Modified starch / polyethyleneimine / curcumin composite indicating film and method and application thereof

Abstract

The invention discloses a modified starch / polyethyleneimine / curcumin composite indicating film and a method and application, and belongs to the technical field of a synthetic method of a novel modified starch / polyethyleneimine composite film material.The method comprises the steps that starch is added into ethyl acetate, pyridine is added, acryloyl chloride is added under stirring, after stirring reaction, suction filtration, washing and vacuum drying are conducted, and the modified starch / polyethyleneimine / curcumin composite indicating film is obtained; the acryloyl esterified starch is obtained; adding water and a glycerol solution into the acryloyl esterified starch, and carrying out gelatinization reaction to obtain an acryloyl esterified starch gelatinization product; and adding a polyethyleneimine solution, 4-mercaptobenzoic acid and a curcumin solution into the acryloyl esterified starch paste, uniformly stirring, pouring into a mold, casting to form a film, and drying to obtain the modified starch / polyethyleneimine / curcumin composite indicating film. The degradable food packaging material has good degradation performance, mechanical property and biocompatibility, can be used as a food packaging material by utilizing color change in solutions with different pH values to represent the spoilage condition of food.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthesis methods of modified starch / polyethyleneimine-based novel composite membrane materials, and specifically relates to a modified starch / polyethyleneimine / curcumin composite indicator membrane, methods and applications thereof. Background Art

[0002] Food packaging, as an indispensable part of the food industry chain, undertakes the key barrier function of blocking the intrusion of oxygen, water vapor and microorganisms, and its performance directly affects the shelf life and quality stability of food. According to industry statistics, about 93% of the annual consumption of global food packaging materials are petroleum-based synthetic plastic products. Such materials need to undergo a degradation cycle of more than 400 years in the natural environment. The resulting microplastic pollution and carbon emission problems caused by landfill and incineration have become global environmental challenges. Developing bio-based packaging materials with a controllable degradation cycle and meeting food-grade safety standards has become an urgent technical need to solve plastic pollution and achieve circular economy.

[0003] At present, biodegradable plastic packaging films have become a global R & D hotspot, and among them, functional food packaging materials based on starch-based biomass degradation materials show significant technical competitiveness. Compared with other biodegradable natural polymer materials, starch has the advantages of wide source, low price, and easy biodegradability. Therefore, starch has great development prospects in the field of biodegradable materials. So, the degradable packaging films prepared by compounding starch with synthetic polymer materials have many unique properties, thus enhancing the value of starch-based degradable plastics in practical applications. Boonsuk P. et al. prepared a series of polyvinyl alcohol / cassava starch composite film materials. When the ratio of polyvinyl alcohol:starch:glycerol was 7:3:1, the mechanical tensile strength of the composite film reached about 25 MPa. (Boonsuk P, et al. Modified cassava starch / poly(vinyl alcohol) blend films plasticized by glycerol: Structure and properties[J].Journal of Applied Polymer Science, 2020, 137(26): 48848.). Polyethyleneimine is a cationic polymer with a high density of amino groups. Its molecular chain contains primary amine, secondary amine, and tertiary amine groups, endowing it with excellent chemical activity and tunable functionality. Compounding modified starch with polyethyleneimine can obtain composite materials with good degradation performance, mechanical properties, biocompatibility, etc. Curcumin is a natural phenolic pigment with antioxidant, anti-inflammatory, antifungal, and antibacterial effects. Curcumin has pH sensitivity. It is yellow when pH < 7, orange when pH = 7 - 8, and red when pH > 8. Therefore, curcumin is selected as an indicator, and incorporating it into food packaging materials can use its easily observable color change to indicate the spoilage of food. Currently, color-changing packaging materials that can show food spoilage changes have become a trend and research hotspot in food packaging. Preparing indicator packaging films using the pH sensitivity of curcumin to timely and accurately reflect food freshness has received people's attention. However, at present, modified starch films cannot be used as food packaging films because they have disadvantages such as poor strength and flexibility, poor hydrophobicity, and easy mildew and spoilage. As food packaging materials, they are prone to breakage, mildew, and spoilage.

[0004] Aiming at the problems existing in the prior art that modified starch films have poor strength, flexibility, hydrophobicity, and high water vapor permeability as food packaging films, and are prone to breakage, mildew, and spoilage as food packaging materials, there is an urgent need to find a new modification method to improve the strength, flexibility, hydrophobicity, barrier property, and anti-mildew performance of modified starch food packaging films. Summary of the Invention

[0005] In order to overcome the disadvantages of the above-mentioned prior art, the object of the present invention is to provide a modified starch / polyethyleneimine / curcumin composite indicator film, method and application, which solve the technical problems existing in the traditional modified starch film, such as poor strength and flexibility, poor hydrophobicity, easy to mildew and deteriorate, and inability to detect the environment and food freshness and quality according to color changes.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions to be realized: The present invention discloses a modified starch / polyethyleneimine / curcumin composite indicator film, and the structural formula of the modified starch / polyethyleneimine / curcumin composite indicator film is as follows: ; Among them, n is 18 - 23.

