Degradable food packaging film as well as preparation method and application thereof

Abstract

The invention discloses a degradable food packaging film as well as a preparation method and application thereof, bacterial cellulose and guanidine salt are covalently cross-linked together to obtain a covalent cross-linked substance OBC-PHGH, and gallic acid is added into a cross-linked solution to obtain a guanidine salt functionalized bacterial cellulose-based film OBC-PHGH / GA. The prepared food packaging film material can prevent the weight loss of strawberries in the storage process, and the shelf life of the strawberries is prolonged from 2 days to 5 days. Therefore, the method is expected to be widely applied in the aspect of green food packaging.

Description

Technical Field

[0001] The invention belongs to the field of food packaging, and relates to a bacterial cellulose-based degradable food packaging film with antibacterial and antioxidant properties, a preparation method thereof, and application in food packaging. Background Art

[0002] Food safety and food waste issues have posed a serious threat to human health, the economy and the environment. Most foods, such as fruits, vegetables and meats, are susceptible to microbial contamination, which can cause serious quality losses, economic losses, and even cause foodborne diseases. Based on the above problems, people have put forward a great demand for the development of food packaging materials to protect food from pathogenic microorganisms and mechanical damage and extend the shelf life of fresh food. At present, most traditional food packaging materials are petroleum-based polymers. This type of food packaging material can protect food, is simple to make and has low cost. However, there are problems such as inability to effectively prevent microbial contamination, non-biodegradability, and non-recyclability, which may lead to serious ecological, economic, environmental and health problems. Therefore, there is an urgent need to develop new food packaging materials to improve the problems existing in traditional packaging materials.

[0003] Bacterial cellulose (BC) is a natural nanocellulose obtained by microbial fermentation. It has the advantages of natural three-dimensional network structure, high mechanical strength, high transparency, high biosafety, and biodegradability. These advantages have attracted more and more attention to bacterial cellulose in food packaging and biomedical applications. However, although bacterial cellulose is an ideal food packaging material, it lacks antibacterial and antioxidant functions, which hinders the potential application of BC in food packaging. Based on this, functionalized bacterial cellulose materials have received widespread attention and have become an important strategy in the field of food packaging. At present, physical doping methods are mainly used to modify bacterial cellulose, but physical doping methods are difficult to obtain uniformly modified BC composites, and may cause the loss of functional components during the washing process of the composite material. Therefore, it is urgent to develop some new strategies for functionalized bacterial cellulose materials.

[0004] Guanidine is an imine urea or carbamate amidine. Guanidine salts can be derived into a variety of guanidine derivatives and have attracted widespread attention due to their strong antibacterial properties. They have broad application prospects in the fields of medicine, packaging, food, agriculture, etc. Guanidine salts (PHGH) can inactivate bacteria by destroying cell membranes. Polyphenols are a group of naturally occurring phytochemicals that are found in large quantities in fruits, vegetables, green tea and other natural products. They have antioxidant, antibacterial and UV protection properties. Among the many natural phenolic compounds found in tea, grapes, berries and other fruits, as well as wine, gallic acid (GA) is a low-cost compound with strong antioxidant and antibacterial properties. At present, gallic acid has been widely used in the preparation of active food packaging materials. Summary of the invention

[0005] Technical problem to be solved: The present invention provides a degradable guanidine-functionalized bacterial cellulose-based active food packaging film with high antibacterial and antioxidant properties, as well as a preparation method and application thereof, to solve the problems of poor antibacterial effect, lack of antioxidant properties, and non-biodegradability of some existing food packaging film materials.

[0006] Technical solution: A method for preparing a food packaging film, wherein bacterial cellulose and guanidine salt are covalently cross-linked to obtain a covalent cross-linked product OBC-PHGH, and gallic acid is added to the cross-linking solution to obtain a guanidine salt functionalized bacterial cellulose-based film OBC-PHGH / GA.

[0007] The bacterial cellulose is obtained from the culture solution of Acetobacter xylinum.

