Preparation method of polylactic acid double-layer food packaging film without adhesive

Abstract

The invention relates to a preparation method of an adhesive-free polylactic acid double-layer food packaging film. The preparation method comprises the following steps: S1, preparing an anthocyanin solution; s2, preparing a chitosan solution; s3, preparing a polylactic acid solution; and S4, preparing the double-layer film. According to the prepared polylactic acid double-layer food packaging film, on one hand, the prepared packaging film integrates the advantages of a polylactic acid film and a chitosan film in a double-layer composite mode, the surface waterproof performance is improved, meanwhile, the air permeability of the film is reduced, and anthocyanin is successfully applied to the polylactic acid film; on the other hand, the prepared packaging film is sensitive to pH response, the double-layer film can display obvious color change when the food contained in the packaging film goes bad, and more convenient and visual commodity quality information can be provided for consumers.

Description

Technical Field

[0001] The invention belongs to the technical field of food packaging materials, and in particular relates to a method for preparing a polylactic acid double-layer food packaging film without adhesive. Background Art

[0002] With the upgrading of the use of packaging in the food industry, plastic has become the main packaging material. Polylactic acid (PLA) is a biodegradable natural polymer obtained by fermentation and polycondensation of agricultural products such as corn and polysaccharides, making it a potential candidate to replace petroleum-based polymers such as polypropylene and polyethylene. It has been widely used in daily life. PLA has high mechanical strength, high transparency and easy processing. On the one hand, PLA film lacks toughness and is brittle, and PLA film often has poor gas barrier properties, which limits its application in food sealing. On the other hand, PLA is only soluble in organic solvents. The packaging film prepared by the cast film method directly wraps the food, which poses the hazard of residual toxic substances migrating to the food. The introduction of other materials and PLA for composite modification has become the current focus of exploration.

[0003] The development of multifunctional food packaging has good application prospects. Smart packaging containing natural pigment indicators can monitor food conditions and provide chemical information about the food inside. Anthocyanins are active compounds extracted from plants and are safer than artificial pigments. Anthocyanins are widely available and have antioxidant, antibacterial, and anti-inflammatory properties, which can provide multifunctionality for packaging films. However, anthocyanins are mostly soluble in solutions such as water and ethanol, and are difficult to dissolve in organic solvents. Natural pigments generally have poor stability and cannot be hot-pressed into films with polylactic acid.

[0004] Chitosan is a cationic natural carbohydrate. Due to the protonation of its amino group, it can be dissolved in weakly acidic solutions. Chitosan has been widely used in cosmetics, food, and pharmaceuticals due to its excellent film-forming properties, durability, flexibility, and antibacterial properties. The positively charged amino groups on the surface of chitosan can produce electrostatic interactions with the carboxylic acid groups on the polylactic acid molecular chain, thereby preparing chitosan / polylactic acid double-layer membranes without adhesives. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for preparing a polylactic acid double-layer food packaging film without an adhesive, so as to prepare a food packaging film with polylactic acid as an outer layer film and chitosan and anthocyanin as an inner layer film. On the one hand, the double-layer composite form enables the prepared packaging film to combine the advantages of polylactic acid film and chitosan film, improve the surface waterproof performance while reducing the air permeability of the film, and enable anthocyanin to be successfully applied to the polylactic acid film; on the other hand, the prepared packaging film is sensitive to pH response, and the double-layer film can show obvious color changes when the food inside is spoiled, which can provide consumers with more convenient and intuitive product quality information.

[0006] The present invention solves the technical problem by the following technical solutions:

[0007] A method for preparing a polylactic acid double-layer food packaging film without adhesive, characterized in that the steps of the method are:

[0008] S1. Weigh an appropriate amount of anthocyanidin and add it to an ethanol solution, stir at a certain temperature until it is completely dissolved, and centrifuge to remove impurities to obtain an anthocyanidin solution for later use;

[0009] S2, weigh a certain mass of chitosan powder, add it to acetic acid solution, stir it at a certain temperature until it is completely dissolved, use an ultrasonic reactor to remove bubbles in the solution, let it stand and cool to room temperature to obtain a chitosan solution for standby use;