[0007] Preferably, the tensile strength of the modified starch / polyethyleneimine / curcumin composite indicator film is 8.4 - 10.5 MPa, the elongation at break is 108.2% - 147.4%, the water contact angle is 103.5 - 113.4°, and the water vapor transmission rate is 1.28×10 −4 ~1.44×10 −4 g / mm·24h.

[0008] The present invention also discloses a preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, including: 1) Add starch to ethyl acetate, then add pyridine, add acryloyl chloride under stirring, after stirring reaction, carry out suction filtration, washing, and vacuum drying to obtain acryloylated starch; 2) Add water and glycerol solution to the acryloylated starch obtained in step 1), and after gelatinization reaction, obtain an acryloylated starch paste; 3) Add the polyethyleneimine solution, 4-mercaptobenzoic acid, and curcumin solution to the acryloylated starch paste obtained in step 2), stir evenly, pour it into a mold and cast a film, and dry it to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0009] Preferably, in step 1), the mass ratio of starch, ethyl acetate, pyridine, and acryloyl chloride is (5 - 10):(30 - 50):(0.5 - 1.5):(0.5 - 0.9); The conditions for the stirring reaction are: stirring reaction at room temperature for 30 - 50 min; the conditions for vacuum drying are: vacuum drying at 50 - 80°C for 10 - 12 h.

[0010] Preferably, in step 2), the mass ratio of acryloylated starch, water, and glycerol solution is (1 - 2):(20 - 30)(0.1 - 0.8); The conditions for the gelatinization reaction are: stirring at 90 - 95°C for 60 - 70 min.

[0011] Preferably, in step 3), the mass ratio of the polyethyleneimine solution, 4-mercaptobenzoic acid, curcumin solution, and acryloylated starch paste is (0.05 - 0.5):(0.01 - 0.05):(1 - 2):(21 - 32); the mass fraction of the polyethyleneimine solution is 50%; the mass concentration of the curcumin solution is 5 mg / ml.

[0012] Preferably, the preparation method of the curcumin solution in step 3) includes: adding turmeric to an ethanol solution, stirring by water bath heating, rotary evaporation, and freeze-drying to obtain curcumin powder; then dissolving the curcumin powder in an ethanol solution with a volume fraction of 60% to obtain the curcumin solution.

[0013] More preferably, the mass ratio of turmeric to the ethanol solution is (1 - 3):(30 - 40); the volume fraction of the ethanol solution is 60%; The conditions for stirring by water bath heating are: stirring by water bath heating at 50 - 60°C for 3 - 3.5 h; The conditions for rotary evaporation are: rotary evaporation in the dark at 60 - 65°C for 1.5 - 2.0 h.

[0014] Preferably, the starch includes wheat starch, potato starch, or corn starch; The mass fractions of ethyl acetate, pyridine, acryloyl chloride, and glycerol are all 99.0%; The polyethyleneimine solution is prepared by dissolving polyethyleneimine in deionized water; the mass fraction of polyethyleneimine is 99.0%, and the average molecular weight is 10,000.

[0015] The present invention also discloses the application of the modified starch / polyethyleneimine / curcumin composite indicator film prepared by the preparation method of the above-mentioned modified starch / polyethyleneimine / curcumin composite indicator film in food packaging materials.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a modified starch / polyethyleneimine / curcumin composite indicator film. The polyethyleneimine, 4-mercaptobenzoic acid, and curcumin molecules in the composite indicator film contain long-chain structures and functional groups such as active amino, hydroxyl, and carboxyl groups, which can form hydrogen bonds and intermolecular forces with each other, effectively improving the strength and toughness of the composite film and meeting the requirements for strength and toughness as a food packaging film. Curcumin has good anti-inflammatory, antifungal, antibacterial, and antioxidant properties, which can endow the modified starch with antibacterial and anti-mildew properties, and also has a significant color change from pink to blue in the pH range of 6 to 9. Since general food spoilage produces alkaline amine substances, the freshness and quality of the packaged food can be determined accordingly. At the same time, the composite film combines the advantages of starch, has good biodegradability and mechanical properties, and can further promote the development of degradable plastic films.

