Preparation method and application of polylactic acid / chitosan double-layer membrane with oxidation resistance

Abstract

The invention discloses a preparation method and application of a polylactic acid / chitosan double-layer membrane with oxidation resistance. The preparation method is characterized in that a polylactic acid / chitosan double-layer film is prepared by a solution casting method, the inner layer is a chitosan film, and the outer layer is a polylactic acid film; glycerol is used as a plasticizer, and one of tea polyphenol, a bamboo leaf antioxidant, phytic acid, a rosemary extract and ascorbic acid is used as an antioxidant to be added into the chitosan solution to prepare a chitosan-based membrane solution; polylactic acid is dissolved in dichloromethane to prepare a polylactic acid membrane solution; and drying and molding the chitosan-based membrane liquid, pouring the chitosan-based membrane liquid into the polylactic acid membrane liquid, and removing the solvent to obtain the polylactic acid / chitosan double-layer membrane. The polylactic acid and the chitosan are used as base materials of the double-layer film, and the natural antioxidant is introduced, so that the mechanical property and the oxidation resistance of the double-layer film are effectively improved.

Description

Technical field:

[0001] The present invention belongs to the technical field of food packaging films, and particularly relates to a polylactic acid / Preparation of chitosan bilayer membranes. Background technology:

[0002] Polylactic acid (PLA) is a renewable biopolymer material with good biocompatibility and biodegradability. It can be completely degraded into water and carbon dioxide, which are harmless to the environment. Therefore, PLA is considered to be a packaging material that can replace petroleum-based plastics. However, the poor plasticity, poor oxygen barrier properties and high production cost of PLA films have affected its application in packaging materials.

[0003] Chitosan (CS) is a natural polymer compound extracted from shrimp and crab shells. It has film-forming properties, non-toxicity, biodegradability, biocompatibility, antioxidant activity and antibacterial activity. It is an important base material for new food packaging materials. However, its insufficient mechanical properties and gas barrier properties still limit its practical application.

[0004] In the existing technology, the use of a single polymer film cannot meet the mechanical properties and gas barrier properties required for food packaging. Therefore, constructing a multilayer film structure or blending treatment to improve the performance of the film is a current research hotspot.

[0005] With the progress of society and the development of science and technology, consumers have higher and higher requirements for food quality and safety. During the production, storage and transportation of food, it is very easy to be affected by environmental factors and undergo oxidation reactions, which leads to the deterioration of food quality. In particular, foods rich in lipids are more susceptible to the attack of active oxygen, which triggers a free radical chain reaction and accelerates the oxidation and deterioration of food. Therefore, it is particularly important to develop antioxidant food packaging materials. Summary of the invention:

[0006] The present invention aims to construct a double-layer film with excellent packaging performance by complementing the properties of polylactic acid and chitosan films; By introducing antioxidants, the antioxidant activity of the double-layer film is enhanced, the shelf life of food is extended and the freshness of food is maintained.

[0007] Therefore, the present invention provides the following technical solution: a method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties, comprising the following steps:

[0008] (1) dissolving chitosan in an acetic acid solution, stirring and centrifuging, adding glycerol and an antioxidant, stirring, and ultrasonically degassing to obtain a chitosan base film solution;

[0009] (2) dissolving polylactic acid in a dichloromethane solution, stirring, and ultrasonically degassing to obtain a polylactic acid membrane solution;

[0010] (3) pouring the chitosan-based film liquid in step (1) into a mold, and pouring the polylactic acid film liquid in step (2) into the mold after drying, volatilizing the solvent, and drying to obtain a double-layer film;

[0011] The mass fraction of chitosan in the chitosan-based membrane solution in step (1) is 0.5-2%, preferably 1%-2%;

[0012] The concentration of the acetic acid solution in step (1) is 0-2%, preferably 1%-2%;

[0013] The mass fraction of glycerol in step (1) is 0-2%, preferably 1%-1.5%;

[0014] The mass fraction of the antioxidant in step (1) is 0 to 0.4%, preferably 0.1% to 0.3%;

[0015] The stirring conditions in step (1) are: stirring speed 700-800 r / min, stirring temperature 20-40° C., Stirring time: 50-60 minutes;

[0016] The ultrasonic conditions in step (1) are as follows: ultrasonic power 200-300 W, ultrasonic time 10-15 min;

[0017] The centrifugal conditions in step (1) are: a speed of 6000 to 8000 r / min and a time of 10 to 15 min;

