption rate (20g / m³ / hour). 2 ).

[0030] 5. The "one-step" compound enzymatic hydrolysis method is adopted, which has a short process flow and mild conditions (pH 5.0–6.5, temperature 50–65℃) to prepare high-performance coatings. The process flow is simple and easy to scale up for production.

[0031] 6. The coating obtained by this invention can be applied to various substrates such as paper and cardboard, and is particularly suitable for food packaging containers, disposable tableware, medical care products and other products with high environmental protection requirements. Attached Figure Description

[0032] Figure 1 This is a comparison of the infrared spectra of the enzymatically modified cassava starch bifunctional coating and the unmodified cassava starch in Example 1. Detailed Implementation

[0033] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but this does not limit the present invention to the scope of the described embodiments. Process parameters not specified in the embodiments of this application can be performed according to conventional methods, and all raw materials used can be obtained through commercial channels.

[0034] Example 1: (1) Acid hydrolysis pretreatment: Take 100g of cassava starch, disperse it in 200mL of 1.5% hydrochloric acid solution, stir and react at 45℃ for 3 hours. Neutralize with 5% NaOH solution to pH=7, filter, wash 3 times with deionized water, dry in an oven at 55℃, and pulverize to obtain acid hydrolyzed cassava starch.

[0035] (2) Compound enzymatic hydrolysis: Weigh 20g of the above acid-hydrolyzed starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase and saccharifying enzyme activity ratio 1:1, the total amount added is 0.15% of the dry starch), and react for 2 hours.

[0036] (3) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0037] (4) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0038] (5) Performance test: The oil resistance was determined according to GB / T22805.2-2008 standard, and the Kit value was level 8; the water vapor absorption result at 50℃ and 80%RH was 20g / m 2 ·h.

[0039] Figure 1 This image shows a comparison of the infrared spectra of cassava starch (or coating) before and after enzymatic hydrolysis. After enzymatic hydrolysis, the structure of the starch granules is disrupted, and the system becomes homogenized, resulting in enhanced vibrational signals of characteristic functional groups such as hydroxyl groups. The infrared absorption intensity is mainly related to the number and vibrational activity of the functional groups: enzymatic hydrolysis breaks down macromolecules into smaller molecules (such as glucose and oligosaccharides), but these smaller molecules still retain the characteristic functional groups of starch (such as hydroxyl groups and glycosidic bonds). After enzymatic hydrolysis, the functional groups of smaller molecules are more exposed (the functional groups of macromolecules may be encapsulated internally, limiting vibration), and the sample dispersion / concentration is more uniform (the system is more homogeneous after enzymatic hydrolysis, and the interaction between infrared light and functional groups is more complete).

[0040] Figure 1The changes in the intensity of the infrared absorption peaks of characteristic functional groups of starch (such as hydroxyl groups -OH and glycosidic bonds -COC) are shown; the absorption peaks after enzymatic hydrolysis (modification) are stronger than those of unmodified starch (or unhydrolyzed starch). This indicates that the "pretreatment + compound enzymatic hydrolysis" successfully disrupts the dense structure of starch granules, fully exposing the functional groups (hydrophilic functional groups such as hydroxyl groups) originally encapsulated within the macromolecules, providing a structural basis for the water absorption function of the hydrophilic units in the coating.

[0041] By comparing the spectral peak positions and intensities before and after modification, it can be indirectly proven that the molecular weight distribution of starch has changed as expected, forming a composite structure of "medium molecular weight dextrin (film-forming backbone) + small molecule sugars (hydrophilic units)". The enhanced peak intensity corresponds to the uniform dispersion of small molecule products, indicating that the enzymatic hydrolysis reaction has achieved the goal of precisely controlling the degree of degradation, rather than random fragmentation.

[0042] Example 2 (1) Oxidation pretreatment: Take 100g of cassava starch, disperse it in 200mL of water, and adjust the pH to 9.0 with 5% NaOH. Slowly add 15mL of sodium hypochlorite solution with an effective chlorine content of 2%, and react at 40℃ for 1.5 hours. Add a small amount of NaHSO3 solution to quench the reaction, adjust the pH to neutral, and the subsequent treatment is the same as in Example 1.

[0043] (2) Compound enzymatic hydrolysis: starch milk concentration 15%, pH=5.5, temperature 55℃, compound enzyme (α-amylase and saccharifying enzyme activity ratio 1:1.2, total addition amount is 0.15% of dry starch), reaction for 2.5 hours.

[0044] (3) Post-treatment: The plasticizer is 12% sorbitol, the crosslinking agent is 4% citric acid, and the rest is the same as in Example 1.

[0045] (4) When coated on 30g food-grade white kraft paper or 210g paper cup base paper, the oil resistance was tested to be level 8. The water vapor absorption was tested at 50℃ and 80%RH, and the water vapor absorption result was 20g / m³. 2 ·h.

[0046] Comparative Example 1 Natural cassava starch was used without pretreatment and was directly enzymatically hydrolyzed (other steps were the same as in Example 1).