[0017] Furthermore, the tensile strength of the modified starch / polyethyleneimine / curcumin composite indicator film is 8.4 - 10.5 MPa, the elongation at break is 108.2% - 147.4%, the water contact angle is 103.5 - 113.4°, and the water vapor transmission rate is 1.28×10 −4 ~1.44×10 −4 g / mm·24h. The relatively high tensile strength endows the film material with good load-bearing capacity, enabling it to withstand a certain amount of external force without being easily broken, and effectively protecting the internal items from damage during packaging, transportation, and storage. The high elongation at break indicates that the film material has good flexibility, and can undergo large deformations without immediate fracture when subjected to external tensile forces. In the field of food packaging, when the packaged food is squeezed or collided, it can not only maintain the structural integrity but also buffer the external force through its own flexibility, reducing the possibility of food damage. The relatively good hydrophobicity makes the surface of the film material not easily wetted by water, effectively preventing the penetration and adsorption of water molecules. In a humid environment, such as a food cold storage or an outdoor humid place, the hydrophobic property can keep the film material dry and clean, reducing the growth of microorganisms and chemical reactions caused by moisture, thereby extending the shelf life of the items inside the package. The relatively low water vapor transmission rate can effectively block the transmission of water vapor, keep the internal environment of the package dry, prevent the entry of external water vapor, and thus extend the shelf life of food and drugs.

[0018] The present invention discloses a preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film. Starch is used as the base material, acryloyl chloride is used to esterify and modify the starch, 4-mercaptobenzoic acid is used as the cross-linking agent, polyethyleneimine is used as the composite material, and at the same time, green and biocompatible glycerol and water-soluble curcumin are used as additives. Then, through the gelatinization of the esterified modified starch and the compounding with polyethyleneimine and additives, a modified starch / polyethyleneimine / curcumin composite film with good mechanical properties, biocompatibility and acid-base indicator properties is obtained. The present invention selects starch and polyethyleneimine as the main raw materials. Starch has the characteristics of being rich, cheap, renewable and inexhaustible, and has the advantages of biodegradability and low pollution; polyethyleneimine has good biocompatibility, is easy to process, has good antibacterial properties, and is easy to form films. Acryloyl chloride is used to esterify and modify the starch. After modification, the acryloyl esterified starch has an ester group functional group, and has better gelatinization ability and film-forming property. By adding pH-sensitive curcumin to the food preservation indicator film, the degree of food spoilage can be judged by the different color changes of the film. By adding the polymer polyethyleneimine with good biocompatibility, the mechanical and other properties of the starch-polyethyleneimine film are further enhanced, so that the flexibility and mechanical strength of the composite starch-polyethyleneimine film are further enhanced. The tensile strength is 8.4~10.5 MPa, the elongation at break is 108.2%~147.4%, the water contact angle is 103.5~113.4°, and the water vapor transmission rate is 1.28×10 −4 ~1.44×10 −4 g / mm·24h. During the film-forming process, there is a good reaction ability between the hydroxyl groups in the starch and the amino groups of polyethyleneimine, and a certain degree of chemical cross-linking reaction will occur during the compounding process. The characteristics of this chemical structure enable the modified starch / polyethyleneimine / curcumin composite indicator film to obtain good mechanical properties. The starch used in the present invention is a natural biomass, with a market price of 3.3 yuan / 500 g, and polyethyleneimine is 0.89 yuan / 100 g, which has the advantages of being cheap and easily available. It can replace expensive and somewhat toxic ordinary plastics in life, solving the environmental protection problem, and thus has great economic benefits.

[0019] The present invention also discloses the application of the above-mentioned modified starch / polyethyleneimine / curcumin composite indicator film in food packaging materials. As a food packaging film, it has good anti-inflammatory, antifungal, antibacterial and antioxidant properties and anti-mildew properties. At the same time, the film has a significant color change from yellow to red in the pH range of 6~9. Generally, alkaline amines are produced during food spoilage. Based on this, the freshness and quality of the packaged food can be judged. Description of the Drawings

[0020] Figure 1This is the mechanism diagram of the modified starch / polyethyleneimine / curcumin composite indicator film of the present invention; Figure 2 This is the infrared spectrum comparison diagram of acryloylated starch and corn starch disclosed in Example 1 of the present invention; Figure 3 This is the infrared spectrum comparison diagram of the modified starch / polyethyleneimine / curcumin composite indicator film disclosed in Example 1 of the present invention, the corn starch film prepared in Comparative Example 1, and the corn starch / polyethyleneimine / curcumin composite film prepared in Comparative Example 2; Figure 4 This is the color change of the modified starch / polyethyleneimine / curcumin composite indicator film disclosed in Example 1 of the present invention immersed in buffer solutions with different pH values. Detailed implementation manners

[0021] Next, the technical solutions of the present invention will be described clearly and completely. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] In the present invention, if there is no special description, all the implementation manners and preferred implementation methods mentioned herein can be combined with each other to form new technical solutions.