[0018] The drying conditions of the polylactic acid in step (2) are as follows: the drying temperature is 40 to 60° C., the drying time is 6 to 12 hours;

[0019] The mass fraction of polylactic acid in the polylactic acid film liquid in step (2) is 1% to 6%, preferably 2% to 4%;

[0020] The stirring conditions in step (2) are: stirring speed 700-800 r / min, stirring temperature 20-40° C., Stirring time: 50-60 minutes;

[0021] The ultrasonic conditions in step (2) are as follows: ultrasonic power 200-300 W, ultrasonic time 10-15 min;

[0022] Preferably, the mass ratio of the chitosan layer to the polylactic acid layer in the double-layer membrane in step (3) is 0-2:2-0. Preferably 0.5-1.5:1.5-0.5;

[0023] The drying and molding conditions in step (3) are as follows: drying temperature 40-60° C., drying time 3-4 h;

[0024] Compared with the prior art, the present invention has the following advantages and effects:

[0025] 1. The preparation method of the present invention is simple to operate, and the prepared polylactic acid / chitosan double-layer film has good mechanical properties and antioxidant activity, and is suitable for the packaging field; at the same time, the materials used for production are easily degradable and have no pollution to the environment, which fully complies with the concept of environmental protection and sustainable development.

[0026] 2. The polylactic acid / chitosan double-layer film with antioxidant properties prepared by the solution casting method of the present invention, When packaging items, the chitosan layer is inside and the polylactic acid layer is outside. The waterproof properties of polylactic acid can be used to prevent external moisture from entering the packaging film, effectively improving the moisture barrier property of the packaging film; the chitosan layer is added with antioxidants, which has antioxidant activity and can better extend the shelf life of food; at the same time, the presence of the chitosan layer increases the tensile strength of the polylactic acid layer, and the presence of the polylactic acid layer increases the elongation at break of the chitosan layer, solving the shortcomings of the two in strength and ductility. Description of the drawings:

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0028] Figure 1 The flow chart of the preparation of double-layer membranes in comparative example 1 and each embodiment of the present invention is as follows

[0029] Figure 2 The antioxidant activity of the double-layer film of Comparative Example 1 and each embodiment of the present invention;

[0030] Figure 3 The peroxide value and anisidine value of the double-layer film-packaged walnut oil in Comparative Example 1 and each embodiment of the present invention;

[0031] Figure 4 The color of the walnut kernels packaged in the double-layer film in Comparative Example 1 and each embodiment of the present invention;

[0032] Figure 5 The antioxidant activity of walnut kernels packaged in double-layer film in Comparative Example 1 and each embodiment of the present invention;

[0033] Figure 6 The peroxide value and anisidine value of the walnut kernels packaged in double-layer film in Comparative Example 1 and each embodiment of the present invention. Specific implementation method:

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with specific embodiments.

[0035] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0036] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0037] The key properties of the double-layer membranes involved in each embodiment and comparative example were measured:

[0038] 1. Mechanical properties

[0039] The tensile strength (TS) and elongation at break (EAB) of the film were tested using an electronic universal testing machine (Instron 5943). The sample was cut into 30 cm × 10 cm strips, and the tensile rate was set to 5 mm / min.

[0040] 2. Water vapor transmission rate

[0041] Seal the film at the mouth of a glass bottle filled with distilled water (100% RH), and then place it in a desiccator containing silica gel (0% RH). Weigh the glass bottle regularly. Calculate the water vapor permeability of the film based on the weight loss of the glass bottle. Where: ΔW is the weight change of the cup (g); X is the film thickness (mm); t is the time (h); S is the effective area of ​​the film (m 2 );Δp is the water vapor pressure difference on both sides of the film (the water vapor pressure difference at 26℃ is 3.3629kpa); WVP is the water vapor transmission rate (g·mm / (m 2 ·h·kpa)).

[0042] 3. Oxygen permeability

[0043] According to the national standard GB / T 1038-2000, the oxygen permeability (OP) of the film was measured by a gas permeability tester (Brugger), and the inner layer of the film faced the high pressure chamber in the instrument, and the results were recorded as OP(cm 3 / (m2 ·d·bar)).

[0044] 4. Color and Opacity

[0045] Select a film sample with a uniform surface and use a colorimeter to measure the L* (brightness) and a* (red / green) of the film. and b*(yellow / blue). Before measurement, a white background was used as a reference (L0=81.4, a0=1.71, b0=10.5). The total color difference ΔE of the sample was calculated using the following formula.

[0046] The opacity is measured by UV spectrophotometer. The size of the film is 10mm*40mm. The film sample is placed close to the inside of the cuvette, and the empty cuvette is used as a blank control. The absorbance of the film is measured at a wavelength of 600nm. The calculation formula is as follows: Wherein, Abs 600 is the absorbance of the membrane at 600 nm, and L is the thickness of the membrane (mm).

[0047] 5. Antioxidant Activity

[0048] DPPH method: 25 mg of membrane was soaked in 3 ml of ethanol solution for 3 h for extraction, 1 ml of the extract was mixed with 3 ml of 0.1 mmol DPPH ethanol solution and placed in a test tube, and reacted in the dark at room temperature for 30 min, and the absorbance was measured at 517 nm;

[0049] ABTS method: 25 mg of membrane was soaked in 3 ml of aqueous solution for 3 h for extraction. 1 ml of the extract was mixed with 3 ml of 0.1 mmol ABTS solution and placed in a test tube. The mixture was reacted in the dark at room temperature for 15 min and the absorbance was measured at 734 nm.

[0050] 6. Ability to delay oxidation of walnut oil and walnut kernels

[0051] The peroxide value and anisidine value of the oil in walnut oil and walnut kernel were determined to evaluate the oxidation degree of walnut oil and walnut kernel. Example 1

[0052] (1) Dissolve chitosan in 2% acetic acid solution, stir for 1 hour, centrifuge at 8000 r / min for 10 minutes to obtain a 1.5% chitosan solution, then add 1% glycerol and 0.2% tea polyphenols (TP) to the chitosan solution, stir for 30 minutes, and ultrasonically degas for 10 minutes to obtain a chitosan base film solution;

[0053] (2) dissolving polylactic acid in a dichloromethane solution, stirring for 1 hour, and ultrasonically degassing for 10 minutes to obtain a 4% polylactic acid film solution;

[0054] (3) Pour the chitosan-based membrane liquid into the mold and dry it in an oven at 60°C for 2 h;

[0055] (4) Pour the polylactic acid film liquid onto the dried chitosan-based film, evaporate the solvent in a fume hood, and then dry in an oven at 40°C for 1 h to form a double-layer film. Example 2

[0056] (1) Chitosan was dissolved in 2% acetic acid solution, stirred for 1 h, and then centrifuged at 8000 r / min for 10 min to obtain a 1.5% chitosan solution. 1% glycerol and 0.2% bamboo leaf antibiotic were added to the chitosan solution. oxide (AOB), stirred for 30 min, and ultrasonically degassed for 10 min to obtain chitosan-based membrane solution;

[0057] (2) dissolving polylactic acid in a dichloromethane solution, stirring for 1 hour, and ultrasonically degassing for 10 minutes to obtain a 4% polylactic acid film solution;

[0058] (3) Pour the chitosan-based membrane liquid into the mold and dry it in an oven at 60°C for 2 h;

[0059] (4) Pour the polylactic acid film liquid onto the dried chitosan-based film, evaporate the solvent in a fume hood, and then dry in an oven at 40°C for 1 h to form a double-layer film. Example 3

[0060] (1) Dissolve chitosan in 2% acetic acid solution, stir for 1 hour, and then centrifuge at 8000 r / min for 10 minutes to obtain a 1.5% chitosan solution, then add 1% glycerol and 0.2% phytic acid (PA) to the chitosan solution, stir for 30 minutes, and ultrasonically degas for 10 minutes to obtain a chitosan base film solution;

[0061] (2) dissolving polylactic acid in a dichloromethane solution, stirring for 1 hour, and ultrasonically degassing for 10 minutes to obtain a 4% polylactic acid film solution;

[0062] (3) Pour the chitosan-based membrane liquid into the mold and dry it in an oven at 60°C for 2 h;

[0063] (4) Pour the polylactic acid film liquid onto the dried chitosan-based film, evaporate the solvent in a fume hood, and then dry in an oven at 40°C for 1 h to form a double-layer film. Comparative Example 1

[0064] Comparative Example 1 is compared with Examples 1 to 3, except that no antioxidant is added, and the steps are as follows:

[0065] (1) Chitosan was dissolved in 2% acetic acid solution, stirred for 1 h, and centrifuged at 8000 r / min for 10 min to obtain a 1.5% chitosan solution. 1% glycerol was then added to the chitosan solution, stirred for 30 min, and ultracentrifuged. Acoustic degassing for 10 min to obtain chitosan-based membrane solution;

[0066] (2) dissolving polylactic acid in a dichloromethane solution, stirring for 1 hour, and ultrasonically degassing for 10 minutes to obtain a 4% polylactic acid film solution;

[0067] (3) Pour the chitosan-based membrane liquid into the mold and dry it in an oven at 60°C for 2 h;

[0068] (4) Pour the polylactic acid film liquid onto the dried chitosan-based film, evaporate the solvent in a fume hood, and then dry in an oven at 40°C for 1 h to form a double-layer film. Comparative Example 2

[0069] Comparative Example 2 is compared with Comparative Example 1, except that the chitosan layer is not included, and the steps are as follows:

[0070] (1) dissolving polylactic acid in a dichloromethane solution, stirring for 1 h, and ultrasonically degassing for 10 min to obtain a 4% polylactic acid film solution;

[0071] (2) Pour the polylactic acid film liquid into the mold, evaporate the solvent in a fume hood, and then dry it in an oven at 40°C 1h to form a polylactic acid film. Comparative Example 3

[0072] Comparative Example 3 is compared with Comparative Example 1, except that the polylactic acid layer is not included, and the steps are as follows:

[0073] (1) Chitosan was dissolved in 2% acetic acid solution, stirred for 1 h, and centrifuged at 8000 r / min for 10 min to obtain a 1.5% chitosan solution. 1% glycerol was then added to the chitosan solution, stirred for 30 min, and ultracentrifuged. Acoustic degassing for 10 min to obtain chitosan-based membrane solution;

[0074] The test results of the mechanical properties and gas barrier properties of the films in each comparative example are shown in Table 1. Table 1

[0075] It can be seen that the pure polylactic acid film has low tensile strength and high elongation at break, which is attributed to the poor plasticity of the polylactic acid film; the pure chitosan film has high tensile strength and low elongation at break, which is attributed to the high brittleness of the chitosan film. The addition of chitosan film improves the disadvantage of the poor plasticity of the polylactic acid film; the addition of polylactic acid film improves the disadvantage of the high brittleness of the chitosan film. The tensile strength of the film in Comparative Example 1 is comparable to that of typical packaging plastics such as polypropylene (PP, 30-40 MPa), showing its potential as a packaging material.

[0076] The water vapor permeability of pure PLA film was lower than that of pure chitosan film, which was attributed to the hygroscopicity of chitosan; The oxygen permeability of pure polylactic acid film is higher than that of pure chitosan film, which is attributed to the large number of amino and hydroxyl groups in the chitosan molecular structure. These functional groups can form intramolecular and intermolecular hydrogen bonds, making the chitosan film have a higher density. The polylactic acid / chitosan double-layer film has good gas barrier properties and is a potential food packaging material.

[0077] The test results of color and opacity of the double-layer films of Comparative Example 1 and Examples 1 to 3 are shown in Table 2: Table 2

[0078] From the color aspect, it can be seen that the color of the film changes after adding antioxidants, among which TP and AOB have the greatest impact on the film color. In terms of opacity, the opacity of the double-layer film increases after adding antioxidants. The reason may be that the addition of water-soluble antioxidants makes the structure of the chitosan layer more compact, and the reduction of voids leads to increased opacity.

[0079] The test results of the antioxidant activity of the double-layer films of Comparative Example 1 and Examples 1 to 3 are as follows Figure 2 As shown:

[0080] It can be seen that the DPPH and ABTS free radical scavenging activities of the double-layer film increased, and the addition of antioxidants significantly improved the antioxidant capacity of the film. These results indicate that the double-layer film with added antioxidants has high antioxidant activity and can be used in active packaging applications to delay the oxidation of food.

[0081] The test results of the peroxide value and anisidine value of the double-layer film packaged walnut oil in Comparative Example 1 and Examples 1 to 3 are as follows: Figure 3 As shown:

[0082] The peroxide value and anisidine value of walnut oil in each group increased with the extension of storage time, indicating that the oxidation degree of walnut oil gradually deepened. The peroxide value and anisidine value of walnut oil packaged with double-layer film containing antioxidants were lower than those of PLA / CS film, and the peroxide value of walnut oil packaged with double-layer film containing tea polyphenols was comparable to that of walnut oil packaged with typical packaging plastics such as polypropylene (PP, 4.04mmol / kg on the 18th day). Therefore, The double-layer film with added antioxidants is suitable for the packaging of walnut oil and can effectively delay the oxidation of walnut oil.

[0083] The color test results of the double-layer film packaged walnut kernels in Comparative Example 1 and Examples 1 to 3 are as follows Figure 4 As shown, Among them, (A) is the L* value, (B) is the a* value, (C) is the b* value, and (D) is the △E value. It can be seen that with the extension of storage time, the L* value of walnut kernels gradually decreases, and the a* and b* values ​​gradually increase, but the △E value of the double-layer film with added antioxidants has a smaller range of change.

[0084] The free radical scavenging ability test results of double-layer film packaging walnut kernels in Comparative Example 1 and Examples 1 to 3 are as follows Figure 5 As shown:

[0085] During storage, the DPPH and ABTS free radical scavenging abilities of walnut kernels gradually decreased, and the antioxidant activity gradually decreased. However, the double-layer film with added antioxidants effectively slowed down the decrease in the free radical scavenging ability of walnut kernels.

[0086] The test results of peroxide value and anisidine value of walnut kernels packaged in double-layer film of Comparative Example 1 and Examples 1 to 3 are as follows: Figure 6 As shown:

[0087] The peroxide value and anisidine value of the oil in each group of walnut kernels increased with the extension of storage time, indicating that the oxidation degree of the walnut kernels gradually deepened. The peroxide value and anisidine value of the walnut kernels packaged with double-layer film containing antioxidants were lower than those of PLA / CS film, and the peroxide value of the walnut kernels packaged with double-layer film containing tea polyphenols was significantly lower than that of typical packaging plastics such as polypropylene (PP, 3.24mmol / kg on the 28th day). Therefore, the double-layer film with antioxidants is suitable for the packaging of walnut kernels and can effectively delay the oxidation of walnut kernels.

Claims

1. A method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties, characterized in that: The steps include: (1) dissolving chitosan in an acetic acid solution, stirring and centrifuging, adding glycerol and an antioxidant, stirring, and ultrasonically degassing to obtain a chitosan base film solution; (2) dissolving polylactic acid in a dichloromethane solution, stirring, and ultrasonically degassing to obtain a polylactic acid membrane solution; (3) pouring the chitosan-based membrane liquid in step (1) into a mold, and pouring the polylactic acid membrane liquid in step (2) into the mold after drying, evaporating the solvent, and drying to obtain a double-layer membrane.

2. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The mass fraction of chitosan in the chitosan-based membrane solution in step (1) is 0.5-2%; The concentration of the acetic acid solution in step (1) is 0-2%; The mass fraction of glycerol in step (1) is 0-2%; The mass fraction of the antioxidant in step (1) is 0 to 0.4%; The mass fraction of polylactic acid in the polylactic acid film liquid in step (2) is 1 to 6%; The mass ratio of the chitosan layer to the polylactic acid layer in the double-layer membrane in step (3) is 0-2:2-0.

3. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The antioxidant in step (1) is one of tea polyphenols, bamboo leaf antioxidants, phytic acid, rosemary extract and ascorbic acid.

4. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The stirring conditions in step (1) are as follows: a stirring speed of 700 to 800 r / min, a stirring temperature of 20 to 40° C., and a stirring time of 50 to 60 min; The ultrasonic conditions in step (1) are as follows: ultrasonic power 200-300 W, ultrasonic time 10-15 min; The centrifugal conditions in step (1) are: a speed of 6000 to 8000 r / min and a time of 10 to 15 min; The stirring conditions in step (2) are: stirring speed 700-800 r / min, stirring temperature 20-40° C., and stirring time 50-60 min; The ultrasonic conditions in step (2) are as follows: ultrasonic power 200-300 W, and ultrasonic time 10-15 min.

5. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The double-layer film drying and molding conditions in step (3) are a drying temperature of 40 to 60° C. and a drying time of 3 to 4 hours.

6. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The molecular weight of the chitosan in step (1) is between 180,000 and 360,000, and the degree of deacetylation is between 75% and 95%.

7. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The molecular weight of the polylactic acid in step (2) is between 200,000 and 4,000,000.

8. The method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties according to claim 1, characterized in that: The thickness of the polylactic acid-chitosan double-layer membrane in step (3) is 40-120 μm.

9. A method for preparing a polylactic acid / chitosan double-layer film with antioxidant properties, characterized in that: It is prepared by the method according to any one of claims 1 to 8.

10. The polylactic acid / chitosan double-layer film with antioxidant properties as claimed in claim 9 can be used as food packaging.

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