A polyphenol acid gradient grafting high amylose starch-based emulsion gel and its preparation and application

Abstract

This invention discloses a polyphenolic acid gradient-grafted high amylose-based emulsion gel, its preparation method, and its application. The preparation method includes: sequentially grafting high amylose with p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid to obtain polyphenolic acid gradient-grafted high amylose; mixing the polyphenolic acid gradient-grafted high amylose with water and performing gelatinization to form an aqueous phase; mixing thymol with an oil solvent to form an oil phase; and mixing the aqueous and oil phases and performing high-speed homogenization, followed by adding agarose and glycerol, mixing, and allowing to stand to solidify, thus obtaining the polyphenolic acid gradient-grafted high amylose-based emulsion gel. The emulsion gel of this invention possesses oil-water interface stability, efficient thymol encapsulation, slow-release antibacterial properties, antioxidant properties, and moisture retention, and can be used for the preservation of fruits and vegetables such as strawberries, blueberries, cherries, and cherry tomatoes.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable preservation technology, specifically relating to a polyphenolic acid gradient grafted high amylose-based emulsion gel and its preparation method and application. Background Technology

[0002] Fruits and vegetables continue to respire and evaporate moisture during post-harvest storage, transportation, and sales, making them susceptible to infection by molds, bacteria, and other microorganisms, leading to rot, softening, dehydration, color deterioration, and a decline in nutritional quality. Berries such as strawberries, blueberries, and cherries, with their soft tissues, thin skins, and high water content, are particularly prone to gray mold, soft rot, and rapid proliferation of surface microorganisms under ambient temperature conditions, resulting in a shorter shelf life. While traditional low-temperature storage and chemical preservatives can delay spoilage to some extent, they present challenges such as high cold chain costs, chemical residues, safety concerns, and consumer acceptance. Therefore, developing safe, edible, and novel fruit and vegetable preservation materials that combine physical barriers with active antibacterial and antioxidant functions is of great significance.

[0003] Active coating preservation technology can form a thin film on the surface of fruits and vegetables, improving storage quality by blocking water evaporation, regulating gas exchange, inhibiting microbial growth, and delaying oxidation. High-amylose corn starch has a high amylose content, good biocompatibility, and gel-forming ability, making it suitable as a food-grade emulsion gel, active delivery carrier, and substrate for fruit and vegetable coatings. However, the surface of natural high-amylose corn starch molecules is mainly composed of hydrophilic hydroxyl groups, lacking hydrophobic structures and exhibiting weak adsorption capacity at the oil-water interface, making it difficult to stably encapsulate hydrophobic active substances such as thymol. Furthermore, it itself possesses almost no antioxidant or antibacterial activity. Direct coating not only has limited preservation effects but may also form a moist polysaccharide layer in high-humidity environments, providing favorable conditions for microbial attachment and reproduction.

[0004] Thymol is a natural monoterpene phenolic active substance with strong broad-spectrum antibacterial activity and certain antioxidant capacity, showing potential for application in fruit and vegetable preservation. However, thymol has low water solubility, high volatility, and a strong pungent odor; direct application easily leads to rapid volatilization or burst release, resulting in short duration of action and low utilization. Encapsulating thymol in an emulsion gel for slow release can improve its stability and long-lasting antibacterial effect, but this system requires the stabilizer to possess good oil-water interface adsorption capacity, gel network construction ability, and active substance immobilization ability. Unmodified high-amylose corn starch is difficult to meet these requirements. Therefore, providing a highly stable, antioxidant, and antibacterial emulsion gel for fruit and vegetable preservation is an urgent problem to be solved. Summary of the Invention

[0005] The main objective of this invention is to provide a polyphenolic acid gradient-grafted high amylose-based emulsion gel, its preparation method, and its application, so as to overcome the shortcomings of the prior art.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention includes: This invention provides a method for preparing a polyphenolic acid gradient-grafted high amylose-based emulsion gel, comprising: High amylose was grafted sequentially with p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid to obtain polyphenolic acid gradient-grafted high amylose; wherein the grafting ratio of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid in the polyphenolic acid gradient-grafted high amylose was (1-3):(1-3):(0.5-2); the total grafting rate of the polyphenolic acid gradient-grafted high amylose was 6%-20%; The polyphenolic acid gradient-grafted high amylose starch was mixed with water and gelatinized to form an aqueous phase; thymol was mixed with an oil solvent to form an oil phase. Furthermore, the aqueous phase and oil phase are mixed and homogenized at high speed, and then agarose and glycerol are added, mixed and allowed to stand to form a polyphenolic acid gradient grafted high amylose-based emulsion gel.

[0007] The present invention also provides a polyphenolic acid gradient grafted high amylose-based emulsion gel prepared by the aforementioned preparation method.

[0008] This invention also provides the application of the aforementioned polyphenolic acid gradient-grafted high amylose-based emulsion gel in fruit and vegetable preservation.

[0009] This invention also provides a method for preserving fruits and vegetables, which includes: uniformly coating the aforementioned polyphenolic acid gradient-grafted high amylose-based emulsion gel onto the surface of fruits and vegetables, thereby achieving the preservation of fruits and vegetables; The fruits and vegetables mentioned include strawberries, blueberries, cherries, cherry tomatoes, grapes, fresh-cut apples, fresh-cut pears, or fresh-cut potatoes.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In this invention, p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid are sequentially used to perform gradient grafting on high amylose corn starch in a specific order, so that the three types of side groups, monophenolic hydroxyl, ortho-diphenolic hydroxyl and pyrogallol, are distributed in a progressive and synergistic manner on the starch skeleton. Compared with unmodified high amylose corn starch, single phenolic acid grafted starch, triphenolic acid physical mixing system, triphenolic acid one-step mixed grafting system and reverse sequence (gallic acid → 3,4-dihydroxybenzoic acid → p-hydroxybenzoic acid) grafting system, the polyphenolic acid gradient grafted high amylose in this invention is significantly better than the above control groups in terms of oil-water interface adsorption, thymol encapsulation rate, release controllability and long-term storage stability of emulsion gel, showing non-additive synergistic effect, indicating that the synergistic effect comes from the "hydrophobic anchoring-hydrogen bond network" amphiphilic interface structure constructed by the specific sequence gradient grafting. (2) The emulsion gel in this invention has a high encapsulation rate of thymol, a low cumulative release rate over 48 hours, a high contact angle, and good stability. When used for the preservation of strawberries at room temperature, it significantly inhibits the growth of total bacterial count, reduces weight loss, and maintains fruit firmness and soluble solids content. Moreover, this invention can achieve antioxidant and antibacterial capabilities comparable to those of single gallic acid grafting at a low gallic acid grafting ratio, demonstrating the improved efficiency of grafting structure function utilization. (3) The emulsion gel in this invention is mainly composed of polyphenolic acid gradient grafted high amylose, food-grade thymol, soybean oil, agarose and glycerin. The coupling reagent used for modification is removed by alcohol precipitation and multiple washing. The final product can be directly applied to the surface of fruits and vegetables as an edible active coating. It has multiple functions such as oil-water interface stability, efficient thymol encapsulation, slow-release antibacterial, antioxidant and moisture retention. It can be used for the preservation of fruits and vegetables such as strawberries, blueberries, cherries and cherry tomatoes. Attached Figure Description