[0023] In the present invention, if there is no special description, all the technical features and preferred features mentioned herein can be combined with each other to form new technical solutions.

[0024] In the present invention, if there is no special description, the percentage (%) or part refers to the mass fraction or weight part relative to the composition.

[0025] In the present invention, if there is no special description, the various components or their preferred components involved can be combined with each other to form new technical solutions.

[0026] In the present invention, unless otherwise stated, the numerical range "a~b" represents the abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "6~22" means that all real numbers between "6~22" have been fully listed herein, and "6~22" is only the abbreviated representation of these numerical combinations.

[0027] The "range" disclosed in the present invention can be in the form of one or more lower limits and one or more upper limits respectively.

[0028] In the present invention, the term "and / or" used herein refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0029] In the present invention, unless otherwise specified, each reaction or operation step can be carried out sequentially or in order. Preferably, the reaction methods herein are carried out sequentially.

[0030] Unless otherwise specified, the professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any methods or materials similar or equivalent to the described content can also be applied to the present invention.

[0031] The present invention discloses a preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film. Using starch as the base material, acryloyl chloride is used to esterify and modify the starch, 4-mercaptobenzoic acid is used as the crosslinking agent, and polyethyleneimine is used as the composite material. At the same time, green and biocompatible glycerol and curcumin are used as additives. Then, through the gelatinization of the acyl chloride-modified starch and the compounding with polyethyleneimine and additives, a modified starch / polyethyleneimine / curcumin composite indicator film with high mechanical properties and biocompatibility is obtained.

[0032] Specifically, it includes the following steps: Step 1: According to the mass fraction (the same hereinafter), add 1-3 parts of crushed turmeric to 30-40 parts of an ethanol solution with a volume fraction of 60%. Heat and stir in a water bath at 50-60 °C for 3-3.5 h. The obtained extract is subjected to rotary evaporation in the dark at 60-65 °C to obtain a concentrated solution, and then freeze-dried to obtain curcumin powder.

[0033] Step 2: Take 5-10 parts of starch and put it into 30-50 parts of ethyl acetate. Then add 0.5-1.5 parts of pyridine. While stirring, dropwise add 0.5-0.9 parts of acryloyl chloride. Continue to stir at room temperature for 30-50 min. Then filter the obtained product, wash it with 20 parts of absolute ethanol each time, wash three times, and vacuum dry at 50-80 °C for 10-12 h to obtain white modified acryloyl esterified starch.

[0034] Step 3: Weigh 1-2 parts of acryloyl esterified starch and add it to 20-30 parts of water and 0.1-0.8 parts of glycerol solution. Heat and stir to 90-95 °C for gelatinization, keep warm and stir for 60-70 min to obtain an acryloyl esterified starch paste. Step 4: Pour 21-32 parts of the acryloyl esterified starch paste into a beaker, add 0.05-0.5 parts of a 50% polyethyleneimine solution, 0.01-0.05 parts of 4-mercaptobenzoic acid, and then add 1-2 parts of a 5 mg / ml curcumin solution. Stir evenly, pour it into a mold and cast a film, and then dry it to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0035] The turmeric selected in Examples 1-5 was obtained from the land and had no other impurities.

[0036] The starch includes wheat starch, potato starch or corn starch.

[0037] The mass fractions of ethyl acetate, pyridine, acryloyl chloride, glycerol and 4-mercaptobenzoic acid are 99.0%.

[0038] The polyethyleneimine solution is prepared by dissolving polyethyleneimine in deionized water; the mass fraction of polyethyleneimine is 99.0% and the average molecular weight is 10,000.

[0039] The modified starch / polyethyleneimine / curcumin composite indicator film prepared by the present invention has a tensile strength of 8.4 - 10.5 MPa, an elongation at break of 108.2% - 147.4%, a water contact angle of 103.5 - 113.4°, and a water vapor transmission rate of 1.28×10 −4 ~1.44×10 −4 g / mm·24h, has an obvious color change within a pH value of 6 - 9, can extend the shelf life of strawberries to more than 7 days at room temperature, and also has biodegradable performance, with good biodegradable performance, mechanical properties, biocompatible properties and the ability to indicate the change of pH related to food spoilage, and can be used as a food packaging material with excellent comprehensive performance.