[0047] (1) Compound enzymatic hydrolysis: Weigh 20g of natural cassava starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase and saccharifying enzyme activity ratio 1:1, the total amount added is 0.15% of the dry starch), and react for 2 hours.

[0048] (2) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0049] (3) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a wire bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0050] Performance testing: The obtained coating liquid has poor film-forming properties, and the coating cracks after drying. The oil resistance is only level 3, and the water vapor absorption is uneven.

[0051] Comparative Example 2 The total amount of enzyme activity added was the same as in Example 1 (0.15% of dry starch) using α-amylase and saccharifying enzyme. (1) Acid hydrolysis pretreatment: Take 100g of cassava starch, disperse it in 200mL of 1.5% hydrochloric acid solution, stir and react at 45℃ for 3 hours. Neutralize with 5% NaOH solution to pH=7, filter, wash 3 times with deionized water, dry in an oven at 55℃, and pulverize to obtain acid hydrolyzed cassava starch.

[0052] (2) Compound enzymatic hydrolysis: Weigh 20g of the above acid-hydrolyzed starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase to saccharifying enzyme activity ratio 1:0.5, the total amount added is 0.15% of the dry starch), and react for 2 hours.

[0053] (3) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0054] (4) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0055] (5) Performance test: The oil resistance was determined according to GB / T22805.2-2008 standard, and the Kit value was level 8; the water vapor absorption result at 50℃ and 80%RH was 10g / m 2 ·h.

[0056] Comparative Example 3 The total amount of enzyme activity added was the same as in Example 1 (0.15% of dry starch) using α-amylase and saccharifying enzyme. (1) Acid hydrolysis pretreatment: Take 100g of cassava starch, disperse it in 200mL of 1.5% hydrochloric acid solution, stir and react at 45℃ for 3 hours. Neutralize with 5% NaOH solution to pH=7, filter, wash 3 times with deionized water, dry in an oven at 55℃, and pulverize to obtain acid hydrolyzed cassava starch.

[0057] (2) Compound enzymatic hydrolysis: Weigh 20g of the above acid-hydrolyzed starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase to saccharifying enzyme activity ratio 1:2, total addition amount is 0.15% of dry starch), and react for 2 hours.

[0058] (3) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0059] (4) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0060] (5) Performance test: The oil resistance was determined according to GB / T22805.2-2008 standard, and the Kit value was level 3; the water vapor absorption result at 50℃ and 80%RH was 20g / m 2 ·h.

[0061] Comparative Example 4: (1) Acid hydrolysis pretreatment: Take 100g of cassava starch, disperse it in 200mL of 1.5% hydrochloric acid solution, stir and react at 45℃ for 3 hours. Neutralize with 5% NaOH solution to pH=7, filter, wash 3 times with deionized water, dry in an oven at 55℃, and pulverize to obtain acid hydrolyzed cassava starch.

[0062] (2) Compound enzymatic hydrolysis: Weigh 20g of the above acid-hydrolyzed starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase and saccharifying enzyme activity ratio 1:1, the total amount added is 0.15% of the dry starch), and react for 1 hour.

[0063] (3) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0064] (4) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0065] (5) Performance test: The coating structure is not dense enough and cannot effectively block grease penetration; the oil resistance was tested according to GB / T22805.2-2008 standard, and the Kit value was level 3; the water vapor absorption result at 50℃ and 80%RH was 10g / m 2 ·h.

[0066] Comparative Example 5: (1) Acid hydrolysis pretreatment: Take 100g of cassava starch, disperse it in 200mL of 1.5% hydrochloric acid solution, stir and react at 45℃ for 3 hours. Neutralize with 5% NaOH solution to pH=7, filter, wash 3 times with deionized water, dry in an oven at 55℃, and pulverize to obtain acid hydrolyzed cassava starch.

[0067] (2) Compound enzymatic hydrolysis: Weigh 20g of the above acid-hydrolyzed starch and prepare 200g of 10% starch milk. Adjust the pH to 6.0 with 1% NaOH solution. Place it in a 60℃ constant temperature water bath, add compound enzyme (α-amylase and saccharifying enzyme activity ratio 1:1, the total amount added is 0.15% of the dry starch), and react for 3 hours.

[0068] (3) Post-treatment: Sterilize the enzymatic hydrolysate in a 90°C water bath for 15 minutes. Then add glycerol (10% of the solid mass of the enzymatic hydrolysate) and citric acid (5% of the solid mass of the enzymatic hydrolysate), and mechanically stir for 30 minutes until the mixture is homogeneous to obtain the coating solution.

[0069] (4) Coating and forming: The above coating liquid is evenly coated onto food-grade white kraft paper with a basis weight of 30g or paper cup base paper with a basis weight of 210g using a bar coater, and the dry coating amount is controlled to be about 10 g / m. 2 Dry in a 100℃ hot air drying oven for 2 minutes.