[0011] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1 This is the infrared spectrum of the PDG-HAMS prepared in Example 1 of this invention; Figure 2 This is the UV-Vis absorption spectrum of the PDG-HAMS prepared in Example 1 of this invention; Figure 3 This is the NMR spectrum of the PDG-HAMS prepared in Example 1 of this invention. Detailed Implementation

[0013] In view of the deficiencies of the prior art, the applicant, through long-term research and extensive practice, has proposed the technical solution of this invention. The technical solution of this invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0014] Specifically, as one aspect of the technical solution of this invention, the method for preparing a polyphenolic acid gradient-grafted high amylose-based emulsion gel includes: High amylose was grafted sequentially with p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid to obtain polyphenolic acid gradient-grafted high amylose; wherein the grafting ratio of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid in the polyphenolic acid gradient-grafted high amylose was (1-3):(1-3):(0.5-2); the total grafting rate of the polyphenolic acid gradient-grafted high amylose was 6%-20%; The polyphenolic acid gradient-grafted high amylose starch was mixed with water and gelatinized to form an aqueous phase; thymol was mixed with an oil solvent to form an oil phase. Furthermore, the aqueous phase and oil phase are mixed and homogenized at high speed, and then agarose and glycerol are added, mixed and allowed to stand to form a polyphenolic acid gradient grafted high amylose-based emulsion gel.

[0015] This invention employs a stepwise sequential grafting strategy of three benzoic acid-type phenolic acids: In the first stage, p-hydroxybenzoic acid is grafted onto the C-6 hydroxyl group of high-amylose corn starch, forming a moderately hydrophobic aromatic interface adsorption site; in the second stage, 3,4-dihydroxybenzoic acid is grafted onto the remaining hydroxyl groups of high-amylose corn starch, introducing ortho-dihydroxy hydrogen bonds and electron transfer sites; in the third stage, gallic acid is grafted onto the remaining hydroxyl groups in a low proportion, introducing strong antioxidant and antibacterial sites of pyrogallol. The "gradient grafting" in this invention does not refer to a strictly linear tandem structure between the three phenolic acid molecules, but rather to a progressive distribution of the composition and functional contribution of monohydroxy, ortho-dihydroxy, and trihydroxy phenolic acid side groups on the starch backbone through stepwise addition and controlled segmented grafting rates. The reaction targets of 3,4-dihydroxybenzoic acid and gallic acid remain the unreacted hydroxyl groups of high-amylose corn starch, rather than the phenolic hydroxyl groups on the side groups of the previous phenolic acid.

[0016] In some preferred embodiments, the total grafting rate of the polyphenolic acid gradient grafted high amylose is 8%-16%.

[0017] In some preferred embodiments, the preparation method specifically includes: High amylose was mixed with an organic solvent and stirred at 70-90°C for 50 minutes, then cooled to 25-35°C to obtain a high amylose solution. Furthermore, the high amylose starch solution is mixed with p-hydroxybenzoic acid, carbodiimide hydrochloride (EDC·HCl), and 4-dimethylaminopyridine (DMAP) and reacted at room temperature for 1-4 h. Then, 3,4-dihydroxybenzoic acid, carbodiimide hydrochloride, and 4-dimethylaminopyridine are added and the reaction is continued at room temperature for 1-4 h. Then, gallic acid, carbodiimide hydrochloride, and 4-dimethylaminopyridine are added and the reaction is continued at room temperature for 1-4 h. Finally, after precipitation, washing, and drying, polyphenolic acid gradient grafted high amylose starch is obtained.

[0018] Furthermore, the high amylose content includes, but is not limited to, high amylose corn starch.

[0019] Furthermore, the amylose content in the high amylose content is 50% to 80%.

[0020] Furthermore, the organic solvent includes, but is not limited to, N,N-dimethylformamide.

[0021] Further, the mass ratio of the high amylose, p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid is 6.0:(0.25-1.10):(0.25-1.10):(0.12-0.65).

[0022] Furthermore, the p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid are used to graft the primary hydroxyl groups at the C-6 position of the glucose units in high amylose starch.

[0023] In some preferred embodiments, the preparation method specifically includes: mixing the polyphenolic acid gradient-grafted high amylose starch with water and subjecting it to high temperature and high pressure treatment for 2 h until complete gelatinization, followed by cooling to 45~50℃ to form an aqueous phase; wherein the temperature of the high temperature and high pressure treatment is 110-130℃ and the pressure is 0.10-0.25 MPa.

[0024] In some preferred embodiments, the oil solvent includes soybean oil, but is not limited thereto.

[0025] In some preferred embodiments, the concentration of polyphenolic acid gradient-grafted high amylose in the aqueous phase is 1-8 wt%.

[0026] In some preferred embodiments, the volume content of thymol in the oil phase is 10-70%.

[0027] In some preferred embodiments, the concentration of agarose in the polyphenolic acid gradient-grafted high amylose-based emulsion gel is 0.2-2.0 wt%, and the concentration of glycerol is 0.1-1 wt%.

[0028] In some preferred embodiments, the volume fraction of the oil phase in the polyphenolic acid gradient-grafted high amylose-based emulsion gel is 10-50%.

[0029] In some preferred embodiments, the volume ratio of the aqueous phase to the oil phase is 9:1 to 5:5.

[0030] In some preferred embodiments, the high-speed homogenization process is performed at a rotation speed of 10,000 to 20,000 r / min for a duration of 5 to 10 min.

[0031] In some preferred embodiments, the static molding temperature is 2~30℃ and the time is 2~12h.

[0032] Another aspect of the present invention provides a polyphenolic acid gradient-grafted high amylose-based emulsion gel prepared by the aforementioned preparation method.