[0040] The present invention utilizes the advantages that the modified starch is easily degradable, the polyethyleneimine has good film-forming property and high strength, and curcumin can be used as an indicator to show the change of food quality, and prepares a modified starch / polyethyleneimine / curcumin composite indicator film, which has the characteristics of a unique preparation method and low cost.

[0041] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components described and shown in the accompanying drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0042] Example 1 A preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, comprising the following steps: Step 1: According to the parts by mass (the same hereinafter), add 2 parts of crushed turmeric to 30 parts of an ethanol solution with a volume fraction of 60%, heat and stir in a water bath at 55 °C for 3.1 h. The obtained extract is rotary evaporated in the dark at 60 °C for 1.5 h to obtain a concentrated solution, which is freeze-dried to obtain curcumin powder.

[0043] Step 2: Put 5 parts of corn starch into 35 parts of ethyl acetate, add 1 part of pyridine, dropwise add 0.5 part of acryloyl chloride under stirring, continue to stir at room temperature for 40 min, then filter the obtained product, wash it with 20 parts of absolute ethanol each time, wash three times, and vacuum dry at 60 °C for 11 h to obtain white modified acryloyl esterified starch.

[0044] Step 3: Weigh 2 parts of acryloyl esterified starch and add it to 25 parts of water and 0.3 part of glycerol solution, heat to 95 °C with stirring for gelatinization, keep warm and stir for 65 min to obtain acryloyl esterified starch paste. Step 4: Pour 21 parts of acryloyl esterified starch paste into a beaker, add 0.2 part of 50% polyethyleneimine solution, 0.04 part of 4-mercaptobenzoic acid, then add 1 part of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0045] Figure 1 This is the mechanism diagram of the modified starch / polyethyleneimine / curcumin composite indicator film of the present invention; it can be seen from the figure that in the modified starch / polyethyleneimine / curcumin composite indicator film, the molecular structures of polyethyleneimine, 4-mercaptobenzoic acid and curcumin contain long-chain structures and functional groups such as active amino groups, hydroxyl groups and carboxyl groups, and hydrogen bonds and intermolecular forces can be formed between them, which can effectively improve the strength and toughness of the composite film and can meet the requirements for the strength and toughness of food packaging films; curcumin in the composite film has good anti-inflammatory, antifungal, antibacterial and antioxidant properties, which can endow the modified starch / polyethyleneimine / curcumin composite indicator film with antibacterial and anti-mildew properties, and at the same time has a significant color change from yellow to red in the pH range of 6-9.

[0046] Figure 2 This is the infrared spectrum comparison chart of acryloyl esterified starch and corn starch disclosed in Example 1 of the present invention; it can be seen from the figure that by comparing with the infrared spectrum of natural starch, it is found that acryloyl esterified starch has a new absorption peak at 1730 cm -1 which is attributed to the characteristic peak of C=O stretching vibration, indicating that a carbonyl group has been successfully introduced onto the starch. In addition, the infrared spectrum of acryloyl esterified starch at 2900 cm -1 also shows characteristic peaks different from those of natural starch, and the characteristic peak of CH stretching vibration exists on the carbon chain of this acylated starch, indicating that the acyl group has been successfully attached to the starch surface. 1730 cm-1 That is, the C=O stretching vibration peak, at 1640 cm -1 is the stretching vibration peak of C=C. From the above, we can see that the esterification reaction of acryloyl chloride and starch is successful, thus preliminarily indicating that the synthesis of acryloyl esterified starch is successful.

[0047] Figure 3 is the infrared spectrum comparison chart of the modified starch / polyethyleneimine / curcumin composite indicator film disclosed in Example 1 of the present invention, the corn starch film prepared in Comparative Example 1, and the corn starch / polyethyleneimine / curcumin composite indicator film prepared in Comparative Example 2; as can be seen from the figure, the corn starch film, the corn starch / polyethyleneimine / curcumin composite indicator film, and the modified starch / polyethyleneimine / curcumin composite indicator film clearly show the stretching vibration absorption band of the -OH bond in the range of 3500-3000 cm⁻¹, which is related to the abundant hydroxyl -OH and -NH in the modified starch, acryloyl esterification, curcumin, 4-mercaptobenzoic acid, and glycerol in the raw materials of the film 2 coincides with the stretching vibration absorption peak. Compared with the corn starch film, new absorption peaks appear at 560 cm⁻¹ and 1416 cm⁻¹ in the corn starch / polyethyleneimine / curcumin composite indicator film and the modified starch / polyethyleneimine / curcumin composite indicator film respectively. First, an obvious absorption peak is observed at 1560 cm⁻¹, which is usually related to the N-H bending vibration in the amide group. In addition, the C=O stretching vibration and the C-N stretching vibration may also produce absorption peaks in this band, due to the amide bond formed by the reaction of the amino group in polyethyleneimine with the carboxyl group in the crosslinking agent 4-mercaptobenzoic acid through amidation reaction. Secondly, the absorption peak at 1416 cm⁻¹ is related to the vibration of the C-S bond, indicating that the sulfhydryl group in 4-mercaptobenzoic acid in the sample may have reacted with the double bond in the esterified starch. Finally, the absorption peak of the modified starch / polyethyleneimine / curcumin composite indicator film at 2400 cm⁻¹ also indicates the presence of the sulfhydryl (C-S) bond, further supporting the conclusion that the sample contains sulfur bonds. These changes indicate that the structure of the composite film has undergone significant chemical modification after adding polyethyleneimine and the crosslinking agent, further enhancing the performance of the film.