[0010] S3, taking an appropriate amount of polylactic acid powder and dissolving it in dichloromethane solvent, stirring at a certain temperature until it is completely dissolved, to obtain a polylactic acid solution for standby use;

[0011] S4. Add the anthocyanin solution of S1 to the chitosan solution of S2, stir and mix evenly under certain conditions, and use an ultrasonic reactor to remove bubbles to obtain a mixed solution, pour the mixed solution into a square glass box, and dry it in an oven to form an inner film; pour and cast the polylactic acid solution of S3 on the surface of the inner film, let it stand and dry to form an outer film, and remove the film to obtain a polylactic acid double-layer food packaging film.

[0012] Moreover, the thickness of the polylactic acid double-layer food packaging film is 0.075±0.001 mm.

[0013] Moreover, the concentration of the chitosan solution in S2 is 2% w / v, and the concentration of the polylactic acid solution in S3 is 1% w / v.

[0014] Moreover, the mass ratio of chitosan to anthocyanin is (1:0.01) to (1:0.04), and the stirring conditions of S1 to S4 are 25 to 60° C. and 1 to 3 hours.

[0015] Moreover, the ultrasonic power of the ultrasonic reactors S2 and S4 is 200 W, and the ultrasonic time is 40 minutes.

[0016] Moreover, the drying temperature of S4 is 65° C., and the drying time is 8 hours.

[0017] Moreover, the mass ratio of the S4 polylactic acid solution to the chitosan is 1:1, and the static drying time is 6 hours.

[0018] The advantages and beneficial effects of the present invention are:

[0019] 1. The surface of the chitosan of the present invention contains positively charged amino groups. The carboxylic acid groups on the polylactic acid molecular chain will be partially ionized under physiological conditions to produce carboxylate ions, which can interact electrostatically with the amino groups, so that the chitosan film and the polylactic acid film layer can be tightly bonded together without introducing other adhesives.

[0020] 2. The present invention adds anthocyanidins to the chitosan film-forming liquid to prepare an inner film, casts a layer of polylactic acid film on the surface of the inner film, and forms a polylactic acid double-layer food packaging film through electrostatic interaction; the prepared double-layer film solves the problem that water-soluble anthocyanidins are difficult to add to the polylactic acid film; on the other hand, the addition of anthocyanidins further improves the tensile strength and antioxidant capacity of the film and improves the hydrophilic properties of the chitosan film, thereby enhancing the multifunctionality of the film.

[0021] 3. The polylactic acid double-layer food packaging film of the present invention has a real-time monitoring effect on the food inside. In cheese application, the prepared packaging film can be observed to have obvious color changes on the third day, and has good visual effects and application prospects in the food field. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a flow chart of the present invention;

[0023] Figure 2 This is a scanning electron microscope image of a cross section of the polylactic acid double-layer food packaging film of the present invention;

[0024] Figure 3 The infrared spectrum analysis diagrams of the chitosan film, polylactic acid film, polylactic acid / chitosan film, and polylactic acid double-layer food packaging film prepared by the present invention;

[0025] Figure 4 The tensile strength and elongation at break of the chitosan film, polylactic acid film, polylactic acid / chitosan film, and polylactic acid double-layer food packaging film prepared by the present invention are shown in FIG.

[0026] Figure 5 The oxygen transmission rate diagrams of the chitosan film, polylactic acid film, polylactic acid / chitosan film, and polylactic acid double-layer food packaging film prepared by the present invention;

[0027] Figure 6 The DPPH free radical scavenging activity graph of the chitosan film, polylactic acid film, polylactic acid / chitosan film, and polylactic acid double-layer food packaging film prepared by the present invention;

[0028] Figure 7 This is a diagram showing the color change of cheese packaged with the polylactic acid double-layer food packaging film prepared by the present invention at room temperature for 4 days;

[0029] Figure 8This is a graph showing the pH change of cheese packaged with the polylactic acid double-layer food packaging film prepared by the present invention at room temperature for 4 days. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0031] like Figure 1 As shown, a method for preparing a polylactic acid double-layer food packaging film without adhesive is disclosed, wherein the packaging film is a double-layer film with polylactic acid as the outer layer and a mixture of chitosan and anthocyanidin as the inner layer; the steps of the method are:

[0032] 1. Weigh 1g of chitosan and place it in 50mL of 1% v / v acetic acid solution. Stir at 60°C for 5 hours until it is completely dissolved. Use an ultrasonic reactor at 200W for 40 minutes to remove bubbles in the film-forming solution. Cool to room temperature and set aside.