[0033] The encapsulation rate of thymol in the emulsion gel of this invention is increased from about 35% in the unmodified system to over 97%, the cumulative release rate after 48 hours is reduced to about 27%, the contact angle is increased to about 82°, and it remains stable after 120 days of storage at 4°C. When used for the room temperature preservation of strawberries, it can reduce the decay rate on the 6th day of storage from about 90% in the blank control to about 16%, and significantly inhibit the growth of total bacterial count, reduce weight loss, and maintain fruit firmness and soluble solids content. Moreover, this invention can achieve antioxidant and antibacterial capabilities comparable to those of single gallic acid grafting at a lower gallic acid grafting ratio, demonstrating the improved efficiency of grafting structure function utilization.

[0034] Another aspect of the present invention provides the application of the aforementioned polyphenolic acid gradient-grafted high amylose-based emulsion gel in fruit and vegetable preservation.

[0035] Furthermore, the fruits and vegetables include, but are not limited to, strawberries, blueberries, cherries, cherry tomatoes, grapes, fresh-cut apples, fresh-cut pears, or fresh-cut potatoes.

[0036] Another aspect of the present invention provides a method for preserving fruits and vegetables, which includes: uniformly coating the aforementioned polyphenolic acid gradient-grafted high amylose-based emulsion gel onto the surface of fruits and vegetables, thereby achieving the preservation of fruits and vegetables. The fruits and vegetables mentioned include strawberries, blueberries, cherries, cherry tomatoes, grapes, fresh-cut apples, fresh-cut pears, or fresh-cut potatoes.

[0037] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0038] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0039] Example 1 6.0 g of high amylose corn starch was dispersed in 150 mL of DMF and stirred at 80 °C for 50 min. The mixture was then cooled to 30 °C to obtain a high amylose corn starch suspension. In the first stage, 0.50 g of p-hydroxybenzoic acid, 2.0 g of EDC·HCl, and 0.45 g of DMAP were added, and the reaction was continued at room temperature for 2 h. In the second stage, 0.55 g of 3,4-dihydroxybenzoic acid, 2.0 g of EDC·HCl, and 0.45 g of DMAP were added, and the reaction continued for another 2 h. In the third stage, 0.30 g of gallic acid, 1.5 g of EDC·HCl, and 0.35 g of DMAP were added, and the reaction continued for another 2 h. After the reaction was complete, the starch was precipitated with 95% ethanol, washed, and freeze-dried to obtain polyphenolic acid gradient-grafted high amylose, denoted as PDG-HAMS. The total grafting rate of the obtained PDG-HAMS was approximately 12%, and the ratio of the segmental grafting rates of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid was approximately 2:2:1.

[0040] The infrared spectrum of the prepared PDG-HAMS is as follows: Figure 1 As shown, PDG-HAMS should have a thickness of 1730–1745 cm⁻¹ compared to unmodified HAMS. - A distinct C=O stretching vibration peak of the ester bond appears within the range of ¹, and reaches 1600 cm⁻¹. - The presence of a C=C skeletal vibration peak near the ¹ indicates that the carboxyl group of the benzoic acid-type phenolic acid has undergone esterification with the starch hydroxyl group, and the aromatic phenolic acid structure has been introduced into the starch skeleton. Simultaneously, at 3200–3600 cm⁻¹... - The broadening of the O–H stretching vibration peaks within the range indicates that the starch hydroxyl groups are partially esterified, and that the phenolic hydroxyl groups in the phenolic acid side groups participate in new hydrogen bonding interactions. Since PDG-HAMS contains three types of side groups—p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid—its ester carbonyl peak and aromatic ring peak should be significantly enhanced compared to starch modified with a single phenolic acid. This provides one of the direct structural evidences of successful polyphenolic acid gradient grafting. The UV-Vis absorption spectrum of the prepared PDG-HAMS is as follows: Figure 2As shown, unmodified HAMS typically exhibits no significant characteristic absorption in the 200–400 nm range, while single phenolic acid grafted samples show absorption peaks around 275 nm for p-hydroxybenzoic acid, around 255 nm for 3,4-dihydroxybenzoic acid, and around 260 nm for gallic acid. PDG-HAMS shows a composite absorption peak covering the aforementioned characteristic bands in the 255–275 nm range, or simultaneously exhibits absorption signals around 275 nm, 255 nm, and 260 nm, indicating the coexistence of the three benzoic acid-type phenolic acid structural units in the same sample. By comparing the UV spectra with Ph-HAMS, Da-HAMS, and Ga-HAMS, and combining standard curves or peak shape decomposition to calculate the segmented grafting rates of the three phenolic acids, a basis is provided for "polyphenolic acid co-grafting" and "controllable grafting ratios."

[0041] The NMR characterization of the prepared PDG-HAMS is as follows: Figure 3 As shown, PDG-HAMS should simultaneously retain the starch skeleton signal and exhibit aromatic carbon signals and ester carbonyl carbon signals in the δ 110–170 ppm range. The aromatic hydrogen and aromatic carbon signals indicate the introduction of benzoic acid-type phenolic acid structures into the system, while the ester carbonyl carbon signal further confirms the formation of covalent ester bonds between the phenolic acid carboxyl group and the starch hydroxyl group. Since PDG-HAMS contains three types of phenolic acid side groups, the aromatic region signals exhibit superposition, broadening, or complex peak shapes, which is a reasonable manifestation of the coexistence of polyphenolic acid side groups. This figure can be corroborated by the infrared and ultraviolet results, indicating that PDG-HAMS is not a simple physical mixture of the three phenolic acids and HAMS, but rather forms a covalent grafted structure of polyphenolic acid side groups.

[0042] Example 2 The method was the same as in Example 1, except that the amounts of the three phenolic acids were adjusted: in the first stage, 0.40 g of p-hydroxybenzoic acid, 1.8 g of EDC·HCl, and 0.40 g of DMAP were added; in the second stage, 0.45 g of 3,4-dihydroxybenzoic acid, 1.8 g of EDC·HCl, and 0.40 g of DMAP were added; and in the third stage, 0.40 g of gallic acid, 1.6 g of EDC·HCl, and 0.35 g of DMAP were added. All other conditions were the same as in Example 1. The total grafting rate of the obtained PDG-HAMS was approximately 8%, and the ratio of the segmental grafting rates of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid was approximately 3:3:2.

[0043] Example 3 The method was the same as in Example 1, except that the amounts of the three phenolic acids were adjusted: in the first stage, 0.30 g of p-hydroxybenzoic acid, 1.6 g of EDC·HCl, and 0.35 g of DMAP were added; in the second stage, 0.75 g of 3,4-dihydroxybenzoic acid, 2.5 g of EDC·HCl, and 0.60 g of DMAP were added; and in the third stage, 0.20 g of gallic acid, 1.2 g of EDC·HCl, and 0.30 g of DMAP were added. All other conditions were the same as in Example 1. The total grafting rate of the obtained PDG-HAMS was approximately 16%, and the ratio of segmented grafting rates was approximately 1:3:0.5.

[0044] Example 4 The method was the same as in Example 1, except that the amounts of the three phenolic acids were adjusted: in the first stage, 0.30 g of p-hydroxybenzoic acid, 1.5 g of EDC·HCl, and 0.30 g of DMAP were added; in the second stage, 0.32 g of 3,4-dihydroxybenzoic acid, 1.5 g of EDC·HCl, and 0.30 g of DMAP were added; and in the third stage, 0.18 g of gallic acid, 1.0 g of EDC·HCl, and 0.20 g of DMAP were added. All other conditions were the same as in Example 1. The total grafting rate of the obtained PDG-HAMS was approximately 6%, and the ratio of segmented grafting rates was approximately 2:2:1.

[0045] Example 5 The method was the same as in Example 1, except that the amounts of the three phenolic acids were adjusted: in the first stage, 0.90 g of p-hydroxybenzoic acid, 2.5 g of EDC·HCl, and 0.60 g of DMAP were added; in the second stage, 1.00 g of 3,4-dihydroxybenzoic acid, 2.5 g of EDC·HCl, and 0.60 g of DMAP were added; and in the third stage, 0.55 g of gallic acid, 2.0 g of EDC·HCl, and 0.50 g of DMAP were added. All other conditions were the same as in Example 1. The total grafting rate of the obtained PDG-HAMS was approximately 20%, and the ratio of segmented grafting rates was approximately 2:2:1.

[0046] Comparative Example 1 Unmodified high amylose corn starch was used, denoted as HAMS.

[0047] Comparative Example 2 The method is the same as in Example 1, except that: high amylose corn starch is mechanically mixed with p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid without reaction, to obtain modified high amylose corn starch, denoted as Mix-HAMS.

[0048] Comparative Example 3 The method is the same as in Example 1, except that p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid are simultaneously mixed and reacted with high amylose corn starch to obtain modified high amylose corn starch, denoted as One-pot-HAMS.

[0049] Comparative Example 4: The order of adding p-hydroxybenzoic acid and gallic acid was interchanged. In the first stage, 0.30 g of gallic acid, 1.5 g of EDC·HCl, and 0.35 g of DMAP were added, and the reaction was carried out at room temperature for 2 h. In the second stage, 0.55 g of 3,4-dihydroxybenzoic acid, 2.0 g of EDC·HCl, and 0.45 g of DMAP were added, and the reaction was continued for 2 h. In the third stage, 0.50 g of p-hydroxybenzoic acid, 2.0 g of EDC·HCl, and 0.45 g of DMAP were added, and the reaction was continued for 2 h. After the reaction was completed, the product was precipitated with 95% ethanol, washed, and freeze-dried to obtain polyphenolic acid gradient-grafted high amylose, denoted as GDP-HAMS. The total grafting rate of the obtained GDP-HAMS was approximately 10%, and the ratio of the segmental grafting rates of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid was approximately 1:3:5.

[0050] Comparative Example 5: Preparation of p-hydroxybenzoic acid-grafted high amylose corn starch 6.0 g of high amylose corn starch was dispersed in 150 mL of DMF and stirred at 80 °C for 50 min, then cooled to 30 °C. 0.8 g of p-hydroxybenzoic acid, 4.0 g of EDC·HCl, and 1.0 g of DMAP were added, and the mixture was reacted at room temperature for 9 h. After the reaction was complete, 95% ethanol was added to precipitate the starch. The precipitate was filtered, washed with ethanol, and freeze-dried to obtain p-hydroxybenzoic acid-grafted high amylose corn starch, denoted as Ph-HAMS.

[0051] Comparative Example 6: Preparation of 3,4-dihydroxybenzoic acid grafted high amylose corn starch The method is the same as that of Comparative Example 5, except that p-hydroxybenzoic acid is replaced with 3,4-dihydroxybenzoic acid in an amount of 1.1 g to obtain 3,4-dihydroxybenzoic acid-grafted high amylose corn starch, denoted as Da-HAMS.

[0052] Comparative Example 7: Preparation of Gallic Acid-Grafted High-Amylose Corn Starch The method is the same as that of Comparative Example 5, except that p-hydroxybenzoic acid is replaced with gallic acid, and the amount used is 1.3 g, to obtain gallic acid-grafted high amylose corn starch, denoted as Ga-HAMS.

[0053] Comparative Example 8: The method is the same as in Example 1, except that the total grafting rate of the polyphenolic acid gradient grafted high amylose starch is 3%. Specifically, the amounts of the three phenolic acids are reduced proportionally: in the first stage, 0.13 g of p-hydroxybenzoic acid, 0.5 g of EDC·HCl, and 0.12 g of DMAP are added; in the second stage, 0.14 g of 3,4-dihydroxybenzoic acid, 0.5 g of EDC·HCl, and 0.12 g of DMAP are added; in the third stage, 0.08 g of gallic acid, 0.4 g of EDC·HCl, and 0.10 g of DMAP are added; the remaining conditions are the same as in Example 1. The grafting ratio of the resulting modified starch is still approximately 2:2:1, and the total grafting rate is approximately 3%, denoted as PDG-HAMS-3%; its structural characterization and performance data are shown in Tables 1-4.

[0054] Comparative Example 9: The method is the same as in Example 1, except that the total grafting rate of the polyphenolic acid gradient grafted onto high amylose is 30%. Specifically, the amounts of the three phenolic acids are increased proportionally—1.25 g of p-hydroxybenzoic acid, 1.40 g of 3,4-dihydroxybenzoic acid, and 0.75 g of gallic acid, with EDC·HCl and DMAP scaled up proportionally; other conditions are the same as in Example 1. The grafting rate ratio remains approximately 2:2:1, and the total grafting rate is approximately 30%, denoted as PDG-HAMS-30%; data are shown in Tables 1-4.

[0055] Comparative Example 10 The method is the same as in Example 1, except that the grafting ratio of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid is 5:2:1. Specifically, 0.95 g of p-hydroxybenzoic acid, 0.35 g of 3,4-dihydroxybenzoic acid, and 0.18 g of gallic acid (and corresponding EDC·HCl and DMAP) were used, with the rest being the same as in Example 1. The grafting ratio was approximately 5:2:1, and the total grafting rate was approximately 12%, denoted as PDG-HAMS-10; the data are shown in Tables 1-4.

[0056] Comparative Example 11 The method is the same as in Example 1, except that the grafting ratio of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid is 0.5:2:3. Specifically, 0.12 g of p-hydroxybenzoic acid, 0.55 g of 3,4-dihydroxybenzoic acid, and 0.85 g of gallic acid (and corresponding EDC·HCl and DMAP) were used, with the rest being the same as in Example 1. The grafting ratio was approximately 0.5:2:3, and the total grafting rate was approximately 12%, denoted as PDG-HAMS-11; the data are shown in Tables 1-4.

[0057] Preparation of emulsion gels: The modified starches prepared in Example 1 and Comparative Examples 1-11 were dispersed in deionized water at 5% (w / v) and subjected to high temperature and high pressure treatment at 121℃ and 0.10 MPa for 2 h to achieve complete gelatinization. After cooling to 50℃, an aqueous phase was obtained. Thymol and soybean oil were mixed at a ratio of 1:1 (v / v) to obtain an oil phase. The oil phase was added to the aqueous phase at a volume ratio of 3:7, and homogenized at 15000 r / min for 6 min to obtain an emulsion. The obtained emulsion was then mixed with 1% (w / v) agarose solution at a volume ratio of 7:3, and 0.5% (v / v) of glycerol was added to the mixture. After mixing, the mixture was allowed to stand at 4℃ to form the modified starch-based emulsion gels corresponding to Example 1 and Comparative Examples 1-11.

[0058] The testing and characterization methods used are as follows: (1) Grafting rate and segmented grafting rate determination: The samples prepared in Example 1 and Comparative Examples 1-11 were respectively prepared as 0.1% (w / v) gelatinized dispersions and scanned in the range of 200-400 nm using a UV-Vis spectrophotometer. Standard curves were established for p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid at their characteristic absorption wavelengths of 275 nm, 255 nm and 260 nm, respectively. The grafting rate of single-grafted samples was calculated according to the corresponding standard curve; the segmented grafting rate of the three phenolic acids in PDG-HAMS samples could be calculated by combining multi-wavelength correction, peak shape decomposition or HPLC hydrolysis analysis. The grafting rate was expressed as the percentage of covalently bound phenolic acid mass to the total mass of the sample.

[0059] (2) Fourier transform infrared spectroscopy (FTIR): Take the dry sample and KBr powder, mix them thoroughly and compress them into a tablet, and then spectroscopy at 4000-400 cm⁻¹. - ¹ Scan within the range, resolution 4 cm - ¹, The number of scans should be no less than 32. Focus on the 1730-1745 cm section. - ¹Near ester bond C=O stretching vibration, 1600 cm⁻¹ - ¹Vibration of the nearby aromatic ring C=C skeleton and 3200-3600 cm - ¹The change in the hydroxyl stretching vibration peak can be used to determine whether phenolic acid is grafted onto the starch backbone in the form of ester bonds.

[0060] (3) Ultraviolet-Vis Spectroscopy (UV-Vis): The sample is fully gelatinized and diluted to a suitable concentration, and the absorption spectrum is measured in the range of 200-400 nm. HAMS usually has no obvious characteristic peaks due to the lack of aromatic chromophores; after phenolic acid grafting, benzene ring π-π* transition absorption peaks can appear at the corresponding wavelengths. If absorption signals appear simultaneously or superimposed near 255 nm, 260 nm and 275 nm in PDG-HAMS, it can be used as one of the evidences of the coexistence of three benzoic acid-type phenolic acid side groups.

[0061] (4) Nuclear Magnetic Resonance Characterization: The structural analysis of the sample was performed using ¹H-NMR and solid-state ¹³C-CP / MAS NMR. In ¹H-NMR, the focus was on observing the aromatic hydrogen signals at δ 6-8 ppm and the H-1, H-2 / H-3 / H-4 / H-5 / H-6 signals of starch glucose units; in solid-state ¹³C-NMR, the focus was on observing the carbonyl carbon signals of ester bonds near δ 160-170 ppm, the aromatic carbon signals at δ 110-160 ppm, and the C-1, C-2 / C-3 / C-5, C-4, and C-6 signals of starch. These signals were used together to demonstrate the covalent introduction of phenolic acid side groups and their main ester bond linkages.

[0062] (5) Contact angle measurement: The dried powder sample was pressed into a flat thin sheet, and the contact angle of water droplets on the sample surface was measured using the static drop method. At least 5 different locations were measured for each sample and the average value was taken. An increase in contact angle indicates that the phenolic acid aromatic structure improves the hydrophobicity of starch and the adsorption potential at the oil-water interface.

[0063] (6) Evaluation of antioxidant activity: The antioxidant capacity of the samples was evaluated using DPPH and ABTS free radical scavenging experiments. In the DPPH method, the gelatinized sample solution was mixed with DPPH ethanol working solution, and the absorbance was measured at 517-519 nm after reacting in the dark for 30 min; in the ABTS method, the sample solution was mixed with ABTS· + The absorbance was measured at 734 nm after the working solution was mixed and reacted for 6 min. The free radical scavenging rate was calculated based on the percentage decrease in absorbance of the blank control.

[0064] (7) Evaluation of antibacterial performance: Based on Escherichia coli (E. coli) E. coli ) and Staphylococcus aureus ( S. aureus) As indicator bacteria, quantitative evaluation was performed using the plate count method. The bacterial suspension was mixed with the sample solution, incubated, and then serially diluted. The mixture was then plated on nutrient agar plates and incubated at 37°C for 18–24 h before colony counting. The inhibition rate was calculated as the percentage difference between the colony counts of the control group and the sample group relative to the total colony count of the control group.

[0065] (8) Evaluation of emulsion gel performance: Modified starch-based emulsion gels prepared in Example 1 and Comparative Examples 1-11 were provided. Macroscopic stability was evaluated by visual observation at 0 d and 120 d; the emulsifying activity index (EAI) was calculated by diluting the absorbance of fresh emulsion; the contents of free thymol and total thymol were determined by ethanol extraction and demulsification extraction, and the encapsulation efficiency (EE) was calculated; if necessary, the cumulative release rate of thymol at 48 h was determined by dialysis.

[0066] (9) Evaluation of fruit and vegetable preservation performance: Using strawberries as a representative fruit and vegetable model, the emulsion gel was evenly coated on the surface of the strawberries and stored at 25℃. The decay rate, total bacterial count (TVC), weight loss rate, firmness retention rate, color, and soluble solids were measured regularly. The decay rate was calculated as the percentage of fruits with visible mold or tissue decay out of the total number of fruits; TVC was expressed as log CFU / g; and weight loss rate was calculated based on the change in weight before and after storage.

[0067] (10) Statistical analysis: All experiments were performed in at least three parallel determinations, and the results were expressed as mean ± standard deviation. One-way ANOVA and multiple comparison tests were used to determine the differences between groups, and p < 0.05 was considered significant.

[0068] The grafting composition, grafting rate and structural characteristics of the modified starch samples prepared in Example 1 and Comparative Examples 1-11 were characterized, and the results are shown in Table 1.

[0069] Table 1. Grafting composition, grafting rate, and structural characteristics of the modified starch samples prepared in Example 1 and Comparative Examples 1-11.

[0070] Table 1 shows that HAMS was not modified, with Ph, Da, and Ga grafting rates all at 0.0%, indicating that it lacks benzoic acid-type phenolic acid side groups, hydrophobic aromatic structures, and phenolic hydroxyl active sites. Therefore, its interfacial adsorption capacity is weak, and it has almost no antioxidant or antibacterial ability. Ph-HAMS, Da-HAMS, and Ga-HAMS are single phenolic acid grafted samples, and their total grafting rates are all controlled at approximately 12%, which can minimize the influence of differences in the total grafting amount on subsequent performance comparisons. Although Mix-HAMS added three phenolic acids, these three phenolic acids did not form covalent bonds with the starch backbone, so its grafting rate is still recorded as 0.0%. This group is mainly used to demonstrate that physical mixing cannot replace covalent grafting. One-pot-HAMS, GDP-HAMS, and PDG-HAMS are all polyphenolic acid grafting systems, with total grafting rates remaining within a similar range, facilitating comparison of the effects of different grafting methods and grafting sequences. PDG-HAMS utilizes a Ph→Da→Ga gradient grafting method to achieve a synergistic distribution of monohydroxy, ortho-dihydroxy, and trihydroxyphenolic acid side groups on the same starch backbone. This is not simply about increasing the total grafting amount, but rather about controlling the segmented grafting rate and grafting sequence to achieve functional complementarity in interfacial adsorption, hydrogen bonding networks, and antibacterial and antioxidant sites. Furthermore, Comparative Examples 8 and 9, while maintaining the Ph→Da→Ga gradient grafting method, reduced the total grafting rate to approximately 3% and increased it to approximately 30%, respectively. Comparative Examples 10 and 11, with a total grafting rate of approximately 12%, adjusted the grafting ratios of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid to 5:2:1 and 0.5:2:3, respectively. These comparative examples, together with Example 1, constitute a controlled gradient across two dimensions: the total grafting rate and the segmented grafting rate ratio. This gradient was used to examine the impact of deviations from the preferred range on interfacial, emulsifying, antioxidant, antibacterial, and preservation properties. In summary, Table 1, by controlling the total grafting rate of each comparable group to a similar level of approximately 12% (GDP-HAMS is slightly lower at approximately 10% due to the reverse order), and by systematically setting up comparisons of grafting methods, grafting order, total grafting amount, and segmentation ratio, ensures that the performance differences reflected in subsequent Tables 2 to 4 are mainly attributable to the grafting structure itself rather than differences in the total grafting amount, thus providing a reliable basis for attributing synergistic effects.

[0071] The contact angles of the modified starch samples prepared in Example 1 and Comparative Examples 1-11, as well as the properties of the corresponding prepared emulsion gels, were characterized, and the results are shown in Table 2.

[0072] Table 2. Contact angles and corresponding performance results of the modified starch samples prepared in Example 1 and Comparative Examples 1-11.

[0073] Table 2 shows that HAMS has a contact angle of only 20.1°, strong hydrophilicity, and weak interfacial adsorption capacity. Therefore, its EAI is only 5.5 m² / g, the thymol encapsulation rate is only 35.5%, and the release rate after 48 h is as high as 83.6%, with rapid stratification, indicating that unmodified HAMS is difficult to stabilize thymol emulsion gels. After grafting Ph-HAMS, Da-HAMS, and Ga-HAMS with a single phenolic acid, both EAI and EE are significantly improved, indicating that phenolic acid grafting can improve the amphiphilicity and emulsion stability of HAMS. Among them, Ph-HAMS has a stronger interfacial adsorption capacity, while Da-HAMS and Ga-HAMS have a stronger limiting effect on thymol release due to the increased number of phenolic hydroxyl groups. However, the long-term stability of the single-grafted system is still insufficient; Ph-HAMS and Da-HAMS show oxidative yellowing, and Ga-HAMS shows partial oil separation. Although Mix-HAMS contains three phenolic acids, the lack of covalent grafting resulted in low EAI, low encapsulation efficiency, high release rate, and rapid stratification, indicating that physical mixing cannot replace covalent modification. One-pot-HAMS showed improvement over Mix-HAMS, but due to the competitive reaction of the three phenolic acids in one-step grafting and the relatively random distribution of side groups, partial oxidation and yellowing still occurred. GDP-HAMS remained relatively stable, indicating that sequential grafting is superior to random grafting, but its encapsulation efficiency and sustained-release effect were still inferior to PDG-HAMS. PDG-HAMS exhibited the highest contact angle, highest EAI, highest EE, and lowest 48-h release rate, and remained stable after 120 days, indicating that Ph→Da→Ga sequential gradient grafting can simultaneously optimize interfacial adsorption, thymol encapsulation, sustained-release control, and long-term storage stability, demonstrating the synergistic effect brought about by a specific grafting sequence. From a mechanistic perspective, the Ph→Da→Ga gradient grafting constructs an amphiphilic interface on the starch backbone that is in equilibrium between a hydrophobic aromatic ring anchoring network and a phenolic hydroxyl hydrogen bond network. First, the grafted p-hydroxybenzoic acid preferentially anchors the oil-water interface with its hydrophobic aromatic ring. Subsequently, the introduced ortho-dihydroxy group and pyrogallol side group further densify the interface film and fix thymol through hydrogen bonding, thereby resulting in the strongest interfacial adsorption, the highest emulsification activity, the strongest encapsulation, and the slowest release. It is worth noting that although Comparative Example 9 (overgrafted) had the highest contact angle (98.3°), its EAI (32.8 m² / g) was actually lower than that of Example 1 (42.5 m² / g). This indicates that the emulsifying ability depends on a proper balance between hydrophilicity and hydrophobicity rather than maximizing hydrophobicity. Overgrafting leads to excessive hydrophobicity of the starch skeleton and aggregation of side groups, which weakens interfacial activity and makes the gel brittle and oil-separating. Comparative Example 8 (undergrafted) had the worst performance in all indicators due to insufficient aromatic side groups. Comparative Example 10 (excessive p-hydroxybenzoic acid ratio) had acceptable interfacial adsorption but insufficient long-term stability. Comparative Example 11 (excessive gallic acid ratio) showed a significant decrease in emulsification and encapsulation abilities.It is evident that only when the total grafting rate and the segmentation ratio are both within the optimal range, and the specific grafting sequence of Ph→Da→Ga is adopted, can the emulsion gel simultaneously achieve optimal interfacial adsorption, thymol encapsulation, sustained release control, and long-term storage stability, demonstrating a synergistic effect that cannot be achieved by physical mixing, one-step grafting, reverse sequential grafting, or systems with unbalanced ratios.

[0074] The antioxidant activity and inhibitory effect on common spoilage-related bacteria of the modified starch samples prepared in Example 1 and Comparative Examples 1-11 were characterized, and the results are shown in Table 3.

[0075] Table 3. Antioxidant activity and inhibitory effect on common spoilage-related bacteria of the modified starch samples prepared in Example 1 and Comparative Examples 1-11.

[0076] Table 3 shows the antioxidant activity and effects on different high amylose corn starch samples. E. coli and S. aureus A comparison of their inhibitory effects. HAMS showed a DPPH radical scavenging rate of only 0.8%, while ABTS showed a scavenging rate of only 3.1%. E. coli and S. aureus The inhibition rates of all three were 0.0%, indicating that unmodified HAMS had almost no antioxidant or antibacterial ability. Ph-HAMS, due to the introduction of a monohydroxybenzoic acid side group, showed improved antioxidant capacity, but its DPPH and ABTS scavenging rates remained low; its inhibition rate against bacteria was significantly higher than that of HAMS, indicating that aromatic phenolic acid side groups can exert a certain antibacterial effect through hydrophobic interactions and membrane perturbation. Da-HAMS contains an ortho-dihydroxy structure, and its phenolic radicals can achieve better stability through conjugation between the ortho-hydroxy group and the aromatic ring, thus significantly improving the DPPH and ABTS scavenging rates. E. coli and S. aureusThe inhibition rate was also significantly enhanced. Ga-HAMS contains a pyrogallol trihydroxy structure and exhibits the strongest antioxidant and antibacterial abilities among the single grafted samples. Mix-HAMS contains free phenolic acids and thus has certain activities. However, due to the non-covalent fixation of phenolic acids, they are prone to migration, diffusion or loss, and the activities are not as stable as those of the covalent grafting system. One-pot-HAMS and GDP-HAMS form polyphenolic acid co-grafted structures, and their activities are higher than those of Mix-HAMS. However, their grafting sequence and side group distribution are not conducive to the full exposure of active sites. PDG-HAMS still exhibits the highest or nearly the highest antioxidant and antibacterial effects under the condition of a relatively low gallic acid segmented grafting rate, indicating that the enhancement of its activity does not solely depend on the increase in gallic acid content but rather stems from the synergistic distribution of three types of side groups, namely Ph, Da, and Ga, on the same starch backbone. From the perspective of the structure-activity relationship, the free radical scavenging ability of benzoic acid-type phenolic acids increases with the increasing number of phenolic hydroxyl groups: p-hydroxybenzoic acid contains only a single phenolic hydroxyl group and has a weak hydrogen donation ability. The ortho-dihydroxy group of 3,4-dihydroxybenzoic acid can stabilize the phenoxyl radical through intramolecular hydrogen bonding, and the pyrogallol structure of gallic acid further enhances the hydrogen donation ability and conjugate stability. Therefore, the DPPH and ABTS scavenging rates show a significant gradient of Ph < Da < Ga. In Comparative Example 8, due to the too low grafting rate and insufficient active sites, the antioxidant and antibacterial activities are significantly weak. In Comparative Example 10, due to the too low proportion of gallic acid, the activity is also significantly weakened. Comparative Example 9 and Comparative Example 11 maintain relatively high activities due to the presence of较多焦棓酚结构而保持较高活性。需要说明的是,PDG-HAMS在没食子酸分段接枝率相对较低的条件下,即可获得与单一没食子酸接枝(Ga-HAMS)相当的抗氧化与抑菌活性,说明其活性来源于对羟基苯甲酸、3,4-二羟基苯甲酸与没食子酸三类侧基在同一骨架上的协同分布与高效利用,而非单纯依赖没食子酸用量的增加。因此,本发明在赋予材料优异界面与递送性能(见表2)的同时,并未牺牲其抗氧化与抑菌活性,为其在果蔬保鲜中的应用奠定了活性基础。

[0077] The effects of the emulsion gel coatings prepared from the modified starches prepared in Example 1 and Comparative Examples 1-11 on the quality deterioration and microbial growth of strawberries stored at 25 °C for 6 days were characterized, and the results are shown in Table 4.

[0078] Table 4 Effects of the emulsion gel coatings prepared from the modified starches prepared in Example 1 and Comparative Examples 1-11 on the quality deterioration and microbial growth of strawberries stored at 25 °C for 6 days

[0079] It should be noted that in the above translation, the part "较多焦棓酚结构而保持较高活性" seems to be an incomplete or incorrect expression in the original Chinese. It might need to be further clarified or corrected in the original text for a more accurate translation.Table 4 shows that the Control group, without effective coating protection, experienced a rot rate of 89.6% and a TVC of 6.45 log CFU / g on day 6, with a weight loss of 24.5%, indicating that strawberries are highly susceptible to microbial spoilage and moisture loss at room temperature. The HAMS-coated group showed a rot rate of 91.3% and a TVC of 6.89 log CFU / g, slightly higher than the Control group, indicating that HAMS coating alone not only fails to effectively preserve freshness but may even promote spoilage under specific high-humidity storage conditions. This is mainly because HAMS itself lacks antibacterial and antioxidant capabilities, and the moist polysaccharide layer formed by the unmodified HAMS coating may provide a favorable microenvironment for microbial attachment, growth, and reproduction, and may even serve as a carbon source usable by some microorganisms. Meanwhile, the weight loss rate of the HAMS-coated group was slightly lower than that of the Control group, indicating that it still has some physical water-blocking effect, but this effect is insufficient to offset the risk of spoilage caused by microbial proliferation. The Ga-HAMS coating group showed a significant reduction in rot rate and TVC, indicating that gallic acid grafting endows the material with strong antioxidant and antibacterial capabilities. Although the Mix-HAMS coating group contained three phenolic acids and thymol, the lack of covalent fixation of the phenolic acids resulted in insufficient coating stability and long-term release capacity, thus limiting its preservation effect. The One-pot-HAMS and GDP-HAMS coating groups showed further improvement, indicating that polyphenolic acid co-grafting is beneficial for improving strawberry preservation, but its effect is still weaker than PDG-HAMS. On day 6, the PDG-HAMS coating group had a rot rate of only 15.7%, TVC reduced to 3.80 log CFU / g, weight loss rate reduced to 5.5%, and maintained the highest firmness and soluble solids content, indicating that it can synergistically delay strawberry spoilage through multiple mechanisms such as slow-release antibacterial effect of thymol, antioxidant effect of polyphenolic acids, water retention of gel network, and coating barrier. This table demonstrates from a practical application perspective that PDG-HAMS is superior to Ga-HAMS, Mix-HAMS, One-pot-HAMS, and GDP-HAMS, serving as key support for the invention's "unexpected comprehensive preservation effect produced by specific Ph→Da→Ga sequential gradient grafting." From a mechanism of action perspective, the preservation efficacy of the emulsion gel stems from the synergistic effect of "thymol sustained-release antibacterial activity—polyphenol side-group antioxidant activity—gel network water retention and barrier." Combining Tables 2 and 3, it can be seen that although the intrinsic antioxidant and antibacterial activities of PDG-HAMS are comparable to those of single-grafted gallic acid, its significantly superior emulsification, encapsulation, and sustained-release properties (highest EE, lowest 48-h release rate, and best long-term stability) allow thymol to be efficiently immobilized and continuously released, thus amplifying the material-level advantages into visible preservation advantages in real fruit and vegetable systems.The rot rate, TVC, and weight loss of the coating groups in Comparative Examples 8 (under-grafted), 9 (over-grafted), 10, and 11 (imbalanced ratio) were all higher than those in Example 1, while the hardness and soluble solids retention were lower. This further indicates that the best synergistic preservation effect can only be achieved when the total grafting rate and segmentation ratio are within the preferred range. In summary, Table 4 confirms from a practical application perspective that the PDG-HAMS-based emulsion gel obtained by specific sequential gradient grafting of Ph→Da→Ga is significantly superior to the control groups in delaying strawberry rot, inhibiting microbial proliferation, reducing water loss, and maintaining textural quality, providing direct application-level evidence for the inventiveness of this invention.

[0080] In addition, the applicant also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0081] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A method for preparing a polyphenolic acid gradient-grafted high amylose-based emulsion gel, characterized in that, include: High amylose was grafted sequentially with p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid to obtain polyphenolic acid gradient-grafted high amylose; wherein the grafting ratio of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid in the polyphenolic acid gradient-grafted high amylose was (1-3):(1-3):(0.5-2); the total grafting rate of the polyphenolic acid gradient-grafted high amylose was 6%-20%; The polyphenolic acid gradient-grafted high amylose starch was mixed with water and gelatinized to form an aqueous phase; thymol was mixed with an oil solvent to form an oil phase. Furthermore, the aqueous phase and oil phase are mixed and homogenized at high speed, and then agarose and glycerol are added, mixed and allowed to stand to form a polyphenolic acid gradient grafted high amylose-based emulsion gel.

2. The preparation method according to claim 1, characterized in that: The total grafting rate of the polyphenolic acid gradient grafted high amylose is 8%-16%.

3. The preparation method according to claim 1, characterized in that, Specifically, it includes: High amylose was mixed with an organic solvent and stirred at 70-90°C for 50 minutes, then cooled to 25-35°C to obtain a high amylose solution. Furthermore, the high amylose starch solution is mixed with p-hydroxybenzoic acid, carbodiimide hydrochloride, and 4-dimethylaminopyridine and reacted at room temperature for 1-4 hours. Then, 3,4-dihydroxybenzoic acid, carbodiimide hydrochloride, and 4-dimethylaminopyridine are added and the reaction is continued at room temperature for 1-4 hours. Then, gallic acid, carbodiimide hydrochloride, and 4-dimethylaminopyridine are added and the reaction is continued at room temperature for 1-4 hours. Finally, after precipitation, washing, and drying, polyphenolic acid gradient grafted high amylose starch is obtained.

4. The preparation method according to claim 3, characterized in that: The high amylose includes high amylose corn starch; And / or, the amylose content in the high amylose content is 50%~80%; And / or, the organic solvent includes N,N-dimethylformamide.

5. The preparation method according to claim 3, characterized in that: The mass ratio of the high amylose, p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and gallic acid is 6.0:(0.25-1.10):(0.25-1.10):(0.12-0.65); And / or, the p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and gallic acid are used to graft the primary hydroxyl group at the C-6 position of the glucose unit in high amylose into high amylose.

6. The preparation method according to claim 1, characterized in that, Specifically, it includes: The polyphenolic acid gradient-grafted high amylose was mixed with water and subjected to high temperature and high pressure treatment for 2 h until it was completely gelatinized. Then it was cooled to 45~50℃ to form an aqueous phase. The temperature of the high temperature and high pressure treatment was 110-130℃ and the pressure was 0.10-0.25 MPa.

7. The preparation method according to claim 1, characterized in that: The oil solvent includes soybean oil; And / or, the concentration of polyphenolic acid gradient-grafted high amylose in the aqueous phase is 1-8 wt%; And / or, the volume content of thymol in the oil phase is 10-70%; And / or, the concentration of agarose in the polyphenolic acid gradient-grafted high amylose-based emulsion gel is 0.2-2.0 wt%, and the concentration of glycerol is 0.1-1 wt%. And / or, the volume fraction of the oil phase in the polyphenolic acid gradient-grafted high amylose-based emulsion gel is 10-50%; And / or, the volume ratio of the aqueous phase to the oil phase is 9:1 to 5:5; And / or, the high-speed homogenization process is performed at a rotation speed of 10,000 to 20,000 r / min for a duration of 5 to 10 min; And / or, the static molding temperature is 2~30℃ and the time is 2~12h.

8. A polyphenolic acid gradient-grafted high amylose-based emulsion gel prepared by the preparation method according to any one of claims 1-7.

9. The application of the polyphenolic acid gradient grafted high amylose-based emulsion gel according to claim 8 in the preservation of fruits and vegetables.

10. A method for preserving fruits and vegetables, characterized in that, include: The polyphenolic acid gradient grafted high amylose emulsion gel of claim 8 is uniformly coated on the surface of fruits and vegetables to achieve the preservation of fruits and vegetables. The fruits and vegetables mentioned include strawberries, blueberries, cherries, cherry tomatoes, grapes, fresh-cut apples, fresh-cut pears, or fresh-cut potatoes.

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