[0008] The preparation method of food packaging film comprises the following steps according to the following ratio: (1) suspending 1g of bacterial cellulose film in 1L of deionized water, adjusting the pH value of the solution to not less than 10 with NaOH, then sequentially adding 2,2,6,6-tetramethylpiperidinium oxide (TEMPO), sodium bromide (NaBr) and sodium hypochlorite (NaClO), and stirring at room temperature for 1h, and obtaining a 0.65wt.% oxidized bacterial cellulose dispersion (OBC) after dispersion; (2) dispersing the above oxidized bacterial cellulose (OBC), 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl), and N-hydroxysuccinimide (NHS) in deionized water at room temperature and 100-1000rpm for 1-3h, adding guanidine salt and reacting overnight at room temperature and 100-1000rpm; (3) mixing gallic acid with the above bacterial cellulose-guanidine salt grafted dispersion, filtering, and drying to obtain an OBC-PHGH / GA film.

[0009] Preferably, the reaction time under the above conditions of 100-1000 rpm is preferably 1 hour.

[0010] Preferably, the guanidine salt is replaced by an antibacterial substance with an amino group, and the antibacterial substance is an antibacterial peptide, chitosan or aminoglycoside antibiotic.

[0011] Preferably, the amount of gallic acid added does not exceed 1 wt.% of the reaction system.

[0012] Preferably, the added amount of the gallic acid is 0.2 wt.%.

[0013] The food packaging film prepared by the above method.

[0014] The food packaging film is used in the preparation of antibacterial, antioxidant and food packaging.

[0015] Beneficial effects: 1. The bacterial cellulose obtained by the present invention is derived from microorganisms, which have a fast growth cycle, cheap and easy-to-obtain raw materials, high product yield, and production is not restricted by natural conditions such as geography, climate, and season. 2. The food packaging film obtained by the present invention, which uses bacterial cellulose as the main material, has good biodegradability and can be degraded by enzymes to produce sugars. The produced sugars can further produce more value-added products and are reusable; at the same time, it avoids pollution to the environment after use. 3. The food packaging film material prepared by the present invention has excellent antioxidant and antibacterial properties, and can kill 100% of Escherichia coli and 99.9% of Staphylococcus aureus. 4. The food packaging film material prepared by the present invention can prevent the weight loss of strawberries during storage and extend the shelf life of strawberries from 2 days to 5 days. Therefore, this method is expected to be widely used in green food packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the scanning electron microscope image of OBC-PHGH / GA in Example 1.

[0017] Figure 2 Schematic diagram of the mechanical properties of OBC-PHGH / GA.

[0018] Figure 3 Schematic diagram of the DPPH radical and AABTS radical scavenging results of OBC-PHGH / GA.

[0019] Figure 4 Schematic diagram of the anti-Escherichia coli and Staphylococcus aureus results of OBC-PHGH / GA.

[0020] Figure 5 Schematic diagram of the degradation results of commercial cling film.

[0021] Figure 6 Schematic diagram of the degradation results of OBC-PHGH / GA.

[0022] Figure 7 Schematic diagram of the preservation effect of strawberries in the unpackaged group.

[0023] Figure 8 Schematic diagram of the strawberry preservation effect in the OBC-PHGH / GA treatment group.

[0024] Fig. 9 This is a curve of weight loss rate of strawberries in the unpackaged group and the OBC-PHGH / GA treatment group during storage.

[0025] Fig.10 Schematic diagram of bacterial colonies in the unpackaged strawberries after storage.

[0026] Fig.11Schematic diagram of fungal colonies in unpackaged strawberries after storage.

[0027] Fig.12 Schematic diagram of bacterial colonies in strawberries treated with OBC-PHGH / GA after storage.

[0028] Fig.13 Schematic diagram of fungal colonies in strawberries treated with OBC-PHGH / GA after storage. DETAILED DESCRIPTION

[0029] Strain activation method: Escherichia coli and Staphylococcus aureus strains stored in a -80°C refrigerator were inoculated into 10 mL LB broth medium at a 3% inoculation rate and cultured at 37°C for 12 hours to activate the strains.