[0033] 2. Weigh 0.4 g of anthocyanidin into 100 mL of 80% v / v ethanol and stir at 25°C until completely dissolved to obtain a 0.4% w / v anthocyanidin solution;

[0034] 3. Weigh 1 g of polylactic acid powder in 60 mL of dichloromethane, stir at 22 °C until completely dissolved, take 50 mL of 2% w / v chitosan solution and mix with 10 mL of 0.4% w / v anthocyanin solution, stir at 25 °C for 2 hours to mix evenly, use an ultrasonic reactor at 200 W for 40 minutes to remove bubbles in the mixed solution to obtain a film-forming solution;

[0035] 4. Pour the film-forming solution into a 150×150×15mm square glass box and dry it in an oven at 65°C for 10 hours to form an inner film. After the film is dried and cooled to room temperature, pour 60mL of polylactic acid solution on its surface and let it stand and dry at room temperature for 6 hours to form an outer film. De-filming is performed to obtain a polylactic acid double-layer food packaging film with a film thickness of 0.075±0.002mm.

[0036] Figure 2 This is a scanning electron microscope image of the cross section of the double-layer smart food packaging film prepared in the present invention. From the cross section, it can be seen that in the absence of an adhesive, the double-layer film is well bonded, has no gap in the middle, and fits tightly.

[0037] The performance of the polylactic acid double-layer food packaging film prepared by the present invention is compared with that of a polylactic acid film and a chitosan film.

[0038] 1. Preparation of single-layer polylactic acid film: Weigh 1g of polylactic acid powder and dissolve it in 60mL of dichloromethane solvent, stir at 20℃ for 1 hour until it is completely dissolved; pour 120mL of polylactic acid solution into a 150×150×15mm square glass box, dry it statically at 25℃ for 6 hours, then remove the film to obtain a polylactic acid film with a film thickness of 0.075±0.002mm.

[0039] 2. Preparation of single-layer chitosan film: Weigh 1g of chitosan and place it in 50mL of 1% v / v acetic acid solution, stir at 60℃ for 5 hours until completely dissolved, and let it stand and cool to room temperature for use. Pour 90mL of chitosan solution into a 150×150×15mm square glass box, dry in an oven at 65℃ for 10 hours, then cool to room temperature and remove the film to obtain a chitosan film with a thickness of 0.075±0.002mm.

[0040] 3. Polylactic acid / chitosan film: Weigh 1g of chitosan and place it in 50mL of 1% v / v acetic acid solution, stir at 60℃ for 5 hours until completely dissolved, use an ultrasonic reactor 200W for 40 minutes to remove bubbles in the film-forming solution, and cool to room temperature for use; weigh 1g of polylactic acid powder in 60mL of dichloromethane, stir at 22℃ until completely dissolved. Pour 50mL of 2% w / v chitosan solution into a 150×150×15mm square glass box, and dry it in an oven at 65℃ for 10 hours to form an inner film. The film is dried and cooled to room temperature, and 60mL of polylactic acid solution is poured on its surface and left to dry at room temperature for 6 hours to form an outer film. De-filming is performed to obtain a polylactic acid / chitosan film with a thickness of 0.075±0.002mm.

[0041] like Figures 3 to 6 In the figure, PLA represents polylactic acid film, CS represents chitosan film, CS / PLA represents polylactic acid / chitosan film, and CS-0.04ACN / PLA represents the polylactic acid double-layer food packaging film of the present invention.

[0042] Figure 3 These are infrared spectrum analysis diagrams of chitosan film, polylactic acid film, and polylactic acid double-layer food packaging film.

[0043] Figure 4 This is a graph of the tensile strength and elongation at break of chitosan film, polylactic acid film, and polylactic acid double-layer food packaging film. It can be seen from the figure that the double-layer film combines the mechanical properties of polylactic acid film and chitosan film.

[0044] Figure 5 The oxygen permeability diagram of the chitosan film, polylactic acid film, and polylactic acid double-layer food packaging film prepared by the present invention shows that the polylactic acid film has better air permeability than the chitosan film at the same thickness, the oxygen permeability decreases after the double-layer composite, and the double-layer film with anthocyanin added has the lowest oxygen permeability.

[0045] Figure 6 This is a graph showing the DPPH free radical scavenging activity of the chitosan film, polylactic acid film, and polylactic acid double-layer food packaging film prepared in the present invention. The double-layer film shows stronger antioxidant capacity than the single-layer film, and the antioxidant capacity of the double-layer film containing anthocyanins is further improved.

[0046] The polylactic acid double-layer food packaging film prepared by the present invention is sensitive to pH response, and the double-layer film can show obvious color changes when the food inside is spoiled, thereby providing consumers with more convenient and intuitive product quality information.

[0047] Application in cheese packaging: Take 12g of cheese and spread it evenly in a culture dish, cut the polylactic acid double-layer food packaging film of the present invention into circles of the same size with a diameter of 60mm, and wrap and seal them on the culture dish, observe and record the color change of the film, the pH value change of the cheese, and the quality change of the cheese every 12 hours, and judge the quality of the food inside according to the color change of the film.

[0048] Figure 7 The color change of cheese packaged with the double-layer intelligent packaging film prepared by the present invention at room temperature for 4 days can be observed that the film gradually changes from dark purple to light and then to orange-red over time, reflecting the degree of deterioration of the cheese.

[0049] Figure 8 The pH change of cheese packaged with the polylactic acid double-layer food packaging film prepared by the present invention at room temperature for 4 days shows that the pH of the cheese gradually decreases over time, indicating that the cheese gradually deteriorates.

[0050] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for preparing a polylactic acid double-layer food packaging film without adhesive, characterized in that: The steps of the method are: S1. Weigh an appropriate amount of anthocyanidin and add it to an ethanol solution, stir at a certain temperature until it is completely dissolved, and centrifuge to remove impurities to obtain an anthocyanidin solution for later use; S2, weigh a certain mass of chitosan powder, add it to acetic acid solution, stir it at a certain temperature until it is completely dissolved, use an ultrasonic reactor to remove bubbles in the solution, let it stand and cool to room temperature to obtain a chitosan solution for standby use; S3, taking an appropriate amount of polylactic acid powder and dissolving it in dichloromethane solvent, stirring at a certain temperature until it is completely dissolved, to obtain a polylactic acid solution for standby use; S4. Add the anthocyanin solution of S1 to the chitosan solution of S2, stir and mix evenly under certain conditions, and use an ultrasonic reactor to remove bubbles to obtain a mixed solution, pour the mixed solution into a square glass box, and dry it in an oven to form an inner film; pour and cast the polylactic acid solution of S3 on the surface of the inner film, let it stand and dry to form an outer film, and remove the film to obtain a polylactic acid double-layer food packaging film.

2. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The thickness of the polylactic acid double-layer food packaging film is 0.075±0.001 mm.

3. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The concentration of the chitosan solution in S2 is 2% w / v, and the concentration of the polylactic acid solution in S3 is 1% w / v.

4. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The mass ratio of chitosan to anthocyanin is (1:0.01) to (1:0.04), and the stirring conditions of S1 to S4 are 25 to 60° C. and 1 to 3 hours.

5. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The ultrasonic power of the ultrasonic reactors S2 and S4 is 200 W, and the ultrasonic time is 40 minutes.

6. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The drying temperature of S4 is 65° C. and the drying time is 8 hours.

7. The method for preparing a polylactic acid double-layer food packaging film without adhesive according to claim 1, characterized in that: The mass ratio of the S4 polylactic acid solution to chitosan is 1:1, and the static drying time is 6 hours.

Images