[0048] Figure 4 is the color change of the modified starch / polyethyleneimine / curcumin composite indicator film disclosed in Example 1 of the present invention immersed in buffer solutions with different pH values; as can be seen from the figure, the indicator film presents different colors at different pH values. When pH < 7, the indicator film is yellow. When pH = 7-8, it is orange. When pH > 8, it is red. The indicator film has excellent pH color development performance. Since the pH will change acid-base during the spoilage process of food, especially during the spoilage of meat products, it can be used as a food packaging film material for antibacterial preservation and freshness indication of food in the future.

[0049] Example 2 A preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, comprising the following steps: Step 1: According to the mass parts (the same hereinafter), add 3 parts of crushed turmeric to 40 parts of an ethanol solution with a volume fraction of 60%, heat and stir in a water bath at 60 °C for 3.5 h, and evaporate the obtained extract under reduced pressure in the dark at 65 °C for 2.0 h to obtain a concentrated solution, and then obtain curcumin powder after freeze-drying.

[0050] Step 2: Put 6 parts of wheat starch into 40 parts of ethyl acetate, add 1.2 parts of pyridine, dropwise add 0.9 part of acryloyl chloride under stirring, continue to stir at room temperature for 45 min, then filter the obtained product, wash it with 20 parts of absolute ethanol each time, wash it three times, and vacuum dry it at 70 °C for 12 h to obtain white modified acryloylated starch.

[0051] Step 3: Weigh 1 part of acryloylated starch and add it to 25 parts of water and 0.4 part of glycerol solution, heat to 90 °C under stirring for gelatinization, keep warm and stir for 65 min to obtain an acryloylated starch paste; Step 4: Pour 25 parts of the acryloylated starch paste in Step 3 into a beaker, add 0.4 part of 50% polyethyleneimine solution, 0.05 part of 4-mercaptobenzoic acid, and then add 1.5 parts of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0052] Example 3 A preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, comprising the following steps: Step 1: According to the mass parts (the same hereinafter), add 2 parts of crushed turmeric to 33 parts of an ethanol solution with a volume fraction of 60%, heat and stir in a water bath at 56 °C for 3.1 h, and evaporate the obtained extract under reduced pressure in the dark at 62 °C for 1.6 h to obtain a concentrated solution, and then obtain curcumin powder after freeze-drying.

[0053] Step 2: Put 7 parts of potato starch into 50 parts of ethyl acetate, add 1.5 parts of pyridine, dropwise add 0.6 part of acryloyl chloride under stirring, continue to stir at room temperature for 50 min, then filter the obtained product, wash it with 20 parts of absolute ethanol each time, wash it three times, and vacuum dry it at 65 °C for 11 h to obtain white modified acryloylated starch.

[0054] Step 3: Weigh 1.5 parts of acryloylated starch and add it to 30 parts of water and 0.2 part of glycerol solution, heat to 93 °C under stirring for gelatinization, keep warm and stir for 70 min to obtain an acryloylated starch paste; Step 4: Pour 28 parts of the acryloylated starch paste obtained in Step 3 into a beaker, add 0.25 part of 50% polyethyleneimine solution and 0.03 part of 4-mercaptobenzoic acid, then add 1 part of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0055] Example 4 A preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, comprising the following steps: Step 1: According to the mass parts (the same hereinafter), add 1 part of crushed turmeric to 30 parts of an ethanol solution with a volume fraction of 60%, heat and stir in a water bath at 50 °C for 3 h, evaporate the obtained extract by rotary evaporation in the dark at 60 °C for 1.7 h to obtain a concentrated solution, and freeze-dry to obtain curcumin powder.

[0056] Step 2: Take 8 parts of corn starch and put it into 30 parts of ethyl acetate, then add 0.5 part of pyridine, dropwise add 0.7 part of acryloyl chloride under stirring, continue to stir at room temperature for 30 min, then filter the obtained product by suction, wash it with 20 parts of absolute ethanol each time, wash it three times, and vacuum dry it at 50 °C for 12 h to obtain white modified acryloylated starch.