[0030] Determination of viable count: The determination of viable count was carried out by dilution plating method, as follows: 0.1 mL of sample solution was pipetted into a sterile centrifuge tube and diluted with sterile saline to a dilution of 10 -1 , 10 -2 , 10 -3 , 10 -4 , 10 -5 , 10 -6 , 10 -7 , 10 -8 , 10 -9 , 10 -10 , 10 -11 , 10 -12 Select a dilution of 10 -4 Up to 10 -12 Spread on LB medium, make three replicates for each dilution, and culture in a 37°C incubator for 24 hours before counting.

[0031] The present invention cross-links bacterial cellulose and guanidine salts by covalent cross-linking to obtain a covalent cross-linked product OBC-PHGH, and adds gallic acid to the cross-linking solution, and obtains a guanidine salt functionalized bacterial cellulose-based film (OBC-PHGH / GA) by suction filtration, and is used for food packaging. The obtained OBC-PHGH film shows a good antibacterial effect, can effectively extend the shelf life of strawberries, and is easily degraded. Therefore, it can not only meet its application in multiple fields such as food, medicine, and cosmetics, but also provide ideas for the development of new food packaging films.

[0032] Example 1

[0033] Synthesis of bacterial cellulose (BC)

[0034] Acetobacter xylinum is inoculated into a culture medium with an inoculation amount of 7wt.%-10wt.%, and statically cultured in a culture dish for 5 days; a bacterial cellulose film (BC) is obtained, and a mixed solution of 0.3wt.% NaOH and 0.09wt.% H2O2 is used for purification, and the mixture is placed in a constant temperature water bath at 80°C for 1-2h, cooled after the treatment, and washed with water until neutral.

[0035] Synthesis of Bacterial Cellulose-Guanidine Salt (OBC-PHGH)

[0036] 1 g of bacterial cellulose film was suspended in 1 L of deionized water and the pH of the solution was adjusted to greater than 10 with NaOH. The bacterial cellulose film suspension was then mixed with 31.25 mg of TEMPO, 1.03 g of NaBr, and 37.2 mL of NaClO and stirred at room temperature for 1 h. The resulting oxidized bacterial cellulose was washed with deionized water to neutrality to remove residual small molecules, thereby obtaining oxidized BC.

[0037] The obtained oxidized BC was dispersed into a 0.65wt.% dispersion using a disperser, and the solution pH was adjusted to less than 6 with HCl, then 30mL of the oxidized bacterial cellulose dispersion was mixed with 400μL of EDC-HCl solution (14mg / mL), stirred at room temperature for 10min, 400μL of NHS solution (13.4mg / mL) was added and incubated at room temperature for 1h at 90rpm. 31.2mg of guanidine salt was added, and the mixture was magnetically stirred at room temperature for 24h. The resulting solution was dialyzed with deionized water (MW=14kDa) for 72h to remove residual guanidine salt, thereby obtaining bacterial cellulose-guanidine salt crosslinked product.

[0038] Synthesis of Gallic Acid-Loaded Bacterial Cellulose-Guanidine Membrane (OBC-PHGH / GA)

[0039] To the OBC-PHGH dispersion obtained above, 0.05wt.%, 0.1wt.%, 0.2wt.% gallic acid solution were added respectively, and the film was formed by suction filtration at a pressure of -0.1MPa. After the water was completely removed, an iron block (2.3kg) was used to press at room temperature for 48h to obtain an OBC-PHGH / GA film.

[0040] Morphological observation of synthesized OBC-PHGH / GA

[0041] The OBC-PHGH / GA obtained above was observed by cold field emission scanning electron microscopy. Figure 1 As shown, OBC-PHGH / GA has a good 3D network structure and the fibers are tightly bound.

[0042] Water vapor permeability of OBC-PHGH / GA

[0043] The water vapor permeability of OBC-PHGH / GA membrane was tested using a water vapor permeability tester (PERME-W3 / 060, Labthink Instrument Co., Ltd., China). The results showed that the water vapor permeability of OBC-PHGH / GA membrane was 17.11 g / m 2 ·day.

[0044] Mechanical properties of OBC-PHGH / GA

[0045] The mechanical properties of OBC-PHGH / GA membrane were analyzed using a universal tensile machine (Shimadzu, AGS-X, Japan). Figure 2 As shown. The results show that with the increase of GA content, the fracture stress of OBC-PHGH / GA increases. When the GA content is 0.2wt.%, the fracture stress of OBC-PHGH / GA is the largest, which is 55.81MPa, and the elongation at break is 3.60%. Therefore, OBC-PHGH / GA has good mechanical properties.

[0046] Antioxidant capacity of OBC-PHGH / GA

[0047] (1) DPPH free radical scavenging experiment

[0048] Immerse the 3cm×3cm OBC-PHGH / GA film in 3mL DPPH methanol solution (5mM) and react for 30min at room temperature in the dark. Measure the absorbance of the mixed solution at 517nm using a UV spectrophotometer, use DPPH methanol solution as a control, and calculate the DPPH free radical scavenging rate using the following equation:

[0049] DPPH free radical scavenging rate % = (control absorbance - sample absorbance) / (control absorbance) × 100% Formula (1)

[0050] (2)ABTS + Free radical scavenging experiment

[0051] ABTS solution (7 mM) was mixed with an equal volume of potassium persulfate solution (2.45 mM) and incubated at 4°C in the dark for 12-16 h to form a molecule containing ABTS free radicals (ABTS +) stock solution, and then dilute it with ethanol until the absorbance at 734nm is 0.7±0.02 to obtain ABTS working solution. Similar to the DPPH free radical scavenging method, 3cm×3cm OBC-PHGH / GA film was immersed in 3mL ABTS working solution and reacted at room temperature in the dark for 5min. The absorbance of the mixed solution at 734nm was measured by UV spectrophotometer, and ABTS working solution was used as a control. The ABTS free radical scavenging rate percentage was calculated using the following equation:

[0052] ABTS free radical scavenging rate % = (control absorbance - sample absorbance) / (control absorbance) × 100% Formula (2)

[0053] The results are as follows Figure 3 As shown in the figure, with the increase of GA content, the antioxidant property of OBC-PHGH / GA film becomes stronger. When the GA content is 0.2wt.%, the antioxidant property of OBC-PHGH / GA film is the best. + The free radical scavenging rates were 92.1% and 99.9%, respectively. Therefore, the OBC-PHGH / GA composite film with GA content of 0.2wt.% is a potential antioxidant active packaging material.

[0054] Antibacterial properties of OBC-PHGH / GA

[0055] Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were used as representatives of Gram-negative bacteria and Gram-positive bacteria, respectively, to calculate the antibacterial activity of the samples. 8 cfu / ml) was dripped on the sample surface and cultured at 37℃ for 24h, then washed thoroughly with sterile saline by vortexing, and the washing solution was diluted to an appropriate multiple and applied to LB medium. Three replicates were made for each dilution, and the samples were cultured in a 37℃ incubator for 24h before counting. The results are shown in Figure 4 As shown, the killing rates of OBC-PHGH / GA against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were 100% and 99.9%, respectively.

[0056] Degradability of OBC-PHGH / GA

[0057] 45mg OBC-PHGH / GA membrane was cut into small pieces (0.5cm×0.5cm) and placed in 27mL HAc / NaAc buffer (50mm, pH=4.8). Then 3mL crude cellulase complex was added to the buffer and incubated at 50℃, 200rpm for 12h. The commercial cling film was used as the control group to observe the degradation of OBC-PHGH / GA membrane. The results are shown in Figure 6 As shown, OBC-PHGH / GA was almost completely degraded after 6 h of cellulase treatment, while the commercial plastic wrap was still in square fragments, indicating that the commercial plastic wrap could not be degraded.

[0058] Test on the preservation effect of OBC-PHGH / GA on strawberries

[0059] Select strawberries of uniform maturity and without damage or mold as experimental raw materials and store them in plastic containers. Cover the prepared OBC-PHGH / GA composite film on the above plastic container, and use a control group without any film. Store at 25°C for 5 days. Weigh and photograph the samples every 24 hours. The weight loss rate is the ratio of the weight lost by the strawberry to the total weight of the strawberry.

[0060] Take 4g of strawberries on the 5th day and mix with 2mL of normal saline to make a homogenate. Dilute the strawberry homogenate to an appropriate multiple, spread 100μL of the diluted solution on LB agar plates and PDA agar plates respectively, and take photos after incubation at 37℃ for 24-48 hours. The results are shown in the figure. On the 5th day of storage, the strawberries in the OBC-PHGH / GA group lost 1.56g, which is much lower than the unpackaged group (3.67g). The unpackaged strawberries had obvious rot and microbial damage on the surface after 5 days of storage ( Figure 7 ). However, the strawberries in the OBC-PHGH / GA group showed good color after 5 days of storage, and there was no rot and microbial loss on the surface ( Figure 8 ). Control group LB plate ( Fig. 9 ) and PDA plates showed a large number of colonies ( Fig.10 No bacterial colonies were found in the OBC-PHGH / GA group ( Fig.11 ), only a few fungal colonies ( Fig.12 ). This indicates that OBC-PHGH / GA membrane can effectively inhibit microbial contamination and extend the shelf life of strawberries.

[0061] The above examples are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit and essence of this invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a food packaging film, characterized in that: The bacterial cellulose was covalently cross-linked with guanidine salt to obtain the covalent cross-linked product OBC-PHGH, and gallic acid was added to the cross-linking solution to obtain the guanidine salt functionalized bacterial cellulose based film OBC-PHGH / GA.

2. The method for preparing the food packaging film according to claim 1, characterized in that: The bacterial cellulose is obtained from the culture broth of Acetobacter xylinum.

3. The method for preparing the food packaging film according to claim 1, characterized in that: According to the following ratio, the steps include: (1) suspending 1 g of bacterial cellulose film in 1 L of deionized water, adjusting the pH value of the solution to not less than 10 with NaOH, then adding 2,2,6,6-tetramethylpiperidinium oxide (TEMPO), sodium bromide (NaBr) and sodium hypochlorite (NaClO) in sequence, and stirring at room temperature for 1 hour to obtain a 0.65wt.% oxidized bacterial cellulose dispersion (OBC) after dispersion; (2) dispersing the above oxidized bacterial cellulose (OBC), 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl), and N-hydroxysuccinimide (NHS) in deionized water at room temperature and 100-1000 rpm for 1-3 hours, adding guanidine salt and reacting at room temperature and 100-1000 rpm overnight; (3) mixing gallic acid with the above bacterial cellulose-guanidine salt grafted dispersion, filtering, and drying to obtain an OBC-PHGH / GA film.

4. The method for preparing the food packaging film according to claim 3, characterized in that: The reaction was carried out at 100-1000 rpm for 1 h.

5. The method for preparing the food packaging film according to claim 3, characterized in that: The guanidine salt is replaced by an antibacterial substance with an amino group, and the antibacterial substance is an antibacterial peptide, chitosan or aminoglycoside antibiotics.

6. The method for preparing the food packaging film according to claim 3, characterized in that: The amount of gallic acid added does not exceed 1 wt.% of the reaction system.

7. The method for preparing the food packaging film according to claim 6, characterized in that: The added amount of the gallic acid is 0.2 wt.%.

8. Food packaging film obtained by the method according to any one of claims 1 to 7.

9. Use of the food packaging film according to claim 8 in the preparation of antibacterial, antioxidant and food packaging.

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