[0070] (5) Performance test: The obtained coating has poor film-forming properties and cracks after drying. The oil resistance was tested according to GB / T22805.2-2008 standard, and the Kit value was level 3. Under the conditions of 50℃ and 80%RH, the water vapor absorption result was 10g / m³. 2 ·h.

[0071] Conclusion: 1. Comparing Examples 1-2 and Comparative Example 1, the coatings obtained from Examples 1-2 after acid hydrolysis / oxidation pretreatment have a dense structure, achieve Kit 8 level oil resistance, and have a water absorption rate of 20 g / m³. 2 •h; Comparative Example 1 without pretreatment, the resulting coating has poor film-forming properties, cracks, and an oil resistance of only level 3, and the water absorption is uneven; it can be seen that pretreatment (acid hydrolysis or oxidation) destroys the starch granule structure, providing better processability for subsequent enzymatic hydrolysis, which is a prerequisite for forming a continuous and stable coating.

[0072] 2. Examples 1-2 (enzyme ratio 1:1 or 1:1.2) yielded coatings with excellent oil resistance and water absorption properties.

[0073] Comparative Example 2, with an enzyme ratio of 1:0.5, showed good oil resistance (level 8), but its water absorption rate was only 10 g / m³. 2 •h, insufficient hydrophilicity. Comparative Example 3: Enzyme ratio 1:2, water absorption rate reached 20 g / m³. 2 However, while the oil resistance is rated at level 3, it does not form a good oil-resistant structure. An enzyme ratio deviating from the preferred range leads to an imbalance between oil resistance and water absorption, preventing the achievement of dual functionality.

[0074] 3. Compared with Comparative Examples 4-5, Example 1 shows that the enzymatic hydrolysis time is a necessary condition for realizing the "skeleton-hydrophilic unit" synergistic structure. If the enzymatic hydrolysis time is too short or too long, the coating structure will fail and the dual functions cannot be achieved.

[0075] This invention is not limited to the above-described embodiments. Any changes in shape or structure are within the scope of protection of this invention. The scope of protection of this invention is defined by the appended claims. Those skilled in the art can make various changes, modifications, substitutions, combinations, and simplifications to these embodiments without departing from the principles and essence of this invention. All such changes and simplifications should be considered equivalent substitutions and fall within the scope of protection of this invention.

Claims

1. A method for preparing a bifunctional coating liquid based on enzymatically modified cassava starch, characterized by, The method comprises the following steps: (1) Pretreatment: modifying cassava starch by acidolysis or oxidation to obtain pretreated cassava starch; (2) Complex enzymatic hydrolysis: preparing the pretreated cassava starch obtained in step (1) into a starch milk with a mass concentration of 5%-20%, adjusting the pH of the system to 5.0-6.5, adding a complex enzyme composed of alpha-amylase and glucoamylase at a temperature of 50-65 ℃ for enzymatic hydrolysis for 2-2.5 hours; the ratio of enzyme activity units of the alpha-amylase to the glucoamylase is 1:0.8-1.5; (3) Post-treatment: performing enzyme inactivation treatment on the enzymatic hydrolysate obtained in step (2), then adding a plasticizer and a crosslinking agent, and stirring uniformly to obtain the dual-functional coating liquid.

2. The production method according to claim 1, characterized by, In step (1), the acidolysis modification is: dispersing cassava starch in a 0.5%-2% hydrochloric acid or sulfuric acid solution, reacting at 40-55 ℃ for 1-4 hours, and washing and drying after neutralization.

3. The production method according to claim 1, characterized by, In step (1), the oxidation pretreatment is: dispersing cassava starch in water, adjusting the pH to 8-10, adding 1%-3% sodium hypochlorite or hydrogen peroxide with effective chlorine content, reacting at 30-50 ℃ for 0.5-2 hours, and washing and drying after quenching.

4. The method of claim 1, wherein, In step (2), the total amount of the complex enzyme added is 0.05%-0.3% of the dry mass of the pretreated cassava starch.

5. The preparation method according to claim 4, characterized in that, In step (2), the total amount of the complex enzyme added is 0.15% of the dry mass of the pretreated cassava starch.

6. The method of claim 1, wherein, In step (3), the plasticizer is at least one of glycerol, sorbitol or polyethylene glycol, and the amount of the plasticizer added is 8%-15% of the mass of the solid content of the enzymatic hydrolysate; The crosslinking agent is citric acid, and the amount of the crosslinking agent added is 3%-8% of the mass of the solid content of the enzymatic hydrolysate.

7. A dual functional coating solution based on enzymatically modified cassava starch, characterized in that, The dual-functional coating material is prepared by the method of any one of claims 1-6.

8. A dual function coating material characterized by, After the substrate surface is coated with the dual-functional coating liquid of claim 7, the dual-functional coating material is formed after drying.

9. The dual function coating material of claim 8, wherein, The substrate is paper or paperboard.

10. Use of the dual-functional coating material of claim 8 or 9 in food packaging or medical supplies.

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