[0057] Step 3: Weigh 2 parts of acryloylated starch and add it to 20 parts of water and 0.1 part of glycerol solution, heat and stir to 92 °C for gelatinization, keep warm and stir for 60 min to obtain an acryloylated starch paste. Step 4: Pour 32 parts of the acryloylated starch paste obtained in Step 3 into a beaker, add 0.05 part of 50% polyethyleneimine solution and 0.01 part of 4-mercaptobenzoic acid, then add 2 parts of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0058] Example 5 A preparation method of a modified starch / polyethyleneimine / curcumin composite indicator film, comprising the following steps: Step 1: According to the mass parts (the same hereinafter), add 1 part of crushed turmeric to 35 parts of an ethanol solution with a volume fraction of 60%, heat and stir in a water bath at 60 °C for 3 h, evaporate the obtained extract by rotary evaporation in the dark at 63 °C for 1.8 h to obtain a concentrated solution, and freeze-dry to obtain curcumin powder.

[0059] Step 2: Take 10 parts of wheat starch and put it into 45 parts of ethyl acetate, then add 0.8 part of pyridine, dropwise add 0.8 part of acryloyl chloride under stirring, continue to stir at room temperature for 40 min, then filter the obtained product by suction, wash it with 20 parts of absolute ethanol each time, wash it three times, and vacuum dry it at 80 °C for 10 h to obtain white modified acryloylated starch.

[0060] Step 3: Weigh 1 portion of acryloylated starch and add it to 28 portions of water and 0.8 portion of glycerol solution. Heat it to 95 °C with stirring for gelatinization, keep warm and stir for 65 min to obtain acryloylated starch paste; Step 4: Pour 32 portions of the acryloylated starch paste obtained in Step 3 into a beaker, add 0.5 portion of 50% polyethyleneimine solution and 0.03 portion of 4-mercaptobenzoic acid, then add 1.3 portions of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

[0061] Control Example 1 A preparation method of a corn starch film includes the following steps: Step 1: Weigh 5 portions of corn starch, put it into 50 portions of water, add 0.4 portion of glycerol, heat it to 95 °C with stirring for gelatinization, keep warm and stir for 60 min to obtain corn starch paste; Step 2: Pour the gelatinized corn starch solution obtained in Step 1 into a beaker, stir evenly, pour it into a mold and cast it into a film, and then dry it to obtain a corn starch film.

[0062] Control Example 2 A preparation method of a corn starch / polyethyleneimine / curcumin composite indicator film includes the following steps: Step 1: Weigh 5 portions of corn starch, put it into 50 portions of water, add 0.4 portion of glycerol, heat it to 95 °C with stirring for gelatinization, keep warm and stir for 60 min to obtain corn starch paste; Step 2: Add 2 portions of crushed turmeric to 30 portions of 60% ethanol solution by volume, heat and stir it in a water bath at 55 °C for 3.1 h, evaporate the obtained extract by rotary evaporation in the dark at 60 °C for 1.5 h to obtain a concentrated solution, and freeze-dry it to obtain curcumin powder; Step 3: Take 21 portions of the corn starch paste prepared in Step 1 and mix it with 0.3 portion of glycerol solution, then add 0.2 portion of 50% polyethyleneimine solution and 0.04 portion of 4-mercaptobenzoic acid, then add 1 portion of 5 mg / ml curcumin solution, stir evenly, pour it into a mold and cast it into a film to obtain a corn starch / polyethyleneimine / curcumin composite indicator film.

[0063] Table 1 Comparison of performance test results of the modified starch / polyethyleneimine / curcumin composite indicator films prepared in Examples 1 - 5, the corn starch film prepared in Control Example 1, and the corn starch / polyethyleneimine / curcumin composite indicator film prepared in Control Example 2

[0064] See Table 1 for the comparison of the performance test results of the modified starch / polyethyleneimine / curcumin composite indicator films prepared in Examples 1-5, the corn starch film prepared in Comparative Example 1, and the corn starch / polyethyleneimine / curcumin composite indicator film prepared in Comparative Example 2; as can be seen from the table, the tensile strength of the modified starch / polyethyleneimine / curcumin composite indicator films prepared in Examples 1-5 of the present invention is 8.4-10.5 MPa, the elongation at break is 108.2%-147.4%, the water contact angle is 103.5-113.4°, and the water vapor transmission rate is 1.28×10 −4 ~1.44×10 −4 g / mm·24 h; there is a significant color change from light yellow to red in the pH range of 6-9, based on which the freshness and quality of the packaged food can be determined. The mechanical properties of Comparative Example 2 are poor and it cannot be used as a food packaging film.

[0065] In summary, the present invention discloses a modified starch / polyethyleneimine / curcumin composite indicator film, a preparation method and an application thereof. Using starch as a base material, the starch is modified with acryloyl chloride, and then compounded with polyethyleneimine, glycerol and curcumin to obtain a modified starch / polyethyleneimine / curcumin composite indicator film with high mechanical properties and acid-base indicator function. The modified starch / polyethyleneimine / curcumin composite indicator film of the present invention can exhibit different color changes in solutions with different pH values, and can use its color changes to indicate the situation and degree of food spoilage. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film of the present invention has good degradation performance, mechanical performance, biocompatibility performance and can indicate the change of pH value related to food spoilage, and can be used as a food packaging material with excellent comprehensive performance, providing a new idea for the high-value utilization of biomass materials.

[0066] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified starch / polyethyleneimine / curcumin composite indicator film, characterized in that: The structural formula of the modified starch / polyethyleneimine / curcumin composite indicator film is as follows: ； Among them, n is 18~23.

2. The modified starch / polyethyleneimine / curcumin composite indicator film according to claim 1, characterized in that: The modified starch / polyethyleneimine / curcumin composite indicator film has a tensile strength of 8.4-10.5 MPa, an elongation at break of 108.2%-147.4%, a water contact angle of 103.5-113.4°, and a water vapor permeability of 1.28×10 −4 ~1.44×10 −4 g / mm·24h.

3. A method for preparing a modified starch / polyethyleneimine / curcumin composite indicator film, characterized in that: include: 1) Add starch to ethyl acetate, then add pyridine, add acryloyl chloride under stirring, stir and react, filter, wash and vacuum dry to obtain acryloyl esterified starch; 2) adding water and glycerol solution to the acryloyl esterified starch obtained in step 1) to obtain acryloyl esterified starch gelatinized material after gelatinization reaction; 3) Adding the polyethyleneimine solution, 4-mercaptobenzoic acid and curcumin solution to the acryloyl esterified starch gelatinized product obtained in step 2), stirring evenly, pouring into a mold to cast into a film, and drying to obtain a modified starch / polyethyleneimine / curcumin composite indicator film.

4. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 3, characterized in that: In step 1), the mass ratio of starch, ethyl acetate, pyridine and acryloyl chloride is (5-10): (30-50): (0.5-1.5): (0.5-0.9); The stirring reaction conditions are: stirring reaction at room temperature for 30-50 minutes; the vacuum drying conditions are: vacuum drying at 50-80° C. for 10-12 hours.

5. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 3, characterized in that: In step 2), the mass ratio of the acryloyl esterified starch, water and glycerol solution is (1-2): (20-30) (0.1-0.8); The gelatinization reaction conditions are: stirring at 90-95° C. for 60-70 min.

6. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 3, characterized in that: In step 3), the mass ratio of the polyethyleneimine solution, 4-mercaptobenzoic acid, curcumin solution and acryloyl esterified starch gelatin is (0.05-0.5):(0.01-0.05):(1-2):(21-32); the mass fraction of the polyethyleneimine solution is 50%; and the mass concentration of the curcumin solution is 5 mg / ml.

7. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 3, characterized in that: In step 3), the method for preparing the curcumin solution comprises: adding turmeric to an ethanol solution, heating and stirring in a water bath, rotary evaporation, and freeze-drying to obtain curcumin powder; and then dissolving the curcumin powder in a 60% by volume ethanol solution to obtain a curcumin solution.

8. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 7, characterized in that: The mass ratio of the turmeric and the ethanol solution is (1-3): (30-40); the volume fraction of the ethanol solution is 60%; The conditions for water bath heating and stirring are: 50-60°C water bath heating and stirring for 3-3.5 h; The rotary evaporation conditions are: rotary evaporation at 60-65° C. in the dark for 1.5-2.0 h.

9. The method for preparing the modified starch / polyethyleneimine / curcumin composite indicator film according to claim 3, characterized in that: The starch includes wheat starch, potato starch or corn starch; The mass fractions of ethyl acetate, pyridine, acryloyl chloride and glycerol are all 99.0%; The polyethyleneimine solution is prepared by dissolving polyethyleneimine in deionized water; the mass fraction of the polyethyleneimine is 99.0% and the average molecular weight is 10,000.

10. Use of the modified starch / polyethyleneimine / curcumin composite indicator film obtained by the preparation method of the modified starch / polyethyleneimine / curcumin composite indicator film according to any one of claims 3 to 9 in food packaging materials